xterm-256color support (proof-of-concept)

[jscheid]

Terminal.app doesn't support 24-bit color, but it does support 256 colors which can make for a decent approximation, see screenshots below.

This needs more polish, for which I actually might not have time myself, but I figured someone might find it useful already as it is.

Before:

After:

[dandavison]

This is great @jscheid, thanks very much for working on it! I think it definitely makes sense to get this functionality into delta.

One question I have is, it looks like bat does something like this. How does what you're doing here compare to what bat is using, i.e. https://docs.rs/ansi_colours/1.0.1/ansi_colours/fn.ansi256_from_rgb.html ?

I'm no expert on the environment variables concerned. I see you're inspecting TERM, whereas bat seems to be looking at COLORTERM (also documented in bat's README). I'll have to read up on them.

In any case, I'd definitely like delta to behave nicely when users don't have 24 bit color support so thanks very much for getting it under way!

[jscheid]

Ah, I was looking for an existing solution like what ansi_colours provides, I wonder why I didn't find it. It probably makes sense for you to go with that instead, if only because the colorspace crate is unpublished (and is APL, not sure if that's an issue for you.)

To answer your question: I believe my solution provides a more accurate approximation because it uses Delta E instead of operating in RGB color space, and takes gamma into account. It comes at the expense of being more compute intensive, however that wouldn't matter in an application like yours where there's a relatively small number of colors, since you can easily cache the results.

I wouldn't worry about accuracy too much though, going with ansi_colour seems like the pragmatic choice here unless it produces wildly different results. Feel free to close this PR.

You're right about COLORTERM, I was just about to fix that but since it looks like you're going a different route anyway I'll leave it as is.

Thanks for a very useful tool btw, cheers!

[dandavison]

Thanks very much @jscheid for getting the work started here. I've merged #111 which adds support for 256-color terminal environments and should serve to replace this PR.

[jscheid]

Awesome, thanks!