Re: Help writing a patch to allow transparent backgrounds?

[Eli Zaretskii <eliz@gnu.org>]

> Cc: emacs-devel@gnu.org
> From: Clément Pit--Claudel <clement.pit@gmail.com>
> Date: Wed, 10 Feb 2016 14:36:28 -0500
> 
> Thanks! I'll be happy to try to implement this feature, with some help to navigate my way around the C source.

Look at xterm.c:x_draw_glyph_string_background.  This is the function
that draws the background of text before that text is drawn in the
foreground color.  The other function to look at is
x_draw_glyph_string_bg_rect in the same file -- this one is used to
draw the background of display elements that are not text: images and
stretches of white space.

A word about "glyph strings": these are structures built by the Emacs
display engine in preparation for actually redrawing portions of the
display which need that.  Each glyph string is the longest possible
collection of glyphs that are to be drawn in the same face, so the
terminal-specific back-end (such as xterm.c) could set up the
necessary settings once, and then draw all of the glyphs of the glyph
string in one go.  For displaying text, the glyph string includes a
run of characters in the same face (font, colors, etc.) specified by
their glyph indices in the font.

HTH