From rgb to xterm256

[nanobowers]

Hello @mrkn , I needed some methods for converting an rgb value into the closest xterm256 color - this package previously only contained the methods for xterm256->to_rgb. I added support as a class method: Colors::Xterm256.from_rgb(rgb), but can move to an instance method on Colors::RGB (e.g. Colors::RGB#to_xterm256) if that is more appropriate.

I plan to use the two way conversion in unicode_plot.rb.

[mrkn]

Thank you for your contribution. But, I cannot merge this now because I still consider that how to find the closest color.
I suspect that Euclidean distance in RGB color space is not appropriate as the distance function to find the closest color.

I guess that one candidate is LUV color space with the appropriate reference white point.

I need more time for studying color theory to decide the appropriate method.

[nanobowers]

Hello @mrkn , thank you for the consideration.

I based the xterm256 approximation on the algorithm found in tmux:
https://github.com/tmux/tmux/blob/master/colour.c

Per the description below, the xterm256 color palette is extremely coarse. Because of that, I was hoping that it would be close enough:

         * Convert an RGB triplet to the xterm(1) 256 colour palette.
         *
         * xterm provides a 6x6x6 colour cube (16 - 231) and 24 greys (232 - 255). We
         * map our RGB colour to the closest in the cube, also work out the closest
         * grey, and use the nearest of the two.
         *
         * Note that the xterm has much lower resolution for darker colours (they are
         * not evenly spread out), so our 6 levels are not evenly spread: 0x0, 0x5f
         * (95), 0x87 (135), 0xaf (175), 0xd7 (215) and 0xff (255). Greys are more
         * evenly spread (8, 18, 28 ... 238).

[mrkn]

I know tmux uses RGB color space to determine the closest color, and I don't want to follow tmux.

In unicode_plot, I would like to derive the closest color using a distance space where the distance between two colors that the human eye perceives as close is small. I guess CIELUV colorspace is more appropriate than the other color spaces.