Analytic AntiAlias for Convex Shapes

src/core/SkAnalyticEdge.h

+/*
+ * Copyright 2006 The Android Open Source Project
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkAnalyticEdge_DEFINED
+#define SkAnalyticEdge_DEFINED
+
+#include "SkEdge.h"
+
+inline SkFixed SkFixedMul_lowprec(SkFixed a, SkFixed b) {
+    SkASSERT(((int64_t)a >> 8) * (b >> 8) <= SK_MaxS32);
+    return (a >> 8) * (b >> 8);
+}
+
+struct SkAnalyticEdge {
+    // Similar to SkEdge, the conic edges will be converted to quadratic edges
+    enum Type {
+        kLine_Type,
+        kQuad_Type,
+        kCubic_Type
+    };
+
+    SkAnalyticEdge* fNext;
+    SkAnalyticEdge* fPrev;
+
+    SkFixed fX;
+    SkFixed fDX;
+    SkFixed fUpperX;        // The x value when y = fUpperY
+    SkFixed fY;             // The current y
+    SkFixed fUpperY;        // The upper bound of y (our edge is from y = fUpperY to y = fLowerY)
+    SkFixed fLowerY;        // The lower bound of y (our edge is from y = fUpperY to y = fLowerY)
+    SkFixed fDY;            // abs(1/fDX); may be SK_MaxS32 when fDX is close to 0.
+                            // fDY is only used for blitting trapezoids.
+
+    int8_t  fCurveCount;    // only used by kQuad(+) and kCubic(-)
+    uint8_t fCurveShift;    // appled to all Dx/DDx/DDDx except for fCubicDShift exception
+    uint8_t fCubicDShift;   // applied to fCDx and fCDy only in cubic
+    int8_t  fWinding;       // 1 or -1
+
+    static const int kDefaultAccuracy = 2; // default accuracy for snapping
+
+    static inline SkFixed snapY(SkFixed y, int accuracy = kDefaultAccuracy) {
+        return SkFixedRoundToFixed(y << accuracy) >> accuracy;
+    }
+
+    // Update fX, fY of this edge so fY = y
+    inline void goY(SkFixed y) {
+        if (y == fY + SK_Fixed1) {
+            fX = fX + fDX;
+            fY = y;
+        } else if (y != fY) {
+            // Drop lower digits as our alpha only has 8 bits
+            // (fDX and y - fUpperY may be greater than SK_Fixed1)
+            fX = fUpperX + SkFixedMul_lowprec(fDX, y - fUpperY);
+            fY = y;
+        }
+    }
+
+    inline bool setLine(const SkPoint& p0, const SkPoint& p1, const SkIRect* clip = nullptr);
+    inline bool updateLine(SkFixed ax, SkFixed ay, SkFixed bx, SkFixed by, SkFixed slope);
+    void chopLineWithClip(const SkIRect& clip);
+
+    inline bool intersectsClip(const SkIRect& clip) const {
+        SkASSERT(SkFixedFloorToInt(fUpperY) < clip.fBottom);
+        return SkFixedCeilToInt(fLowerY) > clip.fTop;
+    }
+
+#ifdef SK_DEBUG
+    void dump() const {
+        SkDebugf("edge: upperY:%d lowerY:%d y:%g x:%g dx:%g w:%d\n",
+                 fUpperY, fLowerY, SkFixedToFloat(fY), SkFixedToFloat(fX),
+                 SkFixedToFloat(fDX), fWinding);
+    }
+
+    void validate() const {
+         SkASSERT(fPrev && fNext);
+         SkASSERT(fPrev->fNext == this);
+         SkASSERT(fNext->fPrev == this);
+
+         SkASSERT(fUpperY < fLowerY);
+         SkASSERT(SkAbs32(fWinding) == 1);
+    }
+#endif
+};
+
+struct SkAnalyticQuadraticEdge : public SkAnalyticEdge {
+    SkQuadraticEdge fQEdge;
+
+    // snap y to integer points in the middle of the curve to accelerate AAA path filling
+    SkFixed fSnappedX, fSnappedY;
+
+    bool setQuadratic(const SkPoint pts[3]);
+    bool updateQuadratic();
+};
+
+struct SkAnalyticCubicEdge : public SkAnalyticEdge {
+    SkCubicEdge fCEdge;
+
+    bool setCubic(const SkPoint pts[4]);
+    bool updateCubic();
+};
+
+bool SkAnalyticEdge::setLine(const SkPoint& p0, const SkPoint& p1, const SkIRect* clip) {
+    // We must set X/Y using the same way (times 4, to FDot6, then to Fixed) as Quads/Cubics.
+    // Otherwise the order of the edge might be wrong due to precision limit.
+    SkFixed x0 = SkFDot6ToFixed(SkScalarToFDot6(p0.fX * 4)) >> 2;
+    SkFixed y0 = snapY(SkFDot6ToFixed(SkScalarToFDot6(p0.fY * 4)) >> 2);
+    SkFixed x1 = SkFDot6ToFixed(SkScalarToFDot6(p1.fX * 4)) >> 2;
+    SkFixed y1 = snapY(SkFDot6ToFixed(SkScalarToFDot6(p1.fY * 4)) >> 2);
+
+    // are we a zero-height line?
+    if (y0 == y1) {
+        return false;
+    }
+
+    int top = SkFixedFloorToInt(y0);
+    int bot = SkFixedCeilToInt(y1);
+
+    // are we completely above or below the clip?
+    if (clip && (top >= clip->fBottom || bot <= clip->fTop)) {
+        return false;
+    }
+
+    int winding = 1;
+
+    if (y0 > y1) {
+        SkTSwap(x0, x1);
+        SkTSwap(y0, y1);
+        winding = -1;
+    }
+
+    SkFixed slope = SkFixedDiv(x1 - x0, y1 - y0);
+
+    fX          = x0;
+    fDX         = slope;
+    fUpperX     = x0;
+    fY          = y0;
+    fUpperY     = y0;
+    fLowerY     = y1;
+    fDY         = x1 != x0 ? SkAbs32(SkFixedDiv(y1 - y0, x1 - x0)) : SK_MaxS32;
+    fCurveCount = 0;
+    fWinding    = SkToS8(winding);
+    fCurveShift = 0;
+
+    if (clip) {
+        this->chopLineWithClip(*clip);
+    }
+    return true;
+}
+
+#endif