Automatic exposure control targeting exposure value (e.g. 18%)

[kofa73]

Darktable currently provides two ways to support exposure adjustments based on the image contents:

• manual exposure, where one can provide a clipping percentage;
• automatic exposure, where the user inputs the desired exposure value (such as -4EV) and a percentile to target.

Neither of those modes is very useful with the scene-referred workflow, requiring one to set exposure correctly for mid-grey.

Describe the solution you'd like
It'd be nice to have a picker that could be used to select an area of the screen, and automatically adjust exposure to set it at 18% (e.g. the clear sky is often quoted as a potential metering target, one example is https://cinematography.com/index.php?/topic/71737-exposing-sky-with-incidental-light-meter-on-film/). Even better, if an additional slider could be used to specify the target exposure (e.g. in 1/3 EV steps), one could place scene elements at pre-determined brightness levels (light skin is often said to be around +2/3 - 1 EV above mid-grey (see e.g. https://www.canon.com.hk/en/club/article/itemDetail.do?itemId=10400 or https://petapixel.com/2012/06/06/use-your-hand-as-a-makeshift-gray-card-for-estimating-exposure/); though as only 18% is kept in place by filmic, this may be less useful).

Alternatives
Currently, one can create a live sample patch in LAB / mean mode, adjust exposure manually to set it to 50%.

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[mbaz]

As a new darktable user, I was thinking along these lines, but with a different interface. The interface I came up would work like this:

• Included in the exposure module.
• Similar to the clipping indicators (the red/blue overlay on a photo), but showing the pixels that are at 18%, maybe with a gray overlay.
• Maybe with a slider for the range (e.g. to see pixels between, say, 17% and 18%).

Such an interface would not be "automatic" as proposed by the OP, but it would allow for precise quantitative measurements, while retaining an artistic/interpretation aspect, since the placement of the midpoint is up to the photographer (using the exposure compensation slider).

Besides being very helpful to "anchor" the midpoint in the photograph, this interface would produce optimal input for subsequent Filmic RGB processing. It would eliminate the "guess work" of trying to find a good midpoint, and it would be very easy to use.

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This issue did not get any activity in the past 60 days and will be closed in 365 days if no update occurs. Please check if the master branch has fixed it and report again or close the issue.

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