CSharpGL(25)一个用raycast实现体渲染VolumeRender的例子

CSharpGL(25)一个用raycast实现体渲染VolumeRender的例子

本文涉及的VolumeRendering相关的C#代码是从（https://github.com/toolchainX/Volume_Rendering_Using_GLSL）的C++代码转换来的。

效果图

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CSharpGL已在GitHub开源，欢迎对OpenGL有兴趣的同学加入（https://github.com/bitzhuwei/CSharpGL）

实现思路

raycast

用一个3D纹理存储整个模型数据。

如下图所示，从画布上的一个像素点出发，垂直于画布的方向上，一条射线穿过3D纹理，每隔一小段距离采样一次颜色，累加起来就是此像素点应有的颜色。

起始点/终点

从射线接触3D纹理的第一个点开始（起始点），到射线离开3D纹理的位置（终点），这段距离就是要采样的范围。

终点

那么如何获取终点的位置？

办法是：渲染一个恰好包围3D纹理的立方体，且只渲染此立方体的背面（用glCullFace(GL_FRONT);）。因为背面就是终点啊。另外，要把这个渲染结果弄到一个2D纹理上。这就需要一个与画布大小相同的2D纹理来记录终点的位置，即需要一个新的FrameBuffer。（详情可参考http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-14-render-to-texture，以后有时间我会把这个翻译一下）

渲染背面的shader非常简单。

 // for raycasting
 #version 

 layout(location = ) in vec3 position;
 layout(location = ) in vec3 color;

 out vec3 passColor;

 uniform mat4 MVP;

 void main()
 {
     passColor = color;
     //passColor = vec4(1, 1, 1, 1);
     gl_Position = MVP * vec4(position, 1.0);
 }

backface.vert

 // for raycasting
 #version 

 in vec3 passColor;
 layout (location = ) out vec4 FragColor;

 void main()
 {
     FragColor = vec4(passColor, 1.0);
 }

backface.frag

渲染时则需要注意启用新的framebuffer。这样就会渲染到指定的2D纹理上。

             // render to texture
             glBindFramebufferEXT(OpenGL.GL_FRAMEBUFFER_EXT, frameBuffer[]);
             OpenGL.Clear(OpenGL.GL_COLOR_BUFFER_BIT | OpenGL.GL_DEPTH_BUFFER_BIT);
             this.backfaceRenderer.Render(arg);
             glBindFramebufferEXT(OpenGL.GL_FRAMEBUFFER_EXT, );

注意，上图中左侧是立方体的前面，右侧是立方体的背面。

起始点/raycast

终点有了，起点也就知道怎么找了：渲染一个恰好包围3D纹理的立方体，且只渲染此立方体的前面（用glCullFace(GL_BACK);）。

起始点和终点都有了，就可以通过累加颜色来计算一个像素点的颜色值了。

 #version 

 in vec3 EntryPoint;
 in vec4 ExitPointCoord;

 uniform sampler2D ExitPoints;
 uniform sampler3D VolumeTex;
 uniform sampler1D TransferFunc;
 uniform float     StepSize = 0.001f;
 uniform vec2      ScreenSize;
 uniform vec4      backgroundColor = vec4(, , , );// value in glClearColor(value);
 layout (location = ) out vec4 FragColor;

 void main()
 {
     // ExitPointCoord is normalized device coordinate
     vec3 exitPoint = texture(ExitPoints, gl_FragCoord.st / ScreenSize).xyz;
     // that will actually give you clip-space coordinates rather than
     // normalised device coordinates, since you're not performing the perspective
     // division which happens during the rasterisation process (between the vertex
     // shader and fragment shader
     // vec2 exitFragCoord = (ExitPointCoord.xy / ExitPointCoord.w + 1.0)/2.0;
     // vec3 exitPoint  = texture(ExitPoints, exitFragCoord).xyz;

     //background need no raycasting
     if (EntryPoint == exitPoint) { discard; }

     vec3 direction = exitPoint - EntryPoint;
     float directionLength = length(direction); // the length from front to back is calculated and used to terminate the ray
     vec3 deltaDirection = direction * (StepSize / directionLength);

     vec3 voxelCoord = EntryPoint;
     vec3 colorAccumulator = vec3(0.0); // The dest color
     float alphaAccumulator = 0.0f;
     float lengthAccumulator = 0.0;
     float intensity;
     vec4 colorSample; // The src color 

     for(int i = ; i < ; i++)
     {
         // get scaler value in the volume data
         intensity =  texture(VolumeTex, voxelCoord).x;
         // get mapped color from 1-D texture
         colorSample = texture(TransferFunc, intensity);
         // modulate the value of colorSample.a
         // front-to-back integration
         if (colorSample.a > 0.0) {
             // accomodate for variable sampling rates (base interval defined by mod_compositing.frag)
             colorSample.a = 1.0 - pow(1.0 - colorSample.a, StepSize * 200.0f);
             colorAccumulator += (1.0 - alphaAccumulator) * colorSample.rgb * colorSample.a;
             alphaAccumulator += (1.0 - alphaAccumulator) * colorSample.a;
         }
         voxelCoord += deltaDirection;
         lengthAccumulator += StepSize;
         if (lengthAccumulator >= directionLength)
         {
             colorAccumulator = colorAccumulator * alphaAccumulator
                 + ( - alphaAccumulator) * backgroundColor.rgb;
             break;  // terminate if opacity > 1 or the ray is outside the volume
         }
         else if (alphaAccumulator > 1.0)
         {
             alphaAccumulator = 1.0;
             break;
         }
     }
     FragColor = vec4(colorAccumulator, alphaAccumulator);
 }

raycast.frag

Raycast所需的vertex shader和backface.vert几乎一样。

 

 #version 

 layout (location = ) in vec3 position;
 // have to use this variable!!!, or it will be very hard to debug for AMD video card
 layout (location = ) in vec3 color;  

 out vec3 EntryPoint;
 out vec4 ExitPointCoord;

 uniform mat4 MVP;

 void main()
 {
     EntryPoint = color;
     gl_Position = MVP * vec4(position,1.0);
     ExitPointCoord = gl_Position;
 }

raycast.vert

而（在CSharpGL中）所需的渲染指令也只需一句话。

 

            this.raycastRenderer.Render(arg);

Miscellaneous

在实现上述过程之前，必须初始化很多东西：3D纹理，附带了2D纹理的frameBuffer，用于渲染背面的shader和立方体模型，用于渲染正面/raycast的shader和立方体模型，从float类型的intensity值到vec3类型的颜色值的转换功能（1D纹理），设置uniform变量。

 

         protected override void DoInitialize()
         {
             InitBackfaceRenderer();

             InitRaycastRenderer();

             initTFF1DTex(@"10RaycastVolumeRender\tff.dat");
             int[] viewport = OpenGL.GetViewport();
             initFace2DTex(viewport[], viewport[]);
             initVol3DTex(@"10RaycastVolumeRender\head256.raw", , , );
             initFrameBuffer(viewport[], viewport[]);

             //this.depthTest = new DepthTestSwitch();

             RaycastingSetupUniforms();
         }

         private void RaycastingSetupUniforms()
         {
             // setting uniforms such as
             // ScreenSize
             // StepSize
             // TransferFunc
             // ExitPoints i.e. the backface, the backface hold the ExitPoints of ray casting
             // VolumeTex the texture that hold the volume data i.e. head256.raw
             int[] viewport = OpenGL.GetViewport();
             this.raycastRenderer.SetUniform("ScreenSize", new vec2(viewport[], viewport[]));
             this.raycastRenderer.SetUniform("StepSize", g_stepSize);
             this.raycastRenderer.SetUniform("TransferFunc", new samplerValue(BindTextureTarget.Texture1D, transferFunc1DTexObj[], OpenGL.GL_TEXTURE0));
             this.raycastRenderer.SetUniform("ExitPoints", new samplerValue(BindTextureTarget.Texture2D, backface2DTexObj[], OpenGL.GL_TEXTURE1));
             this.raycastRenderer.SetUniform("VolumeTex", new samplerValue(BindTextureTarget.Texture3D, vol3DTexObj[], OpenGL.GL_TEXTURE2));
             var clearColor = new float[];
             OpenGL.GetFloat(GetTarget.ColorClearValue, clearColor);
             this.raycastRenderer.SetUniform("backgroundColor", clearColor.ToVec4());
         }

         private void initFrameBuffer(int texWidth, int texHeight)
         {
             // create a depth buffer for our framebuffer
             var depthBuffer = new uint[];
             OpenGL.GetDelegateFor<OpenGL.glGenRenderbuffersEXT>()(, depthBuffer);
             OpenGL.GetDelegateFor<OpenGL.glBindRenderbufferEXT>()(OpenGL.GL_RENDERBUFFER, depthBuffer[]);
             OpenGL.GetDelegateFor<OpenGL.glRenderbufferStorageEXT>()(OpenGL.GL_RENDERBUFFER, OpenGL.GL_DEPTH_COMPONENT, texWidth, texHeight);

             // attach the texture and the depth buffer to the framebuffer
             OpenGL.GetDelegateFor<OpenGL.glGenFramebuffersEXT>()(, frameBuffer);
             OpenGL.GetDelegateFor<OpenGL.glBindFramebufferEXT>()(OpenGL.GL_FRAMEBUFFER_EXT, frameBuffer[]);
             OpenGL.GetDelegateFor<OpenGL.glFramebufferTexture2DEXT>()(OpenGL.GL_FRAMEBUFFER_EXT, OpenGL.GL_COLOR_ATTACHMENT0_EXT, OpenGL.GL_TEXTURE_2D, backface2DTexObj[], );
             OpenGL.GetDelegateFor<OpenGL.glFramebufferRenderbufferEXT>()(OpenGL.GL_FRAMEBUFFER_EXT, OpenGL.GL_DEPTH_ATTACHMENT_EXT, OpenGL.GL_RENDERBUFFER, depthBuffer[]);
             checkFramebufferStatus();
             //OpenGL.Enable(GL_DEPTH_TEST);
         }

         private void checkFramebufferStatus()
         {
             uint complete = OpenGL.GetDelegateFor<OpenGL.glCheckFramebufferStatusEXT>()(OpenGL.GL_FRAMEBUFFER_EXT);
             if (complete != OpenGL.GL_FRAMEBUFFER_COMPLETE_EXT)
             {
                 throw new Exception("framebuffer is not complete");
             }
         }

         private void initVol3DTex(string filename, int width, int height, int depth)
         {
             var data = new UnmanagedArray<byte>(width * height * depth);
             unsafe
             {
                 int index = ;
                 int readCount = ;
                 byte* array = (byte*)data.Header.ToPointer();
                 using (var fs = new FileStream(filename, FileMode.Open, FileAccess.Read))
                 using (var br = new BinaryReader(fs))
                 {
                     int unReadCount = (int)fs.Length;
                     const int cacheSize =  * ;
                     do
                     {
                         int min = Math.Min(cacheSize, unReadCount);
                         var cache = new byte[min];
                         readCount = br.Read(cache, , min);
                         if (readCount != min)
                         { throw new Exception(); }

                         for (int i = ; i < readCount; i++)
                         {
                             array[index++] = cache[i];
                         }
                         unReadCount -= readCount;
                     } while (readCount > );
                 }
             }

             OpenGL.GenTextures(, vol3DTexObj);
             // bind 3D texture target
             OpenGL.BindTexture(OpenGL.GL_TEXTURE_3D, vol3DTexObj[]);
             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_3D, OpenGL.GL_TEXTURE_MAG_FILTER, (int)OpenGL.GL_LINEAR);
             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_3D, OpenGL.GL_TEXTURE_MIN_FILTER, (int)OpenGL.GL_LINEAR);
             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_3D, OpenGL.GL_TEXTURE_WRAP_S, (int)OpenGL.GL_REPEAT);
             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_3D, OpenGL.GL_TEXTURE_WRAP_T, (int)OpenGL.GL_REPEAT);
             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_3D, OpenGL.GL_TEXTURE_WRAP_R, (int)OpenGL.GL_REPEAT);
             // pixel transfer happens here from client to OpenGL server
             OpenGL.PixelStorei(OpenGL.GL_UNPACK_ALIGNMENT, );
             OpenGL.TexImage3D(OpenGL.GL_TEXTURE_3D, , (int)OpenGL.GL_INTENSITY,
                 width, height, depth, ,
                 OpenGL.GL_LUMINANCE, OpenGL.GL_UNSIGNED_BYTE, data.Header);
             data.Dispose();
         }

         private void initFace2DTex(int width, int height)
         {
             OpenGL.GenTextures(, backface2DTexObj);
             OpenGL.BindTexture(OpenGL.GL_TEXTURE_2D, backface2DTexObj[]);
             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_2D, OpenGL.GL_TEXTURE_WRAP_S, (int)OpenGL.GL_REPEAT);
             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_2D, OpenGL.GL_TEXTURE_WRAP_T, (int)OpenGL.GL_REPEAT);
             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_2D, OpenGL.GL_TEXTURE_MIN_FILTER, (int)OpenGL.GL_NEAREST);
             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_2D, OpenGL.GL_TEXTURE_MAG_FILTER, (int)OpenGL.GL_NEAREST);
             OpenGL.TexImage2D(OpenGL.GL_TEXTURE_2D, , OpenGL.GL_RGBA16F, width, height, , OpenGL.GL_RGBA, OpenGL.GL_FLOAT, IntPtr.Zero);
         }

         private void initTFF1DTex(string filename)
         {
             // read in the user defined data of transfer function
             byte[] tff;
             using (var fs = new FileStream(filename, FileMode.Open, FileAccess.Read))
             using (var br = new BinaryReader(fs))
             {
                 tff = br.ReadBytes((int)fs.Length);
             }
             OpenGL.GenTextures(, transferFunc1DTexObj);
             OpenGL.BindTexture(OpenGL.GL_TEXTURE_1D, transferFunc1DTexObj[]);
             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_1D, OpenGL.GL_TEXTURE_WRAP_S, (int)OpenGL.GL_REPEAT);
             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_1D, OpenGL.GL_TEXTURE_MIN_FILTER, (int)OpenGL.GL_NEAREST);
             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_1D, OpenGL.GL_TEXTURE_MAG_FILTER, (int)OpenGL.GL_NEAREST);
             OpenGL.PixelStorei(OpenGL.GL_UNPACK_ALIGNMENT, );
             OpenGL.TexImage1D(OpenGL.GL_TEXTURE_1D, , OpenGL.GL_RGBA8, , , OpenGL.GL_RGBA, OpenGL.GL_UNSIGNED_BYTE, tff);
         }

         private void InitRaycastRenderer()
         {
             var shaderCodes = new ShaderCode[];
             shaderCodes[] = new ShaderCode(File.ReadAllText(@"10RaycastVolumeRender\raycasting.vert"), ShaderType.VertexShader);
             shaderCodes[] = new ShaderCode(File.ReadAllText(@"10RaycastVolumeRender\raycasting.frag"), ShaderType.FragmentShader);
             var map = new PropertyNameMap();
             map.Add("position", "position");
             map.Add("color", "color");
             this.raycastRenderer = new Renderer(model, shaderCodes, map);
             this.raycastRenderer.Initialize();
             this.raycastRenderer.SwitchList.Add(new CullFaceSwitch(CullFaceMode.Back, true));
         }

         private void InitBackfaceRenderer()
         {
             var shaderCodes = new ShaderCode[];
             shaderCodes[] = new ShaderCode(File.ReadAllText(@"10RaycastVolumeRender\backface.vert"), ShaderType.VertexShader);
             shaderCodes[] = new ShaderCode(File.ReadAllText(@"10RaycastVolumeRender\backface.frag"), ShaderType.FragmentShader);
             var map = new PropertyNameMap();
             map.Add("position", "position");
             map.Add("color", "color");
             this.backfaceRenderer = new Renderer(model, shaderCodes, map);
             this.backfaceRenderer.Initialize();
             this.backfaceRenderer.SwitchList.Add(new CullFaceSwitch(CullFaceMode.Front, true));
         }

Initialize

2018-01-16

最近终于解决了在某些电脑上不显示VR的情况。原来是我没有把3个Texture分别绑定到不同的Unit上。

总结

当然，也可以先渲染出起始点，然后再找到终点的时候计算各个像素点的颜色值。

raycast做volume rendering的这个例子中，最耗空间的是3D纹理。但是这是无法避免的。其他空间和时间耗费都是极少的。

欢迎对OpenGL有兴趣的同学关注（https://github.com/bitzhuwei/CSharpGL）