On Mar 2, 2012, at 11:05 AM, Marco Tabini wrote: > My first instinct is to use a memory-mapped file (e.g.: using NSMutableData) > to hold the data while I work on it, but I worry that it will be very slow > and that I'm missing a much simpler solution.
I don’t think you can get around the need to swap pixels in and out of memory from ‘disk’. In general, mmap is a good way to do that because it’s optimized down to the kernel level. The problem is that if you do this in a naive way, with a single huge pixmap, you will have poor locality of reference. Once you get to 1024 RGBA pixels across, every scan-line will occupy its own memory page. So any operation that crosses lots of scan lines but only uses a small fraction of each one (like drawing a vertical line) may involve a lot of paging. The usual workaround to that is to instead break the image up into rectangular tiles, and store each as a separate pixmap. That way most localized graphics operations will only involve a fraction of the total number of tiles. (You can see Photoshop do this, if you run a slow operation or if it’s had to page part of the image out to disk.) I think this will also help with the GPU, because CoreGraphics likes to copy pixmaps to GPU memory so they can be rendered and manipulated faster. (That said, I am not a CoreGraphics guru, and I defer to a better answer from anyone who is.) —Jens
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