SkCanvas interface for drawing a patch.

src/utils/SkPatchUtils.cpp

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkPatchUtils.h"
+
+#include "SkGeometry.h"
+
+// size in pixels of each partition per axis, adjust this knob
+static const int kPartitionSize = 30;
+
+/**
+ * Calculate the approximate arc length given a bezier curve's control points.
+ */
+static SkScalar approx_arc_length(SkPoint* points, int count) {
+    if (count < 2) {
+        return 0;
+    }
+    SkScalar arcLength = 0;
+    for (int i = 0; i < count - 1; i++) {
+        arcLength += SkPoint::Distance(points[i], points[i + 1]);
+    }
+    return arcLength;
+}
+
+SkISize SkPatchUtils::GetLevelOfDetail(const SkPatch& patch, const SkMatrix* matrix) {
+    
+    SkPoint mapPts[12];
+    matrix->mapPoints(mapPts, patch.getControlPoints(), 12);
+    
+    // Approximate length of each cubic.
+    SkPoint pts[4];
+    patch.getTopPoints(pts);
+    matrix->mapPoints(pts, 4);
+    SkScalar topLength = approx_arc_length(pts, 4);
+    
+    patch.getBottomPoints(pts);
+    matrix->mapPoints(pts, 4);
+    SkScalar bottomLength = approx_arc_length(pts, 4);
+    
+    patch.getLeftPoints(pts);
+    matrix->mapPoints(pts, 4);
+    SkScalar leftLength = approx_arc_length(pts, 4);
+    
+    patch.getRightPoints(pts);
+    matrix->mapPoints(pts, 4);
+    SkScalar rightLength = approx_arc_length(pts, 4);
+    
+    // Level of detail per axis, based on the larger side between top and bottom or left and right
+    int lodX = static_cast<int>(SkMaxScalar(topLength, bottomLength) / kPartitionSize);
+    int lodY = static_cast<int>(SkMaxScalar(leftLength, rightLength) / kPartitionSize);
+    
+    return SkISize::Make(SkMax32(4, lodX), SkMax32(4, lodY));
+}