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OpenGL true occlusion culling

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Hi, I need to know wether an object is visible or not. Since we have some issues with openGL and hardware occlusion queries and the support of prev. video cards, I decided to propose another way, and see if this is doable. Maybe I am missing something, or maybe my methods would suck, please give me some feedback! First we render the whole scene to the backbuffer and depthbuffer, without the objects to test. Second we use the depthbuffer from the main scene, use a render target texture, and render all the objects to identify with a color code that can reproduce a number once read back. Third we scale down the rendertexture to a small size. This will remove all "little visible" objects but maintain most very visible once, say we scale with a factor of 4 Forth we read back all the pixels from the scaled down texture and identify the visible objects. How does this sound?

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That requires you to synch up the CPU and GPU, so you'll create a large stall...

Some new console games are actually using a software renderer to rasterize a low-resolution depth buffer, and then do all the occlusion tests on the CPU to avoid the stall ;)

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Hardware occlusion culling isn't used by lots of games, and it definitely shouldn't be the only method of occlusion-culling used.

There's an article on gamasutra (can't find it at the moment) and probably some here on Gamedev too, which describe "occluders" or "occlusion volumes".

Basically, you define a convex polygonal shape for large objects in your scene. You then find the edges of this shape and use them to project an occlusion frustum out into the scene. You can then test the bounding-boxes of small objects on the CPU to see if they lie inside any of these frustums -- if they do, they're hidden.

You should probably also look into sector/portal systems. Portals can be culled using the same method as above (test the portal's BB against occluders, if hidden, don't traverse into the sector linked by the portal).

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before you start with occlusion culling, you shall identify what you want to achive.

occlusion culling is not to make something faster, it's rather to load-balance work. you will have more costs on one side and less on another, the goal is to save time in average by utilizing underutilized resources.

e.g.
-you have damn expensive pixelshaders, in this case your goal #1 is to cull pixels. This could be accomplished by a z-prepass.
-you are vertex limited. in this case your idea is not that bad, you could use the ID-rendertarget in a 2nd pass and identify the visible IDs. resolve them into an ID-buffer (1d texture either set to 0 or 1) and the first instruction in the vertexshader would check the ID and either reject the wohle vertex (dynamic branching) or execute the whole shader (like comple FFT for water or 8weight skinning for some 50k character).
-you are drawcall bound, in this case u need an earlier stage to cull, maybe a software rasterizer or even some artist set anti-portals (which are damn simple to implement) would help already.

sometimes some middleware could help :)

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You should probably also look into sector/portal systems. Portals can be culled using the same method as above (test the portal's BB against occluders, if hidden, don't traverse into the sector linked by the portal).


I would advise against using portals for the following reasons:

1. the are limited to indoor environments

2. you cannot classify where you are in the world without keeping track of every single sector you are in and testing whether you intersects a portal or not every time you move. (you also need to pair yourself with an initial sector) (this goes for all objects, not just the player). You *could* use a BSP tree to classify where you are (like in doom3) but anything involving a bsp is hellish work.

3. portal culling is done in world space, image-space methods should be used as they address the actual problem of occlusion on a raster grid rather than be specific to triangular meshes. e.g. In an image space method - dynamic particles could also be used as occluders.

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2. you cannot classify where you are in the world without keeping track of every single sector you are in and testing whether you intersects a portal or not every time you move. (you also need to pair yourself with an initial sector) (this goes for all objects, not just the player). You *could* use a BSP tree to classify where you are (like in doom3) but anything involving a bsp is hellish work.
that depends on the rooms. if you have room geometry as well, it's fairly easy to check where you're in, and you can assume that "indoor" scene do have those "rooms". in cryengine it seems to be a simple n-gone.

Quote:

3. portal culling is done in world space, image-space methods should be used as they address the actual problem of occlusion on a raster grid rather than be specific to triangular meshes. e.g. In an image space method - dynamic particles could also be used as occluders.


imagespace doesn't strictly imply that a raster grid is used. doing portalculling in imagespace reduces the work to simple 2d-vector-math (rect-rect clipping).

I think a big advantage and at the same time disadvantage is that you need humans to set it up. so they need to be skilled in setting them in the most performant way. portals to cull empty rooms are useless, but also missing portals can be a big performance hit.

but I think the OP was anyway asking for something for dynamic scene, that makes PVS and portals kinda obsolete.

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You can still make use of scene partitioning even if not the entire scene is static. If you know, given a part of the map, what other parts are visible, you can check if the dynamic object is outside the visible parts, and if so, cull it.

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Just to let you guys know:

I need to know if a view specific objects are visible for the viewer or not, hence the idea of checking with occluders. This doesn't need to be 100% pixel perfect, I need to know if an object is visible enough, so there is room for error (thus I can use a primitive object like a box in replacement of the actual objects in the scene)

I thought about using a software rasterizer (idea1, extension) in case of no hardware occlusion support.

Also i thought about using hardware occlusion when possible, either hardware occlusion queries or the method I described in my fipo.

It is not important that the check is done every frame from the game, but it does need to be checked periodically (let's say at least twice each second) so in case of software rasterizing / occlusion check this could be threaded in a second thread, away from the main thread.

Please keep spoofing me with idea's, critism and suggestions, since I am very curious on how to solve this.

Oh and p.s. as in any case, it needs to be as fast as possible :-P

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How is your scene structured? I.e., Is there a good amount of occlusion from just culling dynamic objects behind static objects, or do you really need dynamic objects to cull dynamic objects?

If there are good static occluders (indoor scenes, outdoor scenes with occluding obstacles?), then I suggest looking for a visibility preprocessing algorithm or tool, which will allow you to make very quick decisions using simple queries at run-time.

If you really need to test for occlusion behind dynamic objects, then as another poster said, you should really consider where your bottleneck is, and see if either a z-pass or front-to-back sorting will get you what you need (removing pixel shaded overdraw). If you are geometry limited, then you should probably consider hardware occlusion culling (be careful with this, if you query to frequently, you will sync gpu to cpu and perf will suffer, if you batch too much, then by the definition of your scene type you may not get enough occlusion).

A SW rasterizer is good and all, but it is really difficult to build one with the flexibility of the API geometry pipe (VS and all). Perhaps if you were only using a some simple transforms (single matrices, pass through, etc.)

Something that you haven't really mentioned is if you intend to use this for rendering or for some other purpose (e.g., AI). Is it the former or latter? If the latter, then definitely use occlusion queries, and read back the results after a few frames.

As for the original idea. Just don't do it. :-) Reading back the frame data is a terrible idea (a huge stall, at least in DX9).

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You could render all occluders in the scene without texture mapping, lighting etc. (This could be done in one draw call).

Then get the z buffer and build mip-maps, each mip-map is one fourth the size of the previous. When building a mip map you combine four z values into one by using the maximum z value of the four. At the highest level you would have just one pixel (z value) which is the furthest z value in the scene for this view point.

You could then take your objects 3d bounding box, project all vertices to the screen and build a 2d bounding box around those vertices. You then would use the closest z value of the bounding box and rasterize a simple rectangle to the highest level mip-map. If at any point the rectangles z value is < the z-value in the z buffer you move to a lower level mip until you reach the lowest level, if however no z-values in the buffer are < the rectangles z value you know that the box is not visible and can be culled away.

This is an idea I am going to implement soon, its just a slightly modified HOM's algorithm. I don't know how expensive it would be to lock the z buffer and build mip-maps each frame.

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Original post by crowley9
How is your scene structured? I.e., Is there a good amount of occlusion from just culling dynamic objects behind static objects, or do you really need dynamic objects to cull dynamic objects?

If there are good static occluders (indoor scenes, outdoor scenes with occluding obstacles?), then I suggest looking for a visibility preprocessing algorithm or tool, which will allow you to make very quick decisions using simple queries at run-time.

If you really need to test for occlusion behind dynamic objects, then as another poster said, you should really consider where your bottleneck is, and see if either a z-pass or front-to-back sorting will get you what you need (removing pixel shaded overdraw). If you are geometry limited, then you should probably consider hardware occlusion culling (be careful with this, if you query to frequently, you will sync gpu to cpu and perf will suffer, if you batch too much, then by the definition of your scene type you may not get enough occlusion).

A SW rasterizer is good and all, but it is really difficult to build one with the flexibility of the API geometry pipe (VS and all). Perhaps if you were only using a some simple transforms (single matrices, pass through, etc.)

Something that you haven't really mentioned is if you intend to use this for rendering or for some other purpose (e.g., AI). Is it the former or latter? If the latter, then definitely use occlusion queries, and read back the results after a few frames.

As for the original idea. Just don't do it. :-) Reading back the frame data is a terrible idea (a huge stall, at least in DX9).


Hi,

to remove some confusion, we trying to make different setups for different engines plugin-wise, to check if certain objects were seen by the user, for testing purpose in research. The scene's are dynamic and not persé static, but it would be for lower profile games that do not take the extreme out of your pc just yet ;-)

I read some articles but i forgot to bookmark them since it was a while ago that a lot of engines now days create CPU occlusion maps to optimize scenes, which would be a good idea the other way around, to test if something was visible rather then if something is invisible.

To react on your idea about software rasterizers: nothing fancy is needed, it is just pure for testing occlusion, on a non 100% accurate basis but accurate enough for us to tell that the person beeing tested saw the object good enough. Imagine us hanging a plane with a bord saying "did u see this?" in a room. The whole idea of the plugin probably has some different output later on, that's why i want to try a software solution rather then standing behind it. So nothing fancy, just transformation of the view/projection and draw of an id or something.

I do know about all other techniques to pre process a lot (frustum culling, octtree's etc) and I also even thought about the idea that we can have a maximum radius where visibilty comes in to action. Since we have about 5 objects per scene/level to test, we can make a routine that first checks if those objects are in range, and if so, if they are visible.

And to note: I have no intention to think I can make a plugin that works on any engine that is out there, but I do want to try and make a few fallback theories to support things that are there (hardware occlusion queries, my idea was an idea but as we can see it was crushed and shot down ;) ) and based on that try to make plugins for different engines.

Thanks so much for all your help guys, you are really giving me some idea's and thoughts. I hope I answer any unclear questions/idea's with this post, and I hope you have some energy left ;-)

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Original post by staticVoid2
You could render all occluders in the scene without texture mapping, lighting etc. (This could be done in one draw call).

Then get the z buffer and build mip-maps, each mip-map is one fourth the size of the previous. When building a mip map you combine four z values into one by using the maximum z value of the four. At the highest level you would have just one pixel (z value) which is the furthest z value in the scene for this view point.

You could then take your objects 3d bounding box, project all vertices to the screen and build a 2d bounding box around those vertices. You then would use the closest z value of the bounding box and rasterize a simple rectangle to the highest level mip-map. If at any point the rectangles z value is < the z-value in the z buffer you move to a lower level mip until you reach the lowest level, if however no z-values in the buffer are < the rectangles z value you know that the box is not visible and can be culled away.

This is an idea I am going to implement soon, its just a slightly modified HOM's algorithm. I don't know how expensive it would be to lock the z buffer and build mip-maps each frame.


To me, this sounds very expensive and a lot of work. How would this improve against rendering 3D bounding volumes to an occluder buffer and do a read-back on that? Wether this occluder buffer is through hardware occlusion queries of an offscreen plane or a smart software rasterizer thing, I think making mipmaps and locking the depth buffer would stall a lot, but I am not sure about this?

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To me, this sounds very expensive and a lot of work. How would this improve against rendering 3D bounding volumes to an occluder buffer and do a read-back on that?


it is essentially the same thing although with using mip-maps you avoid expensive fill rates.

you also render a simple rectangle around the bounding box rather than up to a possible 6 triangles.

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Then get the z buffer and build mip-maps, each mip-map is one fourth the size of the previous.

from VMem to system mem? that's probably more expensive than anything you can gain from it. if you want to rely on the hardware, you need to work asynchronously and for that occlusion queries are the way to go.

Quote:

You could then take your objects 3d bounding box, project all vertices to the screen and build a 2d bounding box around those vertices. You then would use the closest z value of the bounding box and rasterize a simple rectangle to the highest level mip-map.
this approauch would be very view dependend. in lot of cases object would be market visible behind walls, just because their rect in screenspace, using nearest-z stick through walls. it would work great with occluders that are coplanar with the nearplane, but you just need to turn the came slightly and you might get an dramatic increase in drawcalls.

maybe that'll be good as an first-pass approach.

Quote:
This is an idea I am going to implement soon, its just a slightly modified HOM's algorithm. I don't know how expensive it would be to lock the z buffer and build mip-maps each frame.
building mipmaps on gpu would probably be fairly simple, but getting the buffer might be damn expensive.
you could maybe experiment with staging buffers from d3d10/11 and map them like 5frames later to be sure u dont stall anything. but then again, occlusion queries would deliver the same result with the same latency.

@FeverGames
so you're making a culling middleware like dPVS by umbra? what exactly do you mean by "we have about 5 objects per scene/level to test"? just 5objects? is it worth to implement culling for 5objects?
but yeah, distance based "culling" is quite common, it's usually combined with LOD systems, so the lowest LOD is just an empty mesh.

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Original post by Krypt0n
Quote:

Then get the z buffer and build mip-maps, each mip-map is one fourth the size of the previous.

from VMem to system mem? that's probably more expensive than anything you can gain from it. if you want to rely on the hardware, you need to work asynchronously and for that occlusion queries are the way to go.

Quote:

You could then take your objects 3d bounding box, project all vertices to the screen and build a 2d bounding box around those vertices. You then would use the closest z value of the bounding box and rasterize a simple rectangle to the highest level mip-map.
this approauch would be very view dependend. in lot of cases object would be market visible behind walls, just because their rect in screenspace, using nearest-z stick through walls. it would work great with occluders that are coplanar with the nearplane, but you just need to turn the came slightly and you might get an dramatic increase in drawcalls.

maybe that'll be good as an first-pass approach.

Quote:
This is an idea I am going to implement soon, its just a slightly modified HOM's algorithm. I don't know how expensive it would be to lock the z buffer and build mip-maps each frame.
building mipmaps on gpu would probably be fairly simple, but getting the buffer might be damn expensive.
you could maybe experiment with staging buffers from d3d10/11 and map them like 5frames later to be sure u dont stall anything. but then again, occlusion queries would deliver the same result with the same latency.

@FeverGames
so you're making a culling middleware like dPVS by umbra? what exactly do you mean by "we have about 5 objects per scene/level to test"? just 5objects? is it worth to implement culling for 5objects?
but yeah, distance based "culling" is quite common, it's usually combined with LOD systems, so the lowest LOD is just an empty mesh.


Well yeah, "just 5". These objects are important to be marked as seen once they have been seen, so yeah, a fair small share of the CPU or GPU can be given away for this. To come to a fair trade of cost against speed, I already said it doesn't need to check every game frame, it can be done in a fixed once in a while time step.

Why would a threaded CPU occlusion rasterizer be not done? With Bounding boxes?

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from VMem to system mem? that's probably more expensive than anything you can gain from it. if you want to rely on the hardware, you need to work asynchronously and for that occlusion queries are the way to go.



you could implement a basic software rasterizer to render the occlusion geometry to a low-resolution z buffer and then build the mip-maps but then it becomes of a problem of determining the set of occluders that are visible as the software rasterizer will likely be to slow to render them all.

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Original post by staticVoid2
Quote:


from VMem to system mem? that's probably more expensive than anything you can gain from it. if you want to rely on the hardware, you need to work asynchronously and for that occlusion queries are the way to go.



you could implement a basic software rasterizer to render the occlusion geometry to a low-resolution z buffer and then build the mip-maps but then it becomes of a problem of determining the set of occluders that are visible as the software rasterizer will likely be to slow to render them all.


I still don't really get what the use of mip maps are in the ideas..

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The objects that I want to know visibility from are always a plane. How about calculating a few rows and columns of points on that plane in world space, and test with rays from the centre of the camera to these points against the view frustum visible objects their bounding volumes?

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Well yeah, "just 5". These objects are important to be marked as seen once they have been seen, so yeah, a fair small share of the CPU or GPU can be given away for this. To come to a fair trade of cost against speed, I already said it doesn't need to check every game frame, it can be done in a fixed once in a while time step.

so you're cost would be kinda 99% for generating an occlusion buffer, 1% for testing some bounding box against it. that's not effective.

Quote:

Why would a threaded CPU occlusion rasterizer be not done? With Bounding boxes?
why that effort if the result is the same as a hardware occlusion query would be?


in a case where you've few variable costs and most is fixcost, you goal needs to be to reduce the fix costs, so get rid of generating an occlusion buffer.
an alternative method to accomplish that can be raycasts. That's a common way to check for visibility e.g. for AI where you have N objects (typically <10) that need to know if there is any visibility inbetween. that would usually mean you have to rasterize N occlusion buffers. it's cheaper to make raycasts in that case.
especially with just 5 objects to test, you'll probably be faster.


Quote:

I still don't really get what the use of mip maps are in the ideas..

Mipmaps are used to have a conservative representation of far more data. it's useful in cases where u assume that most objects will be culled.
Imagin the simplest case, the mip level with just 1zexel. you just need to compare the nearest-z of an object with one depth value an if everything works out, you reject the whole object immediatelly.
it's of course the opposide with fine occlusion that is not that dense e.g. open enviropments where you always have some skybox-depth value that is max.
for reference: http://www.gamasutra.com/view/feature/3394/occlusion_culling_algorithms.php?page=2



Quote:
The objects that I want to know visibility from are always a plane.
you test always 5 planes? i'm really curious what you're doing. with 5 planes you might be able to have the possibility for some fast analytical methode, like projecting the scene triangles on the planes and tesselate it (some kind of 2d bsp). that could be specially fast if you have a low amount of triangles that cause a lot of overdraw.

Quote:

How about calculating a few rows and columns of points on that plane in world space, and test with rays from the centre of the camera to these points against the view frustum visible objects their bounding volumes?

i guess you had the idea already I described above, damn.;)

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      Introduction
      Graphics APIs have come a long way from small set of basic commands allowing limited control of configurable stages of early 3D accelerators to very low-level programming interfaces exposing almost every aspect of the underlying graphics hardware. Next-generation APIs, Direct3D12 by Microsoft and Vulkan by Khronos are relatively new and have only started getting widespread adoption and support from hardware vendors, while Direct3D11 and OpenGL are still considered industry standard. New APIs can provide substantial performance and functional improvements, but may not be supported by older hardware. An application targeting wide range of platforms needs to support Direct3D11 and OpenGL. New APIs will not give any advantage when used with old paradigms. It is totally possible to add Direct3D12 support to an existing renderer by implementing Direct3D11 interface through Direct3D12, but this will give zero benefits. Instead, new approaches and rendering architectures that leverage flexibility provided by the next-generation APIs are expected to be developed.
      There are at least four APIs (Direct3D11, Direct3D12, OpenGL/GLES, Vulkan, plus Apple's Metal for iOS and osX platforms) that a cross-platform 3D application may need to support. Writing separate code paths for all APIs is clearly not an option for any real-world application and the need for a cross-platform graphics abstraction layer is evident. The following is the list of requirements that I believe such layer needs to satisfy:
      Lightweight abstractions: the API should be as close to the underlying native APIs as possible to allow an application leverage all available low-level functionality. In many cases this requirement is difficult to achieve because specific features exposed by different APIs may vary considerably. Low performance overhead: the abstraction layer needs to be efficient from performance point of view. If it introduces considerable amount of overhead, there is no point in using it. Convenience: the API needs to be convenient to use. It needs to assist developers in achieving their goals not limiting their control of the graphics hardware. Multithreading: ability to efficiently parallelize work is in the core of Direct3D12 and Vulkan and one of the main selling points of the new APIs. Support for multithreading in a cross-platform layer is a must. Extensibility: no matter how well the API is designed, it still introduces some level of abstraction. In some cases the most efficient way to implement certain functionality is to directly use native API. The abstraction layer needs to provide seamless interoperability with the underlying native APIs to provide a way for the app to add features that may be missing. Diligent Engine is designed to solve these problems. Its main goal is to take advantages of the next-generation APIs such as Direct3D12 and Vulkan, but at the same time provide support for older platforms via Direct3D11, OpenGL and OpenGLES. Diligent Engine exposes common C++ front-end for all supported platforms and provides interoperability with underlying native APIs. It also supports integration with Unity and is designed to be used as graphics subsystem in a standalone game engine, Unity native plugin or any other 3D application. Full source code is available for download at GitHub and is free to use.
      Overview
      Diligent Engine API takes some features from Direct3D11 and Direct3D12 as well as introduces new concepts to hide certain platform-specific details and make the system easy to use. It contains the following main components:
      Render device (IRenderDevice  interface) is responsible for creating all other objects (textures, buffers, shaders, pipeline states, etc.).
      Device context (IDeviceContext interface) is the main interface for recording rendering commands. Similar to Direct3D11, there are immediate context and deferred contexts (which in Direct3D11 implementation map directly to the corresponding context types). Immediate context combines command queue and command list recording functionality. It records commands and submits the command list for execution when it contains sufficient number of commands. Deferred contexts are designed to only record command lists that can be submitted for execution through the immediate context.
      An alternative way to design the API would be to expose command queue and command lists directly. This approach however does not map well to Direct3D11 and OpenGL. Besides, some functionality (such as dynamic descriptor allocation) can be much more efficiently implemented when it is known that a command list is recorded by a certain deferred context from some thread.
      The approach taken in the engine does not limit scalability as the application is expected to create one deferred context per thread, and internally every deferred context records a command list in lock-free fashion. At the same time this approach maps well to older APIs.
      In current implementation, only one immediate context that uses default graphics command queue is created. To support multiple GPUs or multiple command queue types (compute, copy, etc.), it is natural to have one immediate contexts per queue. Cross-context synchronization utilities will be necessary.
      Swap Chain (ISwapChain interface). Swap chain interface represents a chain of back buffers and is responsible for showing the final rendered image on the screen.
      Render device, device contexts and swap chain are created during the engine initialization.
      Resources (ITexture and IBuffer interfaces). There are two types of resources - textures and buffers. There are many different texture types (2D textures, 3D textures, texture array, cubmepas, etc.) that can all be represented by ITexture interface.
      Resources Views (ITextureView and IBufferView interfaces). While textures and buffers are mere data containers, texture views and buffer views describe how the data should be interpreted. For instance, a 2D texture can be used as a render target for rendering commands or as a shader resource.
      Pipeline State (IPipelineState interface). GPU pipeline contains many configurable stages (depth-stencil, rasterizer and blend states, different shader stage, etc.). Direct3D11 uses coarse-grain objects to set all stage parameters at once (for instance, a rasterizer object encompasses all rasterizer attributes), while OpenGL contains myriad functions to fine-grain control every individual attribute of every stage. Both methods do not map very well to modern graphics hardware that combines all states into one monolithic state under the hood. Direct3D12 directly exposes pipeline state object in the API, and Diligent Engine uses the same approach.
      Shader Resource Binding (IShaderResourceBinding interface). Shaders are programs that run on the GPU. Shaders may access various resources (textures and buffers), and setting correspondence between shader variables and actual resources is called resource binding. Resource binding implementation varies considerably between different API. Diligent Engine introduces a new object called shader resource binding that encompasses all resources needed by all shaders in a certain pipeline state.
      API Basics
      Creating Resources
      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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