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macosperformancecore-graphicsretina-displayvimage

Bad performance of GCContextDrawImage due to it calling suspicious debug functions

A call to GCContextDrawImage turned out to be a bottleneck in my Mac OS X application, especially on retina screens. I managed to mitigate it somewhat by Avoiding colorspace transformations when blitting, Mac OS X 10.11 SDK, but it still seems to be slower than I would expect it to be.

When investigating the stack dump with Instruments I noticed that a lot of time was spent in two functions with highly suspicious names, vImageDebug_CheckDestBuffer which is calling into _ERROR_Buffer_Write__Too_Small_For_Arguments_To_vImage__CheckBacktrace. See the the full stack dump below.

This seems to me like some sort of debug assertion? Am I running a debug version of the vImage library without realising it? Is there something I can do to stop these functions from sucking up all my precious cycles?

Stack trace captured with Instruments

Solution

  • The performance problem was solved by making sure the beginning of the pixel data in each scan line of the source bitmap is aligned to 16 bytes. Doing this seems to make the image drawing considerably faster. Presumably this happens by default if you allocate a new image, but we wrapped a CGImage around an existing pixel buffer which wasn't aligned.