Bt.709 and sRGB have the same primaries.
But different gamma curves. sRGB uses a higher gamma than Bt.709/601/2020.
If I want to routinely mix sRGB and Bt.709, is the right way to do it to convert to linear light using the nominal gamma curves ?
Or simply treat 709 Y'CbCr matrixed to R'G'B' as sRGB ?
How do folk handle this ?
The standards describe gamma as if it was referenced to input linear light, when it is really referenced back from a reference monitor,
viewed under particular ambient lighting. The reference ambient lighting would affect the gamma of the encoded images.
sRGB appears to be (notionally) referenced to well defined viewing conditions. But it's not clear to me what the reference viewing conditions are for Bt.709.
Of course one might make an aesthetic choice to apply an arbitrary tone curve either to the sRGB or the Bt.709...
Photoshop includes sRGB and Rec 709 among its standard profiles, so why don't you just convert your Rec 709 files to sRGB and compost in that space?