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I have some film scans from a half-frame camera, so each scan contains two pictures (side-by-side).

Some of the scans are consistently overexposed, but by different amounts. For example:

enter image description here

I'd like to manually adjust the exposure on each half of the scan without having to cut the picture apart, since I like the diptych effect.

Is there a way to select a rectangular area using darktable's exposure module? Or is there another module I should be using? So far I've only been able to select circles, ellipses, and paths (none of which are exactly right.)

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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: new instance icon, 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/

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You can use the tone curve tool with a mask. Select "drawn mask" (upper green arrow) and you will get the selection of mask drawing tools. One is the "gradient" tool (second arrow). Select that and than move the line around to git your needs. At the top of the screen you'll get the information of what modifiers to click to change compression of the gradient and stuff.

Darktable tone curve with mask

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    Maybe worth noting that masks work with any module. So this is not limited to the tone curve module but would work the same with the exposure module or any other module (e.g. if you also want to adjust colour).
    – luator
    May 13 at 10:46

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