css: Fix computation of pixels occupied by blur radius

These computations were done randomly in lots of places and more often
than not, they were also wrong.
This function was copied (with docs) from Firefox:
  http://lxr.mozilla.org/mozilla-central/source/gfx/2d/Blur.cpp

https://bugzilla.gnome.org/show_bug.cgi?id=723159

diff --git a/gtk/gtkcairoblur.c b/gtk/gtkcairoblur.c
index 3f944e24297d39e42a50ed7b0bedca0085821f34..df989898b2c99ffed4d52ffa094341d002b91782 100644 (file)
--- a/gtk/gtkcairoblur.c
+++ b/gtk/gtkcairoblur.c
@@ -251,3 +251,29 @@ _gtk_cairo_blur_surface (cairo_surface_t* surface,
   /* Inform cairo we altered the surfaces contents. */
   cairo_surface_mark_dirty (surface);
 }
+
+/**
+ * _gtk_cairo_blur_compute_pixels:
+ * @radius: the radius to compute the pixels for
+ *
+ * Computes the number of pixels necessary to extend an image in one
+ * direction to hold the image with shadow.
+ *
+ * This is just the number of pixels added by the blur radius, shadow
+ * offset and spread are not included.
+ * 
+ * Much of this, the 3 * sqrt(2 * pi) / 4, is the known value for
+ * approximating a Gaussian using box blurs.  This yields quite a good
+ * approximation for a Gaussian.  Then we multiply this by 1.5 since our
+ * code wants the radius of the entire triple-box-blur kernel instead of
+ * the diameter of an individual box blur.  For more details, see:
+ * http://www.w3.org/TR/SVG11/filters.html#feGaussianBlurElement
+ * https://bugzilla.mozilla.org/show_bug.cgi?id=590039#c19
+ */
+#define GAUSSIAN_SCALE_FACTOR ((3.0 * sqrt(2 * G_PI) / 4) * 1.5)
+
+int
+_gtk_cairo_blur_compute_pixels (double radius)
+{
+  return floor (radius * GAUSSIAN_SCALE_FACTOR + 0.5);
+}

diff --git a/gtk/gtkcairoblurprivate.h b/gtk/gtkcairoblurprivate.h
index e048bac8cd5c707f0e3d47f7a87d38d6110c130c..d3e2f9ac462ad90693b7e8c4c94a3165c88777b2 100644 (file)
--- a/gtk/gtkcairoblurprivate.h
+++ b/gtk/gtkcairoblurprivate.h
@@ -29,8 +29,9 @@
 
 G_BEGIN_DECLS
 
-void            _gtk_cairo_blur_surface (cairo_surface_t *surface,
-                                         double           radius);
+void            _gtk_cairo_blur_surface         (cairo_surface_t *surface,
+                                                 double           radius);
+int             _gtk_cairo_blur_compute_pixels  (double           radius);
 
 G_END_DECLS
 

diff --git a/gtk/gtkcssshadowsvalue.c b/gtk/gtkcssshadowsvalue.c
index b688594cc1b23ba2b09cde9befcc700f37966aee..eb32471cec0345349f09f8875aa66c5e5c5b4ba1 100644 (file)
--- a/gtk/gtkcssshadowsvalue.c
+++ b/gtk/gtkcssshadowsvalue.c
@@ -21,6 +21,7 @@
 
 #include "gtkcssshadowsvalueprivate.h"
 
+#include "gtkcairoblurprivate.h"
 #include "gtkcssshadowvalueprivate.h"
 
 #include <string.h>
@@ -309,7 +310,7 @@ _gtk_css_shadows_value_get_extents (const GtkCssValue *shadows,
   guint i;
   GtkBorder b = { 0 };
   const GtkCssValue *shadow;
-  gdouble hoffset, voffset, spread, radius;
+  gdouble hoffset, voffset, spread, radius, clip_radius;
 
   g_return_if_fail (shadows->class == &GTK_CSS_VALUE_SHADOWS);
 
@@ -323,11 +324,12 @@ _gtk_css_shadows_value_get_extents (const GtkCssValue *shadows,
       _gtk_css_shadow_value_get_geometry (shadow,
                                           &hoffset, &voffset,
                                           &radius, &spread);
+      clip_radius = _gtk_cairo_blur_compute_pixels (radius);
 
-      b.top = MAX (0, radius + spread - voffset);
-      b.right = MAX (0, radius + spread + hoffset);
-      b.bottom = MAX (0, radius + spread + voffset);
-      b.left = MAX (0, radius + spread - hoffset);
+      b.top = MAX (0, clip_radius + spread - voffset);
+      b.right = MAX (0, clip_radius + spread + hoffset);
+      b.bottom = MAX (0, clip_radius + spread + voffset);
+      b.left = MAX (0, clip_radius + spread - hoffset);
 
       border->top = MAX (border->top, b.top);
       border->right = MAX (border->right, b.right);

diff --git a/gtk/gtkcssshadowvalue.c b/gtk/gtkcssshadowvalue.c
index cdfd8526126552a891b794d016df614ef36c4242..6dcd2dd798186284ab3941a3d35def8f5f153cf9 100644 (file)
--- a/gtk/gtkcssshadowvalue.c
+++ b/gtk/gtkcssshadowvalue.c
@@ -31,10 +31,6 @@
 
 #include <math.h>
 
-/* The blur of _gtk_cairo_blur_surface only approximately ends at radius,
-   so we add an extra pixel to make the clips less dramatic */
-#define CLIP_RADIUS_EXTRA 4
-
 struct _GtkCssValue {
   GTK_CSS_VALUE_BASE
   guint inset :1;
@@ -327,7 +323,7 @@ gtk_css_shadow_value_start_drawing (const GtkCssValue *shadow,
 
   gdk_cairo_get_clip_rectangle (cr, &clip_rect);
 
-  clip_radius = radius + CLIP_RADIUS_EXTRA;
+  clip_radius = _gtk_cairo_blur_compute_pixels (radius);
 
   /* Create a larger surface to center the blur. */
   surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32,
@@ -534,7 +530,7 @@ _gtk_css_shadow_value_paint_box (const GtkCssValue   *shadow,
 
   spread = _gtk_css_number_value_get (shadow->spread, 0);
   radius = _gtk_css_number_value_get (shadow->radius, 0);
-  clip_radius = radius + CLIP_RADIUS_EXTRA;
+  clip_radius = _gtk_cairo_blur_compute_pixels (radius);
   x = _gtk_css_number_value_get (shadow->hoffset, 0);
   y = _gtk_css_number_value_get (shadow->voffset, 0);