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I'd like to break down my image into separate channels, modify them and then recombine them back into a full image.

It does work to a degree following this tutorial: https://docs.krita.org/en/reference_manual/dockers/channels_docker.html

First they suggest selecting a layer and then doing Image->Separate Image. This way I can obtain every individual channel as a layer and edit them. It's possible to combine them back using blend modes where every layer is blended with Misc->Copy Red/Blue/Green options. However, this does not work for alpha as there is no option to have Copy Alpha blend mode.

As a workaround they suggest using "inherit alpha" for RGB channels and then edit transparency of the last channel. However, I want to edit that channel as a grayscale image, not a transparency image. In order to do that they suggest 2 options: by splitting alpha into a mask https://docs.krita.org/en/reference_manual/layers_and_masks/split_alpha.html or by converting a grayscale into alpha. First way is very restrictive as it does not allow basic operations such as pasting into mask (or I can't find how to do that). Second way is inconvenient because I have to convert a layer into alpha after every modification.

Is there a way to do what I want by jumping through less hoops?

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The only "workaround" I found is to use gimp. It has Colors->Components->Decompose/Compose specifically to achieve exactly this. However, it's not perfect either: for some reason "Compose" is often grayed out for me and for some reason it sometimes becomes terribly slow (like, freezes for 10 seconds or so). Also it's not a Krita solution, so I won't mark it as an answer, more of a workaround for those who stumble upon this and just needs things done.

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