[Pixman] [PATCH v13 12/14] pixman-filter: Add description to pixman_filter_create_separable_convolution()
spitzak at gmail.com
spitzak at gmail.com
Wed Feb 10 06:25:30 UTC 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>
Acked-by: Oded Gabbay <oded.gabbay at redhat.com>
pixman/pixman-filter.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 51 insertions(+)
diff --git a/pixman/pixman-filter.c b/pixman/pixman-filter.c
index cb9f72b..cdff78e 100644
@@ -383,6 +383,57 @@ 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, so you must pass
+ * max(size, 1.0). Or use x.IMPULSE for size < 1.0.
+ * x.IMPULSE - Same as IMPULSE.x at size==1, size is ignored.
+ * LINEAR.IMPULSE - Same as the bilinear filter. At size==1 this is the exact
+ * same shape as BOX.BOX and IMPULSE.LINEAR, making it possible
+ * to do seamless transitions between these filters at size==1.
+ * 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".
+ * 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,
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