I was tasked with shooting my sibling’s passport picture. Since I don’t actually have any lighting equipment I just put them in front of a white wall while we had diffuse light bouncing off the room which originally came from the window. This delivered pretty good results, but there is a slight bit of shadow on the left-hand side and the wall is showing a little bit of texture. I think the picture would still be accepted, but I wanted to try learning something new by isolating the background. I was able to create a combined mask which does that but now I don’t know how to fill the background with a uniform colour. The retouch module seems to be able to do that, but I read the documentation and still can’t figure out how the bucket fill actually works.
There is no “fill” command in darktable, it is not a pixel editor. You can either blow out the texture to pure white, use the retouch module to clone it out, or use color balance rgb to tint the whole wall some specific color (that probably won’t get rid of the texture though).
When you select the fill icon, you must thereafter mark the area to be filled, typically by drawing a line round it (third icon from the left). (You have no magic feathering function to help you here as you will have with ordinary masks in most other modules.) Go further down in the module under “shapes” and select “fill mode” as either “erase”, which leaves an initial black hole in the marked area, or “color” where you can fill inn a color either by picking a color from the image by means of the color picker icon to the right, or click on the black color stripe and you get a color palette to chose from.
You may also try the old Colorize module where you have full mask functions and blend modes available.
The texture may possibly also be removed by using a blurring tool, (Diffuse or sharpen preset , censorize or blurs tools), and the shadow may possibly be handled by the graduated density module.
I see you have achieved what you wanted. However, for future reference I would suggest this might be something that GIMP could handle easier because it is a “pixel editor”. DT allows you to export an xcf file designed for further editing in GIMP or GIMP can edit JPG, Tiff, PNG or most file types.
I have no idea what country you are in but it would be a good idea to check the requirements for a photo. I’m in Canada and one of the requirements is that it be taken by a commercial photographer and also they must be unaltered:
