Does H.264 encoded video with BT.709 matrix include any gamma adjustment?

Question

I have read the BT.709 spec a number of times and the thing that is just not clear is should an encoded H.264 bitstream actually apply any gamma curve to the encoded data? N

Answer 1

Your original question: Does H.264 encoded video with BT.709 matrix include any gamma adjustment?

The encoded video only contains gamma adjustment - if you feed the encoder gamma adjusted values.

A H.264 encoder doesn't care about the transfer characteristics. So if you compress linear and then decompress - you'll get linear. So if you compress with gamma and then decompress - you'll get gamma.

Or if your bits are encoded with a Rec. 709 transfer function - the encoder won't change the gamma.

But you can specify the transfer characteristic in the H.264 stream as metadata. (Rec. ITU-T H.264 (04/2017) E.1.1 VUI parameters syntax). So the encoded streams carries the color space information around but it is not used in encoding or decoding.

I would assume that 8 bit video always contains a non linear transfer function. Otherwise you would use the 8 bit fairly unwisely.

If you convert to linear to do effects and composition - I'd recommend increasing the bit depth or linearizing into floats.

A color space consists of primaries, transfer function and matrix coefficients. The gamma adjustment is encoded in the transfer function (and not in the matrix).