Android: Matrix -> what is the different between preconcat and postconcat?

Question

I'm using Matrix to scale and rotate Bitmaps. Now I'm wondering what the difference between preconcat & postconcat is, or more precisely the difference between:

• postRotate
• preRotate
• setRotate

From what I could figure out so far setRotate always overwrites the whole matrix, while with preRotate and postRotate I can apply multiple changes to a matrix (e.g. scaling + rotation). However, either using postRotate or preRotate didn't cause any different results for the cases I used them.