[FFmpeg-user] Curves filter to only affect dark grey/black

[Joshua Grauman]

Hi all,

I am processing some video and would love help with the curves filter. The 
video is shot with a 'black' background, but it is not completely black 
(notice folds in the backdrop). I want to filter the video to make the 
background completely black without affecting any of the rest of the video 
at all. I'm using the following curves filter with some success:

curves=master='0/0 0.10/0 .15/.15 .16/.16 .17/.17 .2/.2 .25/.25 .3/.3 
.4/.4 .5/.5 .8/.8 1/1'

The problem is that the curves filter is affecting (darkening) the dark 
blue shirt as well.

Here is a screenshot of the video I am working on.
http://grauman.com/Screenshot_20180801_143551.png

Is there a way to only affect the levels when the saturation is low 
(gray/black)?

Josh