[Pixman] [PATCH v8 13/14] Added big comment to filter

[spitzak at gmail.com]

From: Bill Spitzak <spitzak at gmail.com>

v8: first version

Signed-off-by: Bill Spitzak <spitzak at gmail.com>
---
 pixman/pixman-filter.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/pixman/pixman-filter.c b/pixman/pixman-filter.c
index 3237036..0961cfc 100644
--- a/pixman/pixman-filter.c
+++ b/pixman/pixman-filter.c
@@ -313,7 +313,28 @@ gnuplot_filter(int width, int samples, const pixman_fixed_t* p)
 #endif
 
 /* Create the parameter list for a SEPARABLE_CONVOLUTION filter
- * with the given kernels and scale parameters
+ *
+ * The reconstruct filter is scaled by 1, while the sample filter is
+ * scaled by scale, and they are convolved together. Note that scale
+ * is the inverse of how the image is being scaled, if the image is
+ * being made smaller, the filter gets larger.
+ *
+ * Some interesting reconstruct.sample combinations:
+ *
+ *  IMPULSE.x - Uses the sample function only. Only works for scale >= 1. This
+ *              is useful for matching other software that does not convolve,
+ *              for large scale it is almost identical to BOX.x but the filter
+ *              is slightly smaller and thus faster.
+ *  x.IMPULSE - Uses the reconstruct function only, scale is ignored.
+ *  LINEAR.IMPULSE - Matches bilinear filtering
+ *  BOX.BOX - Produces a trapazoid-shape. Narrowest possible filter with antialiasing.
+ *            This is equal to LINEAR.IMPULSE at scale == 1. Matches a lot of other
+ *            software, some call it "box", others call it "linear"
+ *  IMPULSE.LINEAR - Some software calls this "linear" or "triangle". Not a good filter.
+ *  LINEAR.LINEAR - non-negative cubic. What some software calls "cubic interpolation".
+ *  IMPULSE.LANCZOS2 - Close to what a lot of other software calls "cubic interpolation"
+ *  IMPULSE.CUBIC - Often called "mitchell" in other software
+ *  IMPULSE.GAUSSIAN - Technically accurate but usually considered too blurry
  */
 PIXMAN_EXPORT pixman_fixed_t *
 pixman_filter_create_separable_convolution (int             *n_values,
-- 
1.9.1