Fixed some floating point inaccuracies in TextContents (#162351)

issue: https://github.com/flutter/flutter/issues/149652

## before
Animating from 0.444x to 0.445x

![before-pos](https://github.com/user-attachments/assets/3b561e35-68ca-49ca-aa3e-e2c224ae06c5)

## after
Animating from 0.444x to 0.445x

![after-pos](https://github.com/user-attachments/assets/3ef67ec9-29ce-481a-83ff-cb27fbe2ddca)

## diff
Diff at 0.444x

![444](https://github.com/user-attachments/assets/2a309f2d-65ad-4be2-bc35-7b9df5639c0f)


## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
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[Features we expect every widget to implement]:
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[CLA]: https://cla.developers.google.com/
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[breaking change policy]:
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[Discord]:
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[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

engine/src/flutter/ci/licenses_golden/excluded_files

@@ -152,6 +152,7 @@
 ../../../flutter/impeller/display_list/dl_golden_unittests.h
 ../../../flutter/impeller/display_list/dl_unittests.cc
 ../../../flutter/impeller/display_list/skia_conversions_unittests.cc
+../../../flutter/impeller/display_list/testing
 ../../../flutter/impeller/docs
 ../../../flutter/impeller/entity/clip_stack_unittests.cc
 ../../../flutter/impeller/entity/contents/filters/blend_filter_contents_unittests.cc

engine/src/flutter/impeller/display_list/BUILD.gn

@@ -78,6 +78,10 @@ template("display_list_unittests_component") {
     "dl_playground.cc",
     "dl_playground.h",
     "dl_unittests.cc",
+    "testing/render_text_in_canvas.cc",
+    "testing/render_text_in_canvas.h",
+    "testing/rmse.cc",
+    "testing/rmse.h",
   ]
   additional_sources = []
   if (defined(invoker.sources)) {

engine/src/flutter/impeller/display_list/aiks_dl_text_unittests.cc

@@ -152,17 +152,27 @@ TEST_P(AiksTest, CanRenderTextFrameWithHalfScaling) {
 }
 
 TEST_P(AiksTest, CanRenderTextFrameWithFractionScaling) {
-  DisplayListBuilder builder;
+  Scalar fine_scale = 0.f;
+  auto callback = [&]() -> sk_sp<DisplayList> {
+    if (AiksTest::ImGuiBegin("Controls", nullptr,
+                             ImGuiWindowFlags_AlwaysAutoResize)) {
+      ImGui::SliderFloat("Fine Scale", &fine_scale, -1, 1);
+      ImGui::End();
+    }
 
-  DlPaint paint;
-  paint.setColor(DlColor::ARGB(1, 0.1, 0.1, 0.1));
-  builder.DrawPaint(paint);
-  builder.Scale(2.625, 2.625);
+    DisplayListBuilder builder;
+    DlPaint paint;
+    paint.setColor(DlColor::ARGB(1, 0.1, 0.1, 0.1));
+    builder.DrawPaint(paint);
+    Scalar scale = 2.625 + fine_scale;
+    builder.Scale(scale, scale);
+    RenderTextInCanvasSkia(GetContext(), builder,
+                           "the quick brown fox jumped over the lazy dog!.?",
+                           "Roboto-Regular.ttf");
+    return builder.Build();
+  };
 
-  ASSERT_TRUE(RenderTextInCanvasSkia(
-      GetContext(), builder, "the quick brown fox jumped over the lazy dog!.?",
-      "Roboto-Regular.ttf"));
-  ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
+  ASSERT_TRUE(OpenPlaygroundHere(callback));
 }
 
 TEST_P(AiksTest, TextFrameSubpixelAlignment) {

engine/src/flutter/impeller/display_list/dl_golden_blur_unittests.cc

@@ -6,58 +6,18 @@
 
 #include "flutter/display_list/dl_builder.h"
 #include "flutter/display_list/effects/dl_mask_filter.h"
+#include "flutter/impeller/display_list/testing/render_text_in_canvas.h"
+#include "flutter/impeller/display_list/testing/rmse.h"
 #include "flutter/impeller/geometry/round_rect.h"
 #include "flutter/impeller/golden_tests/screenshot.h"
 #include "flutter/testing/testing.h"
 #include "gtest/gtest.h"
-#include "impeller/typographer/backends/skia/text_frame_skia.h"
-#include "txt/platform.h"
 
 namespace flutter {
 namespace testing {
 
 using impeller::Font;
 
-namespace {
-struct TextRenderOptions {
-  bool stroke = false;
-  DlScalar font_size = 50;
-  DlColor color = DlColor::kYellow();
-  std::shared_ptr<DlMaskFilter> mask_filter;
-};
-
-bool RenderTextInCanvasSkia(DlCanvas* canvas,
-                            const std::string& text,
-                            const std::string_view& font_fixture,
-                            DlPoint position,
-                            const TextRenderOptions& options = {}) {
-  auto c_font_fixture = std::string(font_fixture);
-  auto mapping = flutter::testing::OpenFixtureAsSkData(c_font_fixture.c_str());
-  if (!mapping) {
-    return false;
-  }
-  sk_sp<SkFontMgr> font_mgr = txt::GetDefaultFontManager();
-  SkFont sk_font(font_mgr->makeFromData(mapping), options.font_size);
-  auto blob = SkTextBlob::MakeFromString(text.c_str(), sk_font);
-  if (!blob) {
-    return false;
-  }
-
-  auto frame = impeller::MakeTextFrameFromTextBlobSkia(blob);
-
-  DlPaint text_paint;
-  text_paint.setColor(options.color);
-  text_paint.setMaskFilter(options.mask_filter);
-  // text_paint.mask_blur_descriptor = options.mask_blur_descriptor;
-  // text_paint.stroke_width = 1;
-  // text_paint.style =
-  //     options.stroke ? Paint::Style::kStroke : Paint::Style::kFill;
-  canvas->DrawTextFrame(frame, position.x, position.y, text_paint);
-  return true;
-}
-
-}  // namespace
-
 TEST_P(DlGoldenTest, TextBlurMaskFilterRespectCTM) {
   impeller::Point content_scale = GetContentScale();
   auto draw = [&](DlCanvas* canvas,
@@ -112,41 +72,6 @@ TEST_P(DlGoldenTest, TextBlurMaskFilterDisrespectCTM) {
   ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
 }
 
-namespace {
-double CalculateDistance(const uint8_t* left, const uint8_t* right) {
-  double diff[4] = {
-      static_cast<double>(left[0]) - right[0],  //
-      static_cast<double>(left[1]) - right[1],  //
-      static_cast<double>(left[2]) - right[2],  //
-      static_cast<double>(left[3]) - right[3]   //
-  };
-  return sqrt((diff[0] * diff[0]) +  //
-              (diff[1] * diff[1]) +  //
-              (diff[2] * diff[2]) +  //
-              (diff[3] * diff[3]));
-}
-
-double RMSE(const impeller::testing::Screenshot* left,
-            const impeller::testing::Screenshot* right) {
-  FML_CHECK(left);
-  FML_CHECK(right);
-  FML_CHECK(left->GetWidth() == right->GetWidth());
-  FML_CHECK(left->GetHeight() == right->GetHeight());
-
-  int64_t samples = left->GetWidth() * left->GetHeight();
-  double tally = 0;
-
-  const uint8_t* left_ptr = left->GetBytes();
-  const uint8_t* right_ptr = right->GetBytes();
-  for (int64_t i = 0; i < samples; ++i, left_ptr += 4, right_ptr += 4) {
-    double distance = CalculateDistance(left_ptr, right_ptr);
-    tally += distance * distance;
-  }
-
-  return sqrt(tally / static_cast<double>(samples));
-}
-}  // namespace
-
 // This is a test to make sure that we don't regress "shimmering" in the
 // gaussian blur. Shimmering is abrupt changes in signal when making tiny
 // changes to the blur parameters.

engine/src/flutter/impeller/display_list/dl_golden_unittests.cc

@@ -8,6 +8,8 @@
 #include "display_list/dl_paint.h"
 #include "display_list/geometry/dl_geometry_types.h"
 #include "flutter/display_list/dl_builder.h"
+#include "flutter/impeller/display_list/testing/render_text_in_canvas.h"
+#include "flutter/impeller/display_list/testing/rmse.h"
 #include "flutter/impeller/geometry/path_builder.h"
 #include "flutter/testing/testing.h"
 #include "gtest/gtest.h"
@@ -350,5 +352,52 @@ TEST_P(DlGoldenTest, SaveLayerAtFractionalValue) {
   ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
 }
 
+namespace {
+int32_t CalculateMaxY(const impeller::testing::Screenshot* img) {
+  const uint32_t* ptr = reinterpret_cast<const uint32_t*>(img->GetBytes());
+  int32_t max_y = 0;
+  for (uint32_t i = 0; i < img->GetHeight(); ++i) {
+    for (uint32_t j = 0; j < img->GetWidth(); ++j) {
+      uint32_t pixel = *ptr++;
+      if ((pixel & 0x00ffffff) != 0) {
+        max_y = std::max(max_y, static_cast<int32_t>(i));
+      }
+    }
+  }
+  return max_y;
+}
+}  // namespace
+
+TEST_P(DlGoldenTest, BaselineHE) {
+  SetWindowSize(impeller::ISize(1024, 200));
+  impeller::Scalar font_size = 300;
+  auto callback = [&](const char* text,
+                      impeller::Scalar scale) -> sk_sp<DisplayList> {
+    DisplayListBuilder builder;
+    DlPaint paint;
+    paint.setColor(DlColor::ARGB(1, 0, 0, 0));
+    builder.DrawPaint(paint);
+    builder.Scale(scale, scale);
+    RenderTextInCanvasSkia(&builder, text, "Roboto-Regular.ttf",
+                           DlPoint::MakeXY(10, 300),
+                           TextRenderOptions{
+                               .font_size = font_size,
+                           });
+    return builder.Build();
+  };
+
+  std::unique_ptr<impeller::testing::Screenshot> right =
+      MakeScreenshot(callback("h", 0.444));
+  if (!right) {
+    GTEST_SKIP() << "making screenshots not supported.";
+  }
+  std::unique_ptr<impeller::testing::Screenshot> left =
+      MakeScreenshot(callback("e", 0.444));
+
+  int32_t left_max_y = CalculateMaxY(left.get());
+  int32_t right_max_y = CalculateMaxY(right.get());
+  int32_t y_diff = std::abs(left_max_y - right_max_y);
+  EXPECT_TRUE(y_diff <= 2) << "y diff: " << y_diff;
+}
 }  // namespace testing
 }  // namespace flutter

engine/src/flutter/impeller/display_list/testing/render_text_in_canvas.cc

@@ -0,0 +1,43 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "impeller/display_list/testing/render_text_in_canvas.h"
+
+#include "flutter/testing/testing.h"
+#include "txt/platform.h"
+
+namespace flutter {
+namespace testing {
+
+bool RenderTextInCanvasSkia(DlCanvas* canvas,
+                            const std::string& text,
+                            const std::string_view& font_fixture,
+                            DlPoint position,
+                            const TextRenderOptions& options) {
+  auto c_font_fixture = std::string(font_fixture);
+  auto mapping = flutter::testing::OpenFixtureAsSkData(c_font_fixture.c_str());
+  if (!mapping) {
+    return false;
+  }
+  sk_sp<SkFontMgr> font_mgr = txt::GetDefaultFontManager();
+  SkFont sk_font(font_mgr->makeFromData(mapping), options.font_size);
+  auto blob = SkTextBlob::MakeFromString(text.c_str(), sk_font);
+  if (!blob) {
+    return false;
+  }
+
+  auto frame = impeller::MakeTextFrameFromTextBlobSkia(blob);
+
+  DlPaint text_paint;
+  text_paint.setColor(options.color);
+  text_paint.setMaskFilter(options.mask_filter);
+  // text_paint.mask_blur_descriptor = options.mask_blur_descriptor;
+  // text_paint.stroke_width = 1;
+  // text_paint.style =
+  //     options.stroke ? Paint::Style::kStroke : Paint::Style::kFill;
+  canvas->DrawTextFrame(frame, position.x, position.y, text_paint);
+  return true;
+}
+}  // namespace testing
+}  // namespace flutter

engine/src/flutter/impeller/display_list/testing/render_text_in_canvas.h

@@ -0,0 +1,34 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef FLUTTER_IMPELLER_DISPLAY_LIST_TESTING_RENDER_TEXT_IN_CANVAS_H_
+#define FLUTTER_IMPELLER_DISPLAY_LIST_TESTING_RENDER_TEXT_IN_CANVAS_H_
+
+#include <memory>
+
+#include "flutter/display_list/dl_canvas.h"
+#include "flutter/display_list/dl_color.h"
+#include "flutter/display_list/effects/dl_mask_filter.h"
+#include "impeller/typographer/backends/skia/text_frame_skia.h"
+
+namespace flutter {
+namespace testing {
+
+struct TextRenderOptions {
+  bool stroke = false;
+  SkScalar font_size = 50;
+  DlColor color = DlColor::kYellow();
+  std::shared_ptr<DlMaskFilter> mask_filter;
+};
+
+bool RenderTextInCanvasSkia(DlCanvas* canvas,
+                            const std::string& text,
+                            const std::string_view& font_fixture,
+                            DlPoint position,
+                            const TextRenderOptions& options = {});
+
+}  // namespace testing
+}  // namespace flutter
+
+#endif  // FLUTTER_IMPELLER_DISPLAY_LIST_TESTING_RENDER_TEXT_IN_CANVAS_H_

engine/src/flutter/impeller/display_list/testing/rmse.cc

@@ -0,0 +1,47 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "impeller/display_list/testing/rmse.h"
+
+#include "flutter/fml/logging.h"
+
+namespace flutter {
+namespace testing {
+namespace {
+double CalculateDistance(const uint8_t* left, const uint8_t* right) {
+  double diff[4] = {
+      static_cast<double>(left[0]) - right[0],  //
+      static_cast<double>(left[1]) - right[1],  //
+      static_cast<double>(left[2]) - right[2],  //
+      static_cast<double>(left[3]) - right[3]   //
+  };
+  return sqrt((diff[0] * diff[0]) +  //
+              (diff[1] * diff[1]) +  //
+              (diff[2] * diff[2]) +  //
+              (diff[3] * diff[3]));
+}
+}  // namespace
+
+double RMSE(const impeller::testing::Screenshot* left,
+            const impeller::testing::Screenshot* right) {
+  FML_CHECK(left);
+  FML_CHECK(right);
+  FML_CHECK(left->GetWidth() == right->GetWidth());
+  FML_CHECK(left->GetHeight() == right->GetHeight());
+
+  int64_t samples = left->GetWidth() * left->GetHeight();
+  double tally = 0;
+
+  const uint8_t* left_ptr = left->GetBytes();
+  const uint8_t* right_ptr = right->GetBytes();
+  for (int64_t i = 0; i < samples; ++i, left_ptr += 4, right_ptr += 4) {
+    double distance = CalculateDistance(left_ptr, right_ptr);
+    tally += distance * distance;
+  }
+
+  return sqrt(tally / static_cast<double>(samples));
+}
+
+}  // namespace testing
+}  // namespace flutter

engine/src/flutter/impeller/display_list/testing/rmse.h

@@ -0,0 +1,16 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef FLUTTER_IMPELLER_DISPLAY_LIST_TESTING_RMSE_H_
+#define FLUTTER_IMPELLER_DISPLAY_LIST_TESTING_RMSE_H_
+
+#include "flutter/impeller/golden_tests/screenshot.h"
+
+namespace flutter {
+namespace testing {
+double RMSE(const impeller::testing::Screenshot* left,
+            const impeller::testing::Screenshot* right);
+}
+}  // namespace flutter
+#endif  // FLUTTER_IMPELLER_DISPLAY_LIST_TESTING_RMSE_H_

engine/src/flutter/impeller/entity/contents/text_contents.cc

@@ -172,8 +172,11 @@ void TextContents::ComputeVertexData(
       Point uv_size = (atlas_glyph_bounds.GetSize() + Point(1, 1)) / atlas_size;
 
       Point unrounded_glyph_position =
-          basis_transform *
-          (glyph_position.position + scaled_bounds.GetLeftTop());
+          // This is for RTL text.
+          (basis_transform.m[0] < 0 ? Matrix::MakeScale({-1, 1, 1})
+                                    : Matrix()) *
+              glyph_bounds.GetLeftTop() +
+          (basis_transform * glyph_position.position);
 
       Point screen_glyph_position =
           (screen_offset + unrounded_glyph_position + subpixel_adjustment)