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Feature #11186

Process and convey alpha channel

Added by Christian Mandel almost 3 years ago.

Status:
New
Priority:
Low
Assignee:
-
Category:
General
Target version:
-
Start date:
09/30/2016
Due date:
% Done:

0%

Estimated time:
Affected Version:
git master branch
System:
all
bitness:
64-bit
hardware architecture:
amd64/x86

Description

I started scanning old slides and negatives recently. The scanner can do an additional infrared scan to detect scratches. The result of the infrared scan ends up in the alpha channel of the resulting TIFF file. Since I want to process the images in darktable and not in the scanner software, I needed some plan what to do with the infrared channel, and I found help on pixls.us (https://discuss.pixls.us/t/scanned-image-scrach-removal-with-ice/2350), using an inpainting algorithm from G'MIC to fill the scratches. There was consent that it would be best to do this as a last processing step, which causes a problem since the alpha channel is lost during processing. That would mean, that I could not do any processing steps that spatially move pixels, e.g. cropping, perspective correction, etc., if I wanted to use the original alpha layer.

Is there any chance that the alpha layer (if it is present at input) can be processed along the RGB channels of the image and that the result can optionally be added as alpha channel to the output?

Test image can be found here: https://filebin.net/gehwnhsxwi48qjb8 (150 MB).

Also available in: Atom PDF

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