[Pixman] [PATCH v13 10/14] pixman-filter: Gaussian fixes

[spitzak at gmail.com]

From: Bill Spitzak <spitzak at gmail.com>

The SIGMA term drops out on simplification.

Expanded the size slightly (from ~4.25 to 5) to make the cutoff less noticable.
Previouly the value at the cutoff was gaussian_filter(sqrt(2)*3/2) = 0.00626
which is larger than the difference between 8-bit pixels (1/255 = 0.003921).
New cutoff is gaussian_filter(2.5) = 0.001089 which is smaller.

v11: added some math to commit message

Signed-off-by: Bill Spitzak <spitzak at gmail.com>
Acked-by: Oded Gabbay <oded.gabbay at gmail.com>
---
 pixman/pixman-filter.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/pixman/pixman-filter.c b/pixman/pixman-filter.c
index e5ef8e6..56e25b1 100644
--- a/pixman/pixman-filter.c
+++ b/pixman/pixman-filter.c
@@ -63,10 +63,7 @@ linear_kernel (double x)
 static double
 gaussian_kernel (double x)
 {
-#define SQRT2 (1.4142135623730950488016887242096980785696718753769480)
-#define SIGMA (SQRT2 / 2.0)
-    
-    return exp (- x * x / (2 * SIGMA * SIGMA)) / (SIGMA * sqrt (2.0 * M_PI));
+    return exp (- x * x) / sqrt (M_PI);
 }
 
 static double
@@ -143,7 +140,7 @@ static const filter_info_t filters[] =
     { PIXMAN_KERNEL_BOX,	        box_kernel,       1.0 },
     { PIXMAN_KERNEL_LINEAR,	        linear_kernel,    2.0 },
     { PIXMAN_KERNEL_CUBIC,		cubic_kernel,     4.0 },
-    { PIXMAN_KERNEL_GAUSSIAN,	        gaussian_kernel,  6 * SIGMA },
+    { PIXMAN_KERNEL_GAUSSIAN,	        gaussian_kernel,  5.0 },
     { PIXMAN_KERNEL_LANCZOS2,	        lanczos2_kernel,  4.0 },
     { PIXMAN_KERNEL_LANCZOS3,	        lanczos3_kernel,  6.0 },
     { PIXMAN_KERNEL_LANCZOS3_STRETCHED, nice_kernel,      8.0 },
-- 
1.9.1