From 160f2c0e1724003767dbbea05bc8046ac5dd78d4 Mon Sep 17 00:00:00 2001
From: "reed@google.com" <reed@google.com@2bbb7eff-a529-9590-31e7-b0007b416f81>
Date: Mon, 21 Mar 2011 20:33:42 +0000
Subject: [PATCH] increase intermediate precision when chopping lines. This
 avoids returning a computed value that is accidentally outside of the
 original range.

git-svn-id: http://skia.googlecode.com/svn/trunk@971 2bbb7eff-a529-9590-31e7-b0007b416f81
---
 src/core/SkLineClipper.cpp | 43 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 41 insertions(+), 2 deletions(-)

diff --git a/src/core/SkLineClipper.cpp b/src/core/SkLineClipper.cpp
index 057f546d77..ab497731cc 100644
--- a/src/core/SkLineClipper.cpp
+++ b/src/core/SkLineClipper.cpp
@@ -6,8 +6,19 @@ static SkScalar sect_with_horizontal(const SkPoint src[2], SkScalar Y) {
     if (SkScalarNearlyZero(dy)) {
         return SkScalarAve(src[0].fX, src[1].fX);
     } else {
+#ifdef SK_SCALAR_IS_FLOAT
+        // need the extra precision so we don't compute a value that exceeds
+        // our original limits
+        double X0 = src[0].fX;
+        double Y0 = src[0].fY;
+        double X1 = src[1].fX;
+        double Y1 = src[1].fY;
+        double result = X0 + ((double)Y - Y0) * (X1 - X0) / (Y1 - Y0);
+        return (float)result;
+#else
         return src[0].fX + SkScalarMulDiv(Y - src[0].fY, src[1].fX - src[0].fX,
                                           dy);
+#endif
     }
 }
 
@@ -17,8 +28,19 @@ static SkScalar sect_with_vertical(const SkPoint src[2], SkScalar X) {
     if (SkScalarNearlyZero(dx)) {
         return SkScalarAve(src[0].fY, src[1].fY);
     } else {
+#ifdef SK_SCALAR_IS_FLOAT
+        // need the extra precision so we don't compute a value that exceeds
+        // our original limits
+        double X0 = src[0].fX;
+        double Y0 = src[0].fY;
+        double X1 = src[1].fX;
+        double Y1 = src[1].fY;
+        double result = Y0 + ((double)X - X0) * (Y1 - Y0) / (X1 - X0);
+        return (float)result;
+#else
         return src[0].fY + SkScalarMulDiv(X - src[0].fX, src[1].fY - src[0].fY,
                                           dx);
+#endif
     }
 }
 
@@ -106,6 +128,19 @@ bool SkLineClipper::IntersectLine(const SkPoint src[2], const SkRect& clip,
     return true;
 }
 
+#ifdef SK_DEBUG
+// return value between the two limits, where the limits are either ascending
+// or descending.
+static bool is_between_unsorted(SkScalar value,
+                                SkScalar limit0, SkScalar limit1) {
+    if (limit0 < limit1) {
+        return limit0 <= value && value <= limit1;
+    } else {
+        return limit1 <= value && value <= limit0;
+    }
+}
+#endif
+
 int SkLineClipper::ClipLine(const SkPoint pts[], const SkRect& clip,
                             SkPoint lines[]) {
     int index0, index1;
@@ -135,9 +170,11 @@ int SkLineClipper::ClipLine(const SkPoint pts[], const SkRect& clip,
     // now compute intersections
     if (pts[index0].fY < clip.fTop) {
         tmp[index0].set(sect_with_horizontal(pts, clip.fTop), clip.fTop);
+        SkASSERT(is_between_unsorted(tmp[index0].fX, pts[0].fX, pts[1].fX));
     }
     if (tmp[index1].fY > clip.fBottom) {
         tmp[index1].set(sect_with_horizontal(pts, clip.fBottom), clip.fBottom);
+        SkASSERT(is_between_unsorted(tmp[index1].fX, pts[0].fX, pts[1].fX));
     }
 
     // Chop it into 1..3 segments that are wholly within the clip in X.
@@ -173,14 +210,16 @@ int SkLineClipper::ClipLine(const SkPoint pts[], const SkRect& clip,
         if (tmp[index0].fX < clip.fLeft) {
             r->set(clip.fLeft, tmp[index0].fY);
             r += 1;
-            r->set(clip.fLeft, sect_with_vertical(pts, clip.fLeft));
+            r->set(clip.fLeft, sect_with_vertical(tmp, clip.fLeft));
+            SkASSERT(is_between_unsorted(r->fY, tmp[0].fY, tmp[1].fY));
         } else {
             *r = tmp[index0];
         }
         r += 1;
 
         if (tmp[index1].fX > clip.fRight) {
-            r->set(clip.fRight, sect_with_vertical(pts, clip.fRight));
+            r->set(clip.fRight, sect_with_vertical(tmp, clip.fRight));
+            SkASSERT(is_between_unsorted(r->fY, tmp[0].fY, tmp[1].fY));
             r += 1;
             r->set(clip.fRight, tmp[index1].fY);
         } else {