Does CGContextSetTextMatrix work for offscreen bitmaps?

Question

I'm creating an image offscreen using a context from CGBitmapContextCreate().

While drawing text, I tried to use the:

CGContextSetTextMatrix(contextRef, CGAffineTransformMake(1.0, 0.0, 0.0, -1.0, 0.0, 0.0));

but my text was still upside down. If I used the standard transforms it was correct:

CGContextTranslateCTM(contextRef, 0.0, contextRect.size.height);
CGContextScaleCTM(contextRef, 1.0, -1.0);

My question is should the CGContextSetTextMatrix work for an offscreen bitmap? Maybe I was doing something wrong.

Answer 1

No. The text matrix, as its name says, only affects text.

All drawing goes through the current transformation matrix, and only text also goes through the text matrix. So, for anything that isn't text, you need to change the CTM.

You can use the CGContextConcatCTM function to concatenate your flip matrix onto the CTM in one function call, although I would find the translate + scale easier to read. Note that concatenating one matrix onto another is not the same as replacing the old matrix with a new one.

There is no function to replace the CTM with a different matrix in Core Graphics; you can only concat onto it. You can get the CTM, invert it, and concat the inverse matrix onto the current matrix to get back to the identity matrix; then, concatenating your desired matrix onto that will result in the matrix being your desired matrix with no other influences. However, there's not much reason to go to all that effort.