EdgeClip demo to show scan-converter clipping behavior

samplecode/SamplePathClip.cpp

Index: samplecode/SamplePathClip.cpp
diff --git a/samplecode/SamplePathClip.cpp b/samplecode/SamplePathClip.cpp
index 27f1f307ef395d92d0ac9aceacf8aa262984df87..09f1c9705c758f11410c35801b07478f88034cda 100644
--- a/samplecode/SamplePathClip.cpp
+++ b/samplecode/SamplePathClip.cpp
@@ -26,17 +26,9 @@ public:
     SkRect fOval;
     SkPoint fCenter;
 
-    PathClipView() {
-        fOval.set(0, 0, SkIntToScalar(200), SkIntToScalar(50));
-        fCenter.set(SkIntToScalar(250), SkIntToScalar(250));
-
-//        test_ats();
-    }
-
-    virtual ~PathClipView() {}
+    PathClipView() : fOval(SkRect::MakeWH(200, 50)), fCenter(SkPoint::Make(250, 250)) {}
 
 protected:
-    // overrides from SkEventSink
     bool onQuery(SkEvent* evt) override {
         if (SampleCode::TitleQ(*evt)) {
             SampleCode::TitleR(evt, "PathClip");
@@ -46,8 +38,8 @@ protected:
     }
 
     void onDrawContent(SkCanvas* canvas) override {
-        SkRect oval = fOval;
-        oval.offset(fCenter.fX - oval.centerX(), fCenter.fY - oval.centerY());
+        const SkRect oval = fOval.makeOffset(fCenter.fX - fOval.centerX(),
+                                             fCenter.fY - fOval.centerY());
 
         SkPaint p;
         p.setAntiAlias(true);
@@ -55,9 +47,7 @@ protected:
         p.setStyle(SkPaint::kStroke_Style);
         canvas->drawOval(oval, p);
 
-        SkRect r;
-        r.set(SkIntToScalar(200), SkIntToScalar(200),
-              SkIntToScalar(300), SkIntToScalar(300));
+        const SkRect r = SkRect::MakeLTRB(200, 200, 300, 300);
         canvas->clipRect(r);
 
         p.setStyle(SkPaint::kFill_Style);
@@ -65,8 +55,6 @@ protected:
         canvas->drawRect(r, p);
 
         p.setColor(0x800000FF);
-        r.set(SkIntToScalar(150), SkIntToScalar(10),
-              SkIntToScalar(250), SkIntToScalar(400));
         canvas->drawOval(oval, p);
     }
 
@@ -83,8 +71,251 @@ protected:
 private:
     typedef SampleView INHERITED;
 };
+DEF_SAMPLE( return new PathClipView; )
 
 //////////////////////////////////////////////////////////////////////////////
 
-static SkView* MyFactory() { return new PathClipView; }
-static SkViewRegister reg(MyFactory);
+static int clip_line(const SkRect& bounds, SkPoint p0, SkPoint p1, SkPoint edges[]) {
+    SkPoint* edgesStart = edges;
+
+    if (p0.fY == p1.fY) {
+        return 0;
+    }
+    
+    if (p0.fY > p1.fY) {
+        SkTSwap(p0, p1);
+    }
+    // now we're monotonic in Y: p0 <= p1
+    if (p1.fY <= bounds.top() || p0.fY >= bounds.bottom()) {
+        return 0;
+    }
+    
+    double dxdy = (double)(p1.fX - p0.fX) / (p1.fY - p0.fY);
+    if (p0.fY < bounds.top()) {
+        p0.fX = SkDoubleToScalar(p0.fX + dxdy * (bounds.top() - p0.fY));
+        p0.fY = bounds.top();
+    }
+    if (p1.fY > bounds.bottom()) {
+        p1.fX = SkDoubleToScalar(p1.fX + dxdy * (bounds.bottom() - p1.fY));
+        p1.fY = bounds.bottom();
+    }
+    
+    // Now p0...p1 is strictly inside bounds vertically, so we just need to clip horizontally
+    
+    if (p0.fX > p1.fX) {
+        SkTSwap(p0, p1);
+    }
+    // now we're left-to-right: p0 .. p1
+    
+    if (p1.fX <= bounds.left()) {   // entirely to the left
+        p0.fX = p1.fX = bounds.left();
+        *edges++ = p0;
+        *edges++ = p1;
+        return 2;
+    }
+    if (p0.fX >= bounds.right()) {  // entirely to the right
+        p0.fX = p1.fX = bounds.right();
+        *edges++ = p0;
+        *edges++ = p1;
+        return 2;
+    }
+    
+    if (p0.fX < bounds.left()) {
+        float y = SkDoubleToScalar(p0.fY + (bounds.left() - p0.fX) / dxdy);
+        *edges++ = SkPoint::Make(bounds.left(), p0.fY);
+        *edges++ = SkPoint::Make(bounds.left(), y);
+        p0.set(bounds.left(), y);
+    }
+    if (p1.fX > bounds.right()) {
+        float y = SkDoubleToScalar(p0.fY + (bounds.right() - p0.fX) / dxdy);
+        *edges++ = p0;
+        *edges++ = SkPoint::Make(bounds.right(), y);
+        *edges++ = SkPoint::Make(bounds.right(), p1.fY);
+    } else {
+        *edges++ = p0;
+        *edges++ = p1;
+    }
+    return SkToInt(edges - edgesStart);
+}
+
+static void draw_clipped_line(SkCanvas* canvas, const SkRect& bounds,
+                              SkPoint p0, SkPoint p1, const SkPaint& paint) {
+    SkPoint verts[6];
+    int count = clip_line(bounds, p0, p1, verts);
+
+    SkPath path;
+    path.addPoly(verts, count, false);
+    canvas->drawPath(path, paint);
+}
+
+// Demonstrate edge-clipping that is used in the scan converter
+//
+class EdgeClipView : public SampleView {
+    enum {
+        N = 3
+    };
+public:
+    SkPoint fPoly[N];
+    SkRect  fClip;
+    SkColor fEdgeColor[N];
+    
+    EdgeClipView() : fClip(SkRect::MakeLTRB(150, 150, 550, 450)) {
+        fPoly[0].set(300, 40);
+        fPoly[1].set(550, 250);
+        fPoly[2].set(40, 450);
+
+        fEdgeColor[0] = 0xFFFF0000;
+        fEdgeColor[1] = 0xFF00FF00;
+        fEdgeColor[2] = 0xFF0000FF;
+    }
+    
+protected:
+    bool onQuery(SkEvent* evt) override {
+        if (SampleCode::TitleQ(*evt)) {
+            SampleCode::TitleR(evt, "EdgeClip");
+            return true;
+        }
+        return this->INHERITED::onQuery(evt);
+    }
+
+    static SkScalar snap(SkScalar x) {
+        return SkScalarRoundToScalar(x * 0.5f) * 2;
+    }
+    static SkPoint snap(const SkPoint& pt) {
+        return SkPoint::Make(snap(pt.x()), snap(pt.y()));
+    }
+    static void snap(SkPoint dst[], const SkPoint src[], int count) {
+        for (int i = 0; i < count; ++i) {
+            dst[i] = snap(src[i]);
+        }
+    }
+
+    void onDrawContent(SkCanvas* canvas) override {
+        SkPath path;
+        path.addPoly(fPoly, N, true);
+
+        // Draw the full triangle, stroked and filled
+        SkPaint p;
+        p.setAntiAlias(true);
+        p.setColor(0xFFE0E0E0);
+        canvas->drawPath(path, p);
+        p.setStyle(SkPaint::kStroke_Style);
+        p.setStrokeWidth(2);
+        for (int i = 0; i < N; ++i) {
+            const int j = (i + 1) % N;
+            p.setColor(fEdgeColor[i]);
+            p.setAlpha(0x88);
+            canvas->drawLine(fPoly[i].x(), fPoly[i].y(), fPoly[j].x(), fPoly[j].y(), p);
+        }
+        p.setStyle(SkPaint::kFill_Style);
+
+        // Draw the clip itself
+        p.setColor(0xFF8888CC);
+        canvas->drawRect(fClip, p);
+
+        // Draw the filled triangle through the clip
+        p.setColor(0xFF88CC88);
+        canvas->save();
+        canvas->clipRect(fClip);
+        canvas->drawPath(path, p);
+        canvas->restore();
+
+        p.setStyle(SkPaint::kStroke_Style);
+        p.setStrokeWidth(6);
+
+        // Draw each of the "Edges" that survived the clipping
+        // We use a layer, so we can PLUS the different edge-colors, showing where two edges
+        // canceled each other out.
+        canvas->saveLayer(nullptr, nullptr);
+        p.setXfermodeMode(SkXfermode::kPlus_Mode);
+        for (int i = 0; i < N; ++i) {
+            const int j = (i + 1) % N;
+            p.setColor(fEdgeColor[i]);
+            draw_clipped_line(canvas, fClip, fPoly[i], fPoly[j], p);
+        }
+        canvas->restore();
+    }
+
+    class MyClick : public Click {
+    public:
+        MyClick(SkView* view) : Click(view) {}
+        virtual void handleMove() = 0;
+    };
+    
+    class VertClick : public MyClick {
+        SkPoint* fPt;
+    public:
+        VertClick(SkView* view, SkPoint* pt) : MyClick(view), fPt(pt) {}
+        void handleMove() override { *fPt = snap(fCurr); }
+    };
+    
+    class DragRectClick : public MyClick {
+        SkRect* fRect;
+    public:
+        DragRectClick(SkView* view, SkRect* rect) : MyClick(view), fRect(rect) {}
+        void handleMove() override { fRect->offset(fCurr.x() - fPrev.x(), fCurr.y() - fPrev.y()); }
+    };
+    
+    class DragPolyClick : public MyClick {
+        SkPoint fSrc[100];
+        SkPoint* fPoly;
+        int fCount;
+    public:
+        DragPolyClick(SkView* view, SkPoint poly[], int count)
+            : MyClick(view), fPoly(poly), fCount(count)
+        {
+            SkASSERT((size_t)count <= SK_ARRAY_COUNT(fSrc));
+            memcpy(fSrc, poly, count * sizeof(SkPoint));
+        }
+        void handleMove() override {
+            const SkScalar dx = fCurr.x() - fOrig.x();
+            const SkScalar dy = fCurr.y() - fOrig.y();
+            for (int i = 0; i < fCount; ++i) {
+                fPoly[i].set(snap(fSrc[i].x() + dx), snap(fSrc[i].y() + dy));
+            }
+        }
+    };
+
+    class DoNothingClick : public MyClick {
+    public:
+        DoNothingClick(SkView* view) : MyClick(view) {}
+        void handleMove() override {}
+    };
+
+    static bool hit_test(const SkPoint& pt, SkScalar x, SkScalar y) {
+        const SkScalar rad = 8;
+        const SkScalar dx = pt.x() - x;
+        const SkScalar dy = pt.y() - y;
+        return dx*dx + dy*dy <= rad*rad;
+    }
+
+    SkView::Click* onFindClickHandler(SkScalar x, SkScalar y, unsigned) override {
+        for (int i = 0; i < N; ++i) {
+            if (hit_test(fPoly[i], x, y)) {
+                return new VertClick(this, &fPoly[i]);
+            }
+        }
+        
+        SkPath path;
+        path.addPoly(fPoly, N, true);
+        if (path.contains(x, y)) {
+            return new DragPolyClick(this, fPoly, N);
+        }
+
+        if (fClip.intersects(SkRect::MakeLTRB(x - 1, y - 1, x + 1, y + 1))) {
+            return new DragRectClick(this, &fClip);
+        }
+        return new DoNothingClick(this);
+    }
+    
+    bool onClick(Click* click) override {
+        ((MyClick*)click)->handleMove();
+        this->inval(nullptr);
+        return false;
+    }
+    
+private:
+    typedef SampleView INHERITED;
+};
+DEF_SAMPLE( return new EdgeClipView; )
+