How to "flatten out" darkest colors in an image

Question

OK so here I have a problem I want solved, but I wouldn't know the beginning of how to correctly formulate it. So bear with me!

Sometimes when I postprocess b&w photographs in darktable, I change the tone curve in such a way that everything below (darker than) a certain level of gray gets "leveled out" to that level of grey. Losing the shadows, that is. (I do this by making a little horizontal "flat piece" in the bottom left corner of the tone curve square, where the left side touches the side of the square.) A prime example of this is the following photo, in which I erased all the shadows right under the bridge, creating that "flat" look that old photographs often have.

• http://www.flickr.com/photos/kphve/12089371644/

Only the problem is, sometimes when I do this, probably just because the tone curve square is not precise enough for me to get it just right, the deepest black pixels stay, as in this photo:

• http://www.flickr.com/photos/kphve/12089278794/in/photostream/

As I apparently am not able to solve this in darktable, I hoped there would be some imagemagick one-liner with which I could just "flatten out" these darkest pixels to the desired gray level. Say, something like:

• convert in.jpg -flatten_up 10 out.jpg

..which would then take all r, g, and b values below 10 and change them to 10, effectively replacing the black pixels by darkgrey.

Answer 1

Try converting "-level 10" to make 10 your black point, thereby losing everything below 10, then converting "-level +10" to move your black point back up to 10.

Answer 2

You can use a parametric masks on darktable in order to only affect the darker areas of your image. That way you can manipulate and highlight the darkest areas without affecting the rest, and if part of the shadows are being affected, just combine a draw mask to select the area that your parametric mask will affect. This masks can be applied almost to any module on darktable. parametric mask + draw mask example of usage