Turn on concave analytic AA

This will allow Skia trybots to exercise analytic AA. But there's still
a guard flag in Chromium that prevents layout tests failure.

Additionally, we
1. fixed nagative shift problem
2. relax the ASSERT when slope is too large: If slope is large, the accuracy of the slope is limited due
to conversion to SkFDot6 and division. Hence we have to relax the constraint.
3. handle the special case where dx != 0 while slope == 0 because of very large dy and limited precision.

BUG=skia:

Change-Id: Ice70164f3f396f0db3896bedc7b96fbd613078dc
Reviewed-on: https://skia-review.googlesource.com/7120
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>

src/core/SkAnalyticEdge.cpp

@@ -39,7 +39,11 @@ bool SkAnalyticEdge::updateLine(SkFixed x0, SkFixed y0, SkFixed x1, SkFixed y1,
     fY          = y0;
     fUpperY     = y0;
     fLowerY     = y1;
+#ifdef SK_SUPPORT_LEGACY_AAA
     fDY         = (absSlope | dx) == 0
+#else
+    fDY         = (dx == 0 || slope == 0)
+#endif
                   ? SK_MaxS32
                   : absSlope < kInverseTableSize
                     ? QuickFDot6Inverse::Lookup(absSlope)
@@ -113,7 +117,7 @@ bool SkAnalyticQuadraticEdge::updateQuadratic() {
 #ifndef SK_SUPPORT_LEGACY_AAA
             SkASSERT(slope == SK_MaxS32 ||
                     SkAbs32(fSnappedX + SkFixedMul(slope, newSnappedY - fSnappedY) - newSnappedX)
-                    < SK_FixedHalf);
+                    < SkFixedMul(SK_Fixed1 >> 4, SkTMax(SK_Fixed1, SkAbs32(slope))));
 #endif
             dx += fQEdge.fQDDx;
             dy += fQEdge.fQDDy;

src/core/SkAnalyticEdge.h

@@ -10,11 +10,6 @@
 
 #include "SkEdge.h"
 
-// Use this to check that we successfully guard the change against Chromium layout tests
-#ifndef  SK_SUPPORT_LEGACY_AAA
-# define SK_SUPPORT_LEGACY_AAA
-#endif
-
 struct SkAnalyticEdge {
     // Similar to SkEdge, the conic edges will be converted to quadratic edges
     enum Type {
@@ -166,7 +161,6 @@ bool SkAnalyticEdge::setLine(const SkPoint& p0, const SkPoint& p1) {
     SkFDot6 dy = SkFixedToFDot6(y1 - y0);
     SkFDot6 dx = SkFixedToFDot6(x1 - x0);
     SkFixed slope = dy ? QuickSkFDot6Div(dx, dy) : SK_MaxS32;
-    SkASSERT(dx == 0 || slope != 0);
     SkFixed absSlope = SkAbs32(slope);
 #endif
 
@@ -179,9 +173,9 @@ bool SkAnalyticEdge::setLine(const SkPoint& p0, const SkPoint& p1) {
 #ifdef SK_SUPPORT_LEGACY_AAA
     fDY         = x1 != x0 ? SkAbs32(SkFixedDiv(y1 - y0, x1 - x0)) : SK_MaxS32;
 #else
-    fDY         = dx == 0 ? SK_MaxS32 : absSlope < kInverseTableSize
+    fDY         = dx == 0 || slope == 0 ? SK_MaxS32 : absSlope < kInverseTableSize
-                                                 ? QuickFDot6Inverse::Lookup(absSlope)
+                                                    ? QuickFDot6Inverse::Lookup(absSlope)
-                                                 : SkAbs32(QuickSkFDot6Div(dy, dx));
+                                                    : SkAbs32(QuickSkFDot6Div(dy, dx));
 #endif
     fCurveCount = 0;
     fWinding    = SkToS8(winding);

src/core/SkScan.h

@@ -23,11 +23,6 @@ class SkPath;
 */
 typedef SkIRect SkXRect;
 
-// Use this to check that we successfully guard the change against Chromium layout tests
-#ifndef  SK_SUPPORT_LEGACY_AAA
-# define SK_SUPPORT_LEGACY_AAA
-#endif
-
 extern std::atomic<bool> gSkUseAnalyticAA;
 extern std::atomic<bool> gSkForceAnalyticAA;
 

src/core/SkScan_AAAPath.cpp

@@ -736,14 +736,14 @@ static void blit_aaa_trapezoid_row(AdditiveBlitter* blitter, int y,
     int uL = SkFixedFloorToInt(ul);
     int lL = SkFixedCeilToInt(ll);
     if (uL + 2 == lL) { // We only need to compute two triangles, accelerate this special case
-        SkFixed first = (uL << 16) + SK_Fixed1 - ul;
+        SkFixed first = SkIntToFixed(uL) + SK_Fixed1 - ul;
         SkFixed second = ll - ul - first;
         SkAlpha a1 = fullAlpha - partialTriangleToAlpha(first, lDY);
         SkAlpha a2 = partialTriangleToAlpha(second, lDY);
         alphas[0] = alphas[0] > a1 ? alphas[0] - a1 : 0;
         alphas[1] = alphas[1] > a2 ? alphas[1] - a2 : 0;
     } else {
-        computeAlphaBelowLine(tempAlphas + uL - L, ul - (uL << 16), ll - (uL << 16),
+        computeAlphaBelowLine(tempAlphas + uL - L, ul - SkIntToFixed(uL), ll - SkIntToFixed(uL),
                 lDY, fullAlpha);
         for (int i = uL; i < lL; ++i) {
             if (alphas[i - L] > tempAlphas[i - L]) {
@@ -757,14 +757,14 @@ static void blit_aaa_trapezoid_row(AdditiveBlitter* blitter, int y,
     int uR = SkFixedFloorToInt(ur);
     int lR = SkFixedCeilToInt(lr);
     if (uR + 2 == lR) { // We only need to compute two triangles, accelerate this special case
-        SkFixed first = (uR << 16) + SK_Fixed1 - ur;
+        SkFixed first = SkIntToFixed(uR) + SK_Fixed1 - ur;
         SkFixed second = lr - ur - first;
         SkAlpha a1 = partialTriangleToAlpha(first, rDY);
         SkAlpha a2 = fullAlpha - partialTriangleToAlpha(second, rDY);
         alphas[len-2] = alphas[len-2] > a1 ? alphas[len-2] - a1 : 0;
         alphas[len-1] = alphas[len-1] > a2 ? alphas[len-1] - a2 : 0;
     } else {
-        computeAlphaAboveLine(tempAlphas + uR - L, ur - (uR << 16), lr - (uR << 16),
+        computeAlphaAboveLine(tempAlphas + uR - L, ur - SkIntToFixed(uR), lr - SkIntToFixed(uR),
                 rDY, fullAlpha);
         for (int i = uR; i < lR; ++i) {
             if (alphas[i - L] > tempAlphas[i - L]) {
@@ -1605,7 +1605,9 @@ static void aaa_walk_edges(SkAnalyticEdge* prevHead, SkAnalyticEdge* nextTail,
                         leftEnds, false, blitter, maskRow, isUsingMask, noRealBlitter,
                         leftClip, rightClip, yShift);
             } else {
-                blit_trapezoid_row(blitter, y >> 16, left, rightClip, leftE->fX, rightClip,
+                blit_trapezoid_row(blitter, y >> 16,
+                        SkTMax(leftClip, left), rightClip,
+                        SkTMax(leftClip, leftE->fX), rightClip,
                         leftDY, 0, fullAlpha, maskRow, isUsingMask,
                         noRealBlitter ||
                                 (fullAlpha == 0xFF && edges_too_close(leftE->fPrev, leftE, nextY)),