Use more color than 16 in sixel graphics

[yoichi]

Problem

In sixel backend, the colors are limited to a very small number 16.
If the image specified by the image option uses more colors, the vividness will be lost.

Fix

Increase color number from 16 to 128.

The color of the beak has been reproduced.

Side Effect

Image rendering time become longer.
On my environment, it takes ~0.24sec with 16 colors -> ~0.4sec with 128 colors

Enironment

• onefetch runs on Ubuntu20.04
• mlterm runs on Windows 10

[shuni64]

The problem with this is that at least xterm isn't required to support more than 16 colors. This isn't because of a flaw with sixel graphics itself, it's because the VT340 doesn't support more than 16 color map entries, so the right behavior from xterm would be to fail above that because it emulates the VT340.
It's possible to increase the amount of colors for emulators that support them, but 16 colors is a safe default that should work on everything emulating the VT340.

The best way to do this would be to query the terminal emulator for the amount of color map entries it supports, but I'm not sure if that's possible.
Alternatively there could be a command line argument to specify that value, that way there shouldn't be any case where sixel images can't be displayed because of a hardcoded value.

[yoichi]

Thanks @CephalonRho for clarifying the reason for the 16 colors.
I understand the existence of terminals that support sixel but not 256 colors and the need for support for them (although I haven't used them myself).

Thanks @spenserblack for suggestion to use environment variable. However, there are situations where that method cannot be used (ssh to target host from the terminal, or docker run from the terminal). We should query the terminal for those cases, but I don't know how to determine color support.

I'll consider a command line argument as @CephalonRho suggested:

Alternatively there could be a command line argument to specify that value, that way there shouldn't be any case where sixel images can't be displayed because of a hardcoded value.

[yoichi]

In 83459a8 I've added a new option --image-colors to specifiy number of colors to be used.
If the option is not given, use 16 colors as before.

[spenserblack]

As far as CLI setup goes, looks good to me! 👍
Just one request to change the grammar in the help setting.

I'm not too familiar with sixel, so I'll be asking @CephalonRho to look at the possible_values setting.

[spenserblack]

@CephalonRho Can you confirm that these are the possible values that should be used?

[shuni64]

Any number above 0 should work. These seem to be chosen arbitrarily, but they are values that should provide a reasonable looking output.
A solution using a custom validator would be perfect, but it's probably not worth the effort.