bug#69782: [PATCH] Fringe bitmaps should respect alpha-background

[Nate Sandy]

Hi,

fringe bitmaps currently don't respect the frame parameter
'alpha-background', i.e., their background is opaque. But since the
fringe itself *does* respect 'alpha-background', fringe bitmaps should
do the same. This patch fixes that. See attached screenshots for the
effect.
The alpha-background used here was set with:
  (set-frame-parameter nil 'alpha-background 50)
  
I am not sure which other platforms support 'alpha-background', and
unfortunately I am not able to test with those. This patch applies to
the pgtk and X Linux builds.

The change is altering the parameter 'respect_alpha_background' in
{pgtk,x}_cr_draw_image - while this name sounds quite generic, I didn't
find any other usages of this function than drawing fringe bitmaps. I
hope I didn't miss any.

I hope that I'm following the conventions here correctly - please let me
know if I can improve anything!

Best
Nate

>From 5f1e653dc53d2838660edc267e6cf4e599a351b7 Mon Sep 17 00:00:00 2001
From: Nate Sandy <nsan@posteo.de>
Date: Wed, 13 Mar 2024 19:58:04 +0100
Subject: [PATCH] Make fringe bitmaps respect alpha-background

---
 src/pgtkterm.c | 2 +-
 src/xterm.c    | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/pgtkterm.c b/src/pgtkterm.c
index 1ec6bfcda4..d891c6a23b 100644
--- a/src/pgtkterm.c
+++ b/src/pgtkterm.c
@@ -2193,7 +2193,7 @@ pgtk_cr_draw_image (struct frame *f, Emacs_GC *gc, 
cairo_pattern_t *image,
     cairo_rectangle (cr, dest_x, dest_y, width, height);
   else
     {
-      pgtk_set_cr_source_with_gc_background (f, gc, false);
+      pgtk_set_cr_source_with_gc_background (f, gc, true);
       cairo_rectangle (cr, dest_x, dest_y, width, height);
       cairo_fill_preserve (cr);
     }
diff --git a/src/xterm.c b/src/xterm.c
index c8a4378556..5e97489751 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -6298,7 +6298,7 @@ x_cr_draw_image (struct frame *f, GC gc, cairo_pattern_t 
*image,
     cairo_rectangle (cr, dest_x, dest_y, width, height);
   else
     {
-      x_set_cr_source_with_gc_background (f, gc, false);
+      x_set_cr_source_with_gc_background (f, gc, true);
       cairo_rectangle (cr, dest_x, dest_y, width, height);
       cairo_fill_preserve (cr);
     }
-- 
2.43.1

fringe-bitmap-current.png
Description: Fringe bitmaps don't respect alpha-background.

fringe-bitmap-respects-alpha-background.png
Description: Fringe bitmaps respect alpha-background (this patch).