feat(image): color channel mask

[PedroVillegas]

Hi Thomas,

I recently found out about tev and have found it to be very handy. The only thing I felt was missing: a color channel masking feature which lets me isolate individual, or multiple, color channels.

Note: I don't have an apple device to make and test changes for the metal shader, so I didn't.

[Tom94]

Hi Pedro, thanks for the PR!

tev can already explode the channels of the currently selected group into individual ones via Ctrl+X. This'll work even when the group isn't made up of RGBA like this PR assumes it to be. But... that feature is not very discoverable; just an entry in the keybindings help window. I assume you didn't see it before working on this PR?

Viewing group subsets is also possible, although admittedly quite a bit more cumbersome: use e.g. tev :R,G image1.png image2.png ... to load just the red and green channels of your images.

So, with these features in mind, I feel like a better approach to the PR would be to

• Add a GUI button for doing what Ctrl+X currently does, and
• (optionally) make it work with subset selection, e.g. using a GUI similar to what the current PR has, but generalized to the current channel group rather than hardcoded for RGBA. Also perhaps worth adding a toggle to make it apply to all currently-open images with the same channel group.
• Possibly both in a popup to avoid clutter. Especially since the current channel group can have arbitrarily many channels in it.

Compared to a pure shader-level approach, that'll also ensure that the histogram tracks the masked channels & that saving via Ctrl+S stays faithful to what's shown.

[PedroVillegas]

Ah, I wasn't aware of the explode channels keybind. Though, the lack of multichannel groups is not ideal and the alternative you pointed out to expose multichannel groups seems like a hassle to use.

For now I'll just use this solution whilst I find the time to implement the changes you suggested.

[Tom94]

I thought about this some more and figured that the feature would actually be nice-to-have as-is after all, my bad. Reason being that shader-level channel masking does behave a bit differently from the channel ungrouping feature the more I think about it.

I made a few small changes and will merge after CI is through:

• Using pure type traits / concepts for bitset overloads (no macros needed)
• Removed special casing on alpha being the only visible channel (that's achievable with the X shortcut; I felt like it'd be nice to have the pure masking behavior at one's disposal as well)
• Inverted the GUI toggle behavior (toggled == masked away)
• Added a GUI button for ungrouping (X behavior) to the right of the RGBA mask buttons
• Use Channel::color(name) instead of hardcoded color values
• Made Ctrl+C and Ctrl+S respect the mask
• Implemented the macos shader