test: Exercise bug in joining together spline segments around cusps

Carl Worth demonstrated a glaring bug in the new stroking code,
introduced in commit 545f30856aac98199 (stroke: Convert the outlines
into contour and then into a polygon), whereby only a bevel join was
being used to connect segments around a sharp inflection point.

This adds the two examples he reported to the test suite.

3 files changed, 97 insertions, 0 deletions

diff --git a/test/Makefile.sources b/test/Makefile.sources
index ddb41b5ce..9aada620c 100644
--- a/test/Makefile.sources
+++ b/test/Makefile.sources
@@ -21,6 +21,7 @@ test_sources = \
 	big-empty-triangle.c				\
 	big-little-box.c				\
 	big-little-triangle.c				\
+	bug-spline.c					\
 	big-trap.c					\
 	bilevel-image.c					\
 	bug-40410.c					\
diff --git a/test/bug-spline.c b/test/bug-spline.c
new file mode 100644
index 000000000..00a915cd3
--- /dev/null
+++ b/test/bug-spline.c
@@ -0,0 +1,96 @@
+/* cc `pkg-config --cflags --libs cairo` cairo-spline-image.c -o cairo-spline-image */
+
+/* Copyright © 2005 Carl Worth
+ * Copyright © 2012 Intel Corporation
+ *
+ * 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.
+ */
+
+#include "cairo-test.h"
+
+#define WIDE_LINE_WIDTH 160
+#define NARROW_LINE_WIDTH 2
+
+/* A spline showing bugs in the "contour-based stroking" in cairo 1.12 */
+static const struct spline {
+    struct { double x, y; } pt[5];
+    double line_width;
+    double rgba[4];
+} splines[] = {
+    {
+	{
+	    { 172.25, 156.185 },
+	    { 177.225, 164.06 },
+	    { 176.5, 157.5 },
+	    { 175.5, 159.5 },
+	},
+	WIDE_LINE_WIDTH,
+	{ 1, 1, 1, 1 },
+    },
+    {
+	{
+	    { 571.25, 247.185 },
+	    { 78.225, 224.06 },
+	    { 129.5, 312.5 },
+	    { 210.5, 224.5 },
+	},
+	NARROW_LINE_WIDTH,
+	{ 1, 0, 0, 1 },
+    }
+};
+#define NUM_SPLINES (sizeof(splines)/sizeof(splines[0]))
+
+static cairo_test_status_t
+draw (cairo_t *cr, int width, int height)
+{
+    unsigned n;
+
+    cairo_set_source_rgb (cr, 0, 0, 0);
+    cairo_paint (cr);
+
+    cairo_set_line_cap(cr, CAIRO_LINE_CAP_SQUARE);
+
+    for (n = 0; n < NUM_SPLINES; n++) {
+	cairo_set_line_width (cr, splines[n].line_width);
+	cairo_set_source_rgba (cr,
+			       splines[n].rgba[0],
+			       splines[n].rgba[1],
+			       splines[n].rgba[2],
+			       splines[n].rgba[3]);
+
+	cairo_move_to (cr, splines[n].pt[0].x, splines[n].pt[0].y);
+	cairo_curve_to (cr,
+			splines[n].pt[1].x, splines[n].pt[1].y,
+			splines[n].pt[2].x, splines[n].pt[2].y,
+			splines[n].pt[3].x, splines[n].pt[3].y);
+
+	cairo_stroke (cr);
+    }
+
+    return CAIRO_TEST_SUCCESS;
+}
+
+CAIRO_TEST (bug_spline,
+	    "Exercises a bug in the stroking of splines",
+	    "spline, stroke", /* keywords */
+	    NULL, /* requirements */
+	    300, 300,
+	    NULL, draw)
diff --git a/test/reference/bug-spline.ref.png b/test/reference/bug-spline.ref.png
new file mode 100644
index 000000000..ebef26144
--- /dev/null
+++ b/test/reference/bug-spline.ref.png