Add line-width-scale test to demonstrate cairo_set_line_width bug

test/.gitignore

@@ -33,6 +33,7 @@ imagediff
 leaky-dash
 leaky-polygon
 line-width
+line-width-scale
 linear-gradient
 mask
 mask-ctm

test/Makefile.am

@@ -28,6 +28,7 @@ gradient-alpha			\
 leaky-dash			\
 leaky-polygon			\
 line-width			\
+line-width-scale		\
 linear-gradient			\
 mask				\
 mask-ctm			\
@@ -144,6 +145,7 @@ leaky-dash-ref.png					\
 leaky-polygon-ref.png					\
 linear-gradient-ref.png					\
 line-width-ref.png					\
+line-width-scale-ref.png				\
 mask-ctm-ref.png					\
 mask-ctm-rgb24-ref.png					\
 mask-ref.png						\

test/line-width-scale.c

@@ -0,0 +1,184 @@
+/*
+ * Copyright © 2006 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: Carl D. Worth <cworth@cworth.org>
+ */
+
+#include "cairo-test.h"
+
+/* This test exercises the various interactions between
+ * cairo_set_line_width and cairo_scale. Specifically it show how
+ * separate transformations can affect the pen for stroking compared
+ * to the path itself.
+ *
+ * This was inspired by an image by Maxim Shemanarev demonstrating the
+ * flexible-pipeline nature of his Antigrain Geometry project:
+ *
+ *	http://antigrain.com/tips/line_alignment/conv_order.gif
+ *
+ * It also uncovered a bug in cairo that cairo_set_line_width was not
+ * transforing the width according the the current CTM, but instead
+ * delaying that transformation until the time of cairo_stroke. See:
+ *
+ *	http://article.gmane.org/gmane.comp.graphics.agg/2518
+ */
+
+#define LINE_WIDTH 13
+#define SPLINE 50.0
+#define XSCALE  0.5
+#define YSCALE  2.0
+#define WIDTH (XSCALE * SPLINE * 6.0)
+#define HEIGHT (YSCALE * SPLINE * 2.0)
+
+cairo_test_t test = {
+    "line-width-scale",
+    "Tests interaction of cairo_set_line_width with cairo_scale",
+    WIDTH, HEIGHT
+};
+
+static void
+spline_path (cairo_t *cr)
+{
+    cairo_save (cr);
+    {
+	cairo_move_to (cr,
+		       - SPLINE, 0);
+	cairo_curve_to (cr,
+			- SPLINE / 4, - SPLINE,
+			  SPLINE / 4,   SPLINE,
+			  SPLINE, 0);
+    }
+    cairo_restore (cr);
+}
+
+/* If we scale before setting the line width or creating the path,
+ * then obviously both will be scaled. */
+static void
+scale_then_set_line_width_and_stroke (cairo_t *cr)
+{
+    cairo_scale (cr, XSCALE, YSCALE);
+    cairo_set_line_width (cr, LINE_WIDTH);
+    spline_path (cr);
+    cairo_stroke (cr);
+}
+
+/* This is used to verify the results of
+ * scale_then_set_line_width_and_stroke.
+ *
+ * It uses save/restore pairs to isolate the scaling of the path and
+ * line_width and ensures that both are scaled.
+ */
+static void
+scale_path_and_line_width (cairo_t *cr)
+{
+    cairo_save (cr);
+    {
+	cairo_scale (cr, XSCALE, YSCALE);
+	spline_path (cr);
+    }
+    cairo_restore (cr);
+
+    cairo_save (cr);
+    {
+	cairo_scale (cr, XSCALE, YSCALE);
+	cairo_set_line_width (cr, LINE_WIDTH);
+	cairo_stroke (cr);
+    }
+    cairo_restore (cr);
+}
+
+/* This one's the bug.
+ *
+ * If we set the line width before scaling, then the path should be
+ * scaled but the line width should not.
+ *
+ * With the bug, the line_width is also being scaled here.
+ */
+static void
+set_line_width_then_scale_and_stroke (cairo_t *cr)
+{
+    cairo_set_line_width (cr, LINE_WIDTH);
+    cairo_scale (cr, XSCALE, YSCALE);
+    spline_path (cr);
+    cairo_stroke (cr);
+}
+
+/* This is used to verify what should be the results of
+ * set_line_width_then_scale_and_stroke (once the bug is fixed).
+ *
+ * It uses save/restore pairs to isolate the scaling of the path and
+ * line_width and ensures that the path is scaled while the line width
+ * is not.
+ */
+static void
+scale_path_not_line_width (cairo_t *cr)
+{
+    cairo_save (cr);
+    {
+	cairo_scale (cr, XSCALE, YSCALE);
+	spline_path (cr);
+    }
+    cairo_restore (cr);
+
+    cairo_save (cr);
+    {
+	cairo_set_line_width (cr, LINE_WIDTH);
+	cairo_stroke (cr);
+    }
+    cairo_restore (cr);
+}
+
+#define ARRAY_SIZE(arr) (sizeof(arr)/sizeof(arr[0]))
+
+static cairo_test_status_t
+draw (cairo_t *cr, int width, int height)
+{
+    int i;
+    typedef void (*figure_t) (cairo_t *cr);
+    figure_t figures[4] = {
+	scale_then_set_line_width_and_stroke,
+	scale_path_and_line_width,
+	set_line_width_then_scale_and_stroke,
+	scale_path_not_line_width
+    };
+
+    cairo_set_source_rgb (cr, 1.0, 1.0, 1.0); /* white */
+    cairo_paint (cr);
+    cairo_set_source_rgb (cr, 0.0, 0.0, 0.0); /* black */
+
+    for (i = 0; i < 4; i++) {
+	cairo_save (cr);
+	cairo_translate (cr,
+			 WIDTH/4  + (i % 2) * WIDTH/2,
+			 HEIGHT/4 + (i / 2) * HEIGHT/2);
+	(figures[i]) (cr);
+	cairo_restore (cr);
+    }
+
+    return CAIRO_TEST_SUCCESS;
+}
+
+int
+main (void)
+{
+    return cairo_test (&test, draw);
+}