What exactly does CVBufferSetAttachment do?

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Created Mar ’23

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I understand the CVBufferSetAttachment simply appends metadata attachment to the sample buffer in a dictionary. But I see there are no errors in appending metadata that is contradictory in nature. For instance, for the sample buffers received from camera in HDR mode which are in YUV422 10 bit biplanar format, both the following succeed:

CVBufferSetAttachment(testPixelBuffer!, kCVImageBufferColorPrimariesKey, kCVImageBufferColorPrimaries_ITU_R_2020, .shouldPropagate)
                
CVBufferSetAttachment(testPixelBuffer!, kCVImageBufferTransferFunctionKey, kCVImageBufferTransferFunction_ITU_R_2100_HLG, .shouldPropagate)
                
CVBufferSetAttachment(testPixelBuffer!, kCVImageBufferYCbCrMatrixKey, kCVImageBufferYCbCrMatrix_ITU_R_2020, .shouldPropagate)

Or

CVBufferSetAttachment(testPixelBuffer!, kCVImageBufferColorPrimariesKey, kCVImageBufferColorPrimaries_ITU_R_709_2, .shouldPropagate)
                 
CVBufferSetAttachment(testPixelBuffer!, kCVImageBufferTransferFunctionKey, kCVImageBufferTransferFunction_ITU_R_709_2, .shouldPropagate)
                    
CVBufferSetAttachment(testPixelBuffer!, kCVImageBufferYCbCrMatrixKey, kCVImageBufferYCbCrMatrix_ITU_R_709_2, .shouldPropagate)

So one could set the color primaries and transfer function to be BT.709 format for sample buffers that are in 10 bit HDR. I see no errors when either sample buffer is appended to AVAssetWriter. I am wondering how attachments actually work and how AVFoundation resolves the contradictions?

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Answer 1

galad87 OP

Mar ’23

I guess it depend where are you using those buffers. If you are encoding them with VideoToolbox or AVFoundation it will probably convert them to the appropriate colourspace set in the encoder, or maybe it will completely ignore it. Try and see.

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