FFMPEG - Using variables in curves video filer expression

Question

I'm tearing my hair out trying to get this to work, i assume it's a syntax issue.

I'm experimenting with the curves video filter in ffmpeg. When I enter the coordinates in the standard form;

ffmpeg -y -i 0.jpg -vf curves=r='0/0 .83/.88 1/1':g='0/0 0.84/0.88 1/1':b='0/0 0.88/0.88 1/1' curves.png;

....I have no issues.

I am trying, however, to use variables vs fixed numbers for the curve points and i cannot in anyway get ffmpeg to accept it.

For example if i simply type;

var3=0.88

and then repeat the code with;

ffmpeg -y -i 0.jpg -vf curves=r='0/0 .83/$var3 1/1':g='0/0 0.84/$var3 1/1':b='0/0 0.88/$var3 1/1' curves.png;

it won't work, it's sees $var3 as being 0.000000

also tried;

ffmpeg -y -i 0.jpg -vf curves=r='0/0 .83/${var3} 1/1':g='0/0 0.84/${var3} 1/1':b='0/0 0.88/${var3} 1/1' curves.png;

and

ffmpeg -y -i 0.jpg -vf curves=r='0/0 .83/`echo $var3` 1/1':g='0/0 0.84/`echo $var3 1/1':b='0/0 0.88/`echo $var3 1/1' curves.png;

I'm out of ideas. Could anyone help in this situation, really need these to be variables vs fixed numbers for my purposes.

Thanks!

Answer 1

Assuming you're using bash or a similar shell, you need double quotes. Single quotes conveys its contents literally.

curves=r="0/0 .83/$var3 1/1":g="0/0 0.84/$var3 1/1":b="0/0 0.88/$var3 1/1"