How can I adjust exposure separately on each half of a half-frame scan in darktable?
Asked 5/7/2022
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2 answers
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I have film scans from a half-frame camera, so each scan contains two side-by-side images that I want to keep together as a diptych. Some scans are overexposed, but each half needs a different correction.
In darktable, is there a way to apply exposure changes to only one rectangular half of the image at a time without splitting the file? I’ve found drawn masks, but I mostly see circles, ellipses, paths, and gradients.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
1
If you're configured to use a scene-referred workflow (default in recent versions of darktable: Preferences | processing | auto-apply pixel workflow defaults = scene-referred) a tone curve will not be available nor recommended.
You can create a separate instance of the exposure module for each half by right-clicking the icon in the module header: 
, use a drawn mask as previously advised on that instance and adjust exposure there for only that part of the image.
It's possible to create an exactly-half mask in one instance and use it again for a subsequent instance / module ("raster" mask) with the opposite polarity. But depending on how familiar you are with darktable, you probably just want to create one mask / instance for each side.
More details can be found in the darktable documentation: https://docs.darktable.org/usermanual/4.0/en/
Exposure module: https://docs.darktable.org/usermanual/4.0/en/module-reference/processing-modules/exposure/
Masking overview: https://docs.darktable.org/usermanual/4.0/en/darkroom/masking-and-blending/overview/
Originally by user106816. Source · Licensed CC BY-SA 4.0
user106816
3y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. In darktable, the usual way is to create separate masked adjustments for each side rather than splitting the image.
If you’re using the default scene-referred workflow, use multiple instances of the exposure module: create one instance for the left half and another for the right half, then apply a drawn mask to each instance and adjust exposure independently.
A gradient mask can work well for dividing the image into halves, and you can position/compress it so the transition is effectively at the center. You can also reuse a mask in another instance and invert it to target the opposite side.
If you’re not using a scene-referred workflow, a tone curve module with a drawn mask can also be used similarly. But in recent darktable versions, separate exposure-module instances with masks are the recommended approach.
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