Pixel formats, CVPixelBufferRefs and glReadPixels

Question

I'm using glReadPixels to read data into a CVPixelBufferRef. I use the CVPixelBufferRef as the input into an AVAssetWriter. Unfortunately the pixel formats seem to be mismatched.

I think glReadPixels is returning pixel data in RGBA format while AVAssetWriter wants pixel data in ARGB format. What's the best way to convert RGBA to ARGB?

Here's what I've tried so far:

• bit manipulation along the lines of argb = (rgba >> 8) | (rgba << 24)
• using a CGImageRef as an intermediate step

The bit manipulation didn't work because CVPixelBufferRef doesn't seem to support subscripts. The CGImageRef intermediate step does work... but I'd prefer not to have 50 extra lines of code that could potentially be a performance hit.

Answer 1

Regarding the bit manipulation, you can get a pointer to the pixel buffer's raw data:

CVPixelBufferLockBaseAddress(buffer, 0);
size_t stride = CVPixelBufferGetBytesPerRow(buffer);
char *data = (char *)CVPixelBufferGetBaseAddress(buffer);
for (size_t y = 0; y < CVPixelBufferGetHeight(buffer); ++y) {
    uint32_t *pixels = (uint32_t *)(data + stride * y);
    for (size_t x = 0; x < CVPixelBufferGetWidth(buffer); ++x)
        pixels[x] = (pixels[x] >> 8) | (pixels[x] << 24);
}
CVPixelBufferUnlockBaseAddress(buffer, 0);