效果圖

下載

CSharpGL已在GitHub開源，歡迎對OpenGL有興趣的同學加入（https://github.com/bitzhuwei/CSharpGL）

實現思路

raycast

用一個3D紋理存儲整個模型數據。

如下圖所示，從畫布上的一個像素點出發，垂直于畫布的方向上，一條射線穿過3D紋理，每隔一小段距離采樣一次顏色，累加起來就是此像素點應有的顏色。

起始點/終點

從射線接觸3D紋理的第一個點開始（起始點），到射線離開3D紋理的位置（終點），這段距離就是要采樣的范圍。

終點

那么如何獲取終點的位置？

辦法是：渲染一個恰好包圍3D紋理的立方體，且只渲染此立方體的背面（用glCullFace(GL_FRONT);）。因為背面就是終點啊。另外，要把這個渲染結果弄到一個2D紋理上。這就需要一個與畫布大小相同的2D紋理來記錄終點的位置，即需要一個新的FrameBuffer。（詳情可參考http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-14-render-to-texture，以后有時間我會把這個翻譯一下）

渲染背面的shader非常簡單。

 1 // for raycasting
 2 #version 400
 3 
 4 layout(location = 0) in vec3 position;
 5 layout(location = 1) in vec3 color;
 6 
 7 out vec3 passColor;
 8 
 9 uniform mat4 MVP;
10 
11 
12 void main()
13 {
14     passColor = color;
15     //passColor = vec4(1, 1, 1, 1);
16     gl_Position = MVP * vec4(position, 1.0);
17 }

backface.vert

 1 // for raycasting
 2 #version 400
 3 
 4 in vec3 passColor;
 5 layout (location = 0) out vec4 FragColor;
 6 
 7 
 8 void main()
 9 {
10     FragColor = vec4(passColor, 1.0);
11 }

backface.frag

渲染時則需要注意啟用新的framebuffer。這樣就會渲染到指定的2D紋理上。

1             // render to texture
2             glBindFramebufferEXT(OpenGL.GL_FRAMEBUFFER_EXT, frameBuffer[0]);
3             OpenGL.Clear(OpenGL.GL_COLOR_BUFFER_BIT | OpenGL.GL_DEPTH_BUFFER_BIT);
4             this.backfaceRenderer.Render(arg);
5             glBindFramebufferEXT(OpenGL.GL_FRAMEBUFFER_EXT, 0);

注意，上圖中左側是立方體的前面，右側是立方體的背面。

起始點/raycast

終點有了，起點也就知道怎么找了：渲染一個恰好包圍3D紋理的立方體，且只渲染此立方體的前面（用glCullFace(GL_BACK);）。

起始點和終點都有了，就可以通過累加顏色來計算一個像素點的顏色值了。

 1 #version 400
 2 
 3 in vec3 EntryPoint;
 4 in vec4 ExitPointCoord;
 5 
 6 uniform sampler2D ExitPoints;
 7 uniform sampler3D VolumeTex;
 8 uniform sampler1D TransferFunc;  
 9 uniform float     StepSize = 0.001f;
10 uniform vec2      ScreenSize;
11 uniform vec4      backgroundColor = vec4(0, 0, 0, 0);// value in glClearColor(value);
12 layout (location = 0) out vec4 FragColor;
13 
14 void main()
15 {
16     // ExitPointCoord is normalized device coordinate
17     vec3 exitPoint = texture(ExitPoints, gl_FragCoord.st / ScreenSize).xyz;
18     // that will actually give you clip-space coordinates rather than
19     // normalised device coordinates, since you're not performing the perspective
20     // division which happens during the rasterisation process (between the vertex
21     // shader and fragment shader
22     // vec2 exitFragCoord = (ExitPointCoord.xy / ExitPointCoord.w + 1.0)/2.0;
23     // vec3 exitPoint  = texture(ExitPoints, exitFragCoord).xyz;
24 
25     //background need no raycasting
26     if (EntryPoint == exitPoint) { discard; }
27 
28     vec3 direction = exitPoint - EntryPoint;
29     float directionLength = length(direction); // the length from front to back is calculated and used to terminate the ray
30     vec3 deltaDirection = direction * (StepSize / directionLength);
31 
32     vec3 voxelCoord = EntryPoint;
33     vec3 colorAccumulator = vec3(0.0); // The dest color
34     float alphaAccumulator = 0.0f;
35     float lengthAccumulator = 0.0;
36     float intensity;
37     vec4 colorSample; // The src color 
38  
39     for(int i = 0; i < 1600; i++)
40     {
41         // get scaler value in the volume data
42         intensity =  texture(VolumeTex, voxelCoord).x;
43         // get mapped color from 1-D texture
44         colorSample = texture(TransferFunc, intensity);
45         // modulate the value of colorSample.a
46         // front-to-back integration
47         if (colorSample.a > 0.0) {
48             // accomodate for variable sampling rates (base interval defined by mod_compositing.frag)
49             colorSample.a = 1.0 - pow(1.0 - colorSample.a, StepSize * 200.0f);
50             colorAccumulator += (1.0 - alphaAccumulator) * colorSample.rgb * colorSample.a;
51             alphaAccumulator += (1.0 - alphaAccumulator) * colorSample.a;
52         }
53         voxelCoord += deltaDirection;
54         lengthAccumulator += StepSize;
55         if (lengthAccumulator >= directionLength)
56         {    
57             colorAccumulator = colorAccumulator * alphaAccumulator 
58                 + (1 - alphaAccumulator) * backgroundColor.rgb;
59             break;  // terminate if opacity > 1 or the ray is outside the volume
60         }    
61         else if (alphaAccumulator > 1.0)
62         {
63             alphaAccumulator = 1.0;
64             break;
65         }
66     }
67     FragColor = vec4(colorAccumulator, alphaAccumulator);
68 }

raycast.frag

Raycast所需的vertex shader和backface.vert幾乎一樣。

 1 #version 400
 2 
 3 
 4 layout (location = 0) in vec3 position;
 5 // have to use this variable!!!, or it will be very hard to debug for AMD video card
 6 layout (location = 1) in vec3 color;  
 7 
 8 
 9 out vec3 EntryPoint;
10 out vec4 ExitPointCoord;
11 
12 uniform mat4 MVP;
13 
14 void main()
15 {
16     EntryPoint = color;
17     gl_Position = MVP * vec4(position,1.0);
18     ExitPointCoord = gl_Position;  
19 }

raycast.vert

而（在CSharpGL中）所需的渲染指令也只需一句話。

            this.raycastRenderer.Render(arg);

Miscellaneous

在實現上述過程之前，必須初始化很多東西：3D紋理，附帶了2D紋理的frameBuffer，用于渲染背面的shader和立方體模型，用于渲染正面/raycast的shader和立方體模型，從float類型的intensity值到vec3類型的顏色值的轉換功能（1D紋理），設置uniform變量。

  1         protected override void DoInitialize()
  2         {
  3             InitBackfaceRenderer();
  4 
  5             InitRaycastRenderer();
  6 
  7             initTFF1DTex(@"10RaycastVolumeRender\tff.dat");
  8             int[] viewport = OpenGL.GetViewport();
  9             initFace2DTex(viewport[2], viewport[3]);
 10             initVol3DTex(@"10RaycastVolumeRender\head256.raw", 256, 256, 225);
 11             initFrameBuffer(viewport[2], viewport[3]);
 12 
 13             //this.depthTest = new DepthTestSwitch();
 14 
 15             RaycastingSetupUniforms();
 16         }
 17 
 18         private void RaycastingSetupUniforms()
 19         {
 20             // setting uniforms such as
 21             // ScreenSize 
 22             // StepSize
 23             // TransferFunc
 24             // ExitPoints i.e. the backface, the backface hold the ExitPoints of ray casting
 25             // VolumeTex the texture that hold the volume data i.e. head256.raw
 26             int[] viewport = OpenGL.GetViewport();
 27             this.raycastRenderer.SetUniform("ScreenSize", new vec2(viewport[2], viewport[3]));
 28             this.raycastRenderer.SetUniform("StepSize", g_stepSize);
 29             this.raycastRenderer.SetUniform("TransferFunc", new samplerValue(BindTextureTarget.Texture1D, transferFunc1DTexObj[0], OpenGL.GL_TEXTURE0));
 30             this.raycastRenderer.SetUniform("ExitPoints", new samplerValue(BindTextureTarget.Texture2D, backface2DTexObj[0], OpenGL.GL_TEXTURE1));
 31             this.raycastRenderer.SetUniform("VolumeTex", new samplerValue(BindTextureTarget.Texture3D, vol3DTexObj[0], OpenGL.GL_TEXTURE2));
 32             var clearColor = new float[4];
 33             OpenGL.GetFloat(GetTarget.ColorClearValue, clearColor);
 34             this.raycastRenderer.SetUniform("backgroundColor", clearColor.ToVec4());
 35         }
 36 
 37         private void initFrameBuffer(int texWidth, int texHeight)
 38         {
 39             // create a depth buffer for our framebuffer
 40             var depthBuffer = new uint[1];
 41             OpenGL.GetDelegateFor<OpenGL.glGenRenderbuffersEXT>()(1, depthBuffer);
 42             OpenGL.GetDelegateFor<OpenGL.glBindRenderbufferEXT>()(OpenGL.GL_RENDERBUFFER, depthBuffer[0]);
 43             OpenGL.GetDelegateFor<OpenGL.glRenderbufferStorageEXT>()(OpenGL.GL_RENDERBUFFER, OpenGL.GL_DEPTH_COMPONENT, texWidth, texHeight);
 44 
 45             // attach the texture and the depth buffer to the framebuffer
 46             OpenGL.GetDelegateFor<OpenGL.glGenFramebuffersEXT>()(1, frameBuffer);
 47             OpenGL.GetDelegateFor<OpenGL.glBindFramebufferEXT>()(OpenGL.GL_FRAMEBUFFER_EXT, frameBuffer[0]);
 48             OpenGL.GetDelegateFor<OpenGL.glFramebufferTexture2DEXT>()(OpenGL.GL_FRAMEBUFFER_EXT, OpenGL.GL_COLOR_ATTACHMENT0_EXT, OpenGL.GL_TEXTURE_2D, backface2DTexObj[0], 0);
 49             OpenGL.GetDelegateFor<OpenGL.glFramebufferRenderbufferEXT>()(OpenGL.GL_FRAMEBUFFER_EXT, OpenGL.GL_DEPTH_ATTACHMENT_EXT, OpenGL.GL_RENDERBUFFER, depthBuffer[0]);
 50             checkFramebufferStatus();
 51             //OpenGL.Enable(GL_DEPTH_TEST); 
 52         }
 53 
 54         private void checkFramebufferStatus()
 55         {
 56             uint complete = OpenGL.GetDelegateFor<OpenGL.glCheckFramebufferStatusEXT>()(OpenGL.GL_FRAMEBUFFER_EXT);
 57             if (complete != OpenGL.GL_FRAMEBUFFER_COMPLETE_EXT)
 58             {
 59                 throw new Exception("framebuffer is not complete");
 60             }
 61         }
 62 
 63         private void initVol3DTex(string filename, int width, int height, int depth)
 64         {
 65             var data = new UnmanagedArray<byte>(width * height * depth);
 66             unsafe
 67             {
 68                 int index = 0;
 69                 int readCount = 0;
 70                 byte* array = (byte*)data.Header.ToPointer();
 71                 using (var fs = new FileStream(filename, FileMode.Open, FileAccess.Read))
 72                 using (var br = new BinaryReader(fs))
 73                 {
 74                     int unReadCount = (int)fs.Length;
 75                     const int cacheSize = 1024 * 1024;
 76                     do
 77                     {
 78                         int min = Math.Min(cacheSize, unReadCount);
 79                         var cache = new byte[min];
 80                         readCount = br.Read(cache, 0, min);
 81                         if (readCount != min)
 82                         { throw new Exception(); }
 83 
 84                         for (int i = 0; i < readCount; i++)
 85                         {
 86                             array[index++] = cache[i];
 87                         }
 88                         unReadCount -= readCount;
 89                     } while (readCount > 0);
 90                 }
 91             }
 92 
 93             OpenGL.GenTextures(1, vol3DTexObj);
 94             // bind 3D texture target
 95             OpenGL.BindTexture(OpenGL.GL_TEXTURE_3D, vol3DTexObj[0]);
 96             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_3D, OpenGL.GL_TEXTURE_MAG_FILTER, (int)OpenGL.GL_LINEAR);
 97             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_3D, OpenGL.GL_TEXTURE_MIN_FILTER, (int)OpenGL.GL_LINEAR);
 98             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_3D, OpenGL.GL_TEXTURE_WRAP_S, (int)OpenGL.GL_REPEAT);
 99             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_3D, OpenGL.GL_TEXTURE_WRAP_T, (int)OpenGL.GL_REPEAT);
100             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_3D, OpenGL.GL_TEXTURE_WRAP_R, (int)OpenGL.GL_REPEAT);
101             // pixel transfer happens here from client to OpenGL server
102             OpenGL.PixelStorei(OpenGL.GL_UNPACK_ALIGNMENT, 1);
103             OpenGL.TexImage3D(OpenGL.GL_TEXTURE_3D, 0, (int)OpenGL.GL_INTENSITY,
104                 width, height, depth, 0,
105                 OpenGL.GL_LUMINANCE, OpenGL.GL_UNSIGNED_BYTE, data.Header);
106             data.Dispose();
107         }
108 
109         private void initFace2DTex(int width, int height)
110         {
111             OpenGL.GenTextures(1, backface2DTexObj);
112             OpenGL.BindTexture(OpenGL.GL_TEXTURE_2D, backface2DTexObj[0]);
113             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_2D, OpenGL.GL_TEXTURE_WRAP_S, (int)OpenGL.GL_REPEAT);
114             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_2D, OpenGL.GL_TEXTURE_WRAP_T, (int)OpenGL.GL_REPEAT);
115             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_2D, OpenGL.GL_TEXTURE_MIN_FILTER, (int)OpenGL.GL_NEAREST);
116             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_2D, OpenGL.GL_TEXTURE_MAG_FILTER, (int)OpenGL.GL_NEAREST);
117             OpenGL.TexImage2D(OpenGL.GL_TEXTURE_2D, 0, OpenGL.GL_RGBA16F, width, height, 0, OpenGL.GL_RGBA, OpenGL.GL_FLOAT, IntPtr.Zero);
118         }
119 
120         private void initTFF1DTex(string filename)
121         {
122             // read in the user defined data of transfer function
123             byte[] tff;
124             using (var fs = new FileStream(filename, FileMode.Open, FileAccess.Read))
125             using (var br = new BinaryReader(fs))
126             {
127                 tff = br.ReadBytes((int)fs.Length);
128             }
129             OpenGL.GenTextures(1, transferFunc1DTexObj);
130             OpenGL.BindTexture(OpenGL.GL_TEXTURE_1D, transferFunc1DTexObj[0]);
131             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_1D, OpenGL.GL_TEXTURE_WRAP_S, (int)OpenGL.GL_REPEAT);
132             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_1D, OpenGL.GL_TEXTURE_MIN_FILTER, (int)OpenGL.GL_NEAREST);
133             OpenGL.TexParameteri(OpenGL.GL_TEXTURE_1D, OpenGL.GL_TEXTURE_MAG_FILTER, (int)OpenGL.GL_NEAREST);
134             OpenGL.PixelStorei(OpenGL.GL_UNPACK_ALIGNMENT, 1);
135             OpenGL.TexImage1D(OpenGL.GL_TEXTURE_1D, 0, OpenGL.GL_RGBA8, 256, 0, OpenGL.GL_RGBA, OpenGL.GL_UNSIGNED_BYTE, tff);
136         }
137 
138         private void InitRaycastRenderer()
139         {
140             var shaderCodes = new ShaderCode[2];
141             shaderCodes[0] = new ShaderCode(File.ReadAllText(@"10RaycastVolumeRender\raycasting.vert"), ShaderType.VertexShader);
142             shaderCodes[1] = new ShaderCode(File.ReadAllText(@"10RaycastVolumeRender\raycasting.frag"), ShaderType.FragmentShader);
143             var map = new PropertyNameMap();
144             map.Add("position", "position");
145             map.Add("color", "color");
146             this.raycastRenderer = new Renderer(model, shaderCodes, map);
147             this.raycastRenderer.Initialize();
148             this.raycastRenderer.SwitchList.Add(new CullFaceSwitch(CullFaceMode.Back, true));
149         }
150 
151         private void InitBackfaceRenderer()
152         {
153             var shaderCodes = new ShaderCode[2];
154             shaderCodes[0] = new ShaderCode(File.ReadAllText(@"10RaycastVolumeRender\backface.vert"), ShaderType.VertexShader);
155             shaderCodes[1] = new ShaderCode(File.ReadAllText(@"10RaycastVolumeRender\backface.frag"), ShaderType.FragmentShader);
156             var map = new PropertyNameMap();
157             map.Add("position", "position");
158             map.Add("color", "color");
159             this.backfaceRenderer = new Renderer(model, shaderCodes, map);
160             this.backfaceRenderer.Initialize();
161             this.backfaceRenderer.SwitchList.Add(new CullFaceSwitch(CullFaceMode.Front, true));
162         }