add support for capped hairlines

Extend the ends of hairline and haircurve segments when the paint is set to square or round, and the line or curve is at the start or end of a contour.

R=reed@google.com
BUG=skia:4599

Review URL: https://codereview.chromium.org/1491843006

1 files changed, 82 insertions, 4 deletions

diff --git a/src/core/SkScan_Hairline.cpp b/src/core/SkScan_Hairline.cpp
index 3917081571..52399c9d78 100644
--- a/src/core/SkScan_Hairline.cpp
+++ b/src/core/SkScan_Hairline.cpp
@@ -348,7 +348,51 @@ static int compute_quad_level(const SkPoint pts[3]) {
     return level;
 }
 
-static void hair_path(const SkPath& path, const SkRasterClip& rclip, SkBlitter* blitter,
+/* Extend the points in the direction of the starting or ending tangent by 1/2 unit to
+   account for a round or square cap. If there's no distance between the end point and
+   the control point, use the next control point to create a tangent. If the curve
+   is degenerate, move the cap out 1/2 unit horizontally. */
+template <SkPaint::Cap capStyle>
+void extend_pts(SkPath::Verb prevVerb, SkPath::Verb nextVerb, SkPoint* pts, int ptCount) {
+    SkASSERT(SkPaint::kSquare_Cap == capStyle || SkPaint::kRound_Cap == capStyle);
+    // The area of a circle is PI*R*R. For a unit circle, R=1/2, and the cap covers half of that.
+    const SkScalar capOutset = SkPaint::kSquare_Cap == capStyle ? 0.5f : SK_ScalarPI / 8;
+    if (SkPath::kMove_Verb == prevVerb) {
+        SkPoint* first = pts;
+        SkPoint* ctrl = first;
+        int controls = ptCount - 1;
+        SkVector tangent;
+        do {
+            tangent = *first - *++ctrl;
+        } while (tangent.isZero() && --controls > 0);
+        if (tangent.isZero()) {
+            tangent.set(1, 0);
+        } else {
+            tangent.normalize();
+        }
+        first->fX += tangent.fX * capOutset;
+        first->fY += tangent.fY * capOutset;
+    }
+    if (SkPath::kMove_Verb == nextVerb || SkPath::kDone_Verb == nextVerb) {
+        SkPoint* last = &pts[ptCount - 1];
+        SkPoint* ctrl = last;
+        int controls = ptCount - 1;
+        SkVector tangent;
+        do {
+            tangent = *last - *--ctrl;
+        } while (tangent.isZero() && --controls > 0);
+        if (tangent.isZero()) {
+            tangent.set(-1, 0);
+        } else {
+            tangent.normalize();
+        }
+        last->fX += tangent.fX * capOutset;
+        last->fY += tangent.fY * capOutset;
+    }
+}
+
+template <SkPaint::Cap capStyle>
+void hair_path(const SkPath& path, const SkRasterClip& rclip, SkBlitter* blitter,
                       SkScan::HairRgnProc lineproc) {
     if (path.isEmpty()) {
         return;
@@ -376,23 +420,35 @@ static void hair_path(const SkPath& path, const SkRasterClip& rclip, SkBlitter*
 
     SkPath::RawIter     iter(path);
     SkPoint             pts[4], firstPt, lastPt;
-    SkPath::Verb        verb;
+    SkPath::Verb        verb, prevVerb;
     SkAutoConicToQuads  converter;
 
+    if (SkPaint::kButt_Cap != capStyle) {
+        prevVerb = SkPath::kDone_Verb;
+    }
     while ((verb = iter.next(pts)) != SkPath::kDone_Verb) {
         switch (verb) {
             case SkPath::kMove_Verb:
                 firstPt = lastPt = pts[0];
                 break;
             case SkPath::kLine_Verb:
+                if (SkPaint::kButt_Cap != capStyle) {
+                    extend_pts<capStyle>(prevVerb, iter.peek(), pts, 2);
+                }
                 lineproc(pts, 2, clip, blitter);
                 lastPt = pts[1];
                 break;
             case SkPath::kQuad_Verb:
+                if (SkPaint::kButt_Cap != capStyle) {
+                    extend_pts<capStyle>(prevVerb, iter.peek(), pts, 3);
+                }
                 hairquad(pts, clip, blitter, compute_quad_level(pts), lineproc);
                 lastPt = pts[2];
                 break;
             case SkPath::kConic_Verb: {
+                if (SkPaint::kButt_Cap != capStyle) {
+                    extend_pts<capStyle>(prevVerb, iter.peek(), pts, 3);
+                }
                 // how close should the quads be to the original conic?
                 const SkScalar tol = SK_Scalar1 / 4;
                 const SkPoint* quadPts = converter.computeQuads(pts,
@@ -406,6 +462,9 @@ static void hair_path(const SkPath& path, const SkRasterClip& rclip, SkBlitter*
                 break;
             }
             case SkPath::kCubic_Verb: {
+                if (SkPaint::kButt_Cap != capStyle) {
+                    extend_pts<capStyle>(prevVerb, iter.peek(), pts, 4);
+                }
                 haircubic(pts, clip, blitter, kMaxCubicSubdivideLevel, lineproc);
                 lastPt = pts[3];
             } break;
@@ -417,15 +476,34 @@ static void hair_path(const SkPath& path, const SkRasterClip& rclip, SkBlitter*
             case SkPath::kDone_Verb:
                 break;
         }
+        if (SkPaint::kButt_Cap != capStyle) {
+            prevVerb = verb;
+        }
     }
 }
 
 void SkScan::HairPath(const SkPath& path, const SkRasterClip& clip, SkBlitter* blitter) {
-    hair_path(path, clip, blitter, SkScan::HairLineRgn);
+    hair_path<SkPaint::kButt_Cap>(path, clip, blitter, SkScan::HairLineRgn);
 }
 
 void SkScan::AntiHairPath(const SkPath& path, const SkRasterClip& clip, SkBlitter* blitter) {
-    hair_path(path, clip, blitter, SkScan::AntiHairLineRgn);
+    hair_path<SkPaint::kButt_Cap>(path, clip, blitter, SkScan::AntiHairLineRgn);
+}
+
+void SkScan::HairSquarePath(const SkPath& path, const SkRasterClip& clip, SkBlitter* blitter) {
+    hair_path<SkPaint::kSquare_Cap>(path, clip, blitter, SkScan::HairLineRgn);
+}
+
+void SkScan::AntiHairSquarePath(const SkPath& path, const SkRasterClip& clip, SkBlitter* blitter) {
+    hair_path<SkPaint::kSquare_Cap>(path, clip, blitter, SkScan::AntiHairLineRgn);
+}
+
+void SkScan::HairRoundPath(const SkPath& path, const SkRasterClip& clip, SkBlitter* blitter) {
+    hair_path<SkPaint::kRound_Cap>(path, clip, blitter, SkScan::HairLineRgn);
+}
+
+void SkScan::AntiHairRoundPath(const SkPath& path, const SkRasterClip& clip, SkBlitter* blitter) {
+    hair_path<SkPaint::kRound_Cap>(path, clip, blitter, SkScan::AntiHairLineRgn);
 }
 
 ///////////////////////////////////////////////////////////////////////////////