I’ve got a very curious issue here. I’m doing image processing on a rogue GPU (iPhone 5S). I’m using a 16bit render target, and rendering in an image, then using a feedback loop and mixing the result back in.
That’s all fine, but there’s a really odd issue when this is left running for quite a while (hundreds of frames). I start seeing “blockiness” in the image - similar to very heavy JPEG compression (the image shows clear blocks of different colours, often with a gradient across the block. Low-contrast parts of the image look perfectly normal, but it might just show up the effect more in higher contrast regions.
The blocks seem to have variable sizes, but the sizes follow a pattern - the smallest is 14 pixels, then 25 pixels, then 51. Larger blocks seem to be multiples of 51.
I thought at first it could be related to the tiler, but I doubt it’s using these strange block sizes! Is there anything else on the GPU that might cause this under extreme circumstances?
Stuff I’ve ruled out so far:
- The source data (scaling the image should show bigger blocks in this case, instead I see a zoomed in image with the same small blocks)
- The rendering setup (at least nothing in there is block based, I can’t imagine anything that would cause it)
- Bad output (the blocking is also seen with uncompressed PNG output).
