[pattern] Optimize away fractional translation for NEAREST patterns.

As identified in bug 15479,
  Unpredictable performance of cairo-xlib with non-integer translations of a
  source surface pattern
  (https://bugs.freedesktop.org/show_bug.cgi?id=15479),
source surfaces with a fractional translation hit slow paths for some
drivers, causing seemingly random performance variations. As a work-around
Owen Taylor proposed that cairo could convert non-integer translations on
NEAREST sources patterns to their integer equivalents.

The messy detail involved here is replicating the rounding mode used by
pixman for the sample offset, but otherwise the conversion is fairly
trivial.

src/cairo-pattern.c

@@ -1756,6 +1756,13 @@ _cairo_pattern_analyze_filter (cairo_surface_pattern_t *pattern,
     return optimized_filter;
 }
 
+
+static double
+_pixman_nearest_sample (double d)
+{
+    return ceil (d - .5);
+}
+
 static cairo_int_status_t
 _cairo_pattern_acquire_surface_for_surface (cairo_surface_pattern_t   *pattern,
 					    cairo_surface_t	       *dst,
@@ -1782,6 +1789,34 @@ _cairo_pattern_acquire_surface_for_surface (cairo_surface_pattern_t   *pattern,
 	attr->x_offset = tx;
 	attr->y_offset = ty;
     }
+    else if (attr->filter == CAIRO_FILTER_NEAREST)
+    {
+	/*
+	 * For NEAREST, we can remove the fractional translation component
+	 * from the transformation - this ensures that the pattern will always
+	 * hit fast-paths in the backends for simple transformations that
+	 * become (almost) identity, without loss of quality.
+	 */
+	attr->matrix = pattern->base.matrix;
+	attr->matrix.x0 = 0;
+	attr->matrix.y0 = 0;
+	if (_cairo_matrix_is_pixel_exact (&attr->matrix)) {
+	    double x1, y1;
+
+	    /* The rounding here is rather peculiar as it needs to match the
+	     * rounding performed on the sample coordinate used by pixman.
+	     */
+	    x1 = _pixman_nearest_sample (pattern->base.matrix.x0);
+	    y1 = _pixman_nearest_sample (pattern->base.matrix.y0);
+	    cairo_matrix_transform_point (&attr->matrix, &x1, &y1);
+	    attr->x_offset = tx = _cairo_lround (x1);
+	    attr->y_offset = ty = _cairo_lround (y1);
+	} else {
+	    attr->matrix = pattern->base.matrix;
+	    attr->x_offset = attr->y_offset = 0;
+	    tx = ty = 0;
+	}
+    }
     else
     {
 	attr->matrix = pattern->base.matrix;

test/.gitignore

@@ -79,6 +79,7 @@ fill-missed-stop
 fill-rule
 filter-bilinear-extents
 filter-nearest-offset
+filter-nearest-transformed
 finer-grained-fallbacks
 ft-text-antialias-none
 ft-font-create-for-ft-face

test/Makefile.am

@@ -64,6 +64,7 @@ fill-missed-stop$(EXEEXT)				\
 fill-rule$(EXEEXT)					\
 filter-bilinear-extents$(EXEEXT)			\
 filter-nearest-offset$(EXEEXT)				\
+filter-nearest-transformed$(EXEEXT)			\
 finer-grained-fallbacks$(EXEEXT)			\
 font-face-get-type$(EXEEXT)				\
 font-matrix-translation$(EXEEXT)			\
@@ -479,6 +480,10 @@ REFERENCE_IMAGES = \
 	filter-nearest-offset-ps3-ref.png	\
 	filter-nearest-offset-svg11-ref.png	\
 	filter-nearest-offset-svg12-ref.png	\
+	filter-nearest-transformed-ref.png	\
+	filter-nearest-transformed-pdf-ref.png	\
+	filter-nearest-transformed-svg11-ref.png	\
+	filter-nearest-transformed-svg12-ref.png	\
 	finer-grained-fallbacks-ref.png			\
 	finer-grained-fallbacks-rgb24-ref.png		\
 	finer-grained-fallbacks-ps2-ref.png	\

test/filter-nearest-transformed.c

@@ -0,0 +1,103 @@
+/*
+ * Copyright © 2008 Chris Wilson
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use, copy,
+ * modify, merge, publish, distribute, sublicense, and/or sell copies
+ * of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+ * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ * Author: Chris Wilson <chris@chris-wilson.co.uk>
+ */
+
+#include "cairo-test.h"
+
+/*
+ * We wish to check the optimization away of non-fractional translations
+ * for NEAREST surface patterns under a few transformations.
+ */
+
+static cairo_test_draw_function_t draw;
+
+static const cairo_test_t test = {
+    "filter-nearest-transformed",
+    "Test sample position when drawing transformed images with FILTER_NEAREST",
+    14, 14,
+    draw
+};
+
+/* A single, black pixel */
+static const uint32_t black_pixel = 0xff000000;
+
+static cairo_test_status_t
+draw (cairo_t *cr, int width, int height)
+{
+    unsigned int i, j, k;
+    cairo_surface_t *surface;
+    cairo_pattern_t *pattern;
+    const cairo_matrix_t transform[] = {
+	{  1, 0, 0,  1,  0, 0 },
+	{ -1, 0, 0,  1,  8, 0 },
+	{  1, 0, 0, -1,  0, 8 },
+	{ -1, 0, 0, -1,  8, 8 },
+    };
+    const double colour[][3] = {
+	{0, 0, 0},
+	{1, 0, 0},
+	{0, 1, 0},
+	{0, 0, 1},
+    };
+    cairo_matrix_t m;
+
+    surface = cairo_image_surface_create_for_data ((uint8_t *) &black_pixel,
+						   CAIRO_FORMAT_ARGB32,
+						   1, 1, 4);
+    pattern = cairo_pattern_create_for_surface (surface);
+    cairo_surface_destroy (surface);
+
+    cairo_pattern_set_filter (pattern, CAIRO_FILTER_NEAREST);
+
+    /* Fill background white */
+    cairo_set_source_rgb (cr, 1, 1, 1);
+    cairo_paint (cr);
+
+    cairo_set_antialias (cr, CAIRO_ANTIALIAS_NONE);
+
+    for (k = 0; k < sizeof (transform) / sizeof (transform[0]); k++) {
+	cairo_set_source_rgb (cr, colour[k][0], colour[k][1], colour[k][2]);
+	for (j = 4; j <= 6; j++) {
+	    for (i = 4; i <= 6; i++) {
+		cairo_matrix_init_translate (&m,
+					     -(2*(i-4) + .1*i),
+					     -(2*(j-4) + .1*j));
+		cairo_matrix_multiply (&m, &m, &transform[k]);
+		cairo_pattern_set_matrix (pattern, &m);
+		cairo_mask (cr, pattern);
+	    }
+	}
+    }
+
+    cairo_pattern_destroy (pattern);
+
+    return CAIRO_TEST_SUCCESS;
+}
+
+int
+main (void)
+{
+    return cairo_test (&test);
+}