fix circular dashing

Path measure cannot use the same code approach for quadratics
and cubics. Subdividing cubics repeatedly does not result in
subdivided t values, e.g. a quarter circle cubic divided in
half twice does not have a t value equivalent to 1/4.

Instead, always compute the cubic segment from a pair of
t values.

When finding the length of the cubic through recursive measures,
it is enough to carry the point at a given t to the next
subdivision.

(Chrome suppression has landed already.)

R=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1602153002

Review URL: https://codereview.chromium.org/1602153002

1 files changed, 78 insertions, 0 deletions

diff --git a/gm/dashcircle.cpp b/gm/dashcircle.cpp
new file mode 100644
index 0000000000..cddd913723
--- /dev/null
+++ b/gm/dashcircle.cpp
@@ -0,0 +1,78 @@
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "gm.h"
+#include "SkPath.h"
+#include "SkDashPathEffect.h"
+
+int dash1[] = { 1, 1 };
+int dash2[] = { 1, 3 };
+int dash3[] = { 1, 1, 3, 3 };
+int dash4[] = { 1, 3, 2, 4 };
+
+struct DashExample {
+    int* pattern;
+    int length;
+} dashExamples[] = {
+    { dash1, SK_ARRAY_COUNT(dash1) },
+    { dash2, SK_ARRAY_COUNT(dash2) },
+    { dash3, SK_ARRAY_COUNT(dash3) },
+    { dash4, SK_ARRAY_COUNT(dash4) }
+};
+
+DEF_SIMPLE_GM(dashcircle, canvas, 900, 1200) {
+    SkPaint refPaint;
+    refPaint.setAntiAlias(true);
+    refPaint.setColor(0xFFbf3f7f);
+    refPaint.setStyle(SkPaint::kStroke_Style);
+    refPaint.setStrokeWidth(1);
+    const SkScalar radius = 125;
+    SkRect oval = SkRect::MakeLTRB(-radius - 20, -radius - 20, radius + 20, radius + 20);
+    SkPath circle;
+    circle.addCircle(0, 0, radius);
+    SkScalar circumference = radius * SK_ScalarPI * 2;
+    int wedges[] = { 6, 12, 36 };
+    canvas->translate(radius + 20, radius + 20);
+    for (int wedge : wedges) {
+        SkScalar arcLength = 360.f / wedge;
+        canvas->save();
+        for (const DashExample& dashExample : dashExamples) {
+            SkPath refPath;
+            int dashUnits = 0;
+            for (int index = 0; index < dashExample.length; ++index) {
+                dashUnits += dashExample.pattern[index];
+            }
+            SkScalar unitLength = arcLength / dashUnits;
+            SkScalar angle = 0;
+            for (int index = 0; index < wedge; ++index) {
+                for (int i2 = 0; i2 < dashExample.length; i2 += 2) {
+                    SkScalar span = dashExample.pattern[i2] * unitLength;
+                    refPath.moveTo(0, 0);
+                    refPath.arcTo(oval, angle, span, false);
+                    refPath.close();
+                    angle += span + (dashExample.pattern[i2 + 1]) * unitLength;
+                }
+            }
+            canvas->drawPath(refPath, refPaint);
+            SkPaint p;
+            p.setAntiAlias(true);
+            p.setStyle(SkPaint::kStroke_Style);
+            p.setStrokeWidth(10);
+            SkScalar intervals[4];
+            int intervalCount = dashExample.length;
+            SkScalar dashLength = circumference / wedge / dashUnits;
+            for (int index = 0; index < dashExample.length; ++index) {
+                intervals[index] = dashExample.pattern[index] * dashLength;
+            }
+            p.setPathEffect(SkDashPathEffect::Create(intervals, intervalCount, 0))->unref();
+            canvas->drawPath(circle, p);
+            canvas->translate(0, radius * 2 + 50);
+        }
+        canvas->restore();
+        canvas->translate(radius * 2  + 50, 0);
+    }
+}