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OpenGL [solved]D3D's RHW and OpenGL's W

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--I've rewritten this topic in hopes that someone will reply as I am in need of help desperetly. ========================================================== Solution: Call glLoadIdentity(); after clearing the buffers at the beginning of the scene fixed my problem (which was not anything to do with RHW, but screen co-ordinates. ========================================================== Though this topic blurs between the two API's I'm asking for an explanation of the difference between the two API's seemingly similar co-ordinate component. Why? In a game I have the source-code to that was made in 1999 and uses Direct3D7, Glide 3 and a Software Rasterizer, the fan community have encountered problems with all three renderers at some stage, most people with older machines with 32-bit CPU's and old nVidia cards (such as my GeForce 6200A) don't have a problem with the D3D7 or Software renderers, but the problems vary and there have been too many reports with a lack of information (from a large number of people who don't even know what the difference between add-on and onboard) for us to narrow down the causes. So what I did to counter the problems was propose to my teammates that we rebuild the Renderer's a Direct3D9 and OpenGL Renderer each, would replace the old ones and hopefully kill all renderer related problems. Also note that the game has all three renderers in the same project and uses an #ifdef pre-processor check to see if the current Build-Target is supposed to use the Render###.CPP file the compiler is checking, it's an old and inefficient way but right now we aren't up to the task of rewriting large portions of code to implement a better solution. Now there are many sources that say a lot of things, so far I've read that Glide and OpenGL are similar and while I agree, it seems only the way you write applicaiton code for them are similar, while OpenGL and Direct3D9 now share a lot of similarities. What I began doing is rewriting the 3Dfx Glide code with OpenGL code and since the original renderers don't make use of the Projection or World matrices, we're stuck with the D3D RHW and Glide's messy equivilent's the z,oow,ooz (oow is for W-Buffering and ooz for Z-Buffering IIRC, but I don't see a purpose for W-Buffering in the game since the code seems to indicate the use of only the Z-Buffer) Now... I've done a bit of googling for resources and come up kind of dry for explanations on the differences between the D3D RHW and OpenGL W values of Vertices and am hoping someone here might be able to clear it up for me as I'd like to write these new renderer's ASAP and there are a lot of people eagerly waiting for modern Hardware support. The best I've managed to scrounge up, sadly, is the following:
Quote:
RHW is often 1 divided by the distance from the origin to the object along the z-axis.
From my understanding (and please correct me if I am wrong) that would mean that RHW is:
float origin_z = 0.f;
float vert_z = 10.0f;

float rhw = 1.0f / (vert_z - origin_z);

result: rhw = 0.1f


Now I've tried using the same data for the OpenGL renderer as the D3D renderer but I'm not getting much more than either a mess of triangles that have a heart attack all over the screen, or no triangles at all. RHW and W equal to 1.0f seem to do the same thing if I provide x,y,z values that lie within the view range (except OpenGL's 2D floating point origin is in the middle of the screen (0.0f,0.0f) where as D3D's is not) Anyway hopefully someone reads this topic this time around and can help me out as I'm stuck Trial and Erroring (which is not working out so well with a complete game engine I'm afraid) ~James [Edited by - RexHunter99 on March 16, 2010 9:05:08 AM]

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-Bump-

I hate bumping but I really need help with this, I've spent the last 3 days trying to figure it out through Trial and Error but nothing beats knowing the correct answer straight up.

I don't know if the topic scared anyone away or if I've just gotten accidentally unnoticed, but the help is very very much needed and appreciated.

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I know Opengl expects a certain triangle winding, counter clock wise by default, for a triangle to be considered forward facing. Directx of course expects the opposite. This could mean that triangles are being culled in opengl that would not be culled in direct X and vise verse. As far as actual coordinates being off I don't know that much about DX OGL differences sorry.

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Why can't you just use view and projection matrices like everybody else? There's no need to mess with all this RHW stuff, just use standard perspective calculation and you're done. AFAIK Triangle winding is the same in DX and OGL because its possible to import models from one to the other without issues.

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I'll post a picture of the OpenGL renderer in action (so you can see what I mean) note that in order to even see the triangles, I had to fix the first vertex to the co-ordinates: 0.0f, 0.0f, 0.0f, 1.0f (x, y, z, w)

^^^ Note that the pixels that are not red (that area to the lower left) is the background color I defined when I cleared the color buffer.)

Quote:
Original post by stonemetal
I know Opengl expects a certain triangle winding, counter clock wise by default, for a triangle to be considered forward facing. Directx of course expects the opposite. This could mean that triangles are being culled in opengl that would not be culled in direct X and vise verse. As far as actual coordinates being off I don't know that much about DX OGL differences sorry.

Yes, OpenGL expects the opposite triangle winding to Direct3D, OpenGl requires Anti-Clock-Wise and Direct3D requires Clock-Wise, I've accommodated for this in the renderers (OpenGl will now take in Direct3D information, for testing purposes right now, I'll properly fix this problem at a later stage)

Quote:
Original post by Momoko_Fan
Why can't you just use view and projection matrices like everybody else? There's no need to mess with all this RHW stuff, just use standard perspective calculation and you're done. AFAIK Triangle winding is the same in DX and OGL because its possible to import models from one to the other without issues.

Oh snappy snappy, you know you could have worded this reply in a nicer way? You make it sound like I'm a stupid idiot. I would have tried to use the 'normal' way to do it, but like I've said, the code is very hard to mess around with, right now I'm only replacing code with updated code that uses Direct3D9 and OpenGL ( my problem lies with the OpenGL though) The game's original creators have already processed all the projection data prior to the rendering stage, or as much of it as possible. I have to deal with this RHW stuff because otherwise I'd have to do a complete code overhaul and my position on the team is the graphical programmer, I work on the 3D code because I can visualize a 3D scene in my head rather easily with the given information.

And you're wrong, D3D and OpenGL have different triangle winding by default, you can change how they deal with that if you enable backface culling and change the winding then, but that's a short-term unpreferred fix. Also OpenGL and Direct3D have different co-ordinate systems, one is Left-Hand and the other is Right-Hand, I think I've accomodated for this already though... not 100% sure because I can't quite tell until this RHW stuff is down.


Just going to say this for anyone else going to tell me to do this the 'normal/modern' way with the Perspective/Ortho matrix functions:
If anyone else wants to be a smart-arse like Momoko_Fan was, then why don't you try rewriting over 20,000 lines of code for me? remember you have to accommodate for at least two different graphics APIs and comment most of the functions as you go along so you/others know what they do?

[Edited by - RexHunter99 on March 12, 2010 9:10:03 AM]

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Quote:
Original post by RexHunter99
Also OpenGL and Direct3D have different co-ordinate systems, one is Left-Hand and the other is Right-Hand, I think I've accomodated for this already though... not 100% sure because I can't quite tell until this RHW stuff is down.


Silly question, but sometimes the silly stuff is what gets us... do you mean you transposed the [4]x[4] D3D matrix in order to get an OpenGL [16] element matrix?

Most of the time thats all I have to do when "translating" matrix operations intended for D3D to OpenGL.

Good Luck.

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No, my quick fix solution was to inverse the Y co-ordinate so it'd appear in the 'correct place' or appear in the same place as it would in Direct3D, anyway that's besides the point, I want to know if there's a difference between OpenGL and Direct3D's W and RHW values and if there is, what the difference is so I can accommodate for it in the game code.

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Quote:
Original post by RexHunter99
No, my quick fix solution was to inverse the Y co-ordinate so it'd appear in the 'correct place' or appear in the same place as it would in Direct3D, anyway that's besides the point, I want to know if there's a difference between OpenGL and Direct3D's W and RHW values and if there is, what the difference is so I can accommodate for it in the game code.


I am not sure if there is a difference, shouldn't be, I usually just leave W as 1 in OpenGL, I think the OpenGL ModelView matrix is 2 separate matrices in D3D, so maybe you should factor that in.

Perhars this would help:
Quote:

9.011 How are coordinates transformed? What are the different coordinate spaces?

Object Coordinates are transformed by the ModelView matrix to produce Eye Coordinates.
Eye Coordinates are transformed by the Projection matrix to produce Clip Coordinates.
Clip Coordinate X, Y, and Z are divided by Clip Coordinate W to produce Normalized Device Coordinates.
Normalized Device Coordinates are scaled and translated by the viewport parameters to produce Window Coordinates.
Object coordinates are the raw coordinates you submit to OpenGL with a call to glVertex*() or glVertexPointer(). They represent the coordinates of your object or other geometry you want to render.
Many programmers use a World Coordinate system. Objects are often modeled in one coordinate system, then scaled, translated, and rotated into the world you're constructing. World Coordinates result from transforming Object Coordinates by the modelling transforms stored in the ModelView matrix. However, OpenGL has no concept of World Coordinates. World Coordinates are purely an application construct.
Eye Coordinates result from transforming Object Coordinates by the ModelView matrix. The ModelView matrix contains both modelling and viewing transformations that place the viewer at the origin with the view direction aligned with the negative Z axis.
Clip Coordinates result from transforming Eye Coordinates by the Projection matrix. Clip Coordinate space ranges from -Wc to Wc in all three axes, where Wc is the Clip Coordinate W value. OpenGL clips all coordinates outside this range.
Perspective division performed on the Clip Coordinates produces Normalized Device Coordinates, ranging from -1 to 1 in all three axes.
Window Coordinates result from scaling and translating Normalized Device Coordinates by the viewport. The parameters to glViewport() and glDepthRange() control this transformation. With the viewport, you can map the Normalized Device Coordinate cube to any location in your window and depth buffer.
For more information, see the OpenGL Specification, Figure 2.6.


If you're dealing with matrices though, you shouldn't just mirror Y, you should transpose the matrices because of how they are accessed, OpenGL defines them as a one dimension array whereas D3D access them as a 4x4 two dimensional array, mapping one to the other doesn't leave the elements on the proper positions, check point 9.005 on the link I posted above.

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Just passing through briefly.. might have some code you can look at later that might help?

Aliens Vs Predator original D3D 5/6 code, the linux OpenGL renderer update for said game and my D3D9 equivelant. Source code repository for linux port doesn't seem to be online at the moment so I can't just link you at the moment.

Out of curiosity, what game is it? If it's one I like i'd be interested in helping :)

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Quote:
Original post by sirlemonhead
Just passing through briefly.. might have some code you can look at later that might help?

Aliens Vs Predator original D3D 5/6 code, the linux OpenGL renderer update for said game and my D3D9 equivelant. Source code repository for linux port doesn't seem to be online at the moment so I can't just link you at the moment.

Out of curiosity, what game is it? If it's one I like i'd be interested in helping :)


D3D 5/6 and D3D7 are quite similar despite how far they came ;) After that D3D just went up exponentionally...

Urm that might be nice actually, thanks, when you can could you show me some code? Would be a great help (I'm just implementing the basic HUD UI function equivalents now, hopefully glDrawPixels isn't too slow to hamper gameplay)

The game is Carnivores 2, created by a company known as Action-Forms, currently, Tatem Games a mobile gaming company has a license to make an iPhone App called Carnivores: Dinosaur Hunter which is set for release this year. Action-Forms gave us the source-code and we were dismayed to find that they'd overwritten the first game's code with the second game's code and also lost the menu code (the game compiles into a .REN file (a renamed .EXE file) that the menu executes after you've selected your level, dinosaurs and weapons.

You may or may not have heard of it ;)

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So I've been tinkering around again and got the Direct3D9 version to work (with extra tinkering) I think I understand somewhat how the z and RHW values work (when the RHW is required eg; when no projection matrix is used)

The z value of a vertice defines the value used within the Z-Buffer, where 1.0f is as close to the camera as possible and 0.0f is as far away as possible. The RHW is typically calculated as 1.0f / z_dist_from_origin (origin is the camera position which in my case always remains 0,0,0)

For some reason, the original D3D renderer defines a value _ZSCALE as -16.0f and then divides it by the vertices z value, then the RHW value is processed where _AZSCALE is equal to 1.0f / 16.0f and RHW is equal to z * _AZSCALE

Also, by default Direct3D's co-ordinate origin is at the top-left of the window and you simply pass the width of the window as a float to a vertex and the vertex will be placed on the right hand side of the window, where as in OpenGl the origin is in the center of the window space, and a value of 1.0f for the x axis will place the vertex on the right side of the window, a value of -1.0f will place it on the left side of the window. For y it's the same, 1.0f will put the y position on the top side of the window and -1.0f will put it on the bottom side.


I'll post some of the data I dumped during a test of mine. I have to work with this data that I have without manipulating it too much ( I can modify it if absolutely necessary but I'd prefer not to)

ev0: 438.974976,371.207733,-19070.726563
ev1: 438.974976,371.207733,-19070.726563
ev2: 438.974976,371.207733,-18559.201172
ev0: 438.974976,371.207733,-19070.726563
ev1: 438.974976,371.207733,-18559.201172
ev2: 438.974976,371.207733,-18558.964844
ev0: 456.157349,371.207733,-19070.726563
ev1: 456.157349,371.207733,-19070.726563
ev2: 456.157349,371.207733,-18559.201172
ev0: 456.157349,371.207733,-19070.726563
ev1: 456.157349,371.207733,-18559.201172
ev2: 456.157349,371.207733,-18559.201172
ev0: 559.253235,364.763855,-19070.019531
ev1: 559.253235,364.763855,-19069.783203
ev2: 559.253235,364.763855,-18558.257813
ev0: 559.253235,364.763855,-19070.019531
ev1: 559.253235,364.763855,-18558.257813
ev2: 559.253235,364.763855,-18558.494141
ev0: 473.339691,371.207733,-19070.726563
ev1: 473.339691,371.207733,-19070.726563
ev2: 473.339691,371.207733,-18559.201172
ev0: 473.339691,371.207733,-19070.726563
ev1: 473.339691,371.207733,-18559.201172
ev2: 473.339691,371.207733,-18559.201172
ev0: 542.069885,366.911865,-19070.253906
ev1: 542.069885,366.911865,-19070.019531
ev2: 542.069885,366.911865,-18558.494141
ev0: 542.069885,366.911865,-19070.253906
ev1: 542.069885,366.911865,-18558.494141
ev2: 542.069885,366.911865,-18558.728516


Left-hand float is the X, the middle float is the Y and the Right-hand float is the Z. These are the same values used for the old D3D and the new D3D9 code.


EDIT:
I think I might be able to solve the origin problem with glFrustum... this will take some testing but the way i'm thinking of it is something along the lines of:

glFrustum(0.f,(float)WinW, 0.f, (float)WinH, 0.1f, 100000.f);

This should set the upper left corner as the origin (or the lower left corner, either one) and then the D3D data should work so long as z and w aren't treated differently from D3D's z and RHW. (again, seems not to be a difference between them other than OpenGl drops the 'rh' prefix.)


[Edited by - RexHunter99 on March 12, 2010 11:56:47 PM]

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Ah! never mind folks... I got it working after I put glLoadIdentity() right after I cleared the Buffers, seems to work 100% the same as the D3D9 version so long as I leave the RHW value in OpenGL as 1.0f o-0

Not really solved the question at hand, but my problem is over for now so I guess this means this is solved somewhat.

Thanks to all who replied.

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Sorry I didn't reply sooner. In the D3D world, the RHW (reciprocal homogenous W) is another way of saying, bypass the projection and modelview transform.

In the GL world, there is no such thing as a bypass. The solution is to set the projection and modelview matrices to identity.

In todays world of GPUs, in other words, a shader world, RHW is a dead concept. The vertex shader always processes ALL vertices.

Quote:
Ah! never mind folks... I got it working after I put glLoadIdentity() right after I cleared the Buffers, seems to work 100% the same as the D3D9 version so long as I leave the RHW value in OpenGL as 1.0f o-0


That is the only solution. Also, as someone said, w is always 1 by default for vertices. It is the same for D3D.

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But my D3D9 Renderer (it's the old D3D7 renderer just initializes D3D9 and calls that) uses RHW fine, infact it works just as it did 'back-in-the-day' it seems. I spent quite a bit of time fiddling around with the RHW in D3D9 in the end I had to inverse the old value to get a correctly working one for D3D9... so either you are wrong or I am doing something that is not done normally.

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It will work with D3D9. The only thing I am saying that it is a dead concept. I haven't gotten into D3D10 but I wonder if they got rid of it. I know that you must do shaders with D3D10.

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      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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