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OpenGL problem writing complex vertex shader

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Greetings, everyone.

 

Recently I've been interested in Warcraft3's model system.

I download the War3ModelEditor source code (from: http://home.magosx.com/index.php?topic=6.0), read it, and rewrite a program witch can render Warcraft3's model using OpenGL ES.

When I run this code on an Android phone, it looks good but, when there're more than 5 models in the screen, the FPS becomes very low.

 

Currently I do all the bone animation(matrix calculation and vertex position calculation) in CPU side.

I think it might be faster if we can do all these works in GPU side.

But I just don't know how to do it sad.png

The Warcraft3's vertex position calculation is complex for me.

 

Let me explain a little more.

In a Warcraft3's model, each vertex is linked to one or moe bone.

Here is how the War3ModelEditor calculate the vertex's position:

step1. for each bone[i], calculate matrix_list[i]
step2. for each vertex
           position = (matrix_list[vertex_bone[0]] * v
                    +  matrix_list[vertex_bone[1]] * v
                    +  ...
                    +  matrix_list[vertex_bone[n]] * v) / n

note: n is the length of 'vertex_bone', each vertex may have a different 'vertex_bone'.

Actually, several vertex can share a same 'vertex_bone' array,

while several other vertex share another 'vertex_bone' array.

For example, a model with 500 vertices may have only 35 different 'vertex_bone' arrays.

But I don't know how can I make use of this, to optimize the performance.

?

 

 

The step1 may be easy. Since a typical Warcraft3 model will have less than 30 bones, we can do this step in CPU side without much performance hit.

But step2 is quite complex.

 

If I write a vertex shader (GLSL) it will be something like this:

uniform mat4 u_matrix_list[50]; /* there might be more ?? */
attribute float a_n;
attribute float a_vertex_bone[4]; /* there might be more ?? */
attribute vec4 a_position;
void main() {
  float i;
  vec4 p = vec4(0.0, 0.0, 0.0, 1.0);
  for (i = 0; i < a_n; ++i) {
    p += u_matrix_list[int(a_vertex_bone[int(i)])] * a_position;
  }
  gl_Position = p / float(a_n);
}

There're some problems.

1. When I compile the vertex shader above (on my laptop, either than an Android phone), it reports 'success' with a warning message 'OpenGL does not allow attributes of type float[4]'.

And some times (when I change the order of the 3 attributes) it cause my program goes down, with a message 'The NVIDIA OpenGL driver lost connection with the display driver due to exceeding the Windows Time-Out limit and is unable to continue.'

2. The book <OpenGL ES 2.0 Programming Guide> page 83, says that 'many OpenGL ES only mandates that array indexing be supported by constant integral expressions (there is an exception to this, which is the indexing of uniform variables in vertex shaders that is discussed in Chapter 8).', so the statement 'a_vertex_bone[int(i)]' might not work on some OpenGL ES hardware.

 

 

Actually I've never write such a complex(?) shader before.

Any one could you give me some advice?

Thank you.

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You're on the right track!  A uniform array of bones, and vertex attributes that index into said array is the common way to handle this. 

 

For your specific problem, I have a solution that should work but will limit you to 4 bones per vertex (I can't imagine this is a problem for WC3 models, but please let me know if it is.)

You could try representing your bone weights as a vec4 instead of an array in the attribute. From there, you could add a second vec4 attribute representing how many bones affect a vertex (such as [1.0, 1.0, 0.0, 0.0] for two bones).

 

Finally, If you take the dot product of this vector with itself, you conveniently enough get the number of bones out! (if we call the vector above v, then dot(v,v) = (1.0*1.0  + 1.0*1.0 + 0.0*0.0 + 0.0*0.0) = 2.0)

 

This would change your attribs to: 

attribute vec4 a_position;
attribute vec4 bone_weights;
attribute vec4 bone_mask;

You would also remove the for loop above, and just say 

vec4 p = vec4(0,0,0,1);
p += u_matrix_list[int(bone_weights.x)]* a_position*bone_mask.x;
p += u_matrix_list[int(bone_weights.y)]* a_position*bone_mask.y;
p += u_matrix_list[int(bone_weights.z)]* a_position*bone_mask.z;
p += u_matrix_list[int(bone_weights.w)]* a_position*bone_mask.w;
gl_Position = p / dot(bone_mask,bone_mask);

Hope this helps!

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Koehler, thank you very much for your reply. It helps me a lot.

Especially the 'dot product', that is wonderful.

 

But let me point out this.

The code "vec4 p = vec4(0,0,0,1);" you wrote, will actually be "vec4 p = vec4(0,0,0,0);". Or the transformation will not be correct.

 

Based on your idea, I've changed my source code.

I'm not very famillar about OpenGL version 2.0 and above. Fortunately I did it with a success:).

And there're still some issues that need to be think about.

 

Let me put my shader source code down here:

(Yes you can see there's something like gl_TextureMatrix and gl_ModelViewProjectionMatrix. That's because the first version of my program is written on an old PC witch only supports OpenGL 1.4. I'll modify these when necessary)

/* vertex shader */
uniform mat4 u_matrix_list[202];
attribute vec3 a_position;
attribute vec2 a_texcoord;
attribute vec4 a_mat_indices;
attribute vec4 a_mat_weights;
varying vec2 v_texcoord;
void main() {
  v_texcoord = (gl_TextureMatrix[0] * vec4(a_texcoord, 0.0, 1.0)).xy;
  vec4 p0 = vec4(a_position, 1.0);
  vec4 p = vec4(0.0, 0.0, 0.0, 0.0);
  p += (u_matrix_list[(int)a_mat_indices[0]] * p0) * a_mat_weights[0];
  p += (u_matrix_list[(int)a_mat_indices[1]] * p0) * a_mat_weights[1];
  p += (u_matrix_list[(int)a_mat_indices[2]] * p0) * a_mat_weights[2];
  p += (u_matrix_list[(int)a_mat_indices[3]] * p0) * a_mat_weights[3];
  p /= dot(a_mat_weights, a_mat_weights);
  gl_Position = gl_ModelViewProjectionMatrix * p;
};

/* fragment shader */
uniform sampler2D tex;
uniform vec4 u_color;
varying vec2 v_texcoord;
void main() {
  gl_FragColor = u_color * texture2D(tex, v_texcoord);
}

Issues:

1. I wrote "uniform mat4 u_matrix_list[202];", this is a very large array for GPU.

    I found that many of Warcraft3's unit model have less than 100 bones. For example a water elemental has 69 bones, and a footman has 49 bones.

    But the buildings' model have many more bones. When I use the model 'AncientOfLore.mdx' for test. I found that it has 202 bones. So I declared such a large array. According to the MDX format, there can be up to 256 nodes(since the node's ID is a BYTE). But when I wrote "uniform mat4 u_matrix_list[256];" the glLinkProgram fails, with an error message "error C6007: Constant register limit exceeded; more than 1024 constant registers needed to compiled program".

   I hear that if we store a mat4 as 3 vec4, it may save some space. But that may not be enough. The OpenGL ES 2.0 only ensure to have 128 vec4 uniform variables (glGetIntegeri with GL_MAX_VERTEX_UNIFORM_VECTORS), so we can only use 128 / 3 = 42 bones or less?

  Or we can try to use a texture to store some more data. The book <OpenGL ES 2.0 Programming Guide> says that "Samplers in a vertex shader are optional". The POWERVR SGX seems to support it. But we need some more information to decide whether or not to use it.

 

2. Yes, the <Warcraft III Art Tools Documention.pdf> says that "Up to four bones can influence one vertex.". So we can use an vec4 attribute to simulate an float[4] array.

    But I found there're some exceptions. For example a water elemetal has some vertices that are influenced by up to 6 bones. This is not very critical because we can add 2 more attribute to fix it.

    In my test I just use the first 4 bones, and ignore the last 2, it looks fine without any obvious problem. So let's just ignore it for now:)

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Here's some snapshot of my test program.

I'd like to share my happy feeling with you. Thank you again.

[attachment=16979:testGL.01.png]

[attachment=16980:testGL.02.png]

[attachment=16981:testGL.03.png]

[attachment=16982:testGL.04.png]

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Glad to see you caught my mistake. I was calling "indices" weights, also. Clearly I didn't test that code :/

 

Those results look good! I am surprised that the ancients have so many bones. If I had to guess, maybe WC3 probably did software skinning so it didn't matter?

 

As an option, maybe you could look through the model and split the mesh based on the bone indices accessed? (half for indices < 110 or something, half for >110) and do two draw calls for the big guys.. This would work best if pieces don't rely on the root bone bones too much.

 

Alternatively you could split the model and duplicate the most shared bones into each of the two smaller models' bone arrays, changing the indices on your vertex data appropriately. It still might let you cut down the number enough to fit into your uniform space. 

Edited by Koehler

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Yes the 'AncientOfLore.mdx' has many bones. When I found this for the first time, I am surprised too.

Once again the Warcraft3's model do not obey the rule they've made in the <Warcraft III Art Tools Documention.pdf>.

According to the documention, a building should have at most 15 bones. And a really big unit should have at most 30 bones.

 

By the way, OpenGL 2.0 spec is released on the year 2004. Warcraft3 is released before that. So I think Warcraft3 is not using a shader to do the bone animation.

 

I've noticed that, not all the bones are used by the mesh. Some of the bones are used for attaching another model, or used by a particle emitter, etc.

For example, when an AncientOfLore tree was badly damaged, some places of the tree body will be on fire. Each place uses a particle emitter to draw the fire, and a particle emitter needs a bone. Simply speaking, 6 places of fire will use 6 bones.

We can ignore these bones when we are loading bone matrices to the shader.

 

There is a concept named "geoset" in the Warcraft3's model. A geoset contains data like vertex positions, texture coords, normals, and the indices of bone matrix. One model may have one or more geoset(s).

Before today I thought that each vertex in each geoset can be linked to any bone of this model. When I see these words "split the mesh" I guess we may make use of the geoset directly, rather than split the mesh by an algorithm.

So I did a simple test.

The 'AncientOfLore.mdx' model has 12 geosets. And in the animation sequence "stand work alternate" there're 6 of them are visible(The documention says that one model should have at most 5 visible geosets!). The number of bones used in each geoset are: 27, 62, 3, 3, 8, 2. All these numbers are much lesser than 202.

But for OpenGL ES, the 62 bones is still too many and will need to split into smaller parts.

 

So if I need to display an 'AncientOfLore.mdx' on my Android phone, I have to design an algorithm to split a geoset into two or more small geosets.

The next step is to design and implement this algorithm. I think that will not be easy for me. But I'll try it.

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      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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