CODEWITHSUNDEEP
iosavassetwriterglreadpixels
MrDatabase
MrDatabase

Reputation: 44455

Pixel formats, CVPixelBufferRefs and glReadPixels

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:

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.

Upvotes: 7

Views: 7625

Answers (5)

gngrwzrd
gngrwzrd

Reputation: 6012

    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);

Upvotes: 1

JP Hribovsek
JP Hribovsek

Reputation: 6707

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

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

Upvotes: 1

Gordon Childs
Gordon Childs

Reputation: 36074

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.

Upvotes: 7

rob mayoff
rob mayoff

Reputation: 385600

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);

Upvotes: 7

rob mayoff
rob mayoff

Reputation: 385600

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.

Upvotes: 2

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