Add custom format variables

[bczhc]

Seems I can't find a way to add custom format variables, for example, for the image exposure value (not the 'bias') calculated by the formula $$\log_2{(100N^2/(\rm{ISO}\cdot S))}$$, I could add a variable called $(EV_AT_ISO100) and use it in lighttable tooltip/overlay text formatting textbox. Or, can I modify the overlay text directly in lua scripts?

[wpferguson]

The overlay text is not accessible from Lua at the current time.

Lua does support almost all of the substitution variables supported by darktable, but it is a separate functionality contained in the Lua scripts string library and is not used by the darktable variable substitutions.

[wpferguson]

I figured out a way to do this...

You could calculate the exposure value for the image. You could create a tag metadata|exposure value|<value>, where <value> would be the computed value and then attach it to the image. You could then use the tag in the variable substitution with $(CATEGORY[0,"metadata|exposure value"])

You could script it checking to see if the tag exists and if not creating and adding it.

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I'm trying to figure out a way for the user to add additional metadata fields that they want monitored and available to use, The above solution with tags is probably the way I'm going to go.