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### #ActualKwizatz

Posted 21 January 2013 - 07:16 PM

In the end I did manage to get it working, I am doing this for animation interpolation, so it does seem justified. I convert to matrix before feeding the transforms to the gpu.
I do think this way is more efficient at least as long as it stays on the CPU, many multiplications can be avoided for example because one of the multiplicands being zero for the translation part and the scale part (what I did was convert the vectors to matrices and assume constants for the non variable elements in the matrix, then simplify the matrix multiplications),
the most expensive operation then becomes rotating the translation vector using the quaternion, since it implies a quaternion to 3x3 matrix conversion and a matrix vector multiplication but still not as many operations as the regular matrix multiplication.
You only need to apply rotation and scale to the translation, I think, because of the nature of translation, a skew in 4D, rather than being a linear transform in 3D, and in any case it makes sense when you think of the translation vector as being in the space of the rotated and scaled axes.

I am leaving SIMD viability as an exercise for later.

### #1Kwizatz

Posted 21 January 2013 - 07:12 PM

In the end I did manage to get it working, I am doing this for animation interpolation, so it does seem justified. I convert to matrix before feeding the transforms to the gpu. I do think this way is more efficient at least as it stays on the CPU, many multiplications can be avoided for example because one of the multiplicands are zero for the translation part and the scale part (what I did was convert the vectors to matrices and assume constants for the non variable elements in the matrix, then simplify the matrix multiplications), the most expensive operation then becomes rotating the translation vector using the quaternion, since it implies a quaternion to 3x3 matrix conversion and a matrix vector multiplication but still not as many operations as the regular matrix multiplication. You only need to apply rotation and scale to the translation, I think, because of the nature of translation, a skew in 4D, rather than being a linear transform in 3D. I am leaving SIMD viability as an exercise for later.

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