Add dash-offset test

Stroking a dash pattern of odd length with a negative offset is broken
(except when the pattern is composed by a single dash).

test/Makefile.am

@@ -312,6 +312,7 @@ REFERENCE_IMAGES = \
 	dash-curve.xlib.ref.png \
 	dash-infinite-loop.ref.png \
 	dash-no-dash.ref.png \
+	dash-offset.ref.png \
 	dash-offset-negative.ref.png \
 	dash-scale.ps2.argb32.ref.png \
 	dash-scale.ps2.rgb24.ref.png \

test/Makefile.sources

@@ -48,6 +48,7 @@ test_sources = \
 	dash-curve.c					\
 	dash-infinite-loop.c				\
 	dash-no-dash.c					\
+	dash-offset.c					\
 	dash-offset-negative.c				\
 	dash-scale.c					\
 	dash-state.c					\

test/dash-offset.c

@@ -0,0 +1,95 @@
+/* -*- Mode: c; c-basic-offset: 4; indent-tabs-mode: t; tab-width: 8; -*- */
+/* cairo - a vector graphics library with display and print output
+ *
+ * Copyright 2009 Andrea Canciani
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it either under the terms of the GNU Lesser General Public
+ * License version 2.1 as published by the Free Software Foundation
+ * (the "LGPL") or, at your option, under the terms of the Mozilla
+ * Public License Version 1.1 (the "MPL"). If you do not alter this
+ * notice, a recipient may use your version of this file under either
+ * the MPL or the LGPL.
+ *
+ * You should have received a copy of the LGPL along with this library
+ * in the file COPYING-LGPL-2.1; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ * You should have received a copy of the MPL along with this library
+ * in the file COPYING-MPL-1.1
+ *
+ * The contents of this file are subject to the Mozilla Public License
+ * Version 1.1 (the "License"); you may not use this file except in
+ * compliance with the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY
+ * OF ANY KIND, either express or implied. See the LGPL or the MPL for
+ * the specific language governing rights and limitations.
+ *
+ * The Original Code is the cairo graphics library.
+ *
+ * The Initial Developer of the Original Code is University of Southern
+ * California.
+ *
+ * Contributor(s):
+ *      Andrea Canciani <ranma42@gmail.com>
+ */
+
+#include "cairo-test.h"
+
+#define ARRAY_SIZE(a) (sizeof (a) / sizeof ((a)[0]))
+
+/* Lengths of the dashes of the dash patterns */
+static const double dashes[] = { 2, 2, 4, 4 };
+/* Dash offset in userspace units
+ * They always grow by 2, so the dash pattern is
+ * should be shifted by the same amount each time */
+static const double frac_offset[] = { 0, 2, 4, 6 };
+/* Dash offset relative to the whole dash pattern
+ * This corresponds to the non-inverted part only if
+ * the dash pattern has odd length, so the expected result
+ * is the same for every int_offset if the pattern has
+ * even lenght, and inverted each time (or shifted by half
+ * period, which is the same) if the pattern has odd length. */
+static const double int_offset[] = { -2, -1, 0, 1, 2 };
+
+#define PAD 6
+#define STROKE_LENGTH 32
+#define IMAGE_WIDTH (PAD + (STROKE_LENGTH + PAD) * ARRAY_SIZE(dashes))
+#define IMAGE_HEIGHT (PAD + PAD * ARRAY_SIZE(int_offset) + PAD * ARRAY_SIZE(frac_offset) * ARRAY_SIZE(int_offset))
+
+
+static cairo_test_status_t
+draw (cairo_t *cr, int width, int height)
+{
+    double total;
+    size_t i, j, k;
+
+    cairo_set_source_rgb (cr, 1, 1, 1);
+    cairo_paint (cr);
+
+    cairo_set_source_rgb (cr, 0, 0, 0);
+    cairo_set_line_width (cr, 2);
+
+    total = 0.0;
+    for (k = 0; k < ARRAY_SIZE(dashes); ++k) {
+	total += dashes[k];
+	for (i = 0; i < ARRAY_SIZE(frac_offset); ++i) {
+	    for (j = 0; j < ARRAY_SIZE(int_offset); ++j) {
+		cairo_set_dash (cr, dashes, k + 1, frac_offset[i] + total * int_offset[j]);
+		cairo_move_to (cr, (STROKE_LENGTH + PAD) * k + PAD, PAD * (i + j + ARRAY_SIZE(frac_offset) * j + 1));
+		cairo_line_to (cr, (STROKE_LENGTH + PAD) * (k + 1), PAD * (i + j + ARRAY_SIZE(frac_offset) * j + 1));
+		cairo_stroke (cr);
+	    }
+	}
+    }
+
+    return CAIRO_TEST_SUCCESS;
+}
+
+CAIRO_TEST (dash_offset,
+	    "Tests dashes of different length with various offsets",
+	    "stroke, dash", /* keywords */
+	    NULL, /* requirements */
+	    IMAGE_WIDTH, IMAGE_HEIGHT,
+	    NULL, draw)