Open Radar

 
Summary: Using UIImageView or CGImageCreateWithJPEGDataProvider to decode a large, progressively encoded JPEG takes much longer than expected with high CPU utilization

Steps to Reproduce:
Using a full screen progressively encoded JPEG for retina resolution on the iPad  is the easiest way to reproduce this issue. I have a test program available on Github that shows three different images getting decoded and displayed. The progressively encoded jpeg is marked.

https://github.com/sujal/Image-Tester

Build and Run the program on an iPad.

Expected Results: The time taken would be longer than a non-progressively encoded JPEG, but not orders of magnitude longer. For example, the baselines encoded JPEG takes around 100-200ms to decode. I would expect, based on my understanding the spec and progressive JPEGs at smaller sizes, that it might take 500-800ms.

Actual Results: My testing shows that it consistently takes 2-3 seconds at least.

Notes: I did not test at very granular sizes of images, but it *feels* like there's a size at which performance suddenly degrades. But, we only have handy images at certain sizes. I only see the issues with the full 2048x2048 jpegs.