The Khronos Group keeps the pace that they set themselves being able to deliver the latest specification of OpenGL less than half year after the revolutionary appearance of OpenGL 4. Abandoning the OpenGL 3.x line of the specification (at least for a while) the new update concentrates on Shader Model 5.0 class GPUs and extensions heavily promoted by the community. Beside all this, the Khronos Group now confessedly opens towards convergence to OpenGL ES making the desktop version of the specification downward compatible with its embedded brother. In this article I would like to present the features introduced with the latest revision of the specification.

At the time of the release of the OpenGL 4 specification I was able to quickly deliver you a thorough presentation of all the new features introduced by that revision of the specification. This time I am already quite late, however I hope that this article will still prove as value for lots of you, especially for those who haven’t had time in the recent past to dig into the details of the new API version.

OpenGL 4.1 is not as revolutionary and feature-rich as its predecessor, however the latest revision was well received by the community as it brought such core extensions to the API that the community was waiting for a long time now. The new revision of the specification was accompanied with the appearance of a couple of other ARB extensions that have not yet been included into core, however I will still talk about some of them as they indicate a slight shift in the force of influence of various vendors and representatives inside the Architecture Review Board (ARB).

New features of OpenGL 4.1

Let’s start with the presentation of the new features arriving with the OpenGL 4.1 specification primarily targeting Shader Model 5.0 hardware. Here you will see a lot of harmonization features as well as community’s choice features that squarely intended to increase OpenGL development efficiency and feedom.

ARB_ES2_compatibility

There have been for a long time rumors about the Khronos Group preparing a convergence between desktop OpenGL and OpenGL ES. This extension of the core specification clearly makes the first step towards this goal by providing an all-in-one specification pack that makes the desktop version of the specification downward compatible with ES. The extension adds support for features of OpenGL ES 2.0 that are missing from OpenGL 3+. According to the extension specification, enabling these features will ease the process of porting applications from OpenGL ES 2.0 to OpenGL.

More precisely, GL_ARB_ES2_compatibility exposes not just all the functions and tokens that weren’t present in the desktop version of the specification but also completes it with all the semantics that were exclusively specified only in the embedded version. Just to mention few of these issues:

• Vertex data format is now extended with the possibility to use 16-bit fixed point values by exposing the GL_FIXED type identifier token.
• Providing possibility to query the precision format used internally by shaders.
• Enable the use of GLSL ES for writing shaders for desktop GL.

While having this extension under the hood does not mean that we can simply pick our last game made for e.g. Symbian and just drop it on our PC, this extension may prove to be great value for GL ES developers migrating their software to desktop platforms.

ARB_get_program_binary

This is one of the most waited additions to the core specification by the developer community. This extension introduces the possibility to acquire some sort of binary format of the compiled and linked shaders that can be later used to specify the program object directly with its binary code thus providing caching possibility to eliminate the need of compilation and linking next time the shader has to be used. This also makes it possible to create an offline GLSL compiler just using the OpenGL API itself.

Still, it has to be mentioned that having this feature in our hand does not necessarily mean that we can simply create our shader binaries offline and then distribute our software without the shader source itself as the binary formats supported by a particular implementation heavily depend on the hardware vendor as well as driver version. This is due to the fact that the shader binary most probably consists of instructions specially generated for the particular GPU-driver combo. The only way to relax this limitation would be to have some sort of cross-platform byte-code for shaders but that would in fact defeat most of the benefits of the extension on its own. Additionally, this extension does not provide any binary formats but leaves this to vendor specific extensions. It only exposes a common infrastructure for acquiring and loading program binaries.

While the usage of this extension does not completely eliminates the need for shader source compilation, it can limit the need for recompilation and relink to an installation time or first-run time compilation instead and use the stored binaries later. It also opens up room for SDK tools providing shader compilers with more aggressive optimization at their disposal being used offline. Such tools can truly be introduced as the specification explicitly mentions that run-time generated binaries by the GL should be interchangeable with those generated by offline SDK tools.

ARB_separate_shader_objects

This is one another extension requested over several forums by the community. This feature has a longer history as it is actually based on the already existing and widely supported extension GL_EXT_separate_shader_objects by NVIDIA. For those who are already familiar with the predecessor of this extension won’t really find too much new stuff reading the specification of the ARB version of the extension, however it is still a must to read for them as well as even though there aren’t too much semantic differences between the functionality of the two, the usage of them still differs quite a lot as the ARB version solved the design issues of its predecessor by introducing a new type of GL object that I will talk about just in a moment.

In a nutshell, this extension provides a way to create program objects using any variation of shaders and bind them together to the current rendering context. Previously there was no way to bind multiple program objects to the context as the program object was designed to be a container for all the shaders forming the rendering pipeline of the context. This was a design decision during the development of GLSL that, before this extension, made the connection between the varyings of subsequent shader stages using a name based binding. As name information is available for shaders latest in the link stage, shaders were tightly coupled meaning that a change in any shader stage code required the relinking of the complete program object.

This proved to be very unpleasant for OpenGL developers as usually every rendering engine has its own set of vertex and fragment shaders (maybe accompanied with other shader types) that are used in various combinations. As an example, let’s take two vertex shaders: a simple MVP matrix based transformation shader and a more complex one that also supports skeletal animation. Also let’s take two fragment shaders: one for diffuse material and one for reflective material. We can have several types of objects: static with diffuse material, static with reflective material, animated with diffuse material and animated with reflective material.

In traditional GLSL the vertex and fragment shaders are bound together at link time rather than at the time they are bound to the context, like it was in case of legacy shaders (GL_ARB_vertex_program, GL_ARB_fragment_program and others). This means that in order to be able to use any of the combinations of vertex and fragment shaders (and maybe some geometry and tesselation shaders as well) we end up with two possible solutions, both having their severe drawbacks:

Link every combination of the shader objects

While this sounds as a viable solution and is still used by most of the developers, it has several problems. First of all, it wastes resources as we now have several copies of the same piece of code and the number of combinations can be pretty high, especially if not just vertex and fragment shaders are in use. While this is already quite a reasonable issue with the solution, the biggest problem arises for the application developer when he or she has to maintain an individual set of uniform locations as well as binding points for vertex attributes, draw buffers and possibly transform feedback buffers. While the GL_ARB_explicit_attrib_location extension already eliminates the need for maintaining binding points for vertex attributes, this solution is still simply unacceptable.

Link the program objects on an on-demand basis

In case of this alternative we are said to link the shader objects only when they are actually needed. While this solution eliminates the need for a possibly huge number of program objects, it introduces a reasonable run-time performance hit due to the additional relink process needed. Additionally, this solution proves to be more inferior even compared to the previous one as the uniform locations are determined at link time so it makes no less headache to the application developer.

This is the rationale behind this extension and why it is included into the core specification. The extension relaxes the strict tightly coupled behavior of the GLSL and adopts a mix-and-match shader stage model allowing multiple different program objects to be bound at once each to an individual set of rendering pipeline stage independently of other stage bindings.

Due to the fact that from now program objects are not the top most containers for the code used currently by the rendering pipeline, the ARB decided to introduce a new container object called a “program pipeline object” that can contain a set of program objects bound to their very own set of shader stages. This is the main difference between the EXT and the ARB version of the extension. I think it was a good decision to introduce this new type of object and the associated semantics as I always thought that the EXT version of the extension doesn’t have a really good design as I’ve seen it kind of a hack to relax the limitations of GLSL. The program pipeline object idea is definitely superior and I hope that the GLSL does not have too much of such annoying design issues hidden within.

ARB_shader_precision

This extension is much more a clarification to the existing specification rather than a new feature. It restricts more clearly the precision requirements of implementations of GLSL. According to the specification, the extension is meant to more precisely define the precision of arithmetic operations (addition, multiplication, etc.), transcendentals (log, exp, pow, etc.), when NaNs (not-a-number) and INFs (infinites) will be accepted and generated and denorm flushing behavior. The precision of the rest of the operations, including trigonometric operations are not addressed by the extension. For further details, please refer to the extension specification.

ARB_vertex_attrib_64bit

This extension trivially introduces 64-bit floating-point types into the list of supported vertex attribute component types. Nominally OpenGL did support this already from the very early stages of its history, however in practice only the latest generation of hardware does really accept vertex attributes in double precision floating-point type. While OpenGL 4 already introduced support for 64-bit floating-point values in GLSL and most of the shaders’ environment, vertex attributes gained the 64-bit precision only with this new extension.

This new feature makes it possible to use high precision for positioning data and other attributes of our geometries. While this sounds pretty awesome and it is actually, still for game developers and other real-time graphics users this shouldn’t mean that they should quickly switch to the new precision only in such cases when the precision requirements of the application really need it as using 64-bit floating-point values for vertex attributes does not just double the memory consumption but also involves a serious hit on performance due to bandwidth limitations and vertex attributes of this type may count double against the implementation-dependent limit on the number of vertex shader attribute vectors.

ARB_viewport_array

Previously, the configuration of the viewport, aka the transformation that generates the screen space coordinates based on the incoming view space coordinates of the vertices, was a global configuration that had effect on all draw commands meaning that in order to draw a primitive into multiple viewports the OpenGL viewport had to be changed between several draw calls. While previously this limitation wasn’t really an issue, due to the introduction of geometry shaders the possibility to amplify geometry and produce multiple output primitives for each primitive input justifies the need of several separately configurable viewports. Why? Because even though one was able to render the output primitives into separate render targets, they still shared the same global viewport.

This extension enhances OpenGL by providing a mechanism to specify multiple viewports and a new ability for the geometry shader being able to select the used viewport on a per-primitive basis. This does not just mean that separate viewports can be used for separate render targets but also enables to use multiple viewports to render to the same render target.

Additionally, the introduction of a viewport array means that we’re gonna have separate scissor rectangle for each viewport in the array as well. This can come handy for deferred shading based renderers that often use the scissor rectangle to limit the number of pixels to be accessed in case of rendering the effect of a light source. Having multiple scissors means that we have to change state less often, thus batching is much less an issue even in case of heavy scissor rectangle usage.

Finally, the new viewport specification commands accept floating point values thus providing additional flexibility to the application developer to define their very own pixel center conventions.

I’m pretty unsure whether this feature depends on any Shader Model 5.0 hardware, maybe others are more aware of this. Anyway, I wouldn’t be surprised if this extension will be supported by a much larger range of graphics cards than just pure SM5 GPUs. Actually this is true for many other extensions introduced by OpenGL 4.1 but let’s not guess but wait for the upcoming drivers to see whether I’m right or wrong.

Some other interesting extensions

So far I presented the new features of the latest revision of the OpenGL specification. While this was the main topic of this article, at about the same time the specification was published, a lot of other ARB extensions just appeared in the registry. While these extensions are not yet included into core and I cannot know whether they will be ever included, I would like to talk about some of them as it made me get to an interesting conclusion.

ARB_shader_stencil_export

The stencil test is a powerful mechanism of OpenGL to selectively discard fragments based on the content of the stencil buffer that is used in a wide variety of rendering techniques including shadow volumes and deferred shading. However, the whole configuration of the stencil test and stencil operations is completely fixed function that is limited to operations such as incrementing, decrementing the existing value, or replacing the existing value in the stencil buffer with a fixed reference value.

This extension provides some programmability to the fixed function stencil operations by enabling the fragment shader to output a stencil reference value on a per-fragment basis. When stencil testing is enabled, this allows the test to be performed against the value generated in the shader. Also, when the stencil operation is set to GL_REPLACE, this allows a value generated in the shader to be written to the stencil buffer directly.

This opens up a lot of possibilities, however, I need to think much more about it as the best use cases of this feature are pretty much not basic ones. Obviously, by using the stencil reference value export inside a fragment shader disables early stencil test in the same style as exporting an new depth value from within a fragment shader disables early depth test.

ARB_debug_output

This extension allows OpenGL to notify the application when various events occur that can come handy during application development and debugging. These events include errors, usage of deprecated functionalities, using configuration that results in undefined behavior, portability or performance issues. The application is notified about these events using a callback function that is defined by passing a function pointer to the appropriate OpenGL command.

While this extension provides a callback mechanism only for debugging purposes, the most revolutionary thing by having such an ARB extension is that this is the first official appearance of a feature that supports callbacks to the application code. Most probably not I’m the only person who would like to see a lot of other callbacks in the future included in the OpenGL API as we can benefit from it by getting notification about e.g. the completion of various asynchronous commands issued previously. This does not just provide a lot of flexibility but may also help in optimizing the rendering code based on the additional information previously available only if we use polling.

Why these extensions are so interesting?

The two extensions presented above already great value on their own but this isn’t why I mentioned them. The reason why I found these extensions so interesting as they are both obviously based on some vendor specific extensions released in the recent past by AMD, namely GL_AMD_shader_stencil_export and GL_AMD_debug_output. This conspicuously reveals that AMD has serious plans with their OpenGL support and this is something that a lot of those crazy folks waited for, who develop OpenGL stuff using ATI cards like me.

I think this also means that the NVIDIA monopoly in the ARB is over and this results in concurency and competition from what OpenGL and its community will definitely benefit in the long run.

Conclusion

The article ran out of control again, like the one I wrote about the previous release of the specification. Again, hope there are at least a few of you who kept up reading and finally got to this last chapter of the article. We can again quote the always recurring question of the community:

Where is direct state access?

Well, it is still not here, however, finally AMD has finished implementing it as well and published it finally. They have been working on it for quite some time but it became officially public only with Catalyst 10.7. Haven’t used it so far so maybe plenty of hidden bugs are still in it but at least they have it. This is one another thing that strengthens my prognostication that AMD committed itself for support OpenGL as previously they barely added support for any other extensions beside core features.

Back to the topic of the OpenGL 4.1 specification, while it is not as revolutionary as we got used to after reading the previous update, OpenGL is still on track and this is thanks to the Khronos Group and obviously to the great community. If OpenGL will get its iterative evolution in this pace like we’ve seen in the last two years, Microsoft will have a difficult time to keep up.

Thanks for reading this not-so-short article!

OpenGL 3.3 - Nature

A few months ago I’ve presented an object culling mechanism that I’ve named Instance Cloud Reduction (ICR) in the article Instance culling using geometry shaders. The technique targets the first generation of OpenGL 3 capable cards and takes advantage of geometry shaders’ capability to reduce the emitted geometry amount in order to get to a fully GPU accelerated algorithm that performs view frustum culling on instanced geometry without the need of OpenCL or any other GPU compute API. After the culling step the reduced set of instance data is fed to the drawing pass in the form of a texture buffers. In this article I will present an improved version of the algorithm that exploits the use of instanced arrays introduced lately in OpenGL 3.3 to further optimize it.

Lets recap the basics of the algorithm before I present the improved technique. The geometry shaders have a very nice feature that they cannot just emit a modified version of the input geometry but can also alter the number of emitted primitives compared to the number of received ones. This is a both-way ability what means that we cannot just increase but also decrease the number of primitives. That is what the technique takes advantage.

In the first pass we feed a simple vertex shader – geometry shader pair with the instance data of the geometries as they’ve been the data of point primitives. The vertex shader then checks whether the actual instance is inside the view frustum or not and sends the result to the geometry shader. If the result is yes then the geometry shader outputs the instance data otherwise discards it. The primitives emitted by the geometry shaders are captured then using transform feedback into a buffer object. Also a query object is needed in order to be able to get the amount of instances that passed the view frustum culling. In the drawing pass we use the result of the query to decide how many instances we have to draw and the captured feedback buffer is used as instance data.

Instance Cloud Reduction - Combined view of Pass 1 + Pass 2

This is a very brief description of the culling mechanism so for a complete specification please read the original article.

Motivation

While Instance Cloud Reduction is a quite robust technique that can severely simplify and speed up the rendering of high amount of instanced geometry its performance is also limited due to some hardware and API restrictions. The most important ones are the following:

• Needs an extra rendering pass to perform the culling.
• Requires the usage of asynchronous queries to determine the number of visible instances.
• Uses texture fetching in the vertex shader of the actual drawing pass.

The first mentioned drawback means that more draw commands are required that use the output of the first pass as input. This and the second disadvantage may cause stalls due to the fact that the CPU has to wait for the data to be ready before issuing the second pass thus the GPU is not used effectively.

What this improvement tries to solve is the third problem. Texture fetching itself is quite fast in the latest generation of hardware, however it causes some slowdowns anyway due to the latency introduced by texture fetches even though GPUs use some latency hiding techniques.

Instanced arrays provide us a way to replace texture fetching with vertex fetching that is usually done by different hardware element that works synchronously with the execution of vertex shaders. I’ve expected quite a reasonable speedup by taking advantage of instanced arrays, however we will see that actual results were far from my initial expectations.

Implementation

Traditional vertex fetching happens in a way that one element is fetched from each enabled input attribute buffer and the vertex shader is issued with these values. One element in a vertex attribute buffer can mean up to four floating point or integer values and for each execution of the vertex shader one set of these elements is used. There is an internal counter that is increased after each fetch and the next vertex attribute fetch will use this counter as an index into the buffer object.

While this mechanism is satisfactory for the most attributes of a vertex, it is not practical for instance data as such data belongs to an instance rather than a vertex. In order to source instance data from vertex attributes in case of traditional vertex fetching, high amount of redundant storage is required in order to get the same information for all the vertices belonging to a particular instance. This is not just waste of memory but also waste of bandwidth and it also defeats the goal of Instance Cloud Reduction.

Compared to traditional vertex fetching, instanced arrays provide a way to increase the internal counter used as the index into the vertex attribute buffer in a different way, in particular one can set the frequency of increase using a vertex attribute divisor that specifies after how many instances the counter shall be increased. This is a per-attribute property and by setting it to one we end up with exactly what we need: one vertex fetch per instance.

This means that actually we need just a very minor change compared to the original technique, more precisely we replace our texture buffer with a vertex attribute buffer that has a divisor of one and use it as the source of instance data in the vertex shader of the drawing pass.

Execution results

As we are not talking about a new technique but just an optimized implementation of the same method, the best way to evaluate it is by comparing the performance of the new version with the original one.

As I’ve mentioned earlier, I expected a reasonable performance increase by replacing texture fetches with vertex fetches, in practice the difference was not so significant. However, the performance difference between the two implementation can heavily depend on the underlying hardware implementation so various cards from various vendors and GPU generations can show more diverging behavior. In fact even driver versions may have an effect on the results.

Performance comparison of the old implementation and the presented one on an AMD Radeon HD5770. Scale is in frames per second (higher value is better).

Due to lack of hardware to use for testing, I’ve checked only with one card, namely a Radeon HD5770 with Catalyst 10.6 drivers. I noticed roughly a 10% speedup as the the new version of the Nature demo showed 100 FPS compared to the 90 FPS observed with the old implementation.

Even though this was not exactly the outcome I’ve expected from the new implementation, maybe the assumption is still valid for older generation of GPUs or for NVIDIA cards. I suspect so because for Shader Model 4.0 cards the hardware implementation of the texture fetching unit and the vertex fetching unit was most probably more differentiated than that of the latest GPUs. Also my guess is that on NVIDIA cards the difference is maybe higher as the vertex fetching hardware in SM 4.0 GeForce cards is less flexible than that of AMD’s taking in consideration that the first HD series Radeons already had some form of tessellation functionality that requires more freedom from the vertex pushing hardware.

In order to get a better picture about how effective the presented optimization is, I would like to ask all the visitors of this post to try the two releases and send me feedback about it.

Conclusion

We’ve seen that how easy it was to take advantage of instanced arrays in an existing implementation of the ICR technique and how does it perform on the latest generation of GPUs compared to the previous version. While this small addition provides some benefits, it also comes at a cost and we have to talk about that as well.

Advantages:

• Eliminates the need for texture fetching in the vertex shader thus improving performance.
• Does not compromise the goal and the implementation architecture of the original method.
• Frees up one texture unit that was previously reserved for the texture buffer containing the instance data.

Disadvantages:

• Requires OpenGL 3.3 or the GL_ARB_instanced_arrays extension in addition to the OpenGL 3.2 features.
• We have to possibly sacrifice multiple vertex input attributes to feed the instance data to the shaders.

Most of the mentioned benefits and drawbacks are self-explanatory, however I would like to say a few words about the last mentioned one…

For the purpose of showcase I used a simple translation factor as instance data that means a single vector of floats. In real life situation one may need more complex transformation data that can only be stored in the matrix. While in the demo the feeding of instance data consumed only one vertex attribute slot, in case of a full transformation matrix it would require four of them (not to mention other possible instance attributes). As the maximum number of input attributes is severely limited, usually to 16, the application of the optimization is restricted to situations when all the vertex and instance attributes fit into this limit.

In case of the original implementation, where a texture buffer was used as input, this did not cause any problem as the vertex shader is free to fetch any number of texels from that (still, performance can be a concern in this case). In order to help situations when input attribute slots are at a premium, in real life scenarios it is recommended to use quaternions instead of transformation matrices as they consume two times less attribute resources. Actually this can be a general recommendation as using quaternions decreases the bandwidth requirements of the instance data fetch thus increasing performance even in situations when there are enough input attribute slots available.

In order to ease the performance comparison for you, you can find download links for both versions of the Nature demo.

Old version binary release

Platform: Windows
Dependency: OpenGL 3.2 capable graphics driver
Download link: nature12_win32.zip (3.58MB)
Comments: This version does NOT include the optimization presented in this article.

Old version source code

Language: C++
Platform: cross-platform
Dependency: GLEW, SFML, GLM
Download link: nature12_src.zip (12.6KB)
Comments: This version does NOT include the optimization presented in this article.

New version binary release

Platform: Windows
Dependency: OpenGL 3.3 capable graphics driver
Download link: nature20_win32.zip (3.58MB)
Comments: This version includes the optimization presented in this article.

New version source code

Language: C++
Platform: cross-platform
Dependency: GLEW, SFML, GLM
Download link: nature20_src.zip (12.8KB)
Comments: This version includes the optimization presented in this article.

The Khronos Group continues the progress of streamlining the OpenGL API. One very important step in this battle has been made just a few days ago by releasing two concurrent core releases of the OpenGL specification, namely version 3.3 and 4.0. This is a major update of the standard containing many revolutionary additions to the tool-set of OpenGL that need careful examination. In this article I would like to talk about these new features trying to point out their importance and touching also some practical use case scenarios.

This is the fourth revision of the OpenGL API standard in the last two years. This fast pace revolution started about one and half years ago with the release of the version 3.0 of the specification. At that time, a great feel of disappointment has overcame the developers due to the lack of the promised rewrite of the whole API. Others, who had to deal with legacy code were also disappointed but they felt so because the new revision of the API threatened them with removing old features. These two opposing forces have put the Khronos Group into a situation where there was very difficult to make a decision that would make everybody happy. After two releases, this issue has been mostly resolved with OpenGL 3.2 and also lots of missing features have been integrated into the core API meanwhile.

Even though great steps has been made in order to fulfill everybody’s needs, the gap between the core functionality of OpenGL and the DirectX API still increased, especially due to the introduction of Shader Model 5.0 hardware. OpenGL was in a position when it had to adopt the features of the new hardware generation and also try to make up leeway in case of Shader Model 4.0 hardware. My personal wish was that there should be two new versions of the API: one that complements the OpenGL 3.x API with the missing features and another that catches up to DirectX 11. Actually my wish became true as the first time in the history of OpenGL we got two new releases of the standard at once, and finally, we got an API that is a really competitive alternative for Microsoft’s DirectX API. I think I can say this in the name of every OpenGL developer: Thank you Khronos!

Okay, but that’s enough about history and acknowledgements. Lets see what’s under the hood of the new API revisions! When I read the good news at OpenGL.org I felt myself like a child at Christmas just taking the first look at the presents under the tree: I was in great ecstasy and started to “open the presents” as fast as I could…

New features of OpenGL 3.3

Let’s start with the new version of the API targeting Shader Model 4.x hardware. It seems that the concentration on the major release 4.0 didn’t capture the attention of the ARB explicitly as we have many interesting features already in the first box…

ARB_blend_func_extended

This is a feature for what I’ve seen many requests on the OpenGL discussion forums. It enables fragment shaders to output an additional color per render target that can be used as a blending factor for either source or destination colors providing an additional degree of freedom to affect the way how fragments are blended into the destination buffers. This is one functionality that is supported by the underlying hardware for a while but without API support it was impossible to take advantage of it. As it is very straightforward how this feature works I would not even talk about it too much. Just one additional comment: surprisingly AMD already supports this extension in its latest graphics drivers which is a remarkable thing taking in consideration that AMD drivers were always a step behind the NVIDIA ones in the race of adopting latest OpenGL features. It seems that now AMD takes seriously the OpenGL support and this is good news for all the developers out there, especially for me, being an ATI fan.

ARB_explicit_attrib_location

Most probably not just for me, the way how the binding of vertex attributes to shader attributes and the binding of shader outputs to render targets happened earlier caused a big headache from both the point of view of modular software design and efficiency. Previously, the application developer had little to no control over how to automatically connect these elements together in a shader independent way. This tight coupling between the host application code and the shaders just make the work of the developers cumbersome. This feature leverages the way how this binding process is done by allowing to globally assign a particular semantic meaning to an attribute location without knowing how that attribute will be named in any particular shader, decoupling the host application from the shaders. This extension is a typical example how design abstractions can ease the life of the developer without any dependency on hardware support.

ARB_occlusion_query2

Well, there isn’t too much to say about this extension as it just adds a new occlusion query type that reports just a boolean value about the visibility of the object rather than the actual samples. It is somewhat equivalent to the occlusion query extensions prior to ARB_occlusion_query. Don’t ask me why this feature is important but they felt that it might be useful. One thing I can think about that with such a query we might get our results about the occlusion query of the proxy object sooner as we have to wait only till the first passed sample but I’m not confident whether such thing is supported by either the hardware or the drivers.

ARB_sampler_objects

This is one another feature that people have been waiting for years. This extension decouples texture image data from sampler state. Previously, if a texture image had to be used with different sampler modes, no matter if we talk about various filtering modes or texture coordinate wrapping, one had to do expensive state changes to modify the sampler state of the texture object, accomplish the needed filtering or wrapping from within shaders or, in worst case, duplicating texture image data in order to have access to the same texture with different sampler parameters. The primary intend of this feature is to solve these problems.

One thing to remark regarding to this extension is that even though it is a long waited addition to the API, several people already expressed their discontent regarding to the fact that the texture unit semantics have been kept. Nevertheless, I also expected that the introduction of this feature should be the point when the texture unit semantics has to go but after seeing the example of ARB_explicit_attrib_location as a way to decouple the shader code from the host application code I tend to agree with Khronos in this decision as we can think about the texture units from now as an adapter layer between GPU and CPU code and as such the decision seems reasonable.

ARB_shader_bit_encoding

This extension adds built-in functions for getting and setting the bit encoding for floating-point values in the OpenGL Shading Language. As it is more like an indicator extension regarding to added functionality in the Shading Language I would rather not go into details as I will talk about the new Shading Language later.

ARB_texture_rgb10_a2ui

Again, an extension that is quite self-explanatory: new texture image format called RGB10_A2 with non-normalized unsigned integers in them. This is nothing more than another hole filled in the gap between hardware and API support.

ARB_texture_swizzle

Especially when using one or two component texture formats, like in the case of shadow maps, the specification was somewhat unclear how these components are finally mapped to RGBA quadruples and provided little to no facilities to control this process. If the developers weren’t already fed up with this, the possibility of a problem increased even further because often the driver implementations behaved differently as well. This issue has been finally clarified with this extension by providing an explicit tool for the application developer to control the swizzling of the components that is done implicitly afterwards in case of every single texture fetch. The new state is introduced as part of texture object state that provides fine grained control over when and how to use the swizzling. According to the extension specification, this feature has a notable role in helping porting issues of legacy OpenGL applications as well as those of the games written for PlayStation 3 as the console provides such functionality already.

ARB_timer_query

Prior to this extension, runtime performance measurements were limited to the use of client side timing information or relying on the use of offline profiling mechanisms like that of AMD’s GPUPerfStudio. During development, this timing information can help identify application, driver or GPU bottlenecks. At runtime, this data can be used to dynamically optimize the scene to achieve reasonable frame rates. While today’s hardware provides a great repertoire of performance measurement metrics there was no API support to access these previously. This feature provides an additional asynchronous query type that enables application developers to measure the driver and GPU time that is required to complete a set of rendering commands, thus providing additional flexibility for both offline and runtime optimizations. While this extension does not guarantee 100% consistency and repeatability, the information gathered with timer queries will definitely make it possible to identify server side bottlenecks and the reasons behind them.

ARB_instanced_arrays

Many people argued with me at the OpenGL discussion forums when I stated that instanced arrays should be included in core OpenGL. Their reasoning was built on the fact that we already have the ARB_draw_instanced extension that provides a shader based thus much more flexible way to handle instanced geometry. While from this point of view I tend to agree with them, there are many non-trivial use cases which prove that my reasoning is not pointless. It seems that Khronos agrees with me regarding to this topic.

In a nutshell, the instanced arrays feature enables the use of vertex attributes as a source of instance data. This is done by introducing a so called “array divisor” that specifies how the corresponding vertex attributes are mapped to instances. Usually a vertex attribute advances on a per-vertex basis. In case of instanced arrays this advance happens only after every Nth conceptual draw calls that is equivalent to  a traditional draw command, excluding instanced draw commands.

One use case can be when one deals with huge number of instances where the per-instance data simply not fits into uniform buffers. While in such cases one can use a texture buffer instead to source the instance data like it was mentioned in my article Uniform Buffers VS Texture Buffers, accepting the additional overhead of using texture fetches may prove to be a not-so-performance-wise decision. Beside standard instancing use cases, there are plenty of nasty tricks that can be efficiently achieved using this feature but that goes far beyond the scope of this article and requires a separate discussion on what I will most probably recap in the near future.

ARB_vertex_type_2_10_10_10_rev

We’ve arrived to the final new extension included in core OpenGL 3.3. This is another gap filling extension to provide two new vertex attribute data formats: a signed and an unsigned format with 10 bits for each significant coordinate. The most typical use of this format is to store vertex normals in the signed-normalized version of the format in order to have a compact (4 bytes per normal) yet high precision (due to 10 bits per component) format that can reduce memory needs and bandwidth requirements while retaining sufficient precision. Previously, there was no way to have such high precision for the vertex attributes in case of a 4-byte footprint.

The OpenGL Shading Language 3.30

The first remarkable thing is the shift in the versioning of the Shading Language. It seems that from now it will be in align with the core specification version. This decision was most probably made because of the introduction of two release branches of the standard specification in order to avoid confusion regarding to the correspondence between API and Shading Language versioning.

As in case of talking about the OpenGL Shading Language it is much more difficult to easily summarize the new features with corresponding use cases I will simply limit my comments to an excerpt from its specification regarding to the features added in this new version:

• Layout qualifiers can be used to declare the location of vertex shader inputs and fragment shader outputs in align with the API functionality provided by ARB_explicit_attrib_location as mentioned before.
• Built-in functions provided to converting floating-point values to integer ones representing their encoding.
• Some clarification of already existing facilities of the language.

New features of OpenGL 4.0

It is very obvious that the major version number change indicates that this revision of the specification is targeting Shader Model 5.0 hardware. To be honest, as I was never really interested in DirectX, I barely know all the features introduced by DX11 but seems that there are some great facilities in OpenGL 4.0 that I’ve never heard that hardware supports it. This can be due to DX11 does not even support such functionalities but it is maybe because I don’t know enough details about DX11. Anyway, let’s see the revolutionary things that we face we checking out the latest version of the OpenGL specification…

ARB_draw_buffers_blend

Using this feature one is able to select individual blend equations and blend functions for each render target. This extension was already exposed for a few months now so most probably everybody heard about it or even if not the functionality is very straightforward. It simply removes some of the restrictions when dealing with multiple render targets (MRT). One interesting thing is still that the Khronos Group decided to include this extension in the 4.0 version of the API but not in 3.3. This is odd as Shader Model 4.0 capable hardware already supports this feature or at least I have the extension on my Radeon HD2600 which raises the question: why only in 4.0? Unfortunately, I don’t know the answer but I hope the ARB has a good reason behind this, as we will see later, there are other features that for some reason were only exposed in the latest version of the API but not in core for Shader Model 4.0 hardware.

ARB_sample_shading

In case of traditional multisample rendering the hardware optimizes the multisampling in a way that the fragment shader is executed only once for each fragment. This can be done as the standard specification relaxes the way how the implementation behaves regarding to feeding color and texture coordinate values for each sample. While this optimization usually does not provide any rendering artifacts and it heavily reduces the amount of pressure on the GPU, there are some situations when this optimization results in aliasing artifacts. One sample use case is when alpha-tested primitives are rendered.

This extension provides a global state for enabling and disabling sample shading and a way to control how fine-grained per-sample shading should be by supplying a minimum number of samples that need to be shaded. Beside this, it also introduces the required language elements to the OpenGL Shading Language to support sample shading.

ARB_shader_subroutine

In my humble opinion, this is one of the most important features introduced in this new version of the API specification. So far, many engine and shader developers faced the problems that where inherently there in the Shading Language that heavily reduced the ability to create a modular shader design in order to separate the independent tasks done in shaders nowadays. One initiative was the idea behind the EXT_separate_shader_objects extension. While that extension removed the dependency between shader stages, it does not address the problem with tight coupling inside one shader stage, also the aforementioned extension defeats some of the design goals of the Shading Language introducing complicated language semantics in order to solve the problem of inter-stage dependency.

Just to emphasize the importance of this new functionality with a very basic example, let’s take a simple rendering engine that supports skeletal animated geometry, materials and lights. In such a use case both the vertex and fragment shaders have multiple roles: the vertex shader has to perform the skeletal animation (property of the geometry) and the view transformation (property of the camera or of the light in case of shadow map rendering), and the fragment shader has to calculate the incident light to the surface point (property of the light) and then calculate the illuminance factor (property of the material). With the traditional tool-set these components of the shaders were tightly coupled and in order to support the combination of any geometry type (animated or not, skeletal or morph animation, etc.), any light type (directional, point, etc.) and material type (diffuse, phong, environment mapped, etc.), one had to compile all possible combinations of the shaders or create uber-shaders that do run-time decisions in order to solve the problem of heterogeneous inputs. Both of these solutions provide additional hardware resource usage and possible runtime overhead.

This extension adds some kind of polymorphism support to shaders. This way a single shader can include many alternative subroutines for a particular task and dynamically select through the API which subroutine is called from each call site. This opens the doors for modular shader designs while retaining most of the performance of specialized shaders.

ARB_tessellation_shader

Yes, this is about the new geometry tessellation mechanism introduced by Shader Model 5.0 hardware. The extension itself introduces three new stages that are roughly situated between the vertex shader and the geometry shader:

• Tessellation Control Shader – This new shader type operates on a patch that is actually nothing more than a fixed-size collection of vertices, each with per-vertex attributes and a number of associated per-patch attributes. Also note that while it operates on a patch, it is invoked on a per-vertex basis. The most important rule of this shader is to perturb the tessellation level for the patch that controls how finely the patch will be tesselated. Usually think about a patch as a triangle or quad. This shader is equivalent to DX11’s hull shader.
• Fixed-function tessellation primitive generator – The role of this new stage is to subdivide the incoming patch based on the tessellation level and related configuration that the unit gets as input.
• Tessellation Evaluation Shader – This new shader type is responsible of calculating the position and other attributes of the vertices produced by the tesselator. This shader is equivalent to DX11’s domain shader.

One important thing to notice is that a new primitive type is introduced, namely a patch. A patch on its own it is not directly or indirectly related to any traditional OpenGL primitive as it cannot be directly rendered. It is used only as the input type for the tesselator, however, a patch supplies the control grid of the geometry to be generated via tessellation so in practice it is most likely to be equivalent with triangles or quads but it is important to remark the difference.

As this is maybe the most well known feature of Shader Model 5.0 hardware I wouldn’t like to talk about it more as everybody knows what is it for and it would be rather long to explain how to use it. Also, it is not the intension of this article to fully cover the usage of all the new features, it is just a quick summarization of the new possibilities.

ARB_texture_buffer_object_rgb32

Yet another extension that introduces an additional format, now for texture buffers. Previously, texture buffers supported only four-component formats, this is extended with three-component formats. As currently there is no any practical use case in my mind when this can be useful, I would rather not come up with one. However, my opinion is that these formats most probably work with reduced performance compared to the four-component ones even though the memory footprint and bandwidth usage is maybe somewhat lower, I have concerns regarding to alignment related performance issues.

ARB_texture_cube_map_array

Those who already use texture arrays to help batching issues and remove unnecessary state changes most probably adore this extension as it enables texture array capabilities also for cube map textures. This comes handy especially in case when many materials use environment cube maps or when shadow cube maps are used for point lights. To come up with even a more concrete example, you can render the shadow cube maps for many hundreds of point lights with a single draw call by taking advantage of the layered rendering capability of geometry shaders and the possibility to bind texture arrays as render targets.

One more thing to notice here is that cube map arrays are already supported by Shader Model 4.1 hardware so the question to the ARB is again there, however, as OpenGL 3.3 still targets Shader Model 4.0 hardware maybe we will see a 3.x version of the specification that will also include this extension. The judgement is up to you whether you agree with me or not.

ARB_texture_gather

Another feature from the repertoire of Shader Model 4.1. This extension introduces new texture fetching functions to the Shading Language that determine a 2×2 footprint of the texture that would be used for linear filtering in a texture lookup and returns a vector consisting of the first component from each of the four texels in the footprint. This is the so called Gather4 texture fetching mode and can be useful to accelerate percentage closer filtering of shadow maps as it can fetch four samples at once. Still, there are some limitations on the use of this fetching mode, one important thing is that a shader cannot use normal and gather fetches on the same sampler. This makes me think about whether this feature is not part of the sampler object state instead of being a Shading Language construct. Anyway, as in typical use cases these limitations does not defeat the goal of the feature, I would not consider this problem a design issue.

ARB_transform_feedback2

The transform feedback mechanism already proved to me that is a great addition to the tool-set of graphics application developers. This feature extends transform feedback with an object type that encapsulates transform feedback related state to enable configuration reuse. Also it provides a way to pause and resume transform feedback mode if, for some reason, some rendering commands should be excluded from the feedback process.

The last and maybe most important benefit of this extension is the ability to draw primitives captured in transform feedback mode without querying the captured primitive count. It is roughly equivalent to DX10’s AutoDraw feature and the purpose of it is to eliminate the need to query the number of previously generated primitives in order to supply it to an OpenGL draw command. This solves the synchronization issues that previously happened between the CPU and the GPU.

One example is when a skeletal animated geometry has to be used in a multipass rendering technique. We can think about traditional forward rendering or when dealing with multiple shadow maps that have to be generated. Anyway, as the calculations needed to perform skeletal animation are rather expensive, it is wastage to perform these calculations in each pass.  A common way to solve this problem is to use transform feedback to capture the geometry emitted by a vertex shader that simply executes the skeletal animation on the input geometry. In subsequent rendering passes this feedback buffer can be used to source the geometry data to eliminate the need to recompute the animation. Without this extension, in such cases the application is most probably stalled until the feedback process ends as it needs to query the number of generated primitives. With this extension, this is solved as we don’t have to know the results of the previous transform feedback in order to issue a draw command that sources the data from the feedback buffer. By the way, this seems to be logical as the information is already on the GPU so why it should ping-pong between the CPU and the GPU?

As I mentioned before, the functionality provided by this extension is equivalent to DX10’s AutoDraw feature. This time my question is really serious: why this feature haven’t been included in OpenGL 3.3? It would provide a great benefit for those who use transform feedback and I don’t see any reason behind not supporting it because, as far as I can tell, it is supported on the corresponding hardware.

ARB_transform_feedback3

Surprisingly, OpenGL 4.0 comes with another transform feedback extension as well but this time a true Shader Model 5.0 feature. The new hardware generation has the ability to emit vertices from the geometry shader to multiple vertex streams. In order to provide clever API support, the ARB decided to relax the previous limitation of transform feedback mode that output can be in either interleaved format or to separate buffers. This new extension enables the use of both together also providing a way to group geometry shader outputs to groups in order to target the individual vertex streams.

The most important benefit of this feature is still that we have separate streams, each with its own primitive emission counter so the outputs should not necessarily have the same granularity. This provide room for very clever rendering techniques. As an example, remember NVIDIA’s Skinned Instancing demo that used one draw call per geometry LOD to sort instance data on a per-LOD basis. Using this extension, this preprocessing step can be done with a single draw call, but the abilities of this feature goes far beyond such a simple use case, I will also talk a bit about another in the next section.

One of my less technical notes is that it seems that the Khronos Group members have good sense of humor. I realized this when I met the “manbearbig” when reading one of the examples in the extension specification.

ARB_draw_indirect

We’ve arrived to the most culminating point in the list of features introduced. It is hard to say such a thing, but in my humble opinion this extension can be the Holy Grail of next generation rendering engines. I will explain why I think so…

The extension provides a way to source the parameters of instanced draw commands from within buffer objects. One naive use case would be to put all the rendering command parameters to a buffer object using the host application and then draw everything with a single command. While this simple method already has its benefits, this feature provides much more flexibility than this. The most revolutionary is that, using this extension, one is able to generate instanced draw commands with the GPU on-the-fly. Together with ARB_transform_feedback3 it is possible to write a completely GPU based scene management system.

Those who remember my Instance Cloud Reduction (ICR) algorithm, presented in the article Instance culling using geometry shaders, know that the required synchronization points between the CPU and the GPU heavily limited the practical utility of the culling technique. By taking advantage of the aforementioned features in case of ICR does not just eliminate the synchronization issues that I’ve spoken of but makes the technique practical also in case of heavily heterogeneous scenes with virtually any number of geometries even if there are multiple number of LOD level for them, and this whole stuff can be done with even less number of draw calls than that of the demo that accompanied my article. As soon as we will see OpenGL 4.0 capable drivers I will write an article about this technique, supplying also a reference implementation.

ARB_texture_query_lod

This extension provides new fragment shader texture functions, namely textureLOD*, that return the results of automatic LOD computations that would be performed if a texture lookup would be performed. These functions return a two-component vector. The X component of the result vector contains information about the mipmap level that would be used if a normal texture lookup would have been made with the same coordinates. This value can be a concrete mipmap level or a value between two levels if trilinear filtering is in use. The Y component of the result holds the computed LOD lambda-prime, see the OpenGL specification in order to check out where it is actually coming from and how it is calculated.

One interesting thing that this extension can be used for is when one implements some shader based filtering and addressing method for textures. As an example, lets take a mega-texture implemented that uses a 3D texture for storage, without actual mipmaps, and the addressing, filtering and mipmapping is done with shader code. As right now this is the only example that came into my mind and this is already awkward enough, I would rather leave the further discussion of the importance of this feature to more competent people.

ARB_gpu_shader5

Basically, this extension is nothing more than a big umbrella feature under what all the additional general or minor API changes go. Just to sum up the miscellaneous features provided by this extension, here is an excerpt from the extension specification:

• Support for indexing into arrays of samplers using non-constant indices.
• Support for indexing into an array of uniform blocks.
• Extending Gather4 with the ability to select any single component of a multi-component texture, to perform per-sample depth comparison, and to specify arbitrary offsets computed at runtime when gathering the 2×2 footprint.
• Support for instanced geometry shaders, where a geometry shader may be run multiple times for each primitive.

For a full list of new facilities introduced by the extension refer to the extension specification.

ARB_gpu_shader_fp64

This extension enables the use of double-precision floating-point data types and arithmetic from within shaders, also providing API entry points for double-precision data where it was missing. While one may think that the added precision is somewhat wastage in case of real-time graphics, it is important to note that GPUs are more and more often used for scientific calculations, not even necessarily in case of graphics related tasks. Taking in consideration this fact, the importance of double-precision floating-point support should not be underestimated. Beside that, maybe standard graphics application developers can also take advantage of the higher precision in some extreme use case scenarios.

The OpenGL Shading Language 4.00

Beside what I’ve already mentioned, there is no important thing to mention regarding to the Shading Language. There where many changes but most of them are simply provide Shading Language support to the API extensions. What I haven’t mentioned so far is the synchronization possibility for tessellation shaders, more implicit conversions, more integer functions, packing and unpacking facilities for floating-point formats and a new qualifier to force precision and disallow optimizations that re-order operations or treat different instances of the same operator with different precision.

Conclusion

I hope that some of you didn’t give up the reading so far. Sorry, but it seems that this article gone wild and still didn’t manage to cover all the topics I intended to talk about. But still, maybe I’ll recap on those subjects later.

Where is direct state access?

The original promise of eliminating the bind-to-modify semantics from the OpenGL API is still not done. The first reaction of many people is still to ask this question. While the bind-to-modify semantics is a rather annoying “feature” of OpenGL, I tend to state that if we are not talking about legacy OpenGL, the importance of direct state access is less and less relevant as we can already heavily reduce the number of state changes and API calls in our applications, thanks to the fast pace evolution of OpenGL. I sincerely think that with a modern rendering engine design built upon the idioms behind the new versions of the OpenGL API one should not face any significant scalability issues due to the outdated bind-to-modify semantics but maybe I’m wrong.

Personally, I have only one problem with the newly released specification versions that I’ve already tried to emphasize several times: the fact that so far many Shader Model 4.x features are missing from the 3.x line of the API specification. Hopefully that will be solved sooner or later, however addressing these issues should happen before the hardware to support will become outdated.

Anyway, we should not have any harsh complains as the Khronos Group did a great job again. They managed to keep again the half-year schedule and they even published two parallel releases at once! If someone still says that the DirectX API is superior compared to OpenGL should think it twice, as it seems that the tendency is that OpenGL just starts to evolve more and more fast. Beside that as now also AMD is being active in the OpenGL world, we can expect good support from both industry and developer community point of view.

My respect for the Khronos Group and thanks for reading the article!

OpenGL 3.2 - Nature

Since the appearance of Shader Model 4.0 people wonder how to take advantage of the newly introduced programmable pipeline stage. The most important feature enabled by geometry shaders is that one can change the amount of emitted primitives inside the pipeline. The first thing that a naive developer would try to do with it is geometry tesselation. However, the new shader performs very bad when used for tesselation in a real life scenario even though there are demos show casting this possibility. If we take a closer look at the new feature we observe that the most revolutionary in it is not that it can raise the number of emitted primitives but that it can discard them. This article would like to present a rendering technique that takes advantage of this aspect of geometry shaders to enable the GPU accelerated culling of higher order primitives.

Geometry shaders can be used for many different advanced rendering techniques that were impossible before the introduction of this flexible programmable shader stage. In this article I would like to present one use case that for me seemed to be one of the most practical application of primitive manipulation possibilities introduced by geometry shaders. As I haven’t seen any whitepaper talking specifically about this particular technique, even if some of them inherently used it, I would dare name the technique myself as Instance Cloud Reduction. I will also present a demo program that shows how to take advantage of the technique in a heavy workload situation.

The idea itself was inspired by AMD’s  tech demo for the Radeon 4800 series cards called March of the Froblins. An almost identical technique presented in this article is used in the mentioned demo for the culling of large amount of animated creatures against the view frustum. Also a somewhat similar technique is used in NVIDIA’s Skinned Instancing demo for determining LOD instance sets. Unfortunately, both demos are for DirectX only and, as far as I can tell, there is no OpenGL demo showing any of the aforementioned rendering techniques.

Motivation

Nowadays, as the computational capabilities of GPUs is growing in a much faster pace than that of CPUs, graphics developers meet more and more optimization problems related to CPU bound applications. More and more focus is on minimizing the number of driver invocations, actually that’s what motivated the restructuring of the two most commonly used graphics APIs. As a result we have now DirectX 10+ and OpenGL 3+. However, even if the introduction of geometry instancing, texture arrays and local memory buffer storage for the most important inputs of the rendering, there is still need for wise decisions from graphics programmers to take full advantage of the horsepower coming with the latest GPUs.

Earlier graphics applications strongly relied on CPU based culling techniques, whether it be the usage of the quite outdated BSPs or the more generic and still heavily applied hierarchical culling techniques. We’ve already reached the point that sometimes even the most efficient CPU based culling techniques seem to be too expensive and usually introduce the small batch problem. Instanced rendering is not an exception.

The applicability of geometry instancing is strongly limited by several factors. One of the most important ones is the culling of instanced geometries. One may choose to cull these objects in the same fashion as others, using the CPU, but that usually breaks the batch and maybe we loose the benefits of geometry instancing. It is more and more imminent to have a GPU based alternative. Without CPU based culling, by sending the whole bunch of instances down the graphics pipeline may choke our vertex processor in case we have high poly geometries and quite large amount of instances of it.

The rendering technique presented in this article will try to achieve this goal. We will use a multi-pass technique that in the first pass culls the object instances against the view frustum using the GPU and in the second pass renders only those instances that are likely to be visible in the final scene. This way we can severely reduce the amount of vertex data sent through the graphics pipeline.

Implementation

For some people it might seem that the promise for such a technique is simply too naive and is most probably relying on very exotic OpenGL features, heavy misuse of some basic features or need of data conversions during the frame rendering. Wondrously, this is not the case as we have all we need in OpenGL 3.2 to implement the object culling method sketched above. All we need are the followings:

• instanced rendering (core since OpenGL 3.1)
• geometry shaders (core since OpenGL 3.2)
• transform feedback (core since OpenGL 3.0)
• uniform or texture buffers (core since OpenGL 3.1)

The method itself is a multi-pass rendering technique, however, unlike other multi-pass rendering techniques it does not produce any fragments in the first pass, instead the first pass does the view frustum culling and processes data entirely only inside buffer objects.

Culling pass

In the first pass we will feed the graphics pipeline with information about the instances that are needed to perform the view frustum culling. For this we need two inputs for the executed shaders in order to be able to perform the required calculations:

1. Instance transformation data (whether it be a simple transformation matrix or quaternions or whatever) -- This preferably comes from one or more buffer objects that are bound as vertex buffers to the context.
2. Object extents information -- Beside the instance positions we have to know the extents of an instance in order to perform correct culling. This can be either a single float representing the object radius if we choose to use bounding spheres for the culling or a three-dimensional extent vector if we would like to use bounding boxes.

Using these as input we can feed in the instance transformation data as attributes of point primitives to our culling shader. The culling shader is composed of a vertex and a geometry shader. In a typical setup the role of each is the following: the vertex shader determines whether the actual object instance’s bounding volume is inside the view frustum and sends a flag about the culling to the geometry shader, that will emit the instance data to the destination buffer if the flag says that the instance is likely to be visible or does not emit anything if it is determined that the object instance is out of view.

Next, transform feedback is used to capture the primitives emitted by the geometry shader into another buffer object that will be used in the actual rendering pass to source instance transformation data. Beside this, we also need to have an asynchronous query to determine the number of primitives generated to know how many instances of the object do we actually need to render. The following figure shows the workflow of the first pass:

Instance Cloud Reduction - Pass 1: Culling

The actual geometry shader implementation needed to perform the actual culling based on the view frustum check performed by the vertex shader should look like the following chunk:

#version 150 core

layout(points) in;
layout(points, max_vertices = 1) out;

in vec4 OrigPosition[1];
flat in int objectVisible[1];

out vec4 CulledPosition;

void main() {

	/* only emit primitive if the object is visible */
	if ( objectVisible[0] == 1 )
	{
		CulledPosition = OrigPosition[0];
		EmitVertex();
		EndPrimitive();
	}
}

In this example we used only simply a four-component position vector for the instance transformation data but the technique works well for transformation matrices and quaternions as well.

One more thing is that beside that we set up transform feedback in a way that we feed our buffer object dedicated for the culled instance data and we also started an asynchronous query to be able to determine the number of primitives written into the buffer object, it is also useful to turn of rasterization as we wouldn’t like to produce any fragments as a result of the first pass.

Rendering pass

In the second pass there is nothing special to do. Simply use whatever rendering setup you would like to use. The only things that need to be changed in this step compared to your already existing rendering path is that the instance data for the rendering must be sourced from the generated culled instance data buffer and, as a result, the number of instances passed for the instanced drawing functions shall be changed in order to render only the visible instances. This number can be read from the asynchronous query’s result that we started in the first pass.

The instance data in the rendering pass can be, of course, sourced from either a uniform or a texture buffer object. This depends on the actual use case and is more clearly explained in the article Uniform Buffers VS Texture Buffers.

Important note is that when one has to deal with several instanced geometries it is recommended to do the culling phase prior to rendering any instanced primitives because of the following reasons:

• The result of the first instance cloud’s culling is more likely to be finished on the GPU so no sync issues arise from reading the asynchronous query result to determine the number of visible instances.
• Probably less state changes are needed as very different setup is required by the two passes.
• Results in tidier renderer design as culling is clearly separated from actual rendering.

Putting everything together, the application of the presented technique would result in the following workflow on the GPU:

Instance Cloud Reduction - Combined view of Pass 1 + Pass 2

Conclusion

We’ve seen that the presented advanced rendering technique is able to help in situations when we have to deal with large number of instanced geometries and how to take advantage of the latest features of graphics cards and OpenGL to perform view frustum culling calculations on the GPU. This prevents us from having to deal with complicated and expensive CPU based object culling methods that break the drawing batches, especially when dealing with dynamic objects. For ease the decision whether to incorporate this technique in your rendering engine I would like to present the advantages and disadvantages of it.

Advantages:

• Heavily reduces the amount of processed data in a naive implementation.
• No need for any space partitioning methods in the host application to handle the culling of dynamic objects.
• Can handle huge amount of instanced objects due to the enormous horsepower of today’s GPUs.
• Scales well with increased number of instances as the per-instance calculation is relatively low.
• Relies strictly on OpenGL 3.2 core features.
• No need for OpenCL capable hardware.

Disadvantages:

• Needs an extra rendering pass to perform the culling.
• Requires the usage of asynchronous queries to determine the number of visible instances.

I hope you agree with me and think about this technique as one more step towards fully GPU based scene management. If you have any remarks or improvement ideas regarding to the rendering technique itself feel free to tell me.

The Demo

As I promised, the technique presented above comes with a live demo that actually took most of my time dedicated to writing this blog in the last two weeks. The demo itself is more like a technical show cast rather than a presentation of a real-life use case scenario.

First of all, I used high polygon count models for the rendering to emphasize the amount of time the culling phase spares from the very valuable time of our GPU. In a real world application one would never do something like this. As a result, the demo is more like a benchmark than an interactive application. However, maybe on high-end graphics cards it can perform pretty well.

The demo scene consists of two object types: trees and grass blocks. The tree model is further divided into two parts as they need different textures: the tree trunk and the tree foliage. Obviously, this additional burden can be prevented by using texture arrays to avoid the need of separate draw calls to render the trunk and the foliage.

The tree trunk consists of 33138 triangles, the tree foliage has 16069 triangles and the faking-free grass block consists of 8961 triangles which I had to model myself as didn’t found any suitable model. Actually this modeling step consumed quite a reasonable amount of my time spent with the demo as I’m not an expert in this domain.As you can see, these models are not the ones that one might use in an interactive real-time application like games. However, they seemed to be very suitable for the purpose of the demonstration.

What really kicks off the boundaries of GPUs is that the demo renders 10,000 trees and 250,000 grass blocks using instancing. This ends up in more than 2.7 billion triangles in the scene. This is far more that a GPU can handle without the aid of some scene management and culling. However, we will use no scene management at all and the only culling method that we will use is the one presented in this article.

The actual results are quite promising. The view frustum culling step usually spares more than 99.9% of the GPU horsepower as the amount of actually rendered triangles after the culling step is far below 2 million triangles. This is still quite much but as we use high polygon count models and we don’t use any LOD techniques this seems reasonable.

Even if the demo scene statistics doesn’t seem like a typical use case scenario, the ease of the implementation and the compelling visual results made me pleased anyway:

www.youtube.com/watch?v=srbOFTLTe8k

On my Radeon HD2600XT I have achieved 6-7 frames per second which is acceptable taking in consideration the huge amount of geometry data still passed to the graphics card. On more recent cards I suppose it should run with good frame rates, however, due to the lack of hardware to test on, these are my only results. If anybody manages to take a better screen capture than mine above then please let me know.

Implementation details

Just to tell a few words about what techniques and tricks I’ve used during the creation of the demo here is a listing of the most important ones:

• Three models are used as mentioned previously with high instance counts with over 2.7 billion of total triangles in the scene as mentioned already.
• Three 512x512 RGBA textures are used for the models that are partially handmade, and again, I’m not a texture artist so sorry if they don’t look flawless.
• The wavefront model and TGA image loader that accompany the demo are very roughly implemented only for the demo so I would strongly encourage you not to use it to any purpose as it handles only a subset of the possibilities of the file formats.
• The vertex data from the wavefront model files is transferred in a very naive way so vertex reuse isn’t taken into account.
• The instance data consists of simple four-component vectors representing the world-space position of the instance. This seemed to be the most simple for the demonstration purposes.
• In the second pass, the instance data is sourced from a texture buffer but not really because the visible instance count exceeded the amount that would fit in a uniform buffer. I used texture buffers because for this simple demonstration they seemed to be a little bit more easy to be integrated.
• The morphing effect that simulated wind blow is done using hard-coded geometry deformation in the vertex shader. It is not physically correct but visually compelling.
• The lighting is a simple directional light using Phong’s shading and reflection model.
• Simple fog is simulated with some awkward formula that I’ve chosen after a few test runs.
• Alpha testing is achieved by using the discard operation in the fragment shader.

Driver issues

During the development of the demonstration program I’ve met several driver related problems as I’ve never used so heavily the latest OpenGL features previously. I’ve worked with Catalyst 9.12 and 10.1 but both seemed to lack of a proper GLSL compiler. Here are some of the issues I’ve met:

• When I’ve forgot to declare the varyings in the geometry shader as arrays like the standard requires then still the driver hasn’t complained about any syntax error but when tried to execute the code the program crashed.
• Except the texture sampler uniform, all other uniforms failed to work when used in the fragment shader only so I’ve put them all in the vertex shader.
• For loops seemed not to work when used inside the geometry shader, that’s why the culling itself is done in the vertex shader in the demo.

All these problems resulted in nasty tricks to make things working and ended up in awful shader code. Sorry for that. At least now it works on my configuration but pretty unsure whether it will work on other graphics card and driver combos. Please report me any success or failure when trying out the demo. Anyway, be sure to have the latest graphics drivers installed as, at least in case of AMD, OpenGL 3.2 drivers came out only at the fall of 2009.

Edit:

Thanks to the information got from Pierre Boudier from AMD I’ve updated both the source and binary releases to support the latest drivers properly. The problem was that I didn’t use attribute location binding as specified in the standard.

Also have to mention that with my new Radeon HD5770 I managed to achieve over 90 frames per second that actually show that this technique can be in fact used for games and other interactive applications.

One more thing in the end. As you know this version of the Nature demo uses a texture buffer to source instance positions. I plan to create another version that will take advantage of the instanced arrays introduced in core with OpenGL 3.4. I expect quite a reasonable speedup as that would eliminate the need for texture fetches in the vertex array by rather dedicating a vertex fetcher for the purpose thus increasing the overall performance of the technique.

Binary release

Platform: Windows
Dependency: OpenGL 3.2 capable graphics driver
Download link: nature12_win32.zip (3.58MB)
Comments: Includes the update that makes it work even with the latest drivers.

Full source code

Language: C++
Platform: cross-platform
Dependency: GLEW, SFML, GLM
Download link: nature12_src.zip (12.6KB)
Comments: Sorry for the many dependencies, however, I would recommend the mentioned libraries for everybody who is doing OpenGL development.