St: Fix blur radius computation to correspond to current CSS draft

The next draft of the CSS Backgrounds and Borders module will actually
define when the blur radius means. Fix our code to use that definition
(2 * standard deviation) rather than using the 1.9 * that we extracted
from what Mozilla was doing.

https://bugzilla.gnome.org/show_bug.cgi?id=632506

diff --git a/src/st/st-private.c b/src/st/st-private.c
index f97c2de7fe8cb9f0151dcb7e34bb624aa9dd1a38..d0aa89bae933dd78a155aafe02f306b8466e4fe3 100644 (file)
--- a/src/st/st-private.c
+++ b/src/st/st-private.c
@@ -435,11 +435,12 @@ blur_pixels (guchar  *pixels_in,
   guchar *pixels_out;
   float   sigma;
 
-  /* we use an approximation of the sigma - blur radius relationship used
-     in Firefox for doing SVG blurs; see
-     http://mxr.mozilla.org/mozilla-central/source/gfx/thebes/src/gfxBlur.cpp#280
-  */
-  sigma = blur / 1.9;
+  /* The CSS specification defines (or will define) the blur radius as twice
+   * the Gaussian standard deviation. See:
+   *
+   * http://lists.w3.org/Archives/Public/www-style/2010Sep/0002.html
+   */
+  sigma = blur / 2.;
 
   if ((guint) blur == 0)
     {