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OpenGL occlusion culling

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Hi, i am back working on on old idea of mine about occlusion culling , i coded a fast zbuffered-concave/convex-polygon-with-holes function ,and i was proceeding with the next step. My biggest concern is about transforming vertices, i am going to do the job twice, opengl renders its own buffer, while in software the code basically do the same. Now i was thinking about this, since i need the normals for the zbuffer,an algorithm might work in this way. I consider the whole world fixed , and i move the observer in the opposite view direction , its just like in the real world the observer rotates and moves, the rest of the world stays fixed, opposing to what normally happens with opengl or directx. Now i think i am going to need a matrix derived from quaternions. Has anyone done the same thing , or is just a stupid idea ? Is there any way to avoid recompunting twice the vertices(sp??) Thanks and sorry for my english.

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If your culling is done on CPU, then of course you need to transform the occluders and ocludees on the CPU as well.
But they must be very low-poly, since you don't wanna do only occlusion, right?

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Original post by Joshua Klint
There's not much reason to do any culling other than frustum culling.
Not true.

When doing occlusion culling on the CPU don't test with the same geometry you are rendering. Use a very low poly version or better yet use a bounding box or AABB. Using an AABB you can cut the transformations down to two vertices (min and max for AABB).

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Original post by Joshua Klint
There's not much reason to do any culling other than frustum culling.


Very untrue. For every single scene, with high geometric complexity, overdraw and occluded objects there is two things one can do to save GPU computing power for a better picture - transfer it from the distant scene to the front via LOD, or remove it from hidden and not visible part of the scene via occlusion culling.
In the end, it almost always saves GPU cycles that can be spent somewhere else. Even for higher FPS...

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It depends on what kind of scene it is. If you are rendering an outdoor scene there is very little you can do that won't be a special trick that only works in small demos that have absolutely no practical use.

I think most of the theory on occlusion is hangons from the Quake BSP days, and it's not really in tune with what modern GPUs can do.

For an indoor scene, a good portal system can make a big difference, but I see beginning programmers who want to draw 10,000 polys on the screen worry about occlusion, and it doesn't make sense unless you are really going to be pushing the GPU. Unless you are saving at least maybe 20,000 polys with occlusion, don't even bother.

For all culling, you should use a sphere test. This is the center of the object, with a radius of the furthest vertex from the center. It is not terribly accurate, but can be made to err on the side of caution. Anything more complicated than that and you will just slow your app down, unless you are running a quad core CPU with a TNT2 GPU.

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Original post by Joshua Klint
It depends on what kind of scene it is. If you are rendering an outdoor scene there is very little you can do that won't be a special trick that only works in small demos that have absolutely no practical use.

Still very untrue. There's been a lot of work done on outdoor occlusion, and there are a lot of practical techniques you can use.

Valve's Source engine uses "occluder frustums" similar to the ones in this article for outdoor scenes.
It might be a special trick, but it definitely has practical use.

Also: Quad-trees/Oct-trees still receive wide-spread use in outdoor visibility culling, and even precomputed-PVS techniques are still being used...

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Original post by Joshua Klint
It depends on what kind of scene it is. If you are rendering an outdoor scene there is very little you can do that won't be a special trick that only works in small demos that have absolutely no practical use.

Incorrect. Although not as effective as in indoor scenes, occlusion culling can still have a huge impact on outdoor scenes. Especially if they're densly populated (bumpy terrain, vegetation, buildings, etc).

Quote:
Original post by Joshua Klint
I think most of the theory on occlusion is hangons from the Quake BSP days, and it's not really in tune with what modern GPUs can do.

I think you don't understand what occlusion culling (according to the modern definition) actually is :) It has strictly nothing to do with BSPs and co, and was never used in Quake. You might be confusing it with PVS, which is indeed a little outdated (a least in the way it was done in Quake). But modern occlusion culling is something completely different than BSP/PVS.

Quote:
Original post by Joshua Klint
For an indoor scene, a good portal system can make a big difference,

Sure it can, but is restricted to a certain type of geometry. In fact, portal rendering is a special type of occlusion culling. More general forms of OC, such as HOM or hardware OC with conditional rendering, can entirely replace a portal system. They're both more effective than portals can ever be (by definition), and can handle all types of scenes without specific requirements such as imposed by portals.

Quote:
Original post by Joshua Klint
but I see beginning programmers who want to draw 10,000 polys on the screen worry about occlusion, and it doesn't make sense unless you are really going to be pushing the GPU. Unless you are saving at least maybe 20,000 polys with occlusion, don't even bother.

That is true, however OC is a vital part of an engine if you increase the poly load over a certain limit, and if you have scenes with high depth complexity. So it is only natural that beginners should learn about it. Especially since hardware support for OC is increasing.

Quote:
Original post by Joshua Klint
For all culling, you should use a sphere test.

No, you should not.

Quote:
Original post by Joshua Klint
This is the center of the object, with a radius of the furthest vertex from the center. It is not terribly accurate, but can be made to err on the side of caution. Anything more complicated than that and you will just slow your app down, unless you are running a quad core CPU with a TNT2 GPU.

Incorrect again. Do not make assumptions if you clearly do not have the expertise to back it up. Spatial culling is an extremely important part of any modern engine. Culling removing depth complexity (means everything from occlusion culling, over LOD, to distance based procedural simplifications like adaptive displacement mapping) is even more important on todays shader heavy scenes. The quickest geometry is the one you don't render.

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All you have done is contradict everything I said. Why don't you lay out a proposal of how you think culling should be handled? Because there are far too many academic opinions on this subject from people who have never actually made anything but a small demo showcasing their idea under extremely controlled conditions.

I have implemented "occluders" as well, but they are only useful in very specific circumstances. Overall I could easily do without them, but I just left them in because I thought they were neat. And yes, the sphere test should be used because you can quickly check to see if it is inside the planes of the occlusion volume, with no transformations required at all.

Outdoor culling is not that important because if you can't render all your geometry fast enough when the player goes up on a hill or something, you're screwed anyways. For static geometry, it is almost always faster to chuck the whole geometry at the GPU, or at least big chunks that can still be frustum-culled.

For dynamic objects, instanced rendering gives you the speed you need. There are not a lot of conditions under which outdoor culling of instanced meshes will provide more frames than it eats up.

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I found GPU occlusion culling worked just fine on my outdoor terrain rendering. Yes, even on top of a hill it is still useful, as some sections of terrain will be overlapping each other in the distance(the big mountains/hills blocking the ones behind them). And if you were to leave the ground and fly up into the sky the terrain would be using a lower LOD anyway. Its fast/cheap and reduces vert count considerably, no reason not to use it.

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Thanks for your input , it gave birth to a new obsession of mine.
Basically i have a way to avoid transforming all the vertices in the scene
and avoiding using a lod for the level itself, it is based around triangle navigation trhough adiacency maps

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Original post by Joshua Klint
All you have done is contradict everything I said.

Because pretty much everything you said was incorrect :) What do you expect me to say ?

Quote:
Original post by Joshua Klint
Why don't you lay out a proposal of how you think culling should be handled? Because there are far too many academic opinions on this subject from people who have never actually made anything but a small demo showcasing their idea under extremely controlled conditions.

In short, the two most important aspects in culling (both CPU and GPU side) is the removal of geometry not visible due to camera projection (ie. frustum culling), and the removal of depth complexity. The former should imnperatively use a form of hierarchical spatial structure. While spheres are a possibility, in practice they usually give far worse results than other structures by being overly conservative. The latter is comprised of occlusion culling, level of detail, and other slightly more exotic culling systems. For very simple scenes (< 100k faces), as the one in your engine (from here, brute forcing everything to the GPU is a possibility. But once you go into more advanced rendering, especially with heavy vertex shaders and state change limitations, some form occlusion culling becomes a must have.

If you are interested in more details about how I think it should be handled, I leave it to you as an exercise to dig out the hundreds of threads where I talked about all aspects of spatial structures, dynamic trees, occlusion systems, performance analysis, etc, in the past. That's all i can offer right now, since my current opinion on the matter is covered by several NDAs :)

Quote:
Original post by Joshua Klint
I have implemented "occluders" as well, but they are only useful in very specific circumstances. Overall I could easily do without them, but I just left them in because I thought they were neat.

This is because your scenes are too simple right now for them to be effective. And also, because your implementation of them is probably far from optimal. In my engine, which is used as a part of a high end commercial and military visualization system, and which features several extremely optimized culling systems, occlusion culling can easily remove up to 90% of geometry in a frame. That means going down from over 5 million visible faces per frame to maybe 500k, in many situations. As you can imagine, this gives us a considerable speedup. The same is true for most modern game 3D engines.

Quote:
Original post by Joshua Klint
And yes, the sphere test should be used because you can quickly check to see if it is inside the planes of the occlusion volume, with no transformations required at all.

As I mentioned above, sphere tests are usually sub-par in terms of efficiency.

Quote:
Original post by Joshua Klint
Outdoor culling is not that important because if you can't render all your geometry fast enough when the player goes up on a hill or something, you're screwed anyways. For static geometry, it is almost always faster to chuck the whole geometry at the GPU, or at least big chunks that can still be frustum-culled.

For dynamic objects, instanced rendering gives you the speed you need. There are not a lot of conditions under which outdoor culling of instanced meshes will provide more frames than it eats up.

Those are very bold statements. Even a seasoned professional would think twice about making such generic statements about rendering and GPU performance behaviour. Now don't take this as an offense, but from your previous posts I think we can safely classify you as a intermediate beginner. As such, you don't yet have the experience of correctly assessing the performance characteristics of modern 3D engine design (which is a very complex field). Just because OC didn't give you a speedup, doesn't mean it won't in the general case. Chances are, that your particular implementation was sub-optimal, or your algorithms were not appropriate for your scenes (as you said yourself, you didn't even know about GPU occlusion culling). Many people in this thread have told you that you were wrong - maybe you should listen to people that have more experience than you, and learn from them.

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If you are interested in more details about how I think it should be handled, I leave it to you as an exercise to dig out the hundreds of threads where I talked about all aspects of spatial structures, dynamic trees, occlusion systems, performance analysis, etc, in the past. That's all i can offer right now, since my current opinion on the matter is covered by several NDAs :)

So I see you don't have a definitive answer, just a lot of smarmy remarks and smily faces. :)

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Original post by Joshua Klint
So I see you don't have a definitive answer, just a lot of smarmy remarks and smily faces. :)

I think I have been quite precise. What do you want more ? Source code ? A step by step guide ? Giving you such detailed answers would invalidate my security clearance and violate at least two NDAs (for those who remember that 'ABT incident' some time ago, yeah, they now make me sign multiple NDAs just to be sure *ahem* ;)

As I said, search the forums. Here, let me help you. Besides that, there is no definitive answer to such a question. It all depends on the specifics of your engine, your target hardware, and the type of scenes you are using. There is no magical algorithm that will make everything super-fast, while all other algorithms are unusable. Fact is, spatial and occlusion culling are vital parts of an engine, if you go over a certain face and shader complexity threshold.

The exact threshold cannot be given, because it depends too much on case-specific circumstances. Same is true for the exact algorithms to use - hardware occlusion culling, HOMs, software renderers, and even PVS. All have their place, and the wise selection of an appropriate algorithm is often more important than its implementation details. This can only be achieved by experience. Trial and error. You should also be aware that algorithms not working for you might very well work for others, because the frame conditions are different - more faces, higher geometrical density, more shader and state bound scenarios, etc.

You'll run into that sooner or later, if you carry on with graphics development. But in the meantime, try not to make generalized statements and assumptions without providing facts, because this will only spread incorrect information.

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Original post by nts

When doing occlusion culling on the CPU don't test with the same geometry you are rendering. Use a very low poly version or better yet use a bounding box or AABB. Using an AABB you can cut the transformations down to two vertices (min and max for AABB).


I've been thinking of coding up a hardware occlusion implementation also. And I had thought to use AABBs instead of the actual geometry. But my question is, wouldn't using AABBs occlude geometry that it shouldn't? If you had a large bounding box - but its model was quite a bit smaller than it's max/min (like a tree trunk for example) - and you had a small object behind this tree but wasn't occluded by the trunk, but was occluded by the tree's AABB, wouldn't this produce artifacts?

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Original post by glaeken
I've been thinking of coding up a hardware occlusion implementation also. And I had thought to use AABBs instead of the actual geometry. But my question is, wouldn't using AABBs occlude geometry that it shouldn't? If you had a large bounding box - but its model was quite a bit smaller than it's max/min (like a tree trunk for example) - and you had a small object behind this tree but wasn't occluded by the trunk, but was occluded by the tree's AABB, wouldn't this produce artifacts?

Yes, this will indeed happen. However, you can use approximate geometry for culling, by making sure that it will only be smaller than (or just as large as) the occluder itself. Think of it as an 'interior bounding box'. Often, such a structure will be more complex than a bounding box, though - a conservative interior hull. Special algorithms exist in order to optimally fit such a volume into an object, but they're unfortunately quite complex. An alternative is to manually model them, and attach them to the object. That's what we did in the past, and it worked pretty well.

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      Device resources are created by the render device. The two main resource types are buffers, which represent linear memory, and textures, which use memory layouts optimized for fast filtering. Graphics APIs usually have a native object that represents linear buffer. Diligent Engine uses IBuffer interface as an abstraction for a native buffer. To create a buffer, one needs to populate BufferDesc structure and call IRenderDevice::CreateBuffer() method as in the following example:
      BufferDesc BuffDesc; BufferDesc.Name = "Uniform buffer"; BuffDesc.BindFlags = BIND_UNIFORM_BUFFER; BuffDesc.Usage = USAGE_DYNAMIC; BuffDesc.uiSizeInBytes = sizeof(ShaderConstants); BuffDesc.CPUAccessFlags = CPU_ACCESS_WRITE; m_pDevice->CreateBuffer( BuffDesc, BufferData(), &m_pConstantBuffer ); While there is usually just one buffer object, different APIs use very different approaches to represent textures. For instance, in Direct3D11, there are ID3D11Texture1D, ID3D11Texture2D, and ID3D11Texture3D objects. In OpenGL, there is individual object for every texture dimension (1D, 2D, 3D, Cube), which may be a texture array, which may also be multisampled (i.e. GL_TEXTURE_2D_MULTISAMPLE_ARRAY). As a result there are nine different GL texture types that Diligent Engine may create under the hood. In Direct3D12, there is only one resource interface. Diligent Engine hides all these details in ITexture interface. There is only one  IRenderDevice::CreateTexture() method that is capable of creating all texture types. Dimension, format, array size and all other parameters are specified by the members of the TextureDesc structure:
      TextureDesc TexDesc; TexDesc.Name = "My texture 2D"; TexDesc.Type = TEXTURE_TYPE_2D; TexDesc.Width = 1024; TexDesc.Height = 1024; TexDesc.Format = TEX_FORMAT_RGBA8_UNORM; TexDesc.Usage = USAGE_DEFAULT; TexDesc.BindFlags = BIND_SHADER_RESOURCE | BIND_RENDER_TARGET | BIND_UNORDERED_ACCESS; TexDesc.Name = "Sample 2D Texture"; m_pRenderDevice->CreateTexture( TexDesc, TextureData(), &m_pTestTex ); If native API supports multithreaded resource creation, textures and buffers can be created by multiple threads simultaneously.
      Interoperability with native API provides access to the native buffer/texture objects and also allows creating Diligent Engine objects from native handles. It allows applications seamlessly integrate native API-specific code with Diligent Engine.
      Next-generation APIs allow fine level-control over how resources are allocated. Diligent Engine does not currently expose this functionality, but it can be added by implementing IResourceAllocator interface that encapsulates specifics of resource allocation and providing this interface to CreateBuffer() or CreateTexture() methods. If null is provided, default allocator should be used.
      Initializing the Pipeline State
      As it was mentioned earlier, Diligent Engine follows next-gen APIs to configure the graphics/compute pipeline. One big Pipelines State Object (PSO) encompasses all required states (all shader stages, input layout description, depth stencil, rasterizer and blend state descriptions etc.). This approach maps directly to Direct3D12/Vulkan, but is also beneficial for older APIs as it eliminates pipeline misconfiguration errors. With many individual calls tweaking various GPU pipeline settings it is very easy to forget to set one of the states or assume the stage is already properly configured when in fact it is not. Using pipeline state object helps avoid these problems as all stages are configured at once.
      Creating Shaders
      While in earlier APIs shaders were bound separately, in the next-generation APIs as well as in Diligent Engine shaders are part of the pipeline state object. The biggest challenge when authoring shaders is that Direct3D and OpenGL/Vulkan use different shader languages (while Apple uses yet another language in their Metal API). Maintaining two versions of every shader is not an option for real applications and Diligent Engine implements shader source code converter that allows shaders authored in HLSL to be translated to GLSL. To create a shader, one needs to populate ShaderCreationAttribs structure. SourceLanguage member of this structure tells the system which language the shader is authored in:
      SHADER_SOURCE_LANGUAGE_DEFAULT - The shader source language matches the underlying graphics API: HLSL for Direct3D11/Direct3D12 mode, and GLSL for OpenGL and OpenGLES modes. SHADER_SOURCE_LANGUAGE_HLSL - The shader source is in HLSL. For OpenGL and OpenGLES modes, the source code will be converted to GLSL. SHADER_SOURCE_LANGUAGE_GLSL - The shader source is in GLSL. There is currently no GLSL to HLSL converter, so this value should only be used for OpenGL and OpenGLES modes. There are two ways to provide the shader source code. The first way is to use Source member. The second way is to provide a file path in FilePath member. Since the engine is entirely decoupled from the platform and the host file system is platform-dependent, the structure exposes pShaderSourceStreamFactory member that is intended to provide the engine access to the file system. If FilePath is provided, shader source factory must also be provided. If the shader source contains any #include directives, the source stream factory will also be used to load these files. The engine provides default implementation for every supported platform that should be sufficient in most cases. Custom implementation can be provided when needed.
      When sampling a texture in a shader, the texture sampler was traditionally specified as separate object that was bound to the pipeline at run time or set as part of the texture object itself. However, in most cases it is known beforehand what kind of sampler will be used in the shader. Next-generation APIs expose new type of sampler called static sampler that can be initialized directly in the pipeline state. Diligent Engine exposes this functionality: when creating a shader, textures can be assigned static samplers. If static sampler is assigned, it will always be used instead of the one initialized in the texture shader resource view. To initialize static samplers, prepare an array of StaticSamplerDesc structures and initialize StaticSamplers and NumStaticSamplers members. Static samplers are more efficient and it is highly recommended to use them whenever possible. On older APIs, static samplers are emulated via generic sampler objects.
      The following is an example of shader initialization:
      ShaderCreationAttribs Attrs; Attrs.Desc.Name = "MyPixelShader"; Attrs.FilePath = "MyShaderFile.fx"; Attrs.SearchDirectories = "shaders;shaders\\inc;"; Attrs.EntryPoint = "MyPixelShader"; Attrs.Desc.ShaderType = SHADER_TYPE_PIXEL; Attrs.SourceLanguage = SHADER_SOURCE_LANGUAGE_HLSL; BasicShaderSourceStreamFactory BasicSSSFactory(Attrs.SearchDirectories); Attrs.pShaderSourceStreamFactory = &BasicSSSFactory; ShaderVariableDesc ShaderVars[] = {     {"g_StaticTexture", SHADER_VARIABLE_TYPE_STATIC},     {"g_MutableTexture", SHADER_VARIABLE_TYPE_MUTABLE},     {"g_DynamicTexture", SHADER_VARIABLE_TYPE_DYNAMIC} }; Attrs.Desc.VariableDesc = ShaderVars; Attrs.Desc.NumVariables = _countof(ShaderVars); Attrs.Desc.DefaultVariableType = SHADER_VARIABLE_TYPE_STATIC; StaticSamplerDesc StaticSampler; StaticSampler.Desc.MinFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MagFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MipFilter = FILTER_TYPE_LINEAR; StaticSampler.TextureName = "g_MutableTexture"; Attrs.Desc.NumStaticSamplers = 1; Attrs.Desc.StaticSamplers = &StaticSampler; ShaderMacroHelper Macros; Macros.AddShaderMacro("USE_SHADOWS", 1); Macros.AddShaderMacro("NUM_SHADOW_SAMPLES", 4); Macros.Finalize(); Attrs.Macros = Macros; RefCntAutoPtr<IShader> pShader; m_pDevice->CreateShader( Attrs, &pShader );
      Creating the Pipeline State Object
      After all required shaders are created, the rest of the fields of the PipelineStateDesc structure provide depth-stencil, rasterizer, and blend state descriptions, the number and format of render targets, input layout format, etc. For instance, rasterizer state can be described as follows:
      PipelineStateDesc PSODesc; RasterizerStateDesc &RasterizerDesc = PSODesc.GraphicsPipeline.RasterizerDesc; RasterizerDesc.FillMode = FILL_MODE_SOLID; RasterizerDesc.CullMode = CULL_MODE_NONE; RasterizerDesc.FrontCounterClockwise = True; RasterizerDesc.ScissorEnable = True; RasterizerDesc.AntialiasedLineEnable = False; Depth-stencil and blend states are defined in a similar fashion.
      Another important thing that pipeline state object encompasses is the input layout description that defines how inputs to the vertex shader, which is the very first shader stage, should be read from the memory. Input layout may define several vertex streams that contain values of different formats and sizes:
      // Define input layout InputLayoutDesc &Layout = PSODesc.GraphicsPipeline.InputLayout; LayoutElement TextLayoutElems[] = {     LayoutElement( 0, 0, 3, VT_FLOAT32, False ),     LayoutElement( 1, 0, 4, VT_UINT8, True ),     LayoutElement( 2, 0, 2, VT_FLOAT32, False ), }; Layout.LayoutElements = TextLayoutElems; Layout.NumElements = _countof( TextLayoutElems ); Finally, pipeline state defines primitive topology type. When all required members are initialized, a pipeline state object can be created by IRenderDevice::CreatePipelineState() method:
      // Define shader and primitive topology PSODesc.GraphicsPipeline.PrimitiveTopologyType = PRIMITIVE_TOPOLOGY_TYPE_TRIANGLE; PSODesc.GraphicsPipeline.pVS = pVertexShader; PSODesc.GraphicsPipeline.pPS = pPixelShader; PSODesc.Name = "My pipeline state"; m_pDev->CreatePipelineState(PSODesc, &m_pPSO); When PSO object is bound to the pipeline, the engine invokes all API-specific commands to set all states specified by the object. In case of Direct3D12 this maps directly to setting the D3D12 PSO object. In case of Direct3D11, this involves setting individual state objects (such as rasterizer and blend states), shaders, input layout etc. In case of OpenGL, this requires a number of fine-grain state tweaking calls. Diligent Engine keeps track of currently bound states and only calls functions to update these states that have actually changed.
      Binding Shader Resources
      Direct3D11 and OpenGL utilize fine-grain resource binding models, where an application binds individual buffers and textures to certain shader or program resource binding slots. Direct3D12 uses a very different approach, where resource descriptors are grouped into tables, and an application can bind all resources in the table at once by setting the table in the command list. Resource binding model in Diligent Engine is designed to leverage this new method. It introduces a new object called shader resource binding that encapsulates all resource bindings required for all shaders in a certain pipeline state. It also introduces the classification of shader variables based on the frequency of expected change that helps the engine group them into tables under the hood:
      Static variables (SHADER_VARIABLE_TYPE_STATIC) are variables that are expected to be set only once. They may not be changed once a resource is bound to the variable. Such variables are intended to hold global constants such as camera attributes or global light attributes constant buffers. Mutable variables (SHADER_VARIABLE_TYPE_MUTABLE) define resources that are expected to change on a per-material frequency. Examples may include diffuse textures, normal maps etc. Dynamic variables (SHADER_VARIABLE_TYPE_DYNAMIC) are expected to change frequently and randomly. Shader variable type must be specified during shader creation by populating an array of ShaderVariableDesc structures and initializing ShaderCreationAttribs::Desc::VariableDesc and ShaderCreationAttribs::Desc::NumVariables members (see example of shader creation above).
      Static variables cannot be changed once a resource is bound to the variable. They are bound directly to the shader object. For instance, a shadow map texture is not expected to change after it is created, so it can be bound directly to the shader:
      PixelShader->GetShaderVariable( "g_tex2DShadowMap" )->Set( pShadowMapSRV ); Mutable and dynamic variables are bound via a new Shader Resource Binding object (SRB) that is created by the pipeline state (IPipelineState::CreateShaderResourceBinding()):
      m_pPSO->CreateShaderResourceBinding(&m_pSRB); Note that an SRB is only compatible with the pipeline state it was created from. SRB object inherits all static bindings from shaders in the pipeline, but is not allowed to change them.
      Mutable resources can only be set once for every instance of a shader resource binding. Such resources are intended to define specific material properties. For instance, a diffuse texture for a specific material is not expected to change once the material is defined and can be set right after the SRB object has been created:
      m_pSRB->GetVariable(SHADER_TYPE_PIXEL, "tex2DDiffuse")->Set(pDiffuseTexSRV); In some cases it is necessary to bind a new resource to a variable every time a draw command is invoked. Such variables should be labeled as dynamic, which will allow setting them multiple times through the same SRB object:
      m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "cbRandomAttribs")->Set(pRandomAttrsCB); Under the hood, the engine pre-allocates descriptor tables for static and mutable resources when an SRB objcet is created. Space for dynamic resources is dynamically allocated at run time. Static and mutable resources are thus more efficient and should be used whenever possible.
      As you can see, Diligent Engine does not expose low-level details of how resources are bound to shader variables. One reason for this is that these details are very different for various APIs. The other reason is that using low-level binding methods is extremely error-prone: it is very easy to forget to bind some resource, or bind incorrect resource such as bind a buffer to the variable that is in fact a texture, especially during shader development when everything changes fast. Diligent Engine instead relies on shader reflection system to automatically query the list of all shader variables. Grouping variables based on three types mentioned above allows the engine to create optimized layout and take heavy lifting of matching resources to API-specific resource location, register or descriptor in the table.
      This post gives more details about the resource binding model in Diligent Engine.
      Setting the Pipeline State and Committing Shader Resources
      Before any draw or compute command can be invoked, the pipeline state needs to be bound to the context:
      m_pContext->SetPipelineState(m_pPSO); Under the hood, the engine sets the internal PSO object in the command list or calls all the required native API functions to properly configure all pipeline stages.
      The next step is to bind all required shader resources to the GPU pipeline, which is accomplished by IDeviceContext::CommitShaderResources() method:
      m_pContext->CommitShaderResources(m_pSRB, COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES); The method takes a pointer to the shader resource binding object and makes all resources the object holds available for the shaders. In the case of D3D12, this only requires setting appropriate descriptor tables in the command list. For older APIs, this typically requires setting all resources individually.
      Next-generation APIs require the application to track the state of every resource and explicitly inform the system about all state transitions. For instance, if a texture was used as render target before, while the next draw command is going to use it as shader resource, a transition barrier needs to be executed. Diligent Engine does the heavy lifting of state tracking.  When CommitShaderResources() method is called with COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES flag, the engine commits and transitions resources to correct states at the same time. Note that transitioning resources does introduce some overhead. The engine tracks state of every resource and it will not issue the barrier if the state is already correct. But checking resource state is an overhead that can sometimes be avoided. The engine provides IDeviceContext::TransitionShaderResources() method that only transitions resources:
      m_pContext->TransitionShaderResources(m_pPSO, m_pSRB); In some scenarios it is more efficient to transition resources once and then only commit them.
      Invoking Draw Command
      The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
      ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
      DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
      Source Code
      Full engine source code is available on GitHub and is free to use. The repository contains two samples, asteroids performance benchmark and example Unity project that uses Diligent Engine in native plugin.
      AntTweakBar sample is Diligent Engine’s “Hello World” example.

       
      Atmospheric scattering sample is a more advanced example. It demonstrates how Diligent Engine can be used to implement various rendering tasks: loading textures from files, using complex shaders, rendering to multiple render targets, using compute shaders and unordered access views, etc.

      Asteroids performance benchmark is based on this demo developed by Intel. It renders 50,000 unique textured asteroids and allows comparing performance of Direct3D11 and Direct3D12 implementations. Every asteroid is a combination of one of 1000 unique meshes and one of 10 unique textures.

      Finally, there is an example project that shows how Diligent Engine can be integrated with Unity.

      Future Work
      The engine is under active development. It currently supports Windows desktop, Universal Windows and Android platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and support for more platforms is planned.
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      I'd really love to hear anyone's thoughts on this, any suggestions on what features you'd want to see in a library like this? 
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