CGContextDrawImage is EXTREMELY slow after large UIImage drawn into it

Question

It seems that CGContextDrawImage(CGContextRef, CGRect, CGImageRef) performs MUCH WORSE when drawing a CGImage that was created by CoreGraphics (i.e. with CGBitmapContextCrea

Answer 1

I had a similar problem. My application has to redraw a picture almost as large as the screen size. The problem came down to drawing as fast as possible two images of the same resolution, neither rotated nor flipped, but scaled and positioned in different places of the screen each time. After all, I was able to get ~15-20 FPS on iPad 1 and ~20-25 FPS on iPad4. So... hope this helps someone:

1. Exactly as typewriter said, you have to use the same pixel format. Using one with AlphaNone gives a speed boost. But even more important, argb32_image call in my case did numerous calls converting pixels from ARGB to BGRA. So the best bitmapInfo value for me was (at the time; there is a probability that Apple can change something here in the future): const CGBitmabInfo g_bitmapInfo = kCGBitmapByteOrder32Little | kCGImageAlphaNoneSkipLast;
2. CGContextDrawImage may work faster if rectangle argument was made integral (by CGRectIntegral). Seems to have more effect when image is scaled by factor close to 1.
3. Using layers actually slowed down things for me. Probably something was changed since 2011 in some internal calls.
4. Setting interpolation quality for the context lower than default (by CGContextSetInterpolationQuality) is important. I would recommend using (IS_RETINA_DISPLAY ? kCGInterpolationNone : kCGInterpolationLow). Macros IS_RETINA_DISPLAY is taken from here.
5. Make sure you get CGColorSpaceRef from CGColorSpaceCreateDeviceRGB() or the like when creating context. Some performance issues were reported for getting fixed color space instead of requesting that of the device.
6. Inheriting view class from UIImageView and simply setting self.image to the image created from context proved useful to me. However, read about using UIImageView first if you want to do this, for it requires some changes in code logic (because drawRect: isn't called anymore).
7. And if you can avoid scaling your image at the time of actual drawing, try to do so. Drawing non-scaled image is significantly faster - unfortunately, for me that was not an option.