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

Answer 1

It's kind of a shot in the dark, but have you tried GL_BGRA for glReadPixels with kCVPixelFormatType_32BGRA for CVPixelBufferCreate?

I suggest this because Technical Q&A QA1501 doesn't list any RGBA format as supported.

Answer 2

Better than using the CPU to swap the components would be to write a simple fragment shader to efficiently do it on the GPU as you render the image.

And the best bestest way is to completely remove the copying stage by using an iOS5 CoreVideo CVOpenGLESTextureCache which allows you to render straight to the CVPixelBufferRef, eliminating the call to glReadPixels.

p.s. I'm pretty sure AVAssetWriter wants data in BGRA format (actually it probably wants it in yuv, but that's another story).

UPDATE: as for links, the doco seems to still be under NDA, but there are two pieces of freely downloadable example code available:

GLCameraRipple and RosyWriter

The header files themselves contain good documentation, and the mac equivalent is very similar (CVOpenGLTextureCache), so you should have plenty to get you started.

Answer 3

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);

Answer 4

glReadPixels(0, 0, w*s, h*s, GL_BGRA, GL_UNSIGNED_BYTE, buffer);

Use GL_BGRA in glReadPixels. It works, just tried it myself.

Answer 5

    glReadPixels(0, 0, width, height, GL_BGRA, GL_UNSIGNED_BYTE, _buffer);
    CVPixelBufferRef buffer = NULL;
    CVReturn ret = CVPixelBufferCreateWithBytes(kCFAllocatorDefault,_size.width,_size.height,kCVPixelFormatType_32BGRA,glubyte,_size.width*4,NULL,NULL,NULL,&buffer);