SkScan_Antihair: assert in debug and potentialy lost last pixels

src/core/SkScan_Antihair.cpp

 
 /*
  * Copyright 2011 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "SkScan.h"
@@ skipping 829 matching lines @@
     // need precise rounding (not just SkAlphaMul) so that values like
     // a=228, b=252 don't overflow the result
     return SkToU8(a + b - SkAlphaMulRound(a, b));
 }
 
 static void inner_scanline(FDot8 L, int top, FDot8 R, U8CPU alpha,
                            SkBlitter* blitter) {
     SkASSERT(L < R);
 
     if ((L >> 8) == ((R - 1) >> 8)) {  // 1x1 pixel
-        blitter->blitV(L >> 8, top, 1, InvAlphaMul(alpha, R - L));
+        FDot8 widClamp = R - L;
+        // border case clamp 256 to 255 instead of going through call_hline_blitter
+        // see skbug/4406
+        widClamp = widClamp - (widClamp >> 8);
+        blitter->blitV(L >> 8, top, 1, InvAlphaMul(alpha, widClamp));
         return;
     }
 
     int left = L >> 8;
     if (L & 0xFF) {
         blitter->blitV(left, top, 1, InvAlphaMul(alpha, L & 0xFF));
         left += 1;
     }
 
     int rite = R >> 8;
@@ skipping 133 matching lines @@
 
 void SkScan::AntiFrameRect(const SkRect& r, const SkPoint& strokeSize,
                            const SkRasterClip& clip, SkBlitter* blitter) {
     if (clip.isBW()) {
         AntiFrameRect(r, strokeSize, &clip.bwRgn(), blitter);
     } else {
         SkAAClipBlitterWrapper wrap(clip, blitter);
         AntiFrameRect(r, strokeSize, &wrap.getRgn(), wrap.getBlitter());
     }
 }