GuCubeIndex.h

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#ifndef GU_CUBE_INDEX_H
#define GU_CUBE_INDEX_H
 
#include "foundation/PxVec3.h"
#include "foundation/PxFPU.h"
 
namespace physx
{
 
    enum CubeIndex
    {
        CUBE_RIGHT,
        CUBE_LEFT,
        CUBE_TOP,
        CUBE_BOTTOM,
        CUBE_FRONT,
        CUBE_BACK,
 
        CUBE_FORCE_DWORD    = 0x7fffffff
    };
 
    /*
        It's pretty straightforwards in concept (though the execution in hardware is
        a bit crufty and complex). You use a 3D texture coord to look up a texel in
        a cube map. First you find which of the axis has the largest value (i.e.
        X,Y,Z), and then the sign of that axis decides which face you are going to
        use. Which is why the faces are called +X, -X, +Y, -Y, +Z, -Z - after their
        principle axis. Then you scale the vector so that the largest value is +/-1.
        Then use the other two as 2D coords to look up your texel (with a 0.5 scale
        & offset).
 
        For example, vector (0.4, -0.2, -0.5). Largest value is the Z axis, and it's
        -ve, so we're reading from the -Z map. Scale so that this Z axis is +/-1,
        and you get the vector (0.8, -0.4, -1.0). So now use the other two values to
        look up your texel. So we look up texel (0.8, -0.4). The scale & offset move
        the -1->+1 range into the usual 0->1 UV range, so we actually look up texel
        (0.9, 0.3). The filtering is extremely complex, especially where three maps
        meet, but that's a hardware problem :-)
    */
 
 
    PX_INLINE CubeIndex     CubemapLookup(const PxVec3& direction, float& u, float& v);
 
    PX_INLINE PxU32 ComputeCubemapOffset(const PxVec3& dir, PxU32 subdiv)
    {
        float u,v;
        const CubeIndex CI = CubemapLookup(dir, u, v);
 
        // Remap to [0, subdiv[
        const float Coeff = 0.5f * float(subdiv-1);
        u += 1.0f;  u *= Coeff;
        v += 1.0f;  v *= Coeff;
 
        // Compute offset
        return PxU32(CI)*(subdiv*subdiv) + PxU32(u)*subdiv + PxU32(v);
    }
 
 
    PX_INLINE PxU32 ComputeCubemapNearestOffset(const PxVec3& dir, PxU32 subdiv)
    {
        float u,v;
        const CubeIndex CI = CubemapLookup(dir, u, v);
 
        // Remap to [0, subdiv]
        const float Coeff = 0.5f * float(subdiv-1);
        u += 1.0f;  u *= Coeff;
        v += 1.0f;  v *= Coeff;
 
        // Compute offset
        return PxU32(CI)*(subdiv*subdiv) + PxU32(u + 0.5f)*subdiv + PxU32(v + 0.5f);
    }
 
 
    PX_INLINE CubeIndex CubemapLookup(const PxVec3& direction, float& u, float& v)
    {
        const PxU32* binary = reinterpret_cast<const PxU32*>(&direction.x);
 
        const PxU32 absPx = binary[0] & ~PX_SIGN_BITMASK;
        const PxU32 absNy = binary[1] & ~PX_SIGN_BITMASK;
        const PxU32 absNz = binary[2] & ~PX_SIGN_BITMASK;
 
        PxU32 Index1 = 0;   //x biggest axis
        PxU32 Index2 = 1;
        PxU32 Index3 = 2;
        if( (absNy > absPx) & (absNy > absNz))
        {
            //y biggest
            Index2 = 2;
            Index3 = 0;
            Index1 = 1;
        }
        else if(absNz > absPx)
        {
            //z biggest
            Index2 = 0;
            Index3 = 1;
            Index1 = 2;
        }
 
        const PxF32* data = &direction.x;
        const float Coeff = 1.0f / fabsf(data[Index1]);
        u = data[Index2] * Coeff;
        v = data[Index3] * Coeff;
 
        const PxU32 Sign = binary[Index1]>>31;
        return CubeIndex(Sign|(Index1+Index1));
    }
 
}
 
#endif