Re: Face transparency attribute

[Nate Sandy]

Po Lu <luangruo@yahoo.com> writes:

> Neither approach is easy or exceptionally challenging, so interested
> individuals are invited to chose whichever they should find more to
> their liking.

Hi Po,

I have written a patch for this for the pgtk backend and went with the
first route - supporting an alpha channel in face colors. I figured it's
nicer since we get alpha channels for all other text decorations as
well.

The way it works is that when painting a background color (one which
respects alpha-background), we instead compose it with the frame
background color via a re-implementation of CAIRO_OPERATOR_OVER. I
didn't find a way to do this with cairo itself without painting the
surface inbetween.

I also made sure that this patch integrates with alpha-background, which
is also my primary use case - fully hiding faces which don't respect
alpha-background.

The only syntax supported for transparency is gtk_parse_rgba's
`rgba(255,255,255,1.0)`. Maybe we would want support for `#ff00ff00` too?

One interesting quirk is that when changing the opacity of the `default`
face to less than 1, the whole background is fully transparent again. I
am not sure whether this is a fault in my code or because of some extra
logic regarding the `default` face.

Attached are a screenshot and the patch. I'd very much appreciate
feedback. If necessary I could also try to implement this for X and/or
terminals, however I don't have access to other platforms.

Best
Nate

>From 63062b45d47f5054fc713f37daf3c0b88a7d5edf Mon Sep 17 00:00:00 2001
From: Nate Sandy <nsan@posteo.de>
Date: Sun, 17 Mar 2024 23:10:53 +0100
Subject: [PATCH] Add rgba support for faces to pgtk

---
 src/dispextern.h |  2 +-
 src/pgtkterm.c   | 56 ++++++++++++++++++++++++++++++++++--------------
 2 files changed, 41 insertions(+), 17 deletions(-)

diff --git a/src/dispextern.h b/src/dispextern.h
index 5387cb4560..1a6fbb2d95 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -50,7 +50,7 @@ #define No_Cursor (None)
 typedef struct
 {
   unsigned long pixel;
-  unsigned short red, green, blue;
+  unsigned short red, green, blue, alpha;
 } Emacs_Color;
 
 #ifndef HAVE_ANDROID
diff --git a/src/pgtkterm.c b/src/pgtkterm.c
index 1ec6bfcda4..b8284ba295 100644
--- a/src/pgtkterm.c
+++ b/src/pgtkterm.c
@@ -1677,6 +1677,7 @@ pgtk_compute_lighter_color (struct frame *f, unsigned 
long *pixel,
   new.red = min (0xffff, factor * color.red);
   new.green = min (0xffff, factor * color.green);
   new.blue = min (0xffff, factor * color.blue);
+  new.alpha = color.alpha;
 
   /* Calculate brightness of COLOR.  */
   bright = (2 * color.red + 3 * color.green + color.blue) / 6;
@@ -1706,7 +1707,8 @@ pgtk_compute_lighter_color (struct frame *f, unsigned 
long *pixel,
        }
     }
 
-  new.pixel = (new.red >> 8 << 16
+  new.pixel = (new.alpha >> 8 << 24
+              | new.red >> 8 << 16
               | new.green >> 8 << 8
               | new.blue >> 8);
 
@@ -1717,7 +1719,8 @@ pgtk_compute_lighter_color (struct frame *f, unsigned 
long *pixel,
       new.red = min (0xffff, delta + color.red);
       new.green = min (0xffff, delta + color.green);
       new.blue = min (0xffff, delta + color.blue);
-      new.pixel = (new.red >> 8 << 16
+      new.pixel = (new.alpha >> 8 << 24
+                  | new.red >> 8 << 16
                   | new.green >> 8 << 8
                   | new.blue >> 8);
     }
@@ -7046,7 +7049,9 @@ pgtk_parse_color (struct frame *f, const char *color_name,
       color->red = rgba.red * 65535;
       color->green = rgba.green * 65535;
       color->blue = rgba.blue * 65535;
-      color->pixel = ((color->red >> 8) << 16
+      color->alpha = rgba.alpha * 65535;
+      color->pixel = ((color->alpha >> 8) << 24
+                     | (color->red >> 8) << 16
                      | (color->green >> 8) << 8
                      | (color->blue >> 8) << 0);
       return 1;
@@ -7066,9 +7071,11 @@ pgtk_query_colors (struct frame *f, Emacs_Color * 
colors, int ncolors)
     {
       unsigned long pixel = colors[i].pixel;
       /* Convert to a 16 bit value in range 0 - 0xffff. */
+#define GetAValue(p) (((p) >> 24) & 0xff)
 #define GetRValue(p) (((p) >> 16) & 0xff)
 #define GetGValue(p) (((p) >> 8) & 0xff)
 #define GetBValue(p) (((p) >> 0) & 0xff)
+      colors[i].alpha = GetAValue (pixel) * 257;
       colors[i].red = GetRValue (pixel) * 257;
       colors[i].green = GetGValue (pixel) * 257;
       colors[i].blue = GetBValue (pixel) * 257;
@@ -7255,6 +7262,18 @@ pgtk_set_cr_source_with_gc_background (struct frame *f, 
Emacs_GC *gc,
                                 respects_alpha_background);
 }
 
+void
+pgtk_cr_operator_over (Emacs_Color a, Emacs_Color b, Emacs_Color *result) {
+  double a_alpha_norm = a.alpha / 65535.0;
+  double b_alpha_norm = b.alpha / 65535.0;
+
+  result->alpha = a_alpha_norm + b_alpha_norm * (1.0 - a_alpha_norm);
+  result->red = (a.red * a_alpha_norm + b.red * b_alpha_norm * (1.0 - 
a_alpha_norm)) / result->alpha;
+  result->green = (a.green * a_alpha_norm + b.green * b_alpha_norm * (1.0 - 
a_alpha_norm)) / result->alpha;
+  result->blue = (a.blue * a_alpha_norm + b.blue * b_alpha_norm * (1.0 - 
a_alpha_norm)) / result->alpha;
+
+}
+
 void
 pgtk_set_cr_source_with_color (struct frame *f, unsigned long color,
                               bool respects_alpha_background)
@@ -7263,19 +7282,24 @@ pgtk_set_cr_source_with_color (struct frame *f, 
unsigned long color,
   col.pixel = color;
   pgtk_query_color (f, &col);
 
-  if (!respects_alpha_background)
-    {
-      cairo_set_source_rgb (FRAME_CR_CONTEXT (f), col.red / 65535.0,
-                           col.green / 65535.0, col.blue / 65535.0);
-      cairo_set_operator (FRAME_CR_CONTEXT (f), CAIRO_OPERATOR_OVER);
-    }
-  else
-    {
-      cairo_set_source_rgba (FRAME_CR_CONTEXT (f), col.red / 65535.0,
-                            col.green / 65535.0, col.blue / 65535.0,
-                            f->alpha_background);
-      cairo_set_operator (FRAME_CR_CONTEXT (f), CAIRO_OPERATOR_SOURCE);
-    }
+  if (respects_alpha_background) {
+    Emacs_Color frame_background;
+    pgtk_query_frame_background_color (f, &frame_background);
+
+    Emacs_Color result;
+    pgtk_cr_operator_over (col, frame_background, &result);
+
+    cairo_set_source_rgba (FRAME_CR_CONTEXT (f), result.red / 65535.0,
+                          result.green / 65535.0, result.blue / 65535.0,
+                          f->alpha_background * result.alpha);
+    cairo_set_operator (FRAME_CR_CONTEXT (f), CAIRO_OPERATOR_SOURCE);
+  } else {
+    cairo_set_source_rgba (FRAME_CR_CONTEXT (f), col.red / 65535.0,
+                          col.green / 65535.0, col.blue / 65535.0,
+                          col.alpha / 65535.0);
+    cairo_set_operator (FRAME_CR_CONTEXT (f), CAIRO_OPERATOR_OVER);
+  }
+
 }
 
 void
-- 
2.43.1

emacs-rgba-pgtk.png
Description: Emacs pgtk with face rgba support