What is the most memory-efficient way of downscaling images on iOS?

Question

In background thread, my application needs to read images from disk, downscale them to the size of screen (1024x768 or 2048x1536) and save them back to disk. Original images are

Answer 1

I would try using a c-based library like leptonica. I'm not sure whether ios optimizes Core Graphics with the relatively new Accelerate Framework, but CoreGraphics probably has more overhead involved just to re-size an image. Finally... If you want to roll your own implementation try using vImageScale_??format?? backed with some memory mapped files, I can't see anything being faster.

http://developer.apple.com/library/ios/#documentation/Performance/Conceptual/vImage/Introduction/Introduction.html

PS. Also make sure to check the compiler optimization flags.