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In Gimp I want to generate a gray scale image of a RGB image defined by this per-pixel formula: value_xy = max(R_xy, G_xy, B_xy).

I didn't find a builtin feature so I wonder if there's some generic filter out there where I can operate on a per-pixel basis by just defining some formula.

Is there something like this already? Would I have to write my own filter for Gimp?

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No need for specific filter...

  • Color>Components>Decompose and use RGB Model, you get a grayscale image where each layer is one of the RGB channels of the original image
  • Set the top two layers to Lighten only mode.

enter image description here

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  • I've changed the title to match your answer :) This is what I've been looking for - but since you seem to know Gimp - do you know the answer to the original Question? Is there a builtin way (or an existing filter) which allows me to calculate color values with functional expressions on a per-pixel basis?
    – frans
    Commented Apr 24, 2017 at 7:10
  • Not as far as I know. Since you are usually dealing with millions of pixels, this is best done with a compiled language, in other words in a C plugin. The other (and likely more palatable option) is to write a filter for the GMIC processor, that works as a Gimp plugin or as command-line processor. But you may also find frameworks to do this kind of job in a command line program/script. For instance, there is the pillow module for Python.
    – xenoid
    Commented Apr 24, 2017 at 8:14

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