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bug#36315: 27.0.50; SVG transparency handling is inaccurate
From: |
Pip Cet |
Subject: |
bug#36315: 27.0.50; SVG transparency handling is inaccurate |
Date: |
Thu, 20 Jun 2019 20:46:20 +0000 |
Oops, typo in the patch. Better patch attached.
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..8e25f1f590 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.green, opacity);
+ blue = MIX (blue, background.blue, opacity);
+#undef MIX
PUT_PIXEL (ximg, x, y, lookup_rgb_color (f, red, green, blue));
}
--
2.20.1
On Thu, Jun 20, 2019 at 8:43 PM Pip Cet <pipcet@gmail.com> wrote:
>
> 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.
0001-SVG-scale-color-values-properly.patch
Description: Source code patch