void buildOffsetTex(inttexSize, intsamplesU, intsamplesV) {int size = texSize;int samples = samplesU * samplesV;int bufSize = size * size * samples * 2;float *data = new float[bufSize];for( int i = 0; i< size; i++ ) {for(int j = 0; j < size; j++ ) {for( int k = 0; k < samples; k += 2 ) {int x1,y1,x2,y2;x1 = k % (samplesU);y1 = (samples - 1 - k) / samplesU;x2 = (k+1) % samplesU;y2 = (samples - 1 - k - 1) / samplesU;vec4 v;// Center on grid and jitterv.x = (x1 + 0.5f) + jitter();v.y = (y1 + 0.5f) + jitter();v.z = (x2 + 0.5f) + jitter();v.w = (y2 + 0.5f) + jitter();// Scale between 0 and 1v.x /= samplesU;v.y /= samplesV;v.z /= samplesU;v.w /= samplesV;// Warp to diskint cell = ((k/2) *size*size + j *size + i) * 4;data[cell+0] = sqrtf(v.y) * cosf(TWOPI*v.x);data[cell+1] = sqrtf(v.y) * sinf(TWOPI*v.x);data[cell+2] = sqrtf(v.w) * cosf(TWOPI*v.z);data[cell+3] = sqrtf(v.w) * sinf(TWOPI*v.z);}}}glActiveTexture(GL_TEXTURE1);GLuint texID;glGenTextures(1, &texID);glBindTexture(GL_TEXTURE_3D, texID);glTexImage3D(GL_TEXTURE_3D, 0, GL_RGBA32F, size, size, samples/2, 0, GL_RGBA, GL_FLOAT, data);glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);delete [] data;
}
// Return random float between -0.5 and 0.5
float jitter() {return ((float)rand() / RAND_MAX) - 0.5f;
}

uniform sampler3D OffsetTex;
uniform vec3 OffsetTexSize; // (width, height, depth)
uniform float Radius;

subroutine (RenderPassType)
void shadeWithShadow()
{vec3 ambient = Light.Intensity * Material.Ka;vec3 diffAndSpec = phongModelDiffAndSpec();ivec3 offsetCoord;offsetCoord.xy = ivec2( mod( gl_FragCoord.xy,OffsetTexSize.xy ) );float sum = 0.0;int samplesDiv2 = int(OffsetTexSize.z);vec4 sc = ShadowCoord;for( int i = 0 ; i< 4; i++ ) {offsetCoord.z = i;vec4 offsets = texelFetch(OffsetTex,offsetCoord,0) *Radius*ShadowCoord.w;sc.xy = ShadowCoord.xy + offsets.xy;sum += textureProj(ShadowMap, sc);sc.xy = ShadowCoord.xy + offsets.zw;sum += textureProj(ShadowMap, sc);}float shadow = sum / 8.0;if( shadow != 1.0 && shadow != 0.0 ) {for( int i = 4; i< samplesDiv2; i++ ) {offsetCoord.z = i;vec4 offsets = texelFetch(OffsetTex, offsetCoord,0)  *Radius * ShadowCoord.w;sc.xy = ShadowCoord.xy + offsets.xy;sum += textureProj(ShadowMap, sc);sc.xy = ShadowCoord.xy + offsets.zw;sum += textureProj(ShadowMap, sc);}shadow =sum/float(samplesDiv2 * 2.0);}FragColor = vec4(diffAndSpec * shadow + ambient, 1.0);
}

就是转换后的坐标。到此为止，我们得到了一个围绕原点的偏移集合。采样时，我们首先在圆的边缘进行，逐步向圆心移动，这样就可以避免完全处于阴影内或阴影外的样本的不必要采集。