From 6da3d1757cfee75c25a86b580834dc49d8b53f05 Mon Sep 17 00:00:00 2001
From: "reed@google.com" <reed@google.com@2bbb7eff-a529-9590-31e7-b0007b416f81>
Date: Wed, 11 Jan 2012 16:41:26 +0000
Subject: [PATCH] improve clamping logic when handling imprecise chopping of
 cubics http://code.google.com/p/skia/issues/detail?id=444

git-svn-id: http://skia.googlecode.com/svn/trunk@3011 2bbb7eff-a529-9590-31e7-b0007b416f81
---
 src/core/SkEdgeClipper.cpp | 39 ++++++++++++++++++++++++---------------
 tests/ClipperTest.cpp      | 19 +++++++++++++++++++
 2 files changed, 43 insertions(+), 15 deletions(-)

diff --git a/src/core/SkEdgeClipper.cpp b/src/core/SkEdgeClipper.cpp
index d8cb75b..d77f6f8 100644
--- a/src/core/SkEdgeClipper.cpp
+++ b/src/core/SkEdgeClipper.cpp
@@ -266,20 +266,21 @@ static bool chopMonoCubicAtX(SkPoint pts[4], SkScalar x, SkScalar* t) {
 
 // Modify pts[] in place so that it is clipped in Y to the clip rect
 static void chop_cubic_in_Y(SkPoint pts[4], const SkRect& clip) {
-    SkScalar t;
-    SkPoint tmp[7]; // for SkChopCubicAt
     
     // are we partially above
     if (pts[0].fY < clip.fTop) {
+        SkScalar t;
         if (chopMonoCubicAtY(pts, clip.fTop, &t)) {
+            SkPoint tmp[7];
             SkChopCubicAt(pts, tmp, t);
-            // given the imprecision of computing t, we just slam our Y coord
-            // to the top of the clip. This also saves us in the bad case where
-            // the t was soooo bad that the entire segment could have been
-            // below fBottom
+
+            // tmp[3, 4, 5].fY should all be to the below clip.fTop, and
+            // still be monotonic in Y. Since we can't trust the numerics of
+            // the chopper, we force those conditions now
             tmp[3].fY = clip.fTop;
-            clamp_ge(tmp[4].fY, clip.fTop);
-            clamp_ge(tmp[5].fY, clip.fTop);
+            tmp[4].fY = SkMaxScalar(tmp[4].fY, clip.fTop);
+            tmp[5].fY = SkMaxScalar(tmp[5].fY, tmp[4].fY);
+
             pts[0] = tmp[3];
             pts[1] = tmp[4];
             pts[2] = tmp[5];
@@ -294,7 +295,9 @@ static void chop_cubic_in_Y(SkPoint pts[4], const SkRect& clip) {
     
     // are we partially below
     if (pts[3].fY > clip.fBottom) {
+        SkScalar t;
         if (chopMonoCubicAtY(pts, clip.fBottom, &t)) {
+            SkPoint tmp[7];
             SkChopCubicAt(pts, tmp, t);
             clamp_le(tmp[1].fY, clip.fBottom);
             clamp_le(tmp[2].fY, clip.fBottom);
@@ -341,18 +344,22 @@ void SkEdgeClipper::clipMonoCubic(const SkPoint src[4], const SkRect& clip) {
         this->appendVLine(clip.fRight, pts[0].fY, pts[3].fY, reverse);
         return;
     }
-    
-    SkScalar t;
-    SkPoint tmp[7];
-    
+
     // are we partially to the left
     if (pts[0].fX < clip.fLeft) {
+        SkScalar t;
         if (chopMonoCubicAtX(pts, clip.fLeft, &t)) {
+            SkPoint tmp[7];
             SkChopCubicAt(pts, tmp, t);
             this->appendVLine(clip.fLeft, tmp[0].fY, tmp[3].fY, reverse);
-            clamp_ge(tmp[3].fX, clip.fLeft);
-            clamp_ge(tmp[4].fX, clip.fLeft);
-            clamp_ge(tmp[5].fX, clip.fLeft);
+
+            // tmp[3, 4, 5].fX should all be to the right of clip.fLeft, and
+            // still be monotonic in X. Since we can't trust the numerics of
+            // the chopper, we force those conditions now
+            tmp[3].fX = clip.fLeft;
+            tmp[4].fX = SkMaxScalar(tmp[4].fX, clip.fLeft);
+            tmp[5].fX = SkMaxScalar(tmp[5].fX, tmp[4].fX);
+
             pts[0] = tmp[3];
             pts[1] = tmp[4];
             pts[2] = tmp[5];
@@ -366,7 +373,9 @@ void SkEdgeClipper::clipMonoCubic(const SkPoint src[4], const SkRect& clip) {
     
     // are we partially to the right
     if (pts[3].fX > clip.fRight) {
+        SkScalar t;
         if (chopMonoCubicAtX(pts, clip.fRight, &t)) {
+            SkPoint tmp[7];
             SkChopCubicAt(pts, tmp, t);
             clamp_le(tmp[1].fX, clip.fRight);
             clamp_le(tmp[2].fX, clip.fRight);
diff --git a/tests/ClipperTest.cpp b/tests/ClipperTest.cpp
index 0f5a615..f6333ba 100644
--- a/tests/ClipperTest.cpp
+++ b/tests/ClipperTest.cpp
@@ -10,6 +10,24 @@
 #include "SkLineClipper.h"
 #include "SkEdgeClipper.h"
 
+static void test_edgeclipper(skiatest::Reporter* reporter) {
+    SkEdgeClipper clipper;
+    
+    const SkPoint pts[] = {
+        { 3.0995476e+010, 42.929779 },
+        { -3.0995163e+010, 51.050385 },
+        { -3.0995157e+010, 51.050392 },
+        { -3.0995134e+010, 51.050400 },
+    };
+
+    const SkRect clip = { 0, 0, 300, 200 };
+
+    // this should not assert, even though our choppers do a poor numerical
+    // job when computing their t values.
+    // http://code.google.com/p/skia/issues/detail?id=444
+    clipper.clipCubic(pts, clip);
+}
+
 static void test_intersectline(skiatest::Reporter* reporter) {
     static const SkScalar L = 0;
     static const SkScalar T = 0;
@@ -90,6 +108,7 @@ static void test_intersectline(skiatest::Reporter* reporter) {
 
 void TestClipper(skiatest::Reporter* reporter) {
     test_intersectline(reporter);
+    test_edgeclipper(reporter);
 }
 
 #include "TestClassDef.h"
-- 
2.7.4