Multiple Clipped Rects to Create Collage in Core Graphics

Question

I am creating a collage combined with elements from other images. Here is some ASCII art to explain what I am doing:

Given images A, B and C,
AAA, BBB, CCC
AAA,         


        

Answer 1

CGContextClipToRect intersects the current clipping rectangle with the argument provided. So the second time you call it, you are effectively turning your clipping region to nothing.

There is no way to restore the clipping region without restoring the graphics state. So, make a call to CGContextSaveGState at the top of your loop and a call to CGContextRestoreGState at the bottom.

The upside-down part can be fixed by adjusting the current transformation matrix: call CGContextTranslateCTM to move the origin and then CGContextScaleCTM to flip the y-axis.