Working with colors in --batch mode

[Kévin Le Gouguec]

Hello Emacs,

For some reason, today finds me playing with color contrast, and having
a devil of a time doing so in --batch mode.

Specifically, I'm stumped by…

$ emacs --batch \
    --eval '(message "color-name-to-rgb: %s"
                     (color-name-to-rgb "#abcdef"))'

… returning…

> color-name-to-rgb: (0.0 1.0 1.0)

… whereas in a non-batch session on an X display, I get something more
relevant to my usual Emacs experience (i.e. (window-system) ↦ x):

> color-name-to-rgb: (0.6705882352941176 0.803921568627451 0.9372549019607843)

The --batch behaviour thwarts my attempts to write a script that calls
modus-themes-contrast, since that function relies on color-name-to-rgb.

I think I've traced this to (color-values COLOR &optional FRAME) taking
a "wrong turn" because emacs --batch does not bother setting up
graphical frames; I tried coaxing Emacs into doing that prior to calling
any color function, with various combinations of…

* --display :0
* --eval "(make-frame '((display . \":0\") (window-system . x)))"
* --eval '(make-frame-on-display ":0")'

… to no avail; Emacs merely tells me "I/O possible" then terminates with
code 157 (-128=29, which tracks with signal(7) mapping this to SIGIO).

It might just be me not finding the right terms, but none of my research
(Emacs & Elisp info manuals; some googling) led me to examples of
handling graphical frames in batch mode.

I'm guessing it's just a matter of finding x such that…

$ emacs --batch \
    --eval "$x" \
    --eval '(message "color-name-to-rgb: %s"
                     (color-name-to-rgb "#abcdef"))'

… sets up enough "graphical terminal" support that display-graphic-p
holds, and (color-values "#abcdef") returns (43947 52685 61423) instead
of (0 65535 65535), but x eludes me thus far.

Alternatively, I could drop --batch and switch to princ +
external-debugging-output + kill-emacs, I guess.


There's still a bunch of things for me to consult before I feel
completely stuck ((elisp) Window Systems, (elisp) Frames, (emacs)
Display X, src/emacs.c, lisp/startup.el, ChatGPT); in the meantime,
bottling up this message and throwing it at the list in case someone
happens to be… sailing by?


Wishing y'all a merry time 🙏