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* bug#36315: 27.0.50; SVG transparency handling is inaccurate
@ 2019-06-20 20:26 Pip Cet
  2020-12-18  0:49 ` bug#36315: Incorrect SVG color Qiantan Hong
  0 siblings, 1 reply; 10+ messages in thread
From: Pip Cet @ 2019-06-20 20:26 UTC (permalink / raw)
  To: 36315

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Evaluate the following in emacs -Q:

(require 'svg)

(defun make-image (color)
  (let ((svg (svg-create 100 100)))
    (svg-rectangle svg 0 0 100 100 :fill color)
    (svg-image svg)))

(set-frame-parameter (window-frame) 'background-color "black")

(insert (propertize " " 'display (make-image "#f00000")))

The expected result is a rectangle (on black background) of color
#f00000. The actual result is a rectangle of color #ef0000. For black
backgrounds, white is no longer representable.

This is related to bug #36304, but much easier to fix.

Patch attached.

[-- Attachment #2: 0001-SVG-scale-color-values-properly.patch --]
[-- Type: text/x-patch, Size: 1707 bytes --]

From 7abe6404d3af04db2f5a503c1c873f80ab86f69e Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet@gmail.com>
Date: Thu, 20 Jun 2019 20:13:12 +0000
Subject: [PATCH] SVG: scale color values properly

* src/image.c (svg_load_image): scale color channel values to proper
range.
---
 src/image.c | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/src/image.c b/src/image.c
index 866323ba6e..6b6235a617 100644
--- a/src/image.c
+++ b/src/image.c
@@ -9658,17 +9658,20 @@ svg_load_image (struct frame *f, struct image *img, char *contents,
       {
 	for (int x = 0; x < width; ++x)
 	  {
-	    int red     = *pixels++;
-	    int green   = *pixels++;
-	    int blue    = *pixels++;
-	    int opacity = *pixels++;
-
-	    red   = ((red * opacity)
-		     + (background.red * ((1 << 8) - opacity)));
-	    green = ((green * opacity)
-		     + (background.green * ((1 << 8) - opacity)));
-	    blue  = ((blue * opacity)
-		     + (background.blue * ((1 << 8) - opacity)));
+	    unsigned int red     = *pixels++;
+	    unsigned int green   = *pixels++;
+	    unsigned int blue    = *pixels++;
+	    unsigned int opacity = *pixels++;
+
+	    /* opacity and the color channel values are in the range {0..255},
+	     * but expected output values are in the range {0..65535}, so scale
+	     * by (256/255)^2. */
+#define MIX(a, b, opacity)						\
+	    (((((a) * opacity) + ((b) * (255 - opacity))) * 65535 + 32512) / 65025)
+	    red   = MIX (red, background.red, opacity);
+	    green = MIX (green, background.red, opacity);
+	    blue  = MIX (blue, background.red, opacity);
+#undef MIX
 
 	    PUT_PIXEL (ximg, x, y, lookup_rgb_color (f, red, green, blue));
 	  }
-- 
2.20.1


^ permalink raw reply related	[flat|nested] 10+ messages in thread
end of thread, other threads:[~2021-10-11 14:24 UTC | newest]

Thread overview: 10+ messages (download: mbox.gz follow: Atom feed
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     [not found] <CA+X8ke6WQrWqVUuE2a9=AqVT-XYov81dXFgPzj4iOXeNtHAK6g@mail.gmail.com>
2021-10-11  7:52 ` bug#36315: Incorrect SVG color Lars Ingebrigtsen
2021-10-11 14:24   ` Matt Huszagh
2019-06-20 20:26 bug#36315: 27.0.50; SVG transparency handling is inaccurate Pip Cet
2020-12-18  0:49 ` bug#36315: Incorrect SVG color Qiantan Hong
2020-12-18 15:00   ` Alan Third
2020-12-18 15:47     ` Qiantan Hong
2020-12-18 15:51     ` Qiantan Hong
2020-12-18 16:42     ` Qiantan Hong
2020-12-18 18:04       ` Alan Third
2020-12-18 18:16         ` Alan Third
2020-12-19  0:24           ` Lars Ingebrigtsen

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