There are already functions to translate(), rotate(), and scale(), but I was looking for a way to mirror coordinates (in 2D), e.g. (x, y) -> (-x, y), or mirroring with respect to an arbitrarily rotated axis.

Finally I found applyMatrix(), which can do this (though some better documentation for the 2D case would be nice, e.g. explain that it takes 6 parameters, and not 9), but the documentation states that:

Quote:

My question is:  Why is it necessary to invert the matrix?  I can't see why it is necessary to have both the matrix and its inverse.  If only the inverse is needed, then why not provide a function that takes the inverse as its argument, so that numerical inversion can be avoided at runtime?

Any explanations or pointers to documentation would be welcome!

EDIT:  Since there are no replies yet, let me clarify a little bit.  First I must say that I know nothing about computer graphics, only a little bit of mathematics.  If I understand correctly, before anything is plotted, all (x, y) coordinates are transformed by multiplying the vector (x, y, 1) with a 3*3 matrix to get (x', y', 1).  This simple operation doesn't require the inverse of the transformation matrix.  So where is the inverse used?

You might not need any math at all. If what you're looking to do is draw at an opposite position, the correct sequence of translate/rotate and then scale negative should do the trick.

If you then need points from that location you can use modelXYZ methods to get them.

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