bug#16992: feature request: background images

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 8 Nov 2024 14:59:20 +0100
> Cc: 16992@debbugs.gnu.org
> From: Cecilio Pardo <cpardo@imayhem.com>
> 
> Here is the incomplete patch that implements background images.

Thanks.  I'm not yet sure I understand the overall picture and its
implications, hence the questions below.

> It works by adding a call to complex_bkg_clear_frame_area on all
> functions that clear the background. These are the functions for w32
> port. For the cairo port, they are parallel:
> 
> w32_clear_frame_area, w32_clear_glyph_string_rect,
> w32_draw_stretch_glyph_string, w32font_draw
> 
> I know I'm missing at least w32_draw_image_glyph_string, but works well
> enough for a test.
> 
> To setup the backgroud image for a frame, you would do something like
> this:
> 
> (progn
> (setq bkg (create-image "icons/hicolor/128x128/apps/emacs.png"))
> (frame-set-background-image (selected-frame) bkg 'center nil)
> (frame-set-background-policy (selected-frame) t))

So showing a vertical line for the fill-column indication would need
to define a background image for a frame?  How do we control the
horizontal coordinate where the line is drawn?

> The image can be placed in the center, or on sides or corners with 'n,
> 's, 'se, 'sw, etc. The background can be filled be repeating the image
> passing t as the last parameter of frame-set-background-image.

Why not let Lisp specify explicit coordinates for the image?

> Apart from the obvious overhead of drawing the image pixels,
> unfortunately the frame contents can't be scrolled to reuse text pixels
> (dispnew.c:scrolling_window). I don't think this can't be fixed without
> big changes.

Hmm.. scroll_run_hook is also called from two redisplay optimizations
in xdisp.c, so those will need to be disabled as well.  Too bad.

But maybe we can make them work if we consider the background image to
be scrolled together with the text? "all we need" is to insert a slice
of the same image from below or from above, depending on the scrolling
direction.

Btw, what happens when text is scrolled horizontally?

Is this only going to work with fixed images?  Then I guess features
like showing vertical lines as indentation indicators, like those
here:

  https://techpress.net/how-to-show-hide-indent-dots-in-vscode/

will not be possible?

> +get_all_live_windows (struct window *w, struct window **dest)
> +{
> +  if (WINDOW_LEAF_P (w))
> +    {

You could perhaps extend window_loop to do whatever the callers of
this function do

Thanks for working on this.