When a PNG is added to an XCode iPhone project, the compiler optimizes it using pngcrush. Once on the device, the image’s rendering performance is very fast.
My problem is that my application downloads its PNGs from an external source at runtime (from Picasa Web albums, using the Google Data APIs). Unfortunately, these images’ performance is quite bad. When I do custom rendering on top of the image, it seems 100x slower than its internally stored counterparts. I strongly suspect this is because the downloaded images haven’t been optimized.
Does anyone know how I can optimize an externally downloaded PNG at runtime on the iPhone? I’m hoping for a class that does this. I even considered adding pngcrush’s source code to my app, which seems drastic. I haven’t been able to find an decent answer myself. I’d be very grateful for any help.
Some folks have suggested that it may be due to the file’s size, but it isn’t. During my tests, I added a toggle button to switch between using the embedded version and the downloaded version of exactly the same PNG. The only difference is that the embedded one was optimized by ‘pngcrush’ during compilation. This does some byte-swapping (from RGBA to BRGA) and pre-multiplication of alpha. (http://iphonedevelopment.blogspot.com/2008/10/iphone-optimized-pngs.html)
Also, the performance I’m referring to isn’t the downloading, but the rendering. I superimpose custom painting on top of the image (overriding the drawRect method of the UIView), and it’s very choppy when the background is the downloaded version, and very smooth when it’s the embedded (and therefore optimized) version. Again, it’s exactly the same file. The only difference is the optimization, which I’m hoping I can perform on the image at runtime, on the device, after downloading it.
Thanks again for everyone’s help!