Decomposing the matrix transform is perhaps not needed; it depends on how you create the original sprite transform matrix. If you just use a rotation and translation, then you can multiply with the scaling matrix, i.e.

M * S( 0.5 ) when using column vectors

S( 0.5 ) * M when using row vectors

should do the job. It works by first scaling the sprite (uniformly) and then applying the original transformation. Have you tried this already?

If I apply a scale matrix to it, the sprites disappear, they don't do the desire effect which is the scaling I want.

If you put the scale matrix on the wrong side, then you scale also the position and a sprite may disappear due to that. So, have you ensured that you've chosen the correct order of multiplication? What happens if you use scaling factors close to 1?

I guess that math-wise, you need to translate to each center and do the scale for each element of the group?

Yes: If p is the position of the center, then translate by -p so that the center comes to rest at 0, do the scaling, then translate back by +p.

Is the drawcall done with an own matrix for each sprite, or with a common matrix for all sprites?

Thanks haegarr.

The problem is that the draw call is done with a common matrix. Any suggestions? Any changes on the vertices are possible?

Thanks for your help!

Regards,

The problem is that the draw call is done with a common matrix. Any suggestions? Any changes on the vertices are possible?

I don't know whether you can change the vertices directly, because you just told nothing about how the vertices are passed.

If you have access to them then of course you can use the CPU to do the job. For example, if all sprites are given by quads then each 4-tuple of vertices are to be used. Or if sprites are passed as 2 triangles, then each 6-tuple is to be used. Or if the vertices are passed by index, then the same goes with indices.

You can then compute the center of each sprite by summing the N vertex positions and divide by N. Then subtract those center position from each of the N vertex positions, half the resulting vector, and re-add the center. E.g.

for each sprite
   vec2 center(0, 0);
   for each of the N vertices of the sprite do
      center += vertex.pos;
   center /= N;
   for each of the N vertices of the sprite do
      vertex.pos = ( vertex.pos + center ) * 0.5;