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Scale texture matrix "around center"

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I've written the following function to scale texture matrices:

void ScaleMatrix( TextureMatrix *matrix, float scaleSpeed, double deltaTime ) const
vec2 scale = 1.0f/scaleSpeed;

matrix->matrix[0][0] *= scale.x;
matrix->matrix[1][1] *= scale.y;

matrix->matrix[0][3] *= scale.x;
matrix->matrix[1][3] *= scale.y;

Now there's two things missing and I can't figure out how to do it:
- When a scaled texture is used to scale texture coordinates, the texture is scaled around the origin (0, 0). I would like them to be scaled around the center (0.5, 0.5) instead. How can I achieve this?
- I'd like to include deltaTime in the computation so that the scaling is continous across several frames.

Thank you! Edited by Vexator

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Please ignore, got it working:

matrix->matrix[3][0] = ( 0.5f - 0.5f * matrix->matrix[0][0] );
matrix->matrix[3][1] = ( 0.5f - 0.5f * matrix->matrix[1][1] );

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