MedicalVisualization

Iso-Contouring with Graphics Hardware

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The pixel-based iso line algorithm is perfect for porting it as a pixel shader onto graphics hardware.

For this purpose we replace the standard vertex and pixel shader by two custom vertex and pixel shaders:

Vertex program that passes texture coordinates to the fragment program: 

!!ARBvp1.0
OPTION ARB_position_invariant;
MOV result.color,vertex.color;
MOV result.texcoord[0],vertex.texcoord[0];
END

Fragment program that samples the texture coordinates and performs a thresholding operation on the distance to the iso line: 

!!ARBfp1.0
PARAM isoval=program.env[0];
PARAM threshold=program.env[1];
TEMP val,col;
TEX val,fragment.texcoord[0],texture[0],2D;
SUB val.x,val.x,isoval.x;
ABS val.x,val.x;
SUB val.x,val.x,threshold.x;
CMP col,val.x,1.0,0.25;
MUL col,col,vertex.color;
MOV result.color,col;
END

With GLSL, a high level language compiler for vertex and pixel shaders, the above shader snippets become more readable:

GLSL vertex shader: 

varying vec4 color;
varying vec2 texcoord;

void main()
{
   // transforming the vertex
   gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;

   // writing varying color and texture coordinate
   color = gl_Color;
   texcoord = gl_MultiTexCoord0;
}

GLSL fragment shader: 

uniform float isovalue, threshold;

varying vec4 color;
varying vec2 texcoord;

uniform sampler2D texture;

void main()
{
   vec4 texcol;

   // sample the texture
   texcol = texture2D(texture, texcoord);

   // calculate distance to iso value and threshold it
   if (abs(texcol.x-isovalue) > threshold)
      texcol *= vec4(0.25,0.25,0.25,1);

   // modulate texture with per-vertex color
   gl_FragColor = texcol * color;
}


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