Thursday, May 10, 2018

JPEG Decoding With the Best

Luke Parham (via Indie iOS Focus Weekly):

That’s right, any time you have a UIImage created from a JPEG, assigning it to be displayed by an image view means that an implicit CATransaction will be created and your image will be copied, decompressed, and rendered on the next display pass triggered by the run loop. […] This all happens on the main thread.


Well you can’t make UIImageView do its decoding more quickly, but if you’re so inclined, you are able to preemptively render the jpeg yourself!


First, there is, unfortunately, no API that’s been exposed for you to tell whether a given UIImage has already been decoded or not. This means that if you’re using this trick, you’ll want to make sure to carefully cache and re-use these images when you can since they take up a lot more memory than their compressed counterparts.

Second, this particular method is technically "offscreen rendering" in that it’s no longer hardware accelerated, but it’s the ok-when-it’s-useful kind of offscreen rendering, not the kind that makes your GPU stall.

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