Question

I have a project where I have to rotate a large image under the users thumb, it represents a dial the use turns to select a day of the year. I am currently using CGContextRotateCTM() and -[UIImage drawInRect:] but the performance on retina display iPads is not good, ironically on pre-retina iPads is works fine (only a 1/4 of the pixels to push around). I have don everything I can to optimise my code, instruments shows me that 87% of the time is in -[UIImage drawInRect:]. Does anybody have any hints on how to get some significant performance improvement, should I be looking a CALayers and rotating them instead.

Answer 1

Put the image in its own UIImageView and set the image view’s transform property to your rotation transform. That will be much, much faster than drawing the image into a CGContext with a rotated CTM.