Remove color range (to black) in Darktable

Question

I'm quite new to DarkTable, I'm trying to learn but I can't spot how to do what I want and can't find it in the tutorials.

I have taken some light painting photos, there is an overcast night sky in part of the background. I want to say "take this region, select its range of colors, and turn them black". Ideally, be able to adjust the upper and lower thresholds of the color parameter (I have that on other software, I'm sure). If I use the color picker, it only picks the color of one pixel - so the surrounding pixels don't get selected. That exact shade of orange gets converted to black, but the 8 pixels around it don't.

If I put a mask over the corner and just drop the contrast, it will affect the streaks that are in a totally different color range.

Yes, I am aware that some orange parts of the light painting will be affected, I think it will be a reasonable compromise to get a pure black background.

Thanks for your help!

Cheers!

Answer 1

You can try a parametric mask for this:

1. Open the 'levels' module
2. Click on 'blend' and choose 'drawn & parametric mask'
3. Draw a mask in the relevant area (e.g. an ellipse roughly covering the area)
4. Click on the 'h' to get the sliders for the hue channel
5. Use the pipette to select the colour you wish to remove. This puts a little vertical line on the sliders.
6. Drag the levels tool black point handle as far right as it goes.
7. Adjust the four thingamabobs on the input slider so that they're close to the little vertical line to get rid of colour.

Here's what I managed in 5 minutes; a better result is no doubt possible with more tinkering. You could also conceivably try a few other modules instead of levels, such as 'exposure'.

Answer 2

A couple of options come to mind.

1. The Colour Look Up Table (CLUT) module.

In Bruce Williams' YouYube video tutorial: darktable ep 052 - The Color Lookup Table module he uses a CLUT to alter the colours on a Item of clothing a model is wearing. The darktable 3.0 manual states:

INTERACTION
To modify the color mapping, you can change source as well as target colors.
The main use case is to change the target colors. You start with an appropriate palette of source colors (either from the presets menu or from a style you download). You can then change lightness (L), green-red (a), blue-yellow (b), or saturation (C) of the patches' target values via sliders.
To change the source color of a patch you select a new color from your image by using the color picker, and Shift+click on the patch you want to replace. You can switch between point and area sampling mode from within the global color picker panel (see Section 3.3.6, “Global color picker”).
To reset a patch, double-click it. Right-click a patch to delete it. Shift+click on empty space to add a new patch (with the currently picked color as source color).

2. Use Gimp.

It is possible to develop your Raw file in darktable then finish the edit in Gimp. You can export a high quality (32-bit float tiff file) from darktable and then open it in Gimp. Alternatively, opening the RAW in Gimp will open an instance of darktable which will apply any edits previously made to the RAW file and allow further changes to be made. When you close darktable the image is imported directly into Gimp in high quality.The Gimp 2.10 manual states:

1.3.3. Adjusting Hue and Saturation
In our experience, if your image has a color cast---too much red, too much blue, etc---the easiest way to correct it is to use the Levels tool, adjusting levels individually on the red, green, and blue channels. If this doesn't work for you, it might be worth your while to try the Color Balance tool or the Curves tool, but these are much more difficult to use effectively. (They are very good for creating certain types of special effects, though.)

Which is more or less a long winded method of achieving what kahovius said in his answer but without the ease of using darktable's parametric maskng.