Support per-channel black level corrections

[junyongjeon]

To crop noise in low grayscale regions, I want to use 'user_black' to crop very low grayscale regions, but since I can only set one int value for 'user_black', I can't subtract a separate value for each color. As a result, it gives the image an awkward color.
Is there any method to provide separate 'user_black' for each color channel?

[letmaik]

libraw (which rawpy is built on) does support per-channel black level corrections via user_cblack[4]. So in principle this could be exposed as a new parameter in rawpy.
Could you give an example of how you would determine the per-channel correction values that you would use in this case?