volume rendering shader

https://stackoverflow.com/questions/9482572/volume-rendering-using-glsl-with-ray-casting-algorithm

vertex shader

void main()
{
    gl_Position = gl_ModelViewProjectionMatrix*gl_Vertex;
    //gl_FrontColor = gl_Color;
    gl_TexCoord[2] = gl_Position;
    gl_TexCoord[0] = gl_MultiTexCoord1;
    gl_TexCoord[1] = gl_Color;
}

fragment shader

uniform sampler2D tex;
uniform sampler3D volume_tex;
uniform float stepsize;

void main()
{
    vec2 texc = ((gl_TexCoord[2].xy/gl_TexCoord[2].w) + 1) / 2;
    vec4 start = gl_TexCoord[0];
    vec4 back_position = texture2D(tex, texc);
    vec3 dir = vec3(0.0);
    dir.x = back_position.x - start.x;
    dir.y = back_position.y - start.y;
    dir.z = back_position.z - start.z;
    float len = length(dir.xyz); // the length from front to back is calculated and used to terminate the ray
    vec3 norm_dir = normalize(dir);
    float delta = stepsize;
    vec3 delta_dir = norm_dir * delta;
    float delta_dir_len = length(delta_dir);
    vec3 vect = start.xyz;
    vec4 col_acc = vec4(0,0,0,0); // The dest color
    float alpha_acc = 0.0;                // The  dest alpha for blending
    float length_acc = 0.0;
    vec4 color_sample; // The src color 
    float alpha_sample; // The src alpha

    for(int i = 0; i < 450; i++)
    {
      color_sample = texture3D(volume_tex,vect);
      //  why multiply the stepsize?
      alpha_sample = color_sample.a*stepsize;
      // why multply 3?
      col_acc   += (1.0 - alpha_acc) * color_sample * alpha_sample*3 ;
      alpha_acc += alpha_sample;
      vect += delta_dir;
      length_acc += delta_dir_len;
      if(length_acc >= len || alpha_acc > 1.0) 
        break; // terminate if opacity > 1 or the ray is outside the volume
    }

    gl_FragColor =  col_acc;
}

https://www.cnblogs.com/bitzhuwei/archive/2016/05/31/5544538.html

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