[lasem] svg_filter: fix blur offset.

[Emmanuel Pacaud <emmanuel src gnome org>]

commit 0ed068b374ac43ced3435aa22487640d536aef67
Author: Emmanuel Pacaud <emmanuel gnome org>
Date:   Thu Oct 18 11:35:38 2012 +0200

    svg_filter: fix blur offset.
    
    For small stdDeviation value, we should use another algorithm, as
    no blur is applied.

 src/lsmcairo.c |   16 ++++++++++++++--
 1 files changed, 14 insertions(+), 2 deletions(-)
---
diff --git a/src/lsmcairo.c b/src/lsmcairo.c
index cde77b9..c1c408c 100644
--- a/src/lsmcairo.c
+++ b/src/lsmcairo.c
@@ -256,8 +256,20 @@ lsm_filter_surface_fast_blur (LsmFilterSurface *input,
 
 	cairo_surface_flush (input->surface);
 
-	kx = floor (sx * 3 * sqrt (2 * M_PI) / 4 + 0.5);
-	ky = floor (sy * 3 * sqrt (2 * M_PI) / 4 + 0.5);
+	/* Original intermediate surface size calculation was:
+	 *
+	 * kx = floor (sx * 3 * sqrt (2 * M_PI) / 4 + 0.5);
+	 * ky = floor (sy * 3 * sqrt (2 * M_PI) / 4 + 0.5);
+	 *
+	 * Which has the drawback to give a surface with even dimensions, leading
+	 * to a blur with a one pixel offset.
+	 * The new dimensions are now always odd.
+	 *
+	 * 0.94 = 3.0 * sqrt(2*M_PI) / 8.0
+	 */
+
+	kx = round (sx * 0.94 - 0.5) * 2.0 + 1.5;
+	ky = round (sy * 0.94 - 0.5) * 2.0 + 1.5;
 
 	if (kx < 1 && ky < 1)
 		return;