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OpenGL backface culling

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I was just wondering if anyone knows exactly how OpenGL performs backface culling. I'm guessing that it looks at the 2d projection and sees if the points are in clockwise or counterclockwise order. If they're counterclockwise, and it's in GL_CW mode, then I think it removes the face. What is the fastest way to tell if a set of points are clockwise or counterclockwise though? Thanks. mike http://www.coolgroups.com/

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I don't know but my guess would be to calculate something similar to a normal for the points and determine whether the resulting vector is pointing towards or away from the viewer.

To add to your question, is there a difference between swapping between back & front culling, or leaving culling mode as it is & swapping between GL_CW & GL_CCW modes performance-wise? Apart from having to specify the vertices in a different order I mean.

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Your guess is correct. As far I know, that's exactly how OpenGL performs backface culling.
As for your second question: I believe there is no other way than to arrange your vertices in the desired order by yourself.


//Clockwise
glBegin(GL_TRIANGLES);
glVertex3f(-1.0f,-1.0f,0);
glVertex3f( 1.0f,-1.0f,0);
glVertex3f( 1.0f, 1.0f,0);
glEnd();

//Counter Clockwise
glBegin(GL_TRIANGLES);
glVertex3f(-1.0f,-1.0f,0);
glVertex3f( 1.0f, 1.0f,0);
glVertex3f( 1.0f,-1.0f,0);
glEnd();



Greets

Chris

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Guest Anonymous Poster
Backface culling is done in hardware, not by OpenGL. I assume the rasterizer can figure out the orientation when it's calculating deltas and stuff anyway in preparations for filling the triangle.

DrewGreen: No, there is no speed difference.

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well, as far as i know, the algorithm is the same as the one used by lighting. it computest the triangle normal vector, then it computes the viewer vector and then it computes a dotproduct between them. if the angle is > 90 degrees, the triangle is invisible to the user.

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If this is true, then why is it that OpenGL makes you do normals yourself for lighting but does them for you for backface culling?

Quote:
Original post by meeshoo
well, as far as i know, the algorithm is the same as the one used by lighting. it computest the triangle normal vector, then it computes the viewer vector and then it computes a dotproduct between them. if the angle is > 90 degrees, the triangle is invisible to the user.


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Quote:
Original post by mike74
If this is true, then why is it that OpenGL makes you do normals yourself for lighting but does them for you for backface culling?

Quote:
Original post by meeshoo
well, as far as i know, the algorithm is the same as the one used by lighting. it computest the triangle normal vector, then it computes the viewer vector and then it computes a dotproduct between them. if the angle is > 90 degrees, the triangle is invisible to the user.


For flexibility, and for the fact that a single triangle can have three different normals for surface approximation, all pointing to different directions.

The winding order of the vertices - in the context of graphics API:s - is almost exclusively used for backface culling.

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The algorithm is very simple

Given a polygon A. B, C, .... you can compute the normal from the first three points.

normal = ( B-A ) ^ ( C-A )


The observer looks always in the opposite z direction ( that is after the modelview transform is like it is)

Now, if the polygon is looking toward the observer the scalar product

normal * observer_direction

is negative otherwise is positive.

// since you need only the z you can 'optimize' the code
bool back_faced = CCW ? ((B-A)^(C-A)).z > 0 : ((B-A)^(C-A)).z < 0;


It's performed by OpenGL before rasterization

EDIT: I confused the sign in back_faced (as I wrote before if the sign is <0 the polygon is front_facing...)

[Edited by - blizzard999 on September 6, 2005 4:05:34 AM]

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Quote:
Original post by blizzard999
The algorithm is very simple
Given a polygon A. B, C, .... you can compute the normal from the first three points.
The observer looks always in the opposite z direction ( that is after the modelview transform is like it is)
Now, if the polygon is looking toward the observer the scalar product
is negative otherwise is positive.


I'm sorey, but you are absolutely wrong )))
Imagine the situation: you have wide viewport with the horizontal range of about 120-150 degrees. Imagine the cube in front of you, which is oriented right to you by one of its faces. Only cube front side is visible to you, the others are got clipped by your algorithm, because they have 0 dot product (the back side has negative dot product, so, it is clipped anyway).
This is right situation and right solution, but until the cube begins to move to the right. It is moving, moving... Dot products remain the same... Now we must see it's left side!!! Imagine it, we MUST see it! But it is still clipped due to your algo.

The only right method to verify it, is to build a plane, including 3 triangle vertices, and to watch, whether the viewing point is lying in positive half-space according to this plane.

So, the algo is:
---------------

A, B, C - points of triangle after modelview transformation

calculate triangle plane:
N = (B - A) x (C - A)
d = A dot N

Now, the equation of plane is : N dot X - d = 0
Equation of positive half-space is : N dot X - d > 0


So, put here X = (0,0,0)
Now, we have -d > 0 to accomplish test, or, d < 0 - triangle is visible

So, if A dot ((B - A) x (C - A)) >= 0 - triangle is got culled.

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Today we use OpenGL, DirectX and even pixel and fragment shaders so we have forgot the ancient art [smile]

You are right!!! And I'm wrong [bawling] but be nice...not totally.

The discriminant is the sign of the distance between the observer position (ie O=(0,0,0) after transform) and the poly plane.

So if the normal is

N = (B-A)^(C-A) in CCW notation

the plane is described by the implicit equation

N * P + d = 0

where d = - N * A ( A is one of the points in the plane )

Now, the distance origin-plane is simply...d = - N * A

If this distance is positive the polygon is front-facing hence 'visible'


float d = - A * ( (B-A)^(C-A) )
bool back_faced = CCW ? d<0 : d>0


This is what you explained. Thanks for the correction. [smile]

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blizzard999, never mind! )))

I know it, only because sometimes )), when I was a young boy, I made total software rendering. Engine was Z-Buffer based, it has features of lightning, texturing, environmental mapping. It has no transparency support at all )))
And, surely, I made back-face culling. The very first realisation was the same, that you described first ))) And it didn't work properly ))) So I had to think a little bit more )))

GL!

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Whoa, this thread has too much misinformation!

In OpenGL, the culling is done in screen-space. That is, the vertices of the triangle are transformed into clip space and divided by w. Then, based on their winding, determines which side of the face(front or back) you're looking at. It uses that result to do the culling if GL_CULL_FACE is enabled, or to determine which material parameters(front or back) will be used if you have two-sided lighting enabled. There is no normals involved or anything like that.

There is the EXT_CULL_VERTEX extension that allows to cull vertices based on their normals(which you must supply as always). When all vertices are culled, then the whole face is culled. You may gain some performance because vertices can be culled without going through the transformation to screen-space.

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mikeman
I've never said here, that I know, how GL does back-face cull )))
I've just made some correction into blixxard999 algoritm )))

Anyway, thnx for the info! Very informative!

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If it doesn't use normals, then how does it determine their winding in screen space?

Quote:
Original post by mikeman
Whoa, this thread has too much misinformation!

In OpenGL, the culling is done in screen-space. That is, the vertices of the triangle are transformed into clip space and divided by w. Then, based on their winding, determines which side of the face(front or back) you're looking at. It uses that result to do the culling if GL_CULL_FACE is enabled, or to determine which material parameters(front or back) will be used if you have two-sided lighting enabled. There is no normals involved or anything like that.

There is the EXT_CULL_VERTEX extension that allows to cull vertices based on their normals(which you must supply as always). When all vertices are culled, then the whole face is culled. You may gain some performance because vertices can be culled without going through the transformation to screen-space.


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Quote:
Original post by mike74
If it doesn't use normals, then how does it determine their winding in screen space?


it checks the sign of the z-component of the cross-product between the two edge-vectors relative to one of the vertices.

something like this:

float x0 = v[2].x - v[0].x;
float y0 = v[2].y - v[0].y;

float x1 = v[2].x - v[1].x;
float y1 = v[2].y - v[1].y;

float cz = x0 * y1 - y0 * x1;

if (cz > 0.f) return GL_CW;
else return GL_CCW;

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Quote:
Original post by mikeman
Whoa, this thread has too much misinformation!

In OpenGL, the culling is done in screen-space. That is, the vertices of the triangle are transformed into clip space and divided by w. Then, based on their winding, determines which side of the face(front or back) you're looking at. It uses that result to do the culling if GL_CULL_FACE is enabled, or to determine which material parameters(front or back) will be used if you have two-sided lighting enabled. There is no normals involved or anything like that.

There is the EXT_CULL_VERTEX extension that allows to cull vertices based on their normals(which you must supply as always). When all vertices are culled, then the whole face is culled. You may gain some performance because vertices can be culled without going through the transformation to screen-space.


The same result can be achieved in space or screen coordinates; however, as the algorithm show, it is more efficient compute it in space coordinates because you can cull a polygon just after modelview transform.
Also you can deduce which side is visible.

Note that the normal we are talking about is the geometric normal of the polygon plane and not the normal(s) used for lighting.

In other words if you use this algorithm ( and it's now correct[smile] ) you will see no difference with the result provided by GL...
It also probable that different GL implementations (video cards, drivers,...) use different algorithms to produce the same result. Why not?

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Quote:

In other words if you use this algorithm ( and it's now correct ) you will see no difference with the result provided by GL...
It also probable that different GL implementations (video cards, drivers,...) use different algorithms to produce the same result. Why not?


It's not a matter of implementation. The OpenGL specs clearly state otherwise:

Quote:

The first step of polygon rasterization is to determine if the polygon is
back facing or front facing. This determination is made by examining the
sign of the area computed by equation 2.7 of section 2.13.1 (including the
possible reversal of this sign as indicated by the last call to FrontFace). If
this sign is positive, the polygon is frontfacing; otherwise, it is back facing.
This determination is used in conjunction with the CullFace enable bit and
mode value to decide whether or not a particular polygon is rasterized.


and in section 2.13.1:

Quote:

The selection between back color and front color depends on the primitive
of which the vertex being lit is a part. If the primitive is a point or a line
segment, the front color is always selected. If it is a polygon, then the
selection is based on the sign of the (clipped or unclipped) polygon's signed
area computed in window coordinates.


Of course using normals will have the same result, it's just not the behaviour OpenGL specs define. As I said, it can be accomplished through an ARB extension.

Also note that the "normals algorithm" would require the normals to be supplied somehow. If the user had to supply precalculated normals, that means that backface culling would require normals in order to work right, which definately breaks OpenGL interface. If we asssume that the card implicitly calculated the normals for each and every face, I don't think it would be faster, since it would have to do it every time a primitive was rendered. Usually, calculating normals is an expensive procedure and you don't do it on the fly.

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What's that stuff about back color and front color? I thought it would just use the glColor3f colors if the polygon is visible. Otherwise, it doesn't show it.

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Quote:
Original post by mike74
What's that stuff about back color and front color? I thought it would just use the glColor3f colors if the polygon is visible. Otherwise, it doesn't show it.


Haven't you seen the first parameter of glMaterial which is GL_FRONT,GL_BACK or GL_FRONT_AND_BACK? You can determine different materials for front and back faces. If you have two-sided lighting enabled(using glLightModel), then OpenGL uses the front or back material for the lighting equation based on which side of the polygon you're seeing(and reverses the normal for back faces).

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I did not found the specification but I found the GL state diagram and...yes...the culling is performed after screen projection.
Now I know why GL, before HW acceleration, was so crappy [smile]

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Quote:
Original post by blizzard999
Today we use OpenGL, DirectX and even pixel and fragment shaders so we have forgot the ancient art [smile]


don't fret, here you go.

http://www.devmaster.net/articles/software-rendering/part1.php

http://www.icarusindie.com/DoItYourSelf/rtsr/ <-- this one is awesome

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Quote:
Original post by blizzard999
I did not found the specification but I found the GL state diagram and...yes...the culling is performed after screen projection.
Now I know why GL, before HW acceleration, was so crappy [smile]

I don't know how one can not find the specification. A Google for, in my oppinion a pretty obvious search phrase, opengl specification returns, as first link, the place where you can download it. And on, in my oppinion an obvious place to look, opengl.orghas a direct link in the left menu on the front page.

Anyway, now I have given you two ways of getting it, so now you know where it is [wink]

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Quote:
Original post by Brother Bob
I don't know how one can not find the specification. A Google for, in my oppinion a pretty obvious search phrase, opengl specification returns, as first link, the place where you can download it. And on, in my oppinion an obvious place to look, opengl.orghas a direct link in the left menu on the front page.

Anyway, now I have given you two ways of getting it, so now you know where it is [wink]


[headshake]

I'm sorry...obviously I found the specification as well as the state diagram (they are on the same page at gl.org !)
What I've not found is the specification about the backface culling (probably with the new version it's no more in the section 2.13 as mikeman reported...)
No problem because if you follow the pipeline on the state diagram you see where backface culling is.

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Quote:
Original post by blizzard999
I'm sorry...obviously I found the specification as well as the state diagram (they are on the same page at gl.org !)
What I've not found is the specification about the backface culling (probably with the new version it's no more in the section 2.13 as mikeman reported...)

If that's what you meant, then I'm sorry for the misunderstanding. If you still want to read about it though, it's around equation (2.6) on page 63 in the OpenGL 2.0 specification. That particular part is about coloring with two sided lighting, where you need to determine what side is visible (to choose front or back material properties). That is about determining whether the back or front is visible, and that is what backface culling in OpenGL is about.

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      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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