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by garik

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I want to invert the brightness in a RAW. I do not want to invert the colors, only the brightness of the colors. Can I do that in darktable 1.4.2, and if yes how?

Edit: By inverting brightness, I mean as described here: http://www.lostinneurons.com/2011/03/inverting-image-brightness.html

I think I understand the idea but don't know if/how I can do this in darktable.

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2 Answers 2

up vote 3 down vote accepted

Open the "tone curve" module and create a straight line from the top left to the bottom right corner. You can just drag the endpoints with the mouse. Then, set "scale chroma" to "manual".

Tone curve in untouched state The applied inverse tone curve should look like this

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I have no idea how to do this in any particular canned app, but the actual manipulations don't sound that hard. I'd start by converting from RGB to intensity, hue, and saturation, then flipping intensity, then converting that back to RGB.

If you look at that math closely, you may be able to find a way to perform the same overall operation directly from RGB to RGB without actually doing the conversion to IHS and back. Conceptually that's what you'd do, but the actual manipulations would be quicker.

Perhaps there is image editing software out there that lets you define manipulations to perform per pixel with a script or by supplying your own subroutine or something.

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