I have an image compressed into NSData using JPEG compression. I access it with [UIImage imageWithContentsOfFile:]. With larger images though, this takes a few seconds. Is there a faster way to load images from the file system, perhaps at the same speed that images are loaded from the bundle? And if not, is there a way to load a lower quality version of the image temporarily while the full quality version loads, other than saving a lower quality version too?
I have an image compressed into NSData using JPEG compression. I access it with
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Although you may be able to build something like this using JPEG 2000 (you’d need to build your own copy of the jpeg library as discussed here, and then hand-write the reading code), I don’t think you’re going to get good return on investment there. The cost of reading the data off disk is still likely to overwhelm everything else.
First, if you’re reading from your bundle, use PNG if at all possible. iOS highly optimizes PNGs stored in the bundle (part of the copying process is to rewrite them in an iOS-specific optimized format).
No matter what you do, if you want a place holder you are probably going to need to provide it somehow yourself, either as a separate file, or as a custom file format that you read and manage yourself. This wouldn’t be an incredibly difficult format to devise, but you’d still need to do all the resizing beforehand somewhere.
The main key is that reading a large image file is expensive and you shouldn’t do it on the main thread. You need to do this stuff on a background queue (GCD or operation) and update the UI when the data becomes available. There’s no really easy way around this fact.