[Pixman] [PATCH v11 12/14] pixman-filter: Add description to pixman_filter_create_separable_convolution()
spitzak at gmail.com
spitzak at gmail.com
Fri Feb 5 08:49:06 CET 2016
From: Bill Spitzak <spitzak at gmail.com>
v9: Described arguments and more filter combinations, fixed some errors.
v11: Further correction, in particular replaced "scale" with "size"
Signed-off-by: Bill Spitzak <spitzak at gmail.com>
---
pixman/pixman-filter.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 50 insertions(+)
diff --git a/pixman/pixman-filter.c b/pixman/pixman-filter.c
index e57b154..b23d348 100644
--- a/pixman/pixman-filter.c
+++ b/pixman/pixman-filter.c
@@ -379,6 +379,56 @@ subsample_bits (int subsample_bits, pixman_kernel_t sample, double size)
/* Create the parameter list for a SEPARABLE_CONVOLUTION filter
* with the given kernels and size parameters
+ *
+ * Returns pointer to the block of numbers
+ *
+ * n_values: pointer to integer to store size of the block
+ *
+ * size_x/y: amount to stretch the sample filter. The best value to
+ * use is the derivative of the sample location. This value is
+ * approximately 1/scale when scaling an image. If size < 1.0 then you
+ * will get square pixels as an image is enlarged, using 1.0 instead
+ * will get the more customary "blurry" enlargement.
+ *
+ * reconstruct_x/y: filter that is not resized.
+ *
+ * sample_x/y: filter that is resized. Result is convolved with reconstruct filter.
+ *
+ * subsample_bits_x/y: If positive there are 2^n subpixel positions computed for
+ * the filter. If negative then -n (rounded up to the next power of 2) positions
+ * are calculated when size==1, but fewer are computed for larger sizes and
+ * more for smaller sizes.
+ *
+ * Some interesting reconstruct.sample combinations:
+ *
+ * IMPULSE.x - Uses the sample function only. This is what many other pieces of
+ * software do. Does not work for size < 1.0.
+ *
+ * x.IMPULSE - Uses the reconstruct function only, size is ignored. This is
+ * what many pieces of software do for size < 1.0.
+ *
+ * LINEAR.IMPULSE - Same as the bilinear filter
+ *
+ * BOX.BOX - Produces a trapazoid-shape. Narrowest possible filter with antialiasing.
+ * Matches a *lot* of other software, some call it "box", others call it
+ * "linear". At size==1 this matches LINEAR.IMPULSE, IMPULSE.LINEAR, and
+ * the bilinear filter, allowing transitions at those points.
+ *
+ * BOX.x - For size < 1.0 this produces square pixels. For size > 1.0 this
+ * approaches IMPULSE.x as size gets larger.
+ *
+ * BOX.LINEAR - At size==1 this is what some software calls a Quadratic filter
+ *
+ * IMPULSE.LINEAR - Some software calls this "linear" or "triangle". Not a good filter.
+ *
+ * LINEAR.LINEAR - At size==1 this is what some software calls "cubic interpolation".
+ *
+ * IMPULSE.LANCZOS2 - Close to what a lot of other software calls "cubic interpolation"
+ *
+ * IMPULSE.CUBIC - A third thing often called "cubic interpolation", sometimes called
+ * "Mitchell" in other software.
+ *
+ * IMPULSE.GAUSSIAN - Best removal of aliasing but usually considered too blurry
*/
PIXMAN_EXPORT pixman_fixed_t *
pixman_filter_create_separable_convolution (int *n_values,
--
1.9.1
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