[Impeller] round scale for glyph atlas cache to 2 decimal places. (flutter#43752)

Fixes flutter#130750

Author: Jonah Williams

impeller/entity/contents/text_contents.cc

@@ -176,8 +176,12 @@ bool TextContents::Render(const ContentContext& renderer,
         size_t vertex_offset = 0;
         for (const auto& run : frame_.GetRuns()) {
           const Font& font = run.GetFont();
+          auto rounded_scale = TextFrame::RoundScaledFontSize(
+              scale_, font.GetMetrics().point_size);
+
           for (const auto& glyph_position : run.GetGlyphPositions()) {
-            FontGlyphPair font_glyph_pair{font, glyph_position.glyph, scale_};
+            FontGlyphPair font_glyph_pair{font, glyph_position.glyph,
+                                          rounded_scale};
             auto maybe_atlas_glyph_bounds =
                 atlas->FindFontGlyphBounds(font_glyph_pair);
             if (!maybe_atlas_glyph_bounds.has_value()) {

impeller/entity/entity_unittests.cc

@@ -2426,6 +2426,13 @@ TEST_P(EntityTest, ColorFilterContentsWithLargeGeometry) {
   ASSERT_TRUE(OpenPlaygroundHere(entity));
 }
 
+TEST_P(EntityTest, TextContentsCeilsGlyphScaleToDecimal) {
+  ASSERT_EQ(TextFrame::RoundScaledFontSize(0.4321111f, 12), 0.43f);
+  ASSERT_EQ(TextFrame::RoundScaledFontSize(0.5321111f, 12), 0.53f);
+  ASSERT_EQ(TextFrame::RoundScaledFontSize(2.1f, 12), 2.1f);
+  ASSERT_EQ(TextFrame::RoundScaledFontSize(0.0f, 12), 0.0f);
+}
+
 }  // namespace testing
 }  // namespace impeller
 

impeller/typographer/text_frame.cc

@@ -79,13 +79,27 @@ bool TextFrame::MaybeHasOverlapping() const {
   return false;
 }
 
+// static
+Scalar TextFrame::RoundScaledFontSize(Scalar scale, Scalar point_size) {
+  return std::round(scale * 100) / 100;
+}
+
 void TextFrame::CollectUniqueFontGlyphPairs(FontGlyphPair::Set& set,
                                             Scalar scale) const {
   for (const TextRun& run : GetRuns()) {
     const Font& font = run.GetFont();
+    auto rounded_scale =
+        RoundScaledFontSize(scale, font.GetMetrics().point_size);
     for (const TextRun::GlyphPosition& glyph_position :
          run.GetGlyphPositions()) {
-      set.insert({font, glyph_position.glyph, scale});
+#if false
+// Glyph size error due to RoundScaledFontSize usage above.
+if (rounded_scale != scale) {
+  auto delta = std::abs(rounded_scale - scale);
+  FML_LOG(ERROR) << glyph_position.glyph.bounds.size * delta;
+}
+#endif
+      set.insert({font, glyph_position.glyph, rounded_scale});
     }
   }
 }

impeller/typographer/text_frame.h

@@ -24,6 +24,8 @@ class TextFrame {
 
   void CollectUniqueFontGlyphPairs(FontGlyphPair::Set& set, Scalar scale) const;
 
+  static Scalar RoundScaledFontSize(Scalar scale, Scalar point_size);
+
   //----------------------------------------------------------------------------
   /// @brief      The conservative bounding box for this text frame.
   ///