SkScan_Antihair: assert in debug and potentialy lost last pixels

alpha: (uint8_t) 256 is clamped to 255 before static cast to
uint8_t in SkToU8.

BUG=4406

Review URL: https://codereview.chromium.org/1424253002

2 files changed, 44 insertions, 1 deletions

diff --git a/src/core/SkScan_Antihair.cpp b/src/core/SkScan_Antihair.cpp
index beddf63202..b14e12bab8 100644
--- a/src/core/SkScan_Antihair.cpp
+++ b/src/core/SkScan_Antihair.cpp
@@ -847,7 +847,11 @@ static void inner_scanline(FDot8 L, int top, FDot8 R, U8CPU alpha,
     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;
     }
 
diff --git a/tests/RectTest.cpp b/tests/RectTest.cpp
index be77a87406..820586c3c6 100644
--- a/tests/RectTest.cpp
+++ b/tests/RectTest.cpp
@@ -46,6 +46,45 @@ static void test_stroke_width_clipping(skiatest::Reporter* reporter) {
     REPORTER_ASSERT(reporter, has_green_pixels(bm));
 }
 
+static void test_skbug4406(skiatest::Reporter* reporter) {
+    SkBitmap bm;
+    bm.allocN32Pixels(10, 10);
+    bm.eraseColor(SK_ColorTRANSPARENT);
+
+    SkCanvas canvas(bm);
+    const SkRect r = { 1.5f, 1, 3.5f, 3 };
+    // draw filled green rect first
+    SkPaint paint;
+    paint.setStyle(SkPaint::kFill_Style);
+    paint.setColor(0xff00ff00);
+    paint.setStrokeWidth(1);
+    paint.setAntiAlias(true);
+    canvas.drawRect(r, paint);
+
+    // paint black with stroke rect (that asserts in bug 4406)
+    // over the filled rect, it should cover it
+    paint.setStyle(SkPaint::kStroke_Style);
+    paint.setColor(0xff000000);
+    paint.setStrokeWidth(1);
+    canvas.drawRect(r, paint);
+    REPORTER_ASSERT(reporter, !has_green_pixels(bm));
+
+    // do it again with thinner stroke
+    paint.setStyle(SkPaint::kFill_Style);
+    paint.setColor(0xff00ff00);
+    paint.setStrokeWidth(1);
+    paint.setAntiAlias(true);
+    canvas.drawRect(r, paint);
+    // paint black with stroke rect (that asserts in bug 4406)
+    // over the filled rect, it doesnt cover it completelly with thinner stroke
+    paint.setStyle(SkPaint::kStroke_Style);
+    paint.setColor(0xff000000);
+    paint.setStrokeWidth(0.99f);
+    canvas.drawRect(r, paint);
+    REPORTER_ASSERT(reporter, has_green_pixels(bm));
+}
+
 DEF_TEST(Rect, reporter) {
     test_stroke_width_clipping(reporter);
+    test_skbug4406(reporter);
 }