aledalgrande aledalgrande - 1 month ago 19
iOS Question

Artifacts when scaling Ycbcr (420f) with Accelerate

I cannot find any documentation or example on how to resize Ycbcr biplanar, supposedly the main format you should use on iOS according to Apple. I tried to resize the two planes like this:

// resize luma
vImage_Buffer originalYBuffer = { CVPixelBufferGetBaseAddressOfPlane(pixelBuffer, 0), CVPixelBufferGetHeightOfPlane(pixelBuffer, 0), CVPixelBufferGetWidthOfPlane(pixelBuffer, 0), CVPixelBufferGetBytesPerRowOfPlane(pixelBuffer, 0) };
vImage_Buffer resizedYBuffer;
vImageBuffer_Init(&resizedYBuffer, IMAGE_HEIGHT, IMAGE_WIDTH, 8 * sizeof(Pixel_8), kvImageNoFlags);
error = vImageScale_Planar8(&originalYBuffer, &resizedYBuffer, NULL, kvImageNoFlags);
assert(!error);
cv::Mat grey(IMAGE_HEIGHT, IMAGE_WIDTH, CV_8UC1, resizedYBuffer.data);

// resize chroma
vImage_Buffer originalUVBuffer = { CVPixelBufferGetBaseAddressOfPlane(pixelBuffer, 1), CVPixelBufferGetHeightOfPlane(pixelBuffer, 1), CVPixelBufferGetWidthOfPlane(pixelBuffer, 1), CVPixelBufferGetBytesPerRowOfPlane(pixelBuffer, 1) };
vImage_Buffer resizedUVBuffer;
vImageBuffer_Init(&resizedUVBuffer, IMAGE_HEIGHT / 2, IMAGE_WIDTH / 2, 8 * sizeof(Pixel_16U), kvImageNoFlags);
error = vImageScale_Planar8(&originalUVBuffer, &resizedUVBuffer, NULL, kvImageNoFlags);
assert(!error);


But the colors are totally borked. The luma channel works by itself, so it's a problem with the chroma. This format is supposed to use 2 bytes for chroma, although not totally sure. If I use
vImageScale_Planar8
I get half of the screen green, else if I use
vImageScale_Planar16U
I get blue/yellow noise all over the image.

Answer

Got the andwer from Apple guys: vImageScale_Planar8 cannot operate on the UV plane, because it is interleaved. The only solution is to split into two independent planes.

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