Don't fold SaveAlphaLayer into DrawTextBlob

SkPaint::drawTextBlob applies alpha on each individual glyph, so
we should keep SaveAlphaLayer to ensure the alpha is applied atomically
on the whole text blob.

Bug: 1006140
Change-Id: I8b1d1868a3423ed4f0ef6541cc8db354773dfb24
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1821944
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699621}

cc/paint/paint_op_buffer.cc

@@ -2373,7 +2373,11 @@ void PaintOpBuffer::PlaybackFoldingIterator::FindNextOp() {
       third = NextUnfoldedOp();
       if (third && third->GetType() == PaintOpType::Restore) {
         auto* save_op = static_cast<const SaveLayerAlphaOp*>(current_op_);
-        if (draw_op->IsPaintOpWithFlags()) {
+        if (draw_op->IsPaintOpWithFlags() &&
+            // SkPaint::drawTextBlob() applies alpha on each glyph so we don't
+            // fold SaveLayerAlpha into DrwaTextBlob to ensure correct alpha
+            // even if some glyphs overlap.
+            draw_op->GetType() != PaintOpType::DrawTextBlob) {
           auto* flags_op = static_cast<const PaintOpWithFlags*>(draw_op);
           if (flags_op->flags.SupportsFoldingAlpha()) {
             current_alpha_ = save_op->alpha;

cc/paint/paint_op_buffer_unittest.cc

@@ -48,7 +48,6 @@ void ValidateOps(PaintOpBuffer* buffer) {
   for (auto* op : PaintOpBuffer::Iterator(buffer))
     EXPECT_TRUE(static_cast<T*>(op)->IsValid());
 }
-
 }  // namespace
 
 class PaintOpSerializationTestUtils {
@@ -240,6 +239,26 @@ TEST(PaintOpBufferTest, SaveDrawRestore) {
   EXPECT_LE(std::abs(expected_alpha - canvas.paint_.getAlpha()), 1.f);
 }
 
+// Verify that we don't optimize SaveLayerAlpha / DrawTextBlob / Restore.
+TEST(PaintOpBufferTest, SaveDrawTextBlobRestore) {
+  PaintOpBuffer buffer;
+
+  uint8_t alpha = 100;
+  buffer.push<SaveLayerAlphaOp>(nullptr, alpha);
+
+  PaintFlags paint_flags;
+  EXPECT_TRUE(paint_flags.SupportsFoldingAlpha());
+  buffer.push<DrawTextBlobOp>(SkTextBlob::MakeFromString("abc", SkFont()), 0, 0,
+                              paint_flags);
+  buffer.push<RestoreOp>();
+
+  SaveCountingCanvas canvas;
+  buffer.Playback(&canvas);
+
+  EXPECT_EQ(1, canvas.save_count_);
+  EXPECT_EQ(1, canvas.restore_count_);
+}
+
 // The same as SaveDrawRestore, but test that the optimization doesn't apply
 // when the drawing op's flags are not compatible with being folded into the
 // save layer with opacity.

third_party/blink/web_tests/external/wpt/css/css-color/opacity-overlapping-letters-ref.html

+<meta name="flags" content="ahem">
+<div style="opacity: 0.5; letter-spacing: -0.6em; font: 100px/1 Ahem; white-space: pre">X X X</div>

third_party/blink/web_tests/external/wpt/css/css-color/opacity-overlapping-letters.html

+<link rel="help" href="http://www.w3.org/TR/css3-color/#transparency">
+<link rel="match" href="opacity-overlapping-letters-ref.html">
+<meta name="flags" content="ahem">
+<meta name="assert" content="Opacity should be apply on the whole text content atomically.">
+<div style="opacity: 0.5; letter-spacing: -0.6em; font: 100px/1 Ahem">XXXXX</div>