Reland of ix for conic fuzz (patchset #1 id:1 of https://codereview.chromium.org/2361473004/ )

Reason for revert:
Landed suppression in Chrome's LayoutTests/TestExpectations

Original issue's description:
> Revert of fix for conic fuzz (patchset #3 id:40001 of https://codereview.chromium.org/2350263003/ )
>
> Reason for revert:
> See if this fixes the layout tests.
>
> Original issue's description:
> > fix for conic fuzz
> >
> > A fuzzer generates a conic that hangs when drawn.
> > The quads that approximate the conics move up and down
> > in y, confusing the renderer.
> >
> > This fix ensures that the split conic maintains the
> > same y direction as the original conic.
> >
> > R=reed@google.com
> > BUG=647922
> > GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2350263003
> >
> > Committed: https://skia.googlesource.com/skia/+/ac78863acdef4b428aaf66985b80c76d1be0fdea
>
> TBR=reed@google.com
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=647922
>
> Committed: https://skia.googlesource.com/skia/+/08b345588414b861af8a55950e7dc21a1bd85a28

TBR=reed@google.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=647922

Review-Url: https://codereview.chromium.org/2359253002

1 files changed, 32 insertions, 0 deletions

diff --git a/src/core/SkGeometry.cpp b/src/core/SkGeometry.cpp
index a3ebfb38a8..027b65f0f1 100644
--- a/src/core/SkGeometry.cpp
+++ b/src/core/SkGeometry.cpp
@@ -1153,6 +1153,12 @@ int SkConic::computeQuadPOW2(SkScalar tol) const {
     return pow2;
 }
 
+// This was originally developed and tested for pathops: see SkOpTypes.h 
+// returns true if (a <= b <= c) || (a >= b >= c)
+static bool between(SkScalar a, SkScalar b, SkScalar c) {
+    return (a - b) * (c - b) <= 0;
+}
+
 static SkPoint* subdivide(const SkConic& src, SkPoint pts[], int level) {
     SkASSERT(level >= 0);
 
@@ -1162,6 +1168,32 @@ static SkPoint* subdivide(const SkConic& src, SkPoint pts[], int level) {
     } else {
         SkConic dst[2];
         src.chop(dst);
+        const SkScalar startY = src.fPts[0].fY;
+        const SkScalar endY = src.fPts[2].fY;
+        if (between(startY, src.fPts[1].fY, endY)) {
+            // If the input is monotonic and the output is not, the scan converter hangs.
+            // Ensure that the chopped conics maintain their y-order.
+            SkScalar midY = dst[0].fPts[2].fY;
+            if (!between(startY, midY, endY)) {
+                // If the computed midpoint is outside the ends, move it to the closer one.
+                SkScalar closerY = SkTAbs(midY - startY) < SkTAbs(midY - endY) ? startY : endY;
+                dst[0].fPts[2].fY = dst[1].fPts[0].fY = closerY;
+            }
+            if (!between(startY, dst[0].fPts[1].fY, dst[0].fPts[2].fY)) {
+                // If the 1st control is not between the start and end, put it at the start.
+                // This also reduces the quad to a line.
+                dst[0].fPts[1].fY = startY;
+            }
+            if (!between(dst[1].fPts[0].fY, dst[1].fPts[1].fY, endY)) {
+                // If the 2nd control is not between the start and end, put it at the end.
+                // This also reduces the quad to a line.
+                dst[1].fPts[1].fY = endY;
+            }
+            // Verify that all five points are in order.
+            SkASSERT(between(startY, dst[0].fPts[1].fY, dst[0].fPts[2].fY));
+            SkASSERT(between(dst[0].fPts[1].fY, dst[0].fPts[2].fY, dst[1].fPts[1].fY));
+            SkASSERT(between(dst[0].fPts[2].fY, dst[1].fPts[1].fY, endY));
+        }
         --level;
         pts = subdivide(dst[0], pts, level);
         return subdivide(dst[1], pts, level);