When solving the cubic line intersection directly fails, use binary search as a fallback.

The cubic line intersection math empirically works 99.99% of the time (fails 3100 out of 1B random tests) but when it fails, an intersection may be missed altogether.

The binary search is may not find a solution if the cubic line failed to find any solutions at all, but so far that case hasn't arisen.

BUG=skia:2504
TBR=reed@google.com

Author: caryclark@google.com

Review URL: https://codereview.chromium.org/266063003

git-svn-id: http://skia.googlecode.com/svn/trunk@14614 2bbb7eff-a529-9590-31e7-b0007b416f81

11 files changed, 538 insertions, 37 deletions

diff --git a/gyp/pathops_unittest.gyp b/gyp/pathops_unittest.gyp
index 51af1e9ac5..35eeabd209 100644
--- a/gyp/pathops_unittest.gyp
+++ b/gyp/pathops_unittest.gyp
@@ -23,6 +23,7 @@
       ],
       'sources': [
         '../tests/PathOpsAngleIdeas.cpp',
+        '../tests/PathOpsCubicLineIntersectionIdeas.cpp',
         '../tests/PathOpsDebug.cpp',
         '../tests/PathOpsOpLoopThreadedTest.cpp',
         '../tests/PathOpsSkpClipTest.cpp',
diff --git a/src/pathops/SkDCubicLineIntersection.cpp b/src/pathops/SkDCubicLineIntersection.cpp
index da4b983d45..41828bb714 100644
--- a/src/pathops/SkDCubicLineIntersection.cpp
+++ b/src/pathops/SkDCubicLineIntersection.cpp
@@ -97,13 +97,27 @@ public:
     int intersectRay(double roots[3]) {
         double adj = fLine[1].fX - fLine[0].fX;
         double opp = fLine[1].fY - fLine[0].fY;
-        SkDCubic r;
+        SkDCubic c;
         for (int n = 0; n < 4; ++n) {
-            r[n].fX = (fCubic[n].fY - fLine[0].fY) * adj - (fCubic[n].fX - fLine[0].fX) * opp;
+            c[n].fX = (fCubic[n].fY - fLine[0].fY) * adj - (fCubic[n].fX - fLine[0].fX) * opp;
         }
         double A, B, C, D;
-        SkDCubic::Coefficients(&r[0].fX, &A, &B, &C, &D);
-        return SkDCubic::RootsValidT(A, B, C, D, roots);
+        SkDCubic::Coefficients(&c[0].fX, &A, &B, &C, &D);
+        int count = SkDCubic::RootsValidT(A, B, C, D, roots);
+        for (int index = 0; index < count; ++index) {
+            SkDPoint calcPt = c.ptAtT(roots[index]);
+            if (!approximately_zero(calcPt.fX)) {
+                for (int n = 0; n < 4; ++n) {
+                    c[n].fY = (fCubic[n].fY - fLine[0].fY) * opp
+                            + (fCubic[n].fX - fLine[0].fX) * adj;
+                }
+                double extremeTs[6];
+                int extrema = SkDCubic::FindExtrema(c[0].fX, c[1].fX, c[2].fX, c[3].fX, extremeTs);
+                count = c.searchRoots(extremeTs, extrema, 0, SkDCubic::kXAxis, roots);
+                break;
+            }
+        }
+        return count;
     }
 
     int intersect() {
@@ -146,11 +160,21 @@ public:
         return fIntersections->used();
     }
 
-    int horizontalIntersect(double axisIntercept, double roots[3]) {
+    static int HorizontalIntersect(const SkDCubic& c, double axisIntercept, double roots[3]) {
         double A, B, C, D;
-        SkDCubic::Coefficients(&fCubic[0].fY, &A, &B, &C, &D);
+        SkDCubic::Coefficients(&c[0].fY, &A, &B, &C, &D);
         D -= axisIntercept;
-        return SkDCubic::RootsValidT(A, B, C, D, roots);
+        int count = SkDCubic::RootsValidT(A, B, C, D, roots);
+        for (int index = 0; index < count; ++index) {
+            SkDPoint calcPt = c.ptAtT(roots[index]);
+            if (!approximately_equal(calcPt.fY, axisIntercept)) {
+                double extremeTs[6];
+                int extrema = SkDCubic::FindExtrema(c[0].fY, c[1].fY, c[2].fY, c[3].fY, extremeTs);
+                count = c.searchRoots(extremeTs, extrema, axisIntercept, SkDCubic::kYAxis, roots);
+                break;
+            }
+        }
+        return count;
     }
 
     int horizontalIntersect(double axisIntercept, double left, double right, bool flipped) {
@@ -158,11 +182,13 @@ public:
         if (fAllowNear) {
             addNearHorizontalEndPoints(left, right, axisIntercept);
         }
-        double rootVals[3];
-        int roots = horizontalIntersect(axisIntercept, rootVals);
-        for (int index = 0; index < roots; ++index) {
-            double cubicT = rootVals[index];
-            SkDPoint pt = fCubic.ptAtT(cubicT);
+        double roots[3];
+        int count = HorizontalIntersect(fCubic, axisIntercept, roots);
+        for (int index = 0; index < count; ++index) {
+            double cubicT = roots[index];
+            SkDPoint pt;
+            pt.fX = fCubic.ptAtT(cubicT).fX;
+            pt.fY = axisIntercept;
             double lineT = (pt.fX - left) / (right - left);
             if (pinTs(&cubicT, &lineT, &pt, kPointInitialized)) {
                 fIntersections->insert(cubicT, lineT, pt);
@@ -174,11 +200,21 @@ public:
         return fIntersections->used();
     }
 
-    int verticalIntersect(double axisIntercept, double roots[3]) {
+    static int VerticalIntersect(const SkDCubic& c, double axisIntercept, double roots[3]) {
         double A, B, C, D;
-        SkDCubic::Coefficients(&fCubic[0].fX, &A, &B, &C, &D);
+        SkDCubic::Coefficients(&c[0].fX, &A, &B, &C, &D);
         D -= axisIntercept;
-        return SkDCubic::RootsValidT(A, B, C, D, roots);
+        int count = SkDCubic::RootsValidT(A, B, C, D, roots);
+        for (int index = 0; index < count; ++index) {
+            SkDPoint calcPt = c.ptAtT(roots[index]);
+            if (!approximately_equal(calcPt.fX, axisIntercept)) {
+                double extremeTs[6];
+                int extrema = SkDCubic::FindExtrema(c[0].fX, c[1].fX, c[2].fX, c[3].fX, extremeTs);
+                count = c.searchRoots(extremeTs, extrema, axisIntercept, SkDCubic::kXAxis, roots);
+                break;
+            }
+        }
+        return count;
     }
 
     int verticalIntersect(double axisIntercept, double top, double bottom, bool flipped) {
@@ -186,11 +222,13 @@ public:
         if (fAllowNear) {
             addNearVerticalEndPoints(top, bottom, axisIntercept);
         }
-        double rootVals[3];
-        int roots = verticalIntersect(axisIntercept, rootVals);
-        for (int index = 0; index < roots; ++index) {
-            double cubicT = rootVals[index];
-            SkDPoint pt = fCubic.ptAtT(cubicT);
+        double roots[3];
+        int count = VerticalIntersect(fCubic, axisIntercept, roots);
+        for (int index = 0; index < count; ++index) {
+            double cubicT = roots[index];
+            SkDPoint pt;
+            pt.fX = axisIntercept;
+            pt.fY = fCubic.ptAtT(cubicT).fY;
             double lineT = (pt.fY - top) / (bottom - top);
             if (pinTs(&cubicT, &lineT, &pt, kPointInitialized)) {
                 fIntersections->insert(cubicT, lineT, pt);
diff --git a/src/pathops/SkPathOpsCubic.cpp b/src/pathops/SkPathOpsCubic.cpp
index a89604f94c..9d70d58ec1 100644
--- a/src/pathops/SkPathOpsCubic.cpp
+++ b/src/pathops/SkPathOpsCubic.cpp
@@ -9,9 +9,58 @@
 #include "SkPathOpsLine.h"
 #include "SkPathOpsQuad.h"
 #include "SkPathOpsRect.h"
+#include "SkTSort.h"
 
 const int SkDCubic::gPrecisionUnit = 256;  // FIXME: test different values in test framework
 
+// give up when changing t no longer moves point
+// also, copy point rather than recompute it when it does change
+double SkDCubic::binarySearch(double min, double max, double axisIntercept,
+        SearchAxis xAxis) const {
+    double t = (min + max) / 2;
+    double step = (t - min) / 2;
+    SkDPoint cubicAtT = ptAtT(t);
+    double calcPos = (&cubicAtT.fX)[xAxis];
+    double calcDist = calcPos - axisIntercept;
+    do {
+        double priorT = t - step;
+        SkASSERT(priorT >= min);
+        SkDPoint lessPt = ptAtT(priorT);
+        if (approximately_equal(lessPt.fX, cubicAtT.fX)
+                && approximately_equal(lessPt.fY, cubicAtT.fY)) {
+            return -1;  // binary search found no point at this axis intercept
+        }
+        double lessDist = (&lessPt.fX)[xAxis] - axisIntercept;
+#if DEBUG_CUBIC_BINARY_SEARCH
+        SkDebugf("t=%1.9g calc=%1.9g dist=%1.9g step=%1.9g less=%1.9g\n", t, calcPos, calcDist,
+                step, lessDist);
+#endif
+        double lastStep = step;
+        step /= 2;
+        if (calcDist > 0 ? calcDist > lessDist : calcDist < lessDist) {
+            t = priorT;
+        } else {
+            double nextT = t + lastStep;
+            SkASSERT(nextT <= max);
+            SkDPoint morePt = ptAtT(nextT);
+            if (approximately_equal(morePt.fX, cubicAtT.fX)
+                    && approximately_equal(morePt.fY, cubicAtT.fY)) {
+                return -1;  // binary search found no point at this axis intercept
+            }
+            double moreDist = (&morePt.fX)[xAxis] - axisIntercept;
+            if (calcDist > 0 ? calcDist <= moreDist : calcDist >= moreDist) {
+                continue;
+            }
+            t = nextT;
+        }
+        SkDPoint testAtT = ptAtT(t);
+        cubicAtT = testAtT;
+        calcPos = (&cubicAtT.fX)[xAxis];
+        calcDist = calcPos - axisIntercept;
+    } while (!approximately_equal(calcPos, axisIntercept));
+    return t;
+}
+
 // FIXME: cache keep the bounds and/or precision with the caller?
 double SkDCubic::calcPrecision() const {
     SkDRect dRect;
@@ -93,6 +142,27 @@ bool SkDCubic::monotonicInY() const {
             && between(fPts[0].fY, fPts[2].fY, fPts[3].fY);
 }
 
+int SkDCubic::searchRoots(double extremeTs[6], int extrema, double axisIntercept,
+        SearchAxis xAxis, double* validRoots) const {
+    extrema += findInflections(&extremeTs[extrema]);
+    extremeTs[extrema++] = 0;
+    extremeTs[extrema] = 1;
+    SkTQSort(extremeTs, extremeTs + extrema);
+    int validCount = 0;
+    for (int index = 0; index < extrema; ) {
+        double min = extremeTs[index];
+        double max = extremeTs[++index];
+        if (min == max) {
+            continue;
+        }
+        double newT = binarySearch(min, max, axisIntercept, xAxis);
+        if (newT >= 0) {
+            validRoots[validCount++] = newT;
+        }
+    }
+    return validCount;
+}
+
 bool SkDCubic::serpentine() const {
 #if 0  // FIXME: enabling this fixes cubicOp114 but breaks cubicOp58d and cubicOp53d
     double tValues[2];
@@ -210,7 +280,7 @@ int SkDCubic::RootsReal(double A, double B, double C, double D, double s[3]) {
         }
         r = A - adiv3;
         *roots++ = r;
-        if (AlmostDequalUlps(R2, Q3)) {
+        if (AlmostDequalUlps((double) R2, (double) Q3)) {
             r = -A / 2 - adiv3;
             if (!AlmostDequalUlps(s[0], r)) {
                 *roots++ = r;
diff --git a/src/pathops/SkPathOpsCubic.h b/src/pathops/SkPathOpsCubic.h
index 500319607d..1037cae4f7 100644
--- a/src/pathops/SkPathOpsCubic.h
+++ b/src/pathops/SkPathOpsCubic.h
@@ -19,21 +19,16 @@ struct SkDCubicPair {
 };
 
 struct SkDCubic {
-    SkDPoint fPts[4];
-
-    void set(const SkPoint pts[4]) {
-        fPts[0] = pts[0];
-        fPts[1] = pts[1];
-        fPts[2] = pts[2];
-        fPts[3] = pts[3];
-    }
-
-    static const int gPrecisionUnit;
+    enum SearchAxis {
+        kXAxis,
+        kYAxis
+    };
 
     const SkDPoint& operator[](int n) const { SkASSERT(n >= 0 && n < 4); return fPts[n]; }
     SkDPoint& operator[](int n) { SkASSERT(n >= 0 && n < 4); return fPts[n]; }
 
     void align(int endIndex, int ctrlIndex, SkDPoint* dstPt) const;
+    double binarySearch(double min, double max, double axisIntercept, SearchAxis xAxis) const;
     double calcPrecision() const;
     SkDCubicPair chopAt(double t) const;
     bool clockwise() const;
@@ -42,9 +37,9 @@ struct SkDCubic {
     SkDVector dxdyAtT(double t) const;
     bool endsAreExtremaInXOrY() const;
     static int FindExtrema(double a, double b, double c, double d, double tValue[2]);
-    int findInflections(double tValues[]) const;
+    int findInflections(double tValues[2]) const;
 
-    static int FindInflections(const SkPoint a[4], double tValues[]) {
+    static int FindInflections(const SkPoint a[4], double tValues[2]) {
         SkDCubic cubic;
         cubic.set(a);
         return cubic.findInflections(tValues);
@@ -56,7 +51,18 @@ struct SkDCubic {
     SkDPoint ptAtT(double t) const;
     static int RootsReal(double A, double B, double C, double D, double t[3]);
     static int RootsValidT(const double A, const double B, const double C, double D, double s[3]);
+
+    int searchRoots(double extremes[6], int extrema, double axisIntercept,
+                    SearchAxis xAxis, double* validRoots) const;
     bool serpentine() const;
+
+    void set(const SkPoint pts[4]) {
+        fPts[0] = pts[0];
+        fPts[1] = pts[1];
+        fPts[2] = pts[2];
+        fPts[3] = pts[3];
+    }
+
     SkDCubic subDivide(double t1, double t2) const;
 
     static SkDCubic SubDivide(const SkPoint a[4], double t1, double t2) {
@@ -81,6 +87,10 @@ struct SkDCubic {
     // utilities callable by the user from the debugger when the implementation code is linked in
     void dump() const;
     void dumpNumber() const;
+
+    static const int gPrecisionUnit;
+
+    SkDPoint fPts[4];
 };
 
 #endif
diff --git a/src/pathops/SkPathOpsDebug.h b/src/pathops/SkPathOpsDebug.h
index 39d5a6dda8..bb54039244 100644
--- a/src/pathops/SkPathOpsDebug.h
+++ b/src/pathops/SkPathOpsDebug.h
@@ -50,6 +50,7 @@
 #define DEBUG_CHECK_TINY 0
 #define DEBUG_CONCIDENT 0
 #define DEBUG_CROSS 0
+#define DEBUG_CUBIC_BINARY_SEARCH 0
 #define DEBUG_FLAT_QUADS 0
 #define DEBUG_FLOW 0
 #define DEBUG_LIMIT_WIND_SUM 0
@@ -84,6 +85,7 @@
 #define DEBUG_CHECK_TINY 1
 #define DEBUG_CONCIDENT 1
 #define DEBUG_CROSS 01
+#define DEBUG_CUBIC_BINARY_SEARCH 1
 #define DEBUG_FLAT_QUADS 0
 #define DEBUG_FLOW 1
 #define DEBUG_LIMIT_WIND_SUM 4
diff --git a/src/pathops/SkPathOpsTypes.cpp b/src/pathops/SkPathOpsTypes.cpp
index dbed086fbd..2c8d778c69 100644
--- a/src/pathops/SkPathOpsTypes.cpp
+++ b/src/pathops/SkPathOpsTypes.cpp
@@ -102,6 +102,13 @@ bool AlmostDequalUlps(float a, float b) {
     return d_equal_ulps(a, b, UlpsEpsilon);
 }
 
+bool AlmostDequalUlps(double a, double b) {
+    if (SkScalarIsFinite(a) || SkScalarIsFinite(b)) {
+        return AlmostDequalUlps(SkDoubleToScalar(a), SkDoubleToScalar(b));
+    }
+    return fabs(a - b) / SkTMax(fabs(a), fabs(b)) < FLT_EPSILON * 16;
+}
+
 bool AlmostEqualUlps(float a, float b) {
     const int UlpsEpsilon = 16;
     return equal_ulps(a, b, UlpsEpsilon, UlpsEpsilon);
diff --git a/src/pathops/SkPathOpsTypes.h b/src/pathops/SkPathOpsTypes.h
index e8054ad476..4f8bd15344 100644
--- a/src/pathops/SkPathOpsTypes.h
+++ b/src/pathops/SkPathOpsTypes.h
@@ -30,9 +30,7 @@ inline bool AlmostEqualUlps(double a, double b) {
 
 // Use Almost Dequal when comparing should not special case denormalized values.
 bool AlmostDequalUlps(float a, float b);
-inline bool AlmostDequalUlps(double a, double b) {
-    return AlmostDequalUlps(SkDoubleToScalar(a), SkDoubleToScalar(b));
-}
+bool AlmostDequalUlps(double a, double b);
 
 bool NotAlmostEqualUlps(float a, float b);
 inline bool NotAlmostEqualUlps(double a, double b) {
diff --git a/tests/PathOpsCubicLineIntersectionIdeas.cpp b/tests/PathOpsCubicLineIntersectionIdeas.cpp
new file mode 100644
index 0000000000..2887a2ccfc
--- /dev/null
+++ b/tests/PathOpsCubicLineIntersectionIdeas.cpp
@@ -0,0 +1,283 @@
+/*
+ * 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 "PathOpsTestCommon.h"
+#include "SkIntersections.h"
+#include "SkPathOpsCubic.h"
+#include "SkPathOpsLine.h"
+#include "SkPathOpsQuad.h"
+#include "SkRandom.h"
+#include "SkReduceOrder.h"
+#include "Test.h"
+
+static bool gPathOpsCubicLineIntersectionIdeasVerbose = false;
+
+static struct CubicLineFailures {
+    SkDCubic c;
+    double t;
+    SkDPoint p;
+} cubicLineFailures[] = {
+    {{{{-164.3726806640625, 36.826904296875}, {-189.045166015625, -953.2220458984375},
+        {926.505859375, -897.36175537109375}, {-139.33489990234375, 204.40771484375}}},
+        0.37329583, {107.54935269006289, -632.13736293162208}},
+    {{{{784.056884765625, -554.8350830078125}, {67.5489501953125, 509.0224609375},
+        {-447.713134765625, 751.375}, {415.7784423828125, 172.22265625}}},
+        0.660005242, {-32.973148967736151, 478.01341797403569}},
+    {{{{-580.6834716796875, -127.044921875}, {-872.8983154296875, -945.54302978515625},
+        {260.8092041015625, -909.34991455078125}, {-976.2125244140625, -18.46551513671875}}},
+        0.578826774, {-390.17910153915489, -687.21144412296007}},
+};
+
+int cubicLineFailuresCount = (int) SK_ARRAY_COUNT(cubicLineFailures);
+
+double measuredSteps[] = {
+    9.15910731e-007, 8.6600277e-007, 7.4122059e-007, 6.92087618e-007, 8.35290245e-007,
+    3.29763199e-007, 5.07547773e-007, 4.41294224e-007, 0, 0,
+    3.76879167e-006, 1.06126249e-006, 2.36873967e-006, 1.62421134e-005, 3.09103599e-005,
+    4.38917976e-005, 0.000112348938, 0.000243149242, 0.000433174114, 0.00170880232,
+    0.00272619724, 0.00518844604, 0.000352621078, 0.00175960064, 0.027875185,
+    0.0351329803, 0.103964925,
+};
+
+/* last output : errors=3121 
+    9.1796875e-007 8.59375e-007 7.5e-007 6.875e-007 8.4375e-007
+    3.125e-007 5e-007 4.375e-007 0 0
+    3.75e-006 1.09375e-006 2.1875e-006 1.640625e-005 3.0859375e-005
+    4.38964844e-005 0.000112304687 0.000243164063 0.000433181763 0.00170898437
+    0.00272619247 0.00518844604 0.000352621078 0.00175960064 0.027875185
+    0.0351329803 0.103964925
+*/
+
+static double binary_search(const SkDCubic& cubic, double step, const SkDPoint& pt, double t,
+        int* iters) {
+    double firstStep = step;
+    do {
+        *iters += 1;
+        SkDPoint cubicAtT = cubic.ptAtT(t);
+        if (cubicAtT.approximatelyEqual(pt)) {
+            break;
+        }
+        double calcX = cubicAtT.fX - pt.fX;
+        double calcY = cubicAtT.fY - pt.fY;
+        double calcDist = calcX * calcX + calcY * calcY;
+        if (step == 0) {
+            SkDebugf("binary search failed: step=%1.9g cubic=", firstStep);
+            cubic.dump();
+            SkDebugf(" t=%1.9g ", t);
+            pt.dump();
+            SkDebugf("\n");
+            return -1;
+        }
+        double lastStep = step;
+        step /= 2;
+        SkDPoint lessPt = cubic.ptAtT(t - lastStep);
+        double lessX = lessPt.fX - pt.fX;
+        double lessY = lessPt.fY - pt.fY;
+        double lessDist = lessX * lessX + lessY * lessY;
+        // use larger x/y difference to choose step
+        if (calcDist > lessDist) {
+            t -= step;
+            t = SkTMax(0., t);
+        } else {
+            SkDPoint morePt = cubic.ptAtT(t + lastStep);
+            double moreX = morePt.fX - pt.fX;
+            double moreY = morePt.fY - pt.fY;
+            double moreDist = moreX * moreX + moreY * moreY;
+            if (calcDist <= moreDist) {
+                continue;
+            }
+            t += step;
+            t = SkTMin(1., t);
+        }
+    } while (true);
+    return t;
+}
+
+#if 0
+static bool r2check(double A, double B, double C, double D, double* R2MinusQ3Ptr) {
+    if (approximately_zero(A)
+            && approximately_zero_when_compared_to(A, B)
+            && approximately_zero_when_compared_to(A, C)
+            && approximately_zero_when_compared_to(A, D)) {  // we're just a quadratic
+        return false;
+    }
+    if (approximately_zero_when_compared_to(D, A)
+            && approximately_zero_when_compared_to(D, B)
+            && approximately_zero_when_compared_to(D, C)) {  // 0 is one root
+        return false;
+    }
+    if (approximately_zero(A + B + C + D)) {  // 1 is one root
+        return false;
+    }
+    double a, b, c;
+    {
+        double invA = 1 / A;
+        a = B * invA;
+        b = C * invA;
+        c = D * invA;
+    }
+    double a2 = a * a;
+    double Q = (a2 - b * 3) / 9;
+    double R = (2 * a2 * a - 9 * a * b + 27 * c) / 54;
+    double R2 = R * R;
+    double Q3 = Q * Q * Q;
+    double R2MinusQ3 = R2 - Q3;
+    *R2MinusQ3Ptr = R2MinusQ3;
+    return true;
+}
+#endif
+
+/* What is the relationship between the accuracy of the root in range and the magnitude of all
+   roots? To find out, create a bunch of cubics, and measure */
+
+DEF_TEST(PathOpsCubicLineRoots, reporter) {
+    if (!gPathOpsCubicLineIntersectionIdeasVerbose) {  // slow; exclude it by default
+        return;
+    }
+    SkRandom ran;
+    double worstStep[256] = {0};
+    int errors = 0;
+    int iters = 0;
+    double smallestR2 = 0;
+    double largestR2 = 0;
+    for (int index = 0; index < 1000000000; ++index) {
+        SkDPoint origin = {ran.nextRangeF(-1000, 1000), ran.nextRangeF(-1000, 1000)};
+        SkDCubic cubic = {{origin,
+                {ran.nextRangeF(-1000, 1000), ran.nextRangeF(-1000, 1000)},
+                {ran.nextRangeF(-1000, 1000), ran.nextRangeF(-1000, 1000)},
+                {ran.nextRangeF(-1000, 1000), ran.nextRangeF(-1000, 1000)}
+        }};
+        // construct a line at a known intersection
+        double t = ran.nextRangeF(0, 1);
+        SkDPoint pt = cubic.ptAtT(t);
+        // skip answers with no intersections (although note the bug!) or two, or more
+        // see if the line / cubic has a fun range of roots
+        double A, B, C, D;
+        SkDCubic::Coefficients(&cubic[0].fY, &A, &B, &C, &D);
+        D -= pt.fY;
+        double allRoots[3] = {0}, validRoots[3] = {0};
+        int realRoots = SkDCubic::RootsReal(A, B, C, D, allRoots);
+        int valid = SkDQuad::AddValidTs(allRoots, realRoots, validRoots);
+        if (valid != 1) {
+            continue;
+        }
+        if (realRoots == 1) {
+            continue;
+        }
+        t = validRoots[0];
+        SkDPoint calcPt = cubic.ptAtT(t);
+        if (calcPt.approximatelyEqual(pt)) {
+            continue;
+        }
+#if 0
+        double R2MinusQ3;
+        if (r2check(A, B, C, D, &R2MinusQ3)) {
+            smallestR2 = SkTMin(smallestR2, R2MinusQ3);
+            largestR2 = SkTMax(largestR2, R2MinusQ3);
+        }
+#endif
+        double largest = SkTMax(fabs(allRoots[0]), fabs(allRoots[1]));
+        if (realRoots == 3) {
+            largest = SkTMax(largest, fabs(allRoots[2]));
+        }
+        int largeBits;
+        if (largest <= 1) {
+#if 0
+            SkDebugf("realRoots=%d (%1.9g, %1.9g, %1.9g) valid=%d (%1.9g, %1.9g, %1.9g)\n",
+                realRoots, allRoots[0], allRoots[1], allRoots[2], valid, validRoots[0],
+                validRoots[1], validRoots[2]);
+#endif
+            double smallest = SkTMin(allRoots[0], allRoots[1]);
+            if (realRoots == 3) {
+                smallest = SkTMin(smallest, allRoots[2]);
+            }
+            SK_ALWAYSBREAK(smallest < 0);
+            SK_ALWAYSBREAK(smallest >= -1);
+            largeBits = 0;
+        } else {
+            frexp(largest, &largeBits);
+            SK_ALWAYSBREAK(largeBits >= 0);
+            SK_ALWAYSBREAK(largeBits < 256);
+        }
+        double step = 1e-6;
+        if (largeBits > 21) {
+            step = 1e-1;
+        } else if (largeBits > 18) {
+            step = 1e-2;
+        } else if (largeBits > 15) {
+            step = 1e-3;
+        } else if (largeBits > 12) {
+            step = 1e-4;
+        } else if (largeBits > 9) {
+            step = 1e-5;
+        }
+        double diff;
+        do {
+            double newT = binary_search(cubic, step, pt, t, &iters);
+            if (newT >= 0) {
+                diff = fabs(t - newT);
+                break;
+            }
+            step *= 1.5;
+            SK_ALWAYSBREAK(step < 1);
+        } while (true);
+        worstStep[largeBits] = SkTMax(worstStep[largeBits], diff);
+#if 0
+        {
+            cubic.dump();
+            SkDebugf("\n");
+            SkDLine line = {{{pt.fX - 1, pt.fY}, {pt.fX + 1, pt.fY}}};
+            line.dump();
+            SkDebugf("\n");
+        }
+#endif
+        ++errors;
+    }
+    SkDebugf("errors=%d avgIter=%1.9g", errors, (double) iters / errors);
+    SkDebugf(" steps: ");
+    int worstLimit = SK_ARRAY_COUNT(worstStep);
+    while (worstStep[--worstLimit] == 0) ;
+    for (int idx2 = 0; idx2 <= worstLimit; ++idx2) {
+        SkDebugf("%1.9g ", worstStep[idx2]);
+    }
+    SkDebugf("\n");
+    SkDebugf("smallestR2=%1.9g largestR2=%1.9g\n", smallestR2, largestR2);
+}
+
+static double testOneFailure(const CubicLineFailures& failure) {
+    const SkDCubic& cubic = failure.c;
+    const SkDPoint& pt = failure.p;
+    double A, B, C, D;
+    SkDCubic::Coefficients(&cubic[0].fY, &A, &B, &C, &D);
+    D -= pt.fY;
+    double allRoots[3] = {0}, validRoots[3] = {0};
+    int realRoots = SkDCubic::RootsReal(A, B, C, D, allRoots);
+    int valid = SkDQuad::AddValidTs(allRoots, realRoots, validRoots);
+    SK_ALWAYSBREAK(valid == 1);
+    SK_ALWAYSBREAK(realRoots != 1);
+    double t = validRoots[0];
+    SkDPoint calcPt = cubic.ptAtT(t);
+    SK_ALWAYSBREAK(!calcPt.approximatelyEqual(pt));
+    int iters = 0;
+    double newT = binary_search(cubic, 0.1, pt, t, &iters);
+    return newT;
+}
+
+DEF_TEST(PathOpsCubicLineFailures, reporter) {
+    return;  // disable for now
+    for (int index = 0; index < cubicLineFailuresCount; ++index) {
+        const CubicLineFailures& failure = cubicLineFailures[index];
+        double newT = testOneFailure(failure);
+        SK_ALWAYSBREAK(newT >= 0);
+    }
+}
+
+DEF_TEST(PathOpsCubicLineOneFailure, reporter) {
+    return;  // disable for now
+    const CubicLineFailures& failure = cubicLineFailures[1];
+    double newT = testOneFailure(failure);
+    SK_ALWAYSBREAK(newT >= 0);
+}
diff --git a/tests/PathOpsCubicLineIntersectionTest.cpp b/tests/PathOpsCubicLineIntersectionTest.cpp
index 1a2e188589..234a53805f 100644
--- a/tests/PathOpsCubicLineIntersectionTest.cpp
+++ b/tests/PathOpsCubicLineIntersectionTest.cpp
@@ -49,6 +49,13 @@ static void testFail(skiatest::Reporter* reporter, int iIndex) {
 }
 
 static lineCubic lineCubicTests[] = {
+    {{{{-634.60540771484375, -481.262939453125}, {266.2696533203125, -752.70867919921875},
+            {-751.8370361328125, -317.37921142578125}, {-969.7427978515625, 824.7255859375}}},
+            {{{-287.9506133720805678, -557.1376476615772617},
+            {-285.9506133720805678, -557.1376476615772617}}}},
+
+    {{{{36.7184372,0.888650894}, {36.7184372,0.888650894}, {35.1233864,0.554015458},
+            {34.5114098,-0.115255356}}}, {{{35.4531212,0}, {31.9375,0}}}},
 
     {{{{421, 378}, {421, 380.209137f}, {418.761414f, 382}, {416, 382}}},
             {{{320, 378}, {421, 378.000031f}}}},
@@ -83,6 +90,32 @@ static lineCubic lineCubicTests[] = {
 
 static const size_t lineCubicTests_count = SK_ARRAY_COUNT(lineCubicTests);
 
+static int doIntersect(SkIntersections& intersections, const SkDCubic& cubic, const SkDLine& line) {
+    int result;
+    bool flipped = false;
+    if (line[0].fX == line[1].fX) {
+        double top = line[0].fY;
+        double bottom = line[1].fY;
+        flipped = top > bottom;
+        if (flipped) {
+            SkTSwap<double>(top, bottom);
+        }
+        result = intersections.vertical(cubic, top, bottom, line[0].fX, flipped);
+    } else if (line[0].fY == line[1].fY) {
+        double left = line[0].fX;
+        double right = line[1].fX;
+        flipped = left > right;
+        if (flipped) {
+            SkTSwap<double>(left, right);
+        }
+        result = intersections.horizontal(cubic, left, right, line[0].fY, flipped);
+    } else {
+        intersections.intersect(cubic, line);
+        result = intersections.used();
+    }
+    return result;
+}
+
 static void testOne(skiatest::Reporter* reporter, int iIndex) {
     const SkDCubic& cubic = lineCubicTests[iIndex].cubic;
     SkASSERT(ValidCubic(cubic));
@@ -102,7 +135,7 @@ static void testOne(skiatest::Reporter* reporter, int iIndex) {
     }
     if (order1 == 4 && order2 == 2) {
         SkIntersections i;
-        int roots = i.intersect(cubic, line);
+        int roots = doIntersect(i, cubic, line);
         for (int pt = 0; pt < roots; ++pt) {
             double tt1 = i[0][pt];
             SkDPoint xy1 = cubic.ptAtT(tt1);
diff --git a/tests/PathOpsOpTest.cpp b/tests/PathOpsOpTest.cpp
index 86baea423c..75b6030aa5 100644
--- a/tests/PathOpsOpTest.cpp
+++ b/tests/PathOpsOpTest.cpp
@@ -3329,10 +3329,30 @@ static void kari1(skiatest::Reporter* reporter, const char* filename) {
     testPathOp(reporter, path1, path2, kDifference_PathOp, filename);
 }
 
+static void issue2504(skiatest::Reporter* reporter, const char* filename) {
+    SkPath path1;
+    path1.moveTo(34.2421875, -5.976562976837158203125);
+    path1.lineTo(35.453121185302734375, 0);
+    path1.lineTo(31.9375, 0);
+    path1.close();
+
+    SkPath path2;
+    path2.moveTo(36.71843719482421875, 0.8886508941650390625);
+    path2.cubicTo(36.71843719482421875, 0.8886508941650390625,
+                  35.123386383056640625, 0.554015457630157470703125,
+                  34.511409759521484375, -0.1152553558349609375);
+    path2.cubicTo(33.899425506591796875, -0.7845261096954345703125,
+                  34.53484344482421875, -5.6777553558349609375,
+                  34.53484344482421875, -5.6777553558349609375);
+    path2.close();
+    testPathOp(reporter, path1, path2, kUnion_PathOp, filename);
+}
+
 static void (*firstTest)(skiatest::Reporter* , const char* filename) = 0;
 static void (*stopTest)(skiatest::Reporter* , const char* filename) = 0;
 
 static struct TestDesc tests[] = {
+    TEST(issue2504),
     TEST(kari1),
     TEST(quadOp10i),
 #if 0  // FIXME: serpentine curve is ordered the wrong way
diff --git a/tools/pathops_sorter.htm b/tools/pathops_sorter.htm
index 59ae2b6d90..41314f62a1 100644
--- a/tools/pathops_sorter.htm
+++ b/tools/pathops_sorter.htm
@@ -867,11 +867,50 @@ op intersect
 {{{900.0235595703125, 551.60284423828125}, {900.06072998046875, 551.29705810546875}, {900.15655517578125, 551.0157470703125}}}
 </div>
 
+<div id="cubicLineMiss1">
+{{{-634.60540771484375, -481.262939453125}, {266.2696533203125, -752.70867919921875}, {-751.8370361328125, -317.37921142578125}, {-969.7427978515625, 824.7255859375}}}
+{{{-287.9506133720805678, -557.1376476615772617}, {-285.9506133720805678, -557.1376476615772617}}}
+</div>
+
+<div id="cubicLineMiss2">
+{{{-818.4456787109375, 248.218017578125}, {944.18505859375, -252.2330322265625}, {957.3946533203125, -45.43280029296875}, {-591.766357421875, 868.6187744140625}}}
+{{{435.1963493079119871, -16.42683763243891093}, {437.1963493079119871, -16.42683763243891093}}}
+</div>
+
+<div id="cubicLineMiss3">
+{{{-818.4456787109375, 248.218017578125}, {944.18505859375, -252.2330322265625}, {957.3946533203125, -45.43280029296875}, {-591.766357421875, 868.6187744140625}}}
+{{{397.5007682490800676, -17.35020084021140008}, {399.5007682490800676, -17.35020084021140008}}}
+</div>
+
+<div id="cubicLineMiss4">
+{{{-652.660888671875, -384.6475830078125}, {-551.7723388671875, -925.5025634765625}, {-321.06658935546875, -813.10345458984375}, {142.6982421875, -47.4503173828125}}}
+{{{-478.4372049758064236, -717.868282575075682}, {-476.4372049758064236, -717.868282575075682}}}
+</div>
+
+<div id="cubicLineErr1">
+{{{-954.4322509765625, 827.2216796875}, {-420.24017333984375, -7.80560302734375}, {799.134765625, -971.4295654296875}, {-556.23486328125, 344.400146484375}}}
+
+{{{58.57411390280688579, -302.8879316712078662}, {60.57411390280688579, -302.8879316712078662}}}
+</div>
+
+<div id="cubicLineErr2">
+{{{-634.60540771484375, -481.262939453125}, {266.2696533203125, -752.70867919921875}, {-751.8370361328125, -317.37921142578125}, {-969.7427978515625, 824.7255859375}}}
+{{{-287.95061337208057, -557.13764766157726}, {-285.95061337208057, -557.13764766157726}}}
+{{{-308.65463091760211, -549.4520029924679} -308.65463091760211, -569.4520029924679
+</div>
+
+
 </div>
 
 <script type="text/javascript">
 
     var testDivs = [
+        cubicLineErr2,
+        cubicLineErr1,
+        cubicLineMiss1,
+        cubicLineMiss2,
+        cubicLineMiss3,
+        cubicLineMiss4,
         skpwww_pindosiya_com_99,
         self1,
         skpwww_seopack_blogspot_com_2153,