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

If either is more efficient of the two then it'll be the former.

When you create a CGImageSource you create just what the name says — some sort of opaque thing from which an image can be obtained. In your case it'll be a reference to a thing on disk. When you ask ImageIO to create a thumbnail you explicitly tell it "do as much as you need to output this many pixels".

Conversely if you draw to a CGBitmapContext then at some point you explicitly bring the whole image into memory.

So the second approach definitely has the whole image in memory at once at some point. Conversely the former needn't necessarily (in practice there'll no doubt be some sort of guesswork within ImageIO as to the best way to proceed). So across all possible implementations of the OS either the former will be advantageous or there'll be no difference between the two.