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POShader code optimization
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copied!<p>I have this code snippet (for cubemap PCF filtering). I would like to optimize it for shader model 2. I tried eliminating the branches with permutation matrices stored in uniforms, but it requires too much (2x24).</p> <pre><code>float3 l = normalize(ldir); float3 al = abs(l); float3 off2, off3, off4; if( al.x < al.y ) { if( al.y < al.z ) { // z is dominant off2 = CubeOffset(l.zxy, float2(0, 1), texelsize).yzx; off3 = CubeOffset(l.zxy, float2(1, 0), texelsize).yzx; off4 = CubeOffset(l.zxy, float2(1, 1), texelsize).yzx; } else { // y is dominant off2 = CubeOffset(l.yxz, float2(0, 1), texelsize).yxz; off3 = CubeOffset(l.yxz, float2(1, 0), texelsize).yxz; off4 = CubeOffset(l.yxz, float2(1, 1), texelsize).yxz; } } else { if( al.x < al.z ) { // z is dominant off2 = CubeOffset(l.zxy, float2(0, 1), texelsize).yzx; off3 = CubeOffset(l.zxy, float2(1, 0), texelsize).yzx; off4 = CubeOffset(l.zxy, float2(1, 1), texelsize).yzx; } else { // x is dominant off2 = CubeOffset(l, float2(0, 1), texelsize); off3 = CubeOffset(l, float2(1, 0), texelsize); off4 = CubeOffset(l, float2(1, 1), texelsize); } } </code></pre> <p>Perhaps a mathematical relation can be found between the comparisons (al.xyy < al.yzz) and the swizzles.</p> <p><strong>UPDATE:</strong> definition of cubeoffset</p> <pre><code>float3 CubeOffset(float3 swiz, float2 off, float2 texelsize) { float3 ret; ret.yz = swiz.yz + 2.0f * off * texelsize; ret.x = sqrt(1.0f - dot(ret.yz, ret.yz)); if( swiz.x < 0 ) ret.x *= -1.0f; return ret; } </code></pre> <p>And the HLSL error when compiling SM 2.0:</p> <pre><code>error X5608: Compiled shader code uses too many arithmetic instruction slots (107). Max. allowed by the target (ps_2_0) is 64. error X5609: Compiled shader code uses too many instruction slots (111). Max. allowed by the target (ps_2_0) is 96. </code></pre> <p>GLSL handles it fine. The goal is backward compatibility.</p> <p>(btw. the algorithm is faulty, but that's not an issue right now)</p>

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