GM (and fix) for drawArc capping issue

https://codereview.chromium.org/18271003/



git-svn-id: http://skia.googlecode.com/svn/trunk@9928 2bbb7eff-a529-9590-31e7-b0007b416f81

gm/arcofzorro.cpp

@@ -0,0 +1,83 @@
+/*
+ * Copyright 2013 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 "SkRandom.h"
+
+namespace skiagm {
+
+// This GM draws a lot of arcs in a 'Z' shape. It particularly exercises 
+// the 'drawArc' code near a singularly of its processing (i.e., near the
+// edge of one of its underlying quads).
+class ArcOfZorroGM : public GM {
+public:
+    ArcOfZorroGM() {
+        this->setBGColor(0xFFCCCCCC);
+    }
+
+protected:
+    virtual SkString onShortName() SK_OVERRIDE {
+        return SkString("arcofzorro");
+    }
+
+    virtual SkISize onISize() SK_OVERRIDE {
+        return make_isize(1000, 1000);
+    }
+
+    virtual void onDraw(SkCanvas* canvas) SK_OVERRIDE {
+        SkMWCRandom rand;
+
+        SkRect rect = SkRect::MakeXYWH(10, 10, 200, 200);
+
+        SkPaint p;
+
+        p.setStyle(SkPaint::kStroke_Style);
+        p.setStrokeWidth(35);
+        int xOffset = 0, yOffset = 0;
+        int direction = 0;
+
+        for (float arc = 134.0f; arc < 136.0f; arc += 0.01f) {
+            SkColor color = rand.nextU();
+            color |= 0xff000000;
+            p.setColor(color);
+
+            canvas->save();
+            canvas->translate(SkIntToScalar(xOffset), SkIntToScalar(yOffset));
+            canvas->drawArc(rect, 0, arc, false, p);
+            canvas->restore();
+
+            switch (direction) {
+            case 0:
+                xOffset += 10;
+                if (xOffset >= 700) {
+                    direction = 1;
+                }
+                break;
+            case 1:
+                xOffset -= 10;
+                yOffset += 10;
+                if (xOffset < 50) {
+                    direction = 2;
+                }
+                break;
+            case 2:
+                xOffset += 10;
+                break;
+            }
+        }
+
+    }
+
+private:
+    typedef GM INHERITED;
+};
+
+//////////////////////////////////////////////////////////////////////////////
+
+DEF_GM( return SkNEW(ArcOfZorroGM); )
+
+}

gyp/gmslides.gypi

@@ -4,6 +4,7 @@
     '../gm/aaclip.cpp',
     '../gm/aarectmodes.cpp',
     '../gm/alphagradients.cpp',
+    '../gm/arcofzorro.cpp',
     '../gm/arithmode.cpp',
     '../gm/bicubicfilter.cpp',
     '../gm/bigmatrix.cpp',

src/core/SkGeometry.cpp

@@ -1205,11 +1205,13 @@ static SkScalar quad_solve(SkScalar a, SkScalar b, SkScalar c, SkScalar d)
     return count == 1 ? roots[0] : 0;
 }
 
-/*  given a quad-curve and a point (x,y), chop the quad at that point and return
+/*  given a quad-curve and a point (x,y), chop the quad at that point and place
-    the new quad's offCurve point. Should only return false if the computed pos
+    the new off-curve point and endpoint into 'dest'. The new end point is used
-    is the start of the curve (i.e. root == 0)
+    (rather than (x,y)) to compensate for numerical inaccuracies.
+    Should only return false if the computed pos is the start of the curve 
+    (i.e. root == 0)
 */
-static bool quad_pt2OffCurve(const SkPoint quad[3], SkScalar x, SkScalar y, SkPoint* offCurve)
+static bool truncate_last_curve(const SkPoint quad[3], SkScalar x, SkScalar y, SkPoint* dest)
 {
     const SkScalar* base;
     SkScalar        value;
@@ -1230,7 +1232,8 @@ static bool quad_pt2OffCurve(const SkPoint quad[3], SkScalar x, SkScalar y, SkPo
     {
         SkPoint tmp[5];
         SkChopQuadAt(quad, tmp, t);
-        *offCurve = tmp[1];
+        dest[0] = tmp[1];
+        dest[1] = tmp[2];
         return true;
     } else {
         /*  t == 0 means either the value triggered a root outside of [0, 1)
@@ -1247,7 +1250,8 @@ static bool quad_pt2OffCurve(const SkPoint quad[3], SkScalar x, SkScalar y, SkPo
         if ((base[0] < base[4] && value > base[2]) ||
             (base[0] > base[4] && value < base[2]))   // should root have been 1
         {
-            *offCurve = quad[1];
+            dest[0] = quad[1];
+            dest[1].set(x, y);
             return true;
         }
     }
@@ -1360,9 +1364,8 @@ int SkBuildQuadArc(const SkVector& uStart, const SkVector& uStop,
         memcpy(quadPoints, gQuadCirclePts, (wholeCount + 1) * sizeof(SkPoint));
 
         const SkPoint* arc = &gQuadCirclePts[wholeCount];
-        if (quad_pt2OffCurve(arc, x, y, &quadPoints[wholeCount + 1]))
+        if (truncate_last_curve(arc, x, y, &quadPoints[wholeCount + 1]))
         {
-            quadPoints[wholeCount + 2].set(x, y);
             wholeCount += 2;
         }
         pointCount = wholeCount + 1;