Comment and clean up the gradient computation. (_cairo_linear_pattern_classify): Determine if a linear gradient is horizontal or vertical. (_cairo_pattern_acquire_surface_for_gradient): Optimize horizontal/vertical gradients with a repeating surface.

Test case for linear gradients at angles and with a rotated pattern matrix.

8 files changed, 497 insertions, 38 deletions

diff --git a/ChangeLog b/ChangeLog
index b99fa62cb..ea84a3ed9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,15 @@
+2005-03-06  Owen Taylor  <otaylor@redhat.com>
+
+	* src/cairo_pattern.c (_cairo_image_data_set_linear): Comment
+	and clean up the gradient computation.
+	(_cairo_linear_pattern_classify): Determine if a linear
+	gradient is horizontal or vertical.
+	(_cairo_pattern_acquire_surface_for_gradient): Optimize
+	horizontal/vertical gradients with a repeating surface.
+
+	* test/linear_gradient.c: Test case for linear gradients
+	at angles and with a rotated pattern matrix.
+
 2005-03-06  David Reveman  <davidr@novell.com>
 
 	* src/cairo_glitz_surface.c (_cairo_glitz_pattern_acquire_surface): 
diff --git a/src/cairo-pattern.c b/src/cairo-pattern.c
index 49c878cd1..283c36dbd 100644
--- a/src/cairo-pattern.c
+++ b/src/cairo-pattern.c
@@ -642,9 +642,8 @@ _cairo_image_data_set_linear (cairo_linear_pattern_t *pattern,
 {
     int x, y;
     cairo_point_double_t point0, point1;
-    double px, py, ex, ey;
     double a, b, c, d, tx, ty;
-    double length, start, angle, fx, fy, factor;
+    double scale, start, dx, dy, factor;
     cairo_shader_op_t op;
     cairo_status_t status;
 
@@ -652,6 +651,16 @@ _cairo_image_data_set_linear (cairo_linear_pattern_t *pattern,
     if (status)
 	return status;
 
+    /* We compute the position in the linear gradient for
+     * a point q as:
+     *
+     *  [q . (p1 - p0) - p0 . (p1 - p0)] / (p1 - p0) ^ 2
+     *
+     * The computation is done in pattern space. The 
+     * calculation could be heavily optimized by using the
+     * fact that 'factor' increases linearly in both
+     * directions.
+     */
     point0.x = pattern->point0.x;
     point0.y = pattern->point0.y;
     point1.x = pattern->point1.x;
@@ -659,28 +668,24 @@ _cairo_image_data_set_linear (cairo_linear_pattern_t *pattern,
 
     cairo_matrix_get_affine (&pattern->base.base.matrix,
 			     &a, &b, &c, &d, &tx, &ty);
-    
-    length = sqrt ((point1.x - point0.x) * (point1.x - point0.x) +
-		   (point1.y - point0.y) * (point1.y - point0.y));
-    length = (length) ? 1.0 / length : CAIRO_MAXSHORT;
 
-    angle = -atan2 (point1.y - point0.y, point1.x - point0.x);
-    fx = cos (angle);
-    fy = -sin (angle);
-    
-    start = fx * point0.x;
-    start += fy * point0.y;
+    dx = point1.x - point0.x;
+    dy = point1.y - point0.y;
+    scale = dx * dx + dy * dy;
+    scale = (scale) ? 1.0 / scale : 1.0;
+
+    start = dx * point0.x + dy * point0.y;
 
     for (y = 0; y < height; y++) {
 	for (x = 0; x < width; x++) {
-	    px = x + offset_x;
-	    py = y + offset_y;
+	    double qx_device = x + offset_x;
+	    double qy_device = y + offset_y;
 		
-	    /* transform fragment */
-	    ex = a * px + c * py + tx;
-	    ey = b * px + d * py + ty;
+	    /* transform fragment into pattern space */
+	    double qx = a * qx_device + c * qy_device + tx;
+	    double qy = b * qx_device + d * qy_device + ty;
 	    
-	    factor = ((fx * ex + fy * ey) - start) * length;
+	    factor = ((dx * qx + dy * qy) - start) * scale;
 
 	    _cairo_pattern_calc_color_at_pixel (&op, factor * 65536, pixels++);
 	}
@@ -691,6 +696,64 @@ _cairo_image_data_set_linear (cairo_linear_pattern_t *pattern,
     return CAIRO_STATUS_SUCCESS;
 }
 
+static void
+_cairo_linear_pattern_classify (cairo_linear_pattern_t *pattern,
+				double		       offset_x,
+				double		       offset_y,
+				int		       width,
+				int		       height,
+				cairo_bool_t           *is_horizontal,
+				cairo_bool_t           *is_vertical)
+{
+    cairo_point_double_t point0, point1;
+    double a, b, c, d, tx, ty;
+    double scale, start, dx, dy;
+    cairo_fixed_t factors[3];
+    int i;
+
+    /* To classidy a pattern as horizontal or vertical, we first
+     * compute the (fixed point) factors at the corners of the
+     * pattern. We actually only need 3/4 corners, so we skip the
+     * fourth.
+     */
+    point0.x = pattern->point0.x;
+    point0.y = pattern->point0.y;
+    point1.x = pattern->point1.x;
+    point1.y = pattern->point1.y;
+
+    cairo_matrix_get_affine (&pattern->base.base.matrix,
+			     &a, &b, &c, &d, &tx, &ty);
+
+    dx = point1.x - point0.x;
+    dy = point1.y - point0.y;
+    scale = dx * dx + dy * dy;
+    scale = (scale) ? 1.0 / scale : 1.0;
+
+    start = dx * point0.x + dy * point0.y;
+
+    for (i = 0; i < 3; i++) {
+	double qx_device = (i % 2) * (width - 1) + offset_x;
+	double qy_device = (i / 2) * (height - 1) + offset_y;
+
+	/* transform fragment into pattern space */
+	double qx = a * qx_device + c * qy_device + tx;
+	double qy = b * qx_device + d * qy_device + ty;
+	
+	factors[i] = _cairo_fixed_from_double (((dx * qx + dy * qy) - start) * scale);
+    }
+
+    /* We consider a pattern to be vertical if the fixed point factor
+     * at the two upper corners is the same. We could accept a small
+     * change, but determining what change is acceptable would require
+     * sorting the stops in the pattern and looking at the differences.
+     *
+     * Horizontal works the same way with the two left corners.
+     */
+
+    *is_vertical = factors[1] == factors[0];
+    *is_horizontal = factors[2] == factors[0];
+}
+
 static cairo_status_t
 _cairo_image_data_set_radial (cairo_radial_pattern_t *pattern,
 			      double		     offset_x,
@@ -828,6 +891,24 @@ _cairo_pattern_acquire_surface_for_gradient (cairo_gradient_pattern_t *pattern,
     cairo_image_surface_t *image;
     cairo_status_t	  status;
     uint32_t		  *data;
+    cairo_bool_t          repeat = FALSE;
+
+    if (pattern->base.type == CAIRO_PATTERN_LINEAR) {
+	cairo_bool_t is_horizontal;
+	cairo_bool_t is_vertical;
+
+	_cairo_linear_pattern_classify ((cairo_linear_pattern_t *)pattern,
+					x, y, width, height,
+					&is_horizontal, &is_vertical);
+	if (is_horizontal) {
+	    height = 1;
+	    repeat = TRUE;
+	}
+	if (is_vertical) {
+	    width = 1;
+	    repeat = TRUE;
+	}
+    }
     
     data = malloc (width * height * 4);
     if (!data)
@@ -873,7 +954,7 @@ _cairo_pattern_acquire_surface_for_gradient (cairo_gradient_pattern_t *pattern,
     attr->x_offset = -x;
     attr->y_offset = -y;
     cairo_matrix_set_identity (&attr->matrix);
-    attr->extend = CAIRO_EXTEND_NONE;
+    attr->extend = repeat ? CAIRO_EXTEND_REPEAT : CAIRO_EXTEND_NONE;
     attr->filter = CAIRO_FILTER_NEAREST;
     attr->acquired = FALSE;
     
diff --git a/src/cairo_pattern.c b/src/cairo_pattern.c
index 49c878cd1..283c36dbd 100644
--- a/src/cairo_pattern.c
+++ b/src/cairo_pattern.c
@@ -642,9 +642,8 @@ _cairo_image_data_set_linear (cairo_linear_pattern_t *pattern,
 {
     int x, y;
     cairo_point_double_t point0, point1;
-    double px, py, ex, ey;
     double a, b, c, d, tx, ty;
-    double length, start, angle, fx, fy, factor;
+    double scale, start, dx, dy, factor;
     cairo_shader_op_t op;
     cairo_status_t status;
 
@@ -652,6 +651,16 @@ _cairo_image_data_set_linear (cairo_linear_pattern_t *pattern,
     if (status)
 	return status;
 
+    /* We compute the position in the linear gradient for
+     * a point q as:
+     *
+     *  [q . (p1 - p0) - p0 . (p1 - p0)] / (p1 - p0) ^ 2
+     *
+     * The computation is done in pattern space. The 
+     * calculation could be heavily optimized by using the
+     * fact that 'factor' increases linearly in both
+     * directions.
+     */
     point0.x = pattern->point0.x;
     point0.y = pattern->point0.y;
     point1.x = pattern->point1.x;
@@ -659,28 +668,24 @@ _cairo_image_data_set_linear (cairo_linear_pattern_t *pattern,
 
     cairo_matrix_get_affine (&pattern->base.base.matrix,
 			     &a, &b, &c, &d, &tx, &ty);
-    
-    length = sqrt ((point1.x - point0.x) * (point1.x - point0.x) +
-		   (point1.y - point0.y) * (point1.y - point0.y));
-    length = (length) ? 1.0 / length : CAIRO_MAXSHORT;
 
-    angle = -atan2 (point1.y - point0.y, point1.x - point0.x);
-    fx = cos (angle);
-    fy = -sin (angle);
-    
-    start = fx * point0.x;
-    start += fy * point0.y;
+    dx = point1.x - point0.x;
+    dy = point1.y - point0.y;
+    scale = dx * dx + dy * dy;
+    scale = (scale) ? 1.0 / scale : 1.0;
+
+    start = dx * point0.x + dy * point0.y;
 
     for (y = 0; y < height; y++) {
 	for (x = 0; x < width; x++) {
-	    px = x + offset_x;
-	    py = y + offset_y;
+	    double qx_device = x + offset_x;
+	    double qy_device = y + offset_y;
 		
-	    /* transform fragment */
-	    ex = a * px + c * py + tx;
-	    ey = b * px + d * py + ty;
+	    /* transform fragment into pattern space */
+	    double qx = a * qx_device + c * qy_device + tx;
+	    double qy = b * qx_device + d * qy_device + ty;
 	    
-	    factor = ((fx * ex + fy * ey) - start) * length;
+	    factor = ((dx * qx + dy * qy) - start) * scale;
 
 	    _cairo_pattern_calc_color_at_pixel (&op, factor * 65536, pixels++);
 	}
@@ -691,6 +696,64 @@ _cairo_image_data_set_linear (cairo_linear_pattern_t *pattern,
     return CAIRO_STATUS_SUCCESS;
 }
 
+static void
+_cairo_linear_pattern_classify (cairo_linear_pattern_t *pattern,
+				double		       offset_x,
+				double		       offset_y,
+				int		       width,
+				int		       height,
+				cairo_bool_t           *is_horizontal,
+				cairo_bool_t           *is_vertical)
+{
+    cairo_point_double_t point0, point1;
+    double a, b, c, d, tx, ty;
+    double scale, start, dx, dy;
+    cairo_fixed_t factors[3];
+    int i;
+
+    /* To classidy a pattern as horizontal or vertical, we first
+     * compute the (fixed point) factors at the corners of the
+     * pattern. We actually only need 3/4 corners, so we skip the
+     * fourth.
+     */
+    point0.x = pattern->point0.x;
+    point0.y = pattern->point0.y;
+    point1.x = pattern->point1.x;
+    point1.y = pattern->point1.y;
+
+    cairo_matrix_get_affine (&pattern->base.base.matrix,
+			     &a, &b, &c, &d, &tx, &ty);
+
+    dx = point1.x - point0.x;
+    dy = point1.y - point0.y;
+    scale = dx * dx + dy * dy;
+    scale = (scale) ? 1.0 / scale : 1.0;
+
+    start = dx * point0.x + dy * point0.y;
+
+    for (i = 0; i < 3; i++) {
+	double qx_device = (i % 2) * (width - 1) + offset_x;
+	double qy_device = (i / 2) * (height - 1) + offset_y;
+
+	/* transform fragment into pattern space */
+	double qx = a * qx_device + c * qy_device + tx;
+	double qy = b * qx_device + d * qy_device + ty;
+	
+	factors[i] = _cairo_fixed_from_double (((dx * qx + dy * qy) - start) * scale);
+    }
+
+    /* We consider a pattern to be vertical if the fixed point factor
+     * at the two upper corners is the same. We could accept a small
+     * change, but determining what change is acceptable would require
+     * sorting the stops in the pattern and looking at the differences.
+     *
+     * Horizontal works the same way with the two left corners.
+     */
+
+    *is_vertical = factors[1] == factors[0];
+    *is_horizontal = factors[2] == factors[0];
+}
+
 static cairo_status_t
 _cairo_image_data_set_radial (cairo_radial_pattern_t *pattern,
 			      double		     offset_x,
@@ -828,6 +891,24 @@ _cairo_pattern_acquire_surface_for_gradient (cairo_gradient_pattern_t *pattern,
     cairo_image_surface_t *image;
     cairo_status_t	  status;
     uint32_t		  *data;
+    cairo_bool_t          repeat = FALSE;
+
+    if (pattern->base.type == CAIRO_PATTERN_LINEAR) {
+	cairo_bool_t is_horizontal;
+	cairo_bool_t is_vertical;
+
+	_cairo_linear_pattern_classify ((cairo_linear_pattern_t *)pattern,
+					x, y, width, height,
+					&is_horizontal, &is_vertical);
+	if (is_horizontal) {
+	    height = 1;
+	    repeat = TRUE;
+	}
+	if (is_vertical) {
+	    width = 1;
+	    repeat = TRUE;
+	}
+    }
     
     data = malloc (width * height * 4);
     if (!data)
@@ -873,7 +954,7 @@ _cairo_pattern_acquire_surface_for_gradient (cairo_gradient_pattern_t *pattern,
     attr->x_offset = -x;
     attr->y_offset = -y;
     cairo_matrix_set_identity (&attr->matrix);
-    attr->extend = CAIRO_EXTEND_NONE;
+    attr->extend = repeat ? CAIRO_EXTEND_REPEAT : CAIRO_EXTEND_NONE;
     attr->filter = CAIRO_FILTER_NEAREST;
     attr->acquired = FALSE;
     
diff --git a/test/Makefile.am b/test/Makefile.am
index aa3d0604a..252dcbca9 100644
--- a/test/Makefile.am
+++ b/test/Makefile.am
@@ -3,6 +3,7 @@ TESTS = 		\
 fill_rule		\
 leaky_polygon		\
 line_width		\
+linear_gradient		\
 move_to_show_surface	\
 text_cache_crash	\
 text_rotate		\
@@ -17,6 +18,7 @@ EXTRA_DIST =			\
 fill_rule-ref.png		\
 leaky_polygon-ref.png		\
 line_width-ref.png		\
+linear_gradient-ref.png		\
 move_to_show_surface-ref.png	\
 coverage-ref.png		\
 clip_twice-ref.png		\
@@ -68,6 +70,7 @@ xmalloc.h
 fill_rule_SOURCES = fill_rule.c $(cairo_test_lib)
 leaky_polygon_SOURCES = leaky_polygon.c $(cairo_test_lib)
 line_width_SOURCES = line_width.c $(cairo_test_lib)
+linear_gradient_SOURCES = linear_gradient.c $(cairo_test_lib)
 move_to_show_surface_SOURCES = move_to_show_surface.c $(cairo_test_lib)
 text_cache_crash_SOURCES = text_cache_crash.c $(cairo_test_lib)
 text_rotate_SOURCES = text_rotate.c $(cairo_test_lib)
diff --git a/test/linear-gradient-ref.png b/test/linear-gradient-ref.png
new file mode 100644
index 000000000..77904144d
--- /dev/null
+++ b/test/linear-gradient-ref.png
diff --git a/test/linear-gradient.c b/test/linear-gradient.c
new file mode 100644
index 000000000..189b50065
--- /dev/null
+++ b/test/linear-gradient.c
@@ -0,0 +1,141 @@
+/*
+ * Copyright © 2005 Red Hat, Inc.
+ *
+ * Permission to use, copy, modify, distribute, and sell this software
+ * and its documentation for any purpose is hereby granted without
+ * fee, provided that the above copyright notice appear in all copies
+ * and that both that copyright notice and this permission notice
+ * appear in supporting documentation, and that the name of
+ * Red Hat, Inc. not be used in advertising or publicity pertaining to
+ * distribution of the software without specific, written prior
+ * permission. Red Hat, Inc. makes no representations about the
+ * suitability of this software for any purpose.  It is provided "as
+ * is" without express or implied warranty.
+ *
+ * RED HAT, INC. DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS
+ * SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS, IN NO EVENT SHALL RED HAT, INC. BE LIABLE FOR ANY SPECIAL,
+ * INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER
+ * RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR
+ * IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ * Author: Owen Taylor <otaylor@redhat.com>
+ */
+
+#include "cairo_test.h"
+#include "stdio.h"
+
+/* The test matrix is
+ *
+ * A) Horizontal   B) 5°          C) 45°          D) Vertical
+ * 1) Rotated 0°   2) Rotated 45° C) Rotated 90°
+ * a) 2 stop       b) 3 stop
+ *
+ *  A1a   B1a  C1a  D1a
+ *  A2a   B2a  C2a  D2a
+ *  A3a   B3a  C3a  D3a
+ *  A1b   B1b  C1b  D1b
+ *  A2b   B2b  C2b  D2b
+ *  A3b   B3b  C3b  D3b
+ */
+
+static const double gradient_angles[] = { 0, 45, 90 };
+#define N_GRADIENT_ANGLES 3
+static const double rotate_angles[] = { 0, 45, 90 };
+#define N_ROTATE_ANGLES 3
+static const int n_stops[] = { 2, 3 };
+#define N_N_STOPS 2
+
+#define UNIT_SIZE 75
+#define UNIT_SIZE 75
+#define PAD 5
+
+#define WIDTH  N_GRADIENT_ANGLES * UNIT_SIZE + (N_GRADIENT_ANGLES + 1) * PAD
+#define HEIGHT N_N_STOPS * N_ROTATE_ANGLES * UNIT_SIZE + (N_N_STOPS * N_ROTATE_ANGLES + 1) * PAD
+
+cairo_test_t test = {
+    "linear_gradient",
+    "Tests the drawing of linear gradients",
+    WIDTH, HEIGHT
+};
+
+static void
+draw_unit (cairo_t *cr,
+	   double   gradient_angle,
+	   double   rotate_angle,
+	   int      n_stops)
+{
+    cairo_pattern_t *pattern;
+
+    cairo_rectangle (cr, 0, 0, 1, 1);
+    cairo_clip (cr);
+    cairo_new_path(cr);
+    
+    cairo_set_rgb_color (cr, 0.0, 0.0, 0.0);
+    cairo_rectangle (cr, 0, 0, 1, 1);
+    cairo_fill (cr);
+    
+    cairo_translate (cr, 0.5, 0.5);
+    cairo_scale (cr, 1 / 1.5, 1 / 1.5);
+    cairo_rotate (cr, rotate_angle);
+    
+    pattern = cairo_pattern_create_linear (-0.5 * cos (gradient_angle),  -0.5 * sin (gradient_angle),
+ 					    0.5 * cos (gradient_angle),   0.5 * sin (gradient_angle));
+
+    if (n_stops == 2) {
+	cairo_pattern_add_color_stop (pattern, 0.,
+				      0.3, 0.3, 0.3,
+				      1.0);
+	cairo_pattern_add_color_stop (pattern, 1.,
+				      1.0, 1.0, 1.0,
+				      1.0);
+    } else {
+	cairo_pattern_add_color_stop (pattern, 0.,
+				      1.0, 0.0, 0.0,
+				      1.0);
+	cairo_pattern_add_color_stop (pattern, 0.5,
+				      1.0, 1.0, 1.0,
+				      1.0);
+	cairo_pattern_add_color_stop (pattern, 1.,
+				      0.0, 0.0, 1.0,
+				      1.0);
+    }
+
+    cairo_set_pattern (cr, pattern);
+    cairo_pattern_destroy (pattern);
+    cairo_rectangle (cr, -0.5, -0.5, 1, 1);
+    cairo_fill (cr);
+}
+
+static void
+draw (cairo_t *cr, int width, int height)
+{
+    int i, j, k;
+
+    cairo_set_rgb_color (cr, 0.5, 0.5, 0.5);
+    cairo_rectangle (cr, 0, 0, width, height);
+    cairo_fill (cr);
+
+    for (i = 0; i < N_GRADIENT_ANGLES; i++)
+	for (j = 0; j < N_ROTATE_ANGLES; j++)
+	  for (k = 0; k < N_N_STOPS; k++) {
+		cairo_save (cr);
+		cairo_translate (cr,
+				 PAD + (PAD + UNIT_SIZE) * i,
+				 PAD + (PAD + UNIT_SIZE) * (N_ROTATE_ANGLES * k + j));
+		cairo_scale (cr, UNIT_SIZE, UNIT_SIZE);
+		
+		draw_unit (cr,
+			   gradient_angles[i] * M_PI / 180.,
+			   rotate_angles[j] * M_PI / 180.,
+			   n_stops[k]);
+		cairo_restore (cr);
+	    }
+}
+
+int
+main (void)
+{
+    return cairo_test (&test, draw);
+}
diff --git a/test/linear_gradient-ref.png b/test/linear_gradient-ref.png
new file mode 100644
index 000000000..77904144d
--- /dev/null
+++ b/test/linear_gradient-ref.png
diff --git a/test/linear_gradient.c b/test/linear_gradient.c
new file mode 100644
index 000000000..189b50065
--- /dev/null
+++ b/test/linear_gradient.c
@@ -0,0 +1,141 @@
+/*
+ * Copyright © 2005 Red Hat, Inc.
+ *
+ * Permission to use, copy, modify, distribute, and sell this software
+ * and its documentation for any purpose is hereby granted without
+ * fee, provided that the above copyright notice appear in all copies
+ * and that both that copyright notice and this permission notice
+ * appear in supporting documentation, and that the name of
+ * Red Hat, Inc. not be used in advertising or publicity pertaining to
+ * distribution of the software without specific, written prior
+ * permission. Red Hat, Inc. makes no representations about the
+ * suitability of this software for any purpose.  It is provided "as
+ * is" without express or implied warranty.
+ *
+ * RED HAT, INC. DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS
+ * SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS, IN NO EVENT SHALL RED HAT, INC. BE LIABLE FOR ANY SPECIAL,
+ * INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER
+ * RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR
+ * IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ * Author: Owen Taylor <otaylor@redhat.com>
+ */
+
+#include "cairo_test.h"
+#include "stdio.h"
+
+/* The test matrix is
+ *
+ * A) Horizontal   B) 5°          C) 45°          D) Vertical
+ * 1) Rotated 0°   2) Rotated 45° C) Rotated 90°
+ * a) 2 stop       b) 3 stop
+ *
+ *  A1a   B1a  C1a  D1a
+ *  A2a   B2a  C2a  D2a
+ *  A3a   B3a  C3a  D3a
+ *  A1b   B1b  C1b  D1b
+ *  A2b   B2b  C2b  D2b
+ *  A3b   B3b  C3b  D3b
+ */
+
+static const double gradient_angles[] = { 0, 45, 90 };
+#define N_GRADIENT_ANGLES 3
+static const double rotate_angles[] = { 0, 45, 90 };
+#define N_ROTATE_ANGLES 3
+static const int n_stops[] = { 2, 3 };
+#define N_N_STOPS 2
+
+#define UNIT_SIZE 75
+#define UNIT_SIZE 75
+#define PAD 5
+
+#define WIDTH  N_GRADIENT_ANGLES * UNIT_SIZE + (N_GRADIENT_ANGLES + 1) * PAD
+#define HEIGHT N_N_STOPS * N_ROTATE_ANGLES * UNIT_SIZE + (N_N_STOPS * N_ROTATE_ANGLES + 1) * PAD
+
+cairo_test_t test = {
+    "linear_gradient",
+    "Tests the drawing of linear gradients",
+    WIDTH, HEIGHT
+};
+
+static void
+draw_unit (cairo_t *cr,
+	   double   gradient_angle,
+	   double   rotate_angle,
+	   int      n_stops)
+{
+    cairo_pattern_t *pattern;
+
+    cairo_rectangle (cr, 0, 0, 1, 1);
+    cairo_clip (cr);
+    cairo_new_path(cr);
+    
+    cairo_set_rgb_color (cr, 0.0, 0.0, 0.0);
+    cairo_rectangle (cr, 0, 0, 1, 1);
+    cairo_fill (cr);
+    
+    cairo_translate (cr, 0.5, 0.5);
+    cairo_scale (cr, 1 / 1.5, 1 / 1.5);
+    cairo_rotate (cr, rotate_angle);
+    
+    pattern = cairo_pattern_create_linear (-0.5 * cos (gradient_angle),  -0.5 * sin (gradient_angle),
+ 					    0.5 * cos (gradient_angle),   0.5 * sin (gradient_angle));
+
+    if (n_stops == 2) {
+	cairo_pattern_add_color_stop (pattern, 0.,
+				      0.3, 0.3, 0.3,
+				      1.0);
+	cairo_pattern_add_color_stop (pattern, 1.,
+				      1.0, 1.0, 1.0,
+				      1.0);
+    } else {
+	cairo_pattern_add_color_stop (pattern, 0.,
+				      1.0, 0.0, 0.0,
+				      1.0);
+	cairo_pattern_add_color_stop (pattern, 0.5,
+				      1.0, 1.0, 1.0,
+				      1.0);
+	cairo_pattern_add_color_stop (pattern, 1.,
+				      0.0, 0.0, 1.0,
+				      1.0);
+    }
+
+    cairo_set_pattern (cr, pattern);
+    cairo_pattern_destroy (pattern);
+    cairo_rectangle (cr, -0.5, -0.5, 1, 1);
+    cairo_fill (cr);
+}
+
+static void
+draw (cairo_t *cr, int width, int height)
+{
+    int i, j, k;
+
+    cairo_set_rgb_color (cr, 0.5, 0.5, 0.5);
+    cairo_rectangle (cr, 0, 0, width, height);
+    cairo_fill (cr);
+
+    for (i = 0; i < N_GRADIENT_ANGLES; i++)
+	for (j = 0; j < N_ROTATE_ANGLES; j++)
+	  for (k = 0; k < N_N_STOPS; k++) {
+		cairo_save (cr);
+		cairo_translate (cr,
+				 PAD + (PAD + UNIT_SIZE) * i,
+				 PAD + (PAD + UNIT_SIZE) * (N_ROTATE_ANGLES * k + j));
+		cairo_scale (cr, UNIT_SIZE, UNIT_SIZE);
+		
+		draw_unit (cr,
+			   gradient_angles[i] * M_PI / 180.,
+			   rotate_angles[j] * M_PI / 180.,
+			   n_stops[k]);
+		cairo_restore (cr);
+	    }
+}
+
+int
+main (void)
+{
+    return cairo_test (&test, draw);
+}