For any given file data size, I want to be able to resize (or compress) a UIImage to fit within that data limit. This question is NOT about how to resize, or how to check file sizes… it is about an algorithm to getting this in a performant way.
Searching here already, I found this thread which talks about stepping down the image jpeg quality in a linear, or binary algorithm. This isn’t very performant, taking dozens of seconds at best.
I am working on iOS so images can be close to 10MB (from iPhone 4S). My target, although variable, is currently 3145728 bytes.
I am currently using UIImageJPEGRepresentation to compress a little, but to get to my low target it appears I would have to lose much quality for such a large photo. Is there a relation between UIImage size and NSData size? Is there some function where I can say something like:
area * X = dataSize
…and solve for a scale factor so I can resize in one shot?
One idea I just had after looking at the thread you linked to: compressing a 10MB image is going to be relatively slow. How about resizing to be much smaller (so that compression is much faster), then performing the compression algorithm (from the link). This can then be used as a guide to the size of compressing the 10MB image? The idea being that the compression ratio should be similar for the same image, independent of size.
Let’s say 1000×1000 pixels compressed is 10MB, target size is 3MB.
Then say smaller 100×100 pixels (for example), compressed with same quality, is C MB. Then perform the binary search alg on the 100×100 image until size = C * (3/10). Then use this compression quality for the 1000×1000 image to get ~3MB image.
Note: I have no idea how well this will work – it’s just a suggestion. What size to pick (I’ve used 100×100) for the smaller-sized image is also just a guess and something would need to be experimented with.