I’m digitizing drawings/artwork photographed under controlled conditions: two Ra93 lights at about 45° on opposite sides, DIY tracing-paper diffuser, and a fixed phone-arm setup. The lighting is still slightly uneven, but all images were shot the same way, so I can reuse a common history stack.
My goal is to make the paper neutral/white while preserving the artwork colors for printing.
Current modules:
rotate and perspective
exposure / tone equalizer
color calibration
crop
My problem is that if I push the background toward white, the colors often get too bright or start to fade. If I protect the colors, the paper stays too dark.
What workflow or module combination would you recommend for this kind of artwork reproduction in darktable? In particular, I’d like advice on:
correcting slight lighting unevenness
neutralizing paper color
preserving color saturation and contrast in the drawing
building a reusable edit for a batch shot in identical conditions
I feel there are a few ways of doing this. If shooting in RAW then using AgX I would try and adjust the white relative exposure slider to brighten the white paper. If the colors fade I would use color zones to darken colors. Maybe if you share an image as playraw people could try different methods. Even a scribble onto the paper in question if you can’t share the artwork.
For uneven lighting, a common method is to photograph your drawing, and photograph a blank sheet of paper. Then make a new image that has each channel of each pixel set to the drawing divided by the blank sheet, normalised to [0…1]. This compensates for uneven density, and uneven colour, of the lighting.
Where the two inputs are the same (both blank), the output will be white.
I think Colour Look up Table might help with this, especially with drawn/parametric masks. You can protect the drawing colours both with masks and by setting custom patch colours in that module.
Colour Balance RGB might be good for desaturating/brightening the paper if necessary, with its shadows/midtones/highlights masks.
RGB curves might help with uniformity. Filmic RGB might be a good idea too, to compress/desaturate uneven highlights and use its highlight inpainting.
Colour Zones as Terry said is also an idea; you can adjust lightness/saturation based on chroma value (instead of hue).
Censorise, colourise come to mind as well.
All of these combined with darktable masks and maybe blending modes like lightness/chromaticity.
Blurs, Diffuse or Sharpen and Contrast Equaliser would have a tendency to bleed drawing colours into the paper area. Worth a try with lightness blend mode though.
This might be possible with the Composite module if you want to keep it in DT. Set uniform blending with divide mode (might need to be in display blending space for that)
Unfortunately, I can’t really share the artwork for now because I’m helping my partner with digitalizing her art for a competition; and by the rules of the competition we can’t publish anything before the final results. If she agrees I would like to share either a scribble onto the paper and/or a full drawing once the competition is over; so we can learn better how to preserve / adjust the colors in the future.
That’s actually an excellent idea, both mathematically beautiful and extremely simple to implement in my case. I tried what you suggested: photographed a blank sheet of paper, and then the drawing, each time placing the sheet exactly at the same position, and it worked really well.
I still need to perform some color adjustments, but otherwise I think this is by far the best result I got, and it’s so simple.
Thank you @gwbarn as well. I’ll try that, so that I may be able to do everything in darktable (although I love the ImageMagick solution too).
Also thanks for modules. I didn’t know a lot of them. I tried Filmic RGB, but it didn’t go anywhere, I think I might be holding it wrong. I’ll play around with all that and see if I can get somewhere nice.