From 4f62eabb8b6920333453df353700dc92526c040b Mon Sep 17 00:00:00 2001
From: Brandon DeRosier <bdero@google.com>
Date: Fri, 12 May 2023 15:28:19 -0700
Subject: [PATCH] [Impeller] Ignore the Z basis when scaling text (#41995)

Fixes https://github.com/flutter/flutter/issues/126510.

`GetMaxBasisLength()` includes the Z basis vector, which happens to not
be getting scaled down along with the X and Y for this reproduction
case. And so given the 2D nature of text and the intent of this scale
parameter, it's returning a value that's too large here.

(cherry picked from commit 6dc9b28677b7d3e65594864347a2c1e3c1b19481)
---
 impeller/display_list/display_list_dispatcher.cc |  2 +-
 impeller/geometry/geometry_unittests.cc          | 15 +++++++++++++++
 impeller/geometry/matrix.cc                      |  8 ++++++++
 impeller/geometry/matrix.h                       |  2 ++
 4 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/impeller/display_list/display_list_dispatcher.cc b/impeller/display_list/display_list_dispatcher.cc
index c50d0d00ea00a..53f4e67357c6a 100644
--- a/impeller/display_list/display_list_dispatcher.cc
+++ b/impeller/display_list/display_list_dispatcher.cc
@@ -1283,7 +1283,7 @@ void DisplayListDispatcher::drawDisplayList(
 void DisplayListDispatcher::drawTextBlob(const sk_sp<SkTextBlob> blob,
                                          SkScalar x,
                                          SkScalar y) {
-  Scalar scale = canvas_.GetCurrentTransformation().GetMaxBasisLength();
+  Scalar scale = canvas_.GetCurrentTransformation().GetMaxBasisLengthXY();
   canvas_.DrawTextFrame(TextFrameFromTextBlob(blob, scale),  //
                         impeller::Point{x, y},               //
                         paint_                               //
diff --git a/impeller/geometry/geometry_unittests.cc b/impeller/geometry/geometry_unittests.cc
index 54625b85b9617..a904c07e92124 100644
--- a/impeller/geometry/geometry_unittests.cc
+++ b/impeller/geometry/geometry_unittests.cc
@@ -336,6 +336,21 @@ TEST(GeometryTest, MatrixGetMaxBasisLength) {
   }
 }
 
+TEST(GeometryTest, MatrixGetMaxBasisLengthXY) {
+  {
+    auto m = Matrix::MakeScale({3, 1, 1});
+    ASSERT_EQ(m.GetMaxBasisLengthXY(), 3);
+
+    m = m * Matrix::MakeSkew(0, 4);
+    ASSERT_EQ(m.GetMaxBasisLengthXY(), 5);
+  }
+
+  {
+    auto m = Matrix::MakeScale({-3, 4, 7});
+    ASSERT_EQ(m.GetMaxBasisLengthXY(), 4);
+  }
+}
+
 TEST(GeometryTest, MatrixMakeOrthographic) {
   {
     auto m = Matrix::MakeOrthographic(Size(100, 200));
diff --git a/impeller/geometry/matrix.cc b/impeller/geometry/matrix.cc
index 70ede5866aee8..0819465591496 100644
--- a/impeller/geometry/matrix.cc
+++ b/impeller/geometry/matrix.cc
@@ -202,6 +202,14 @@ Scalar Matrix::GetMaxBasisLength() const {
   return std::sqrt(max);
 }
 
+Scalar Matrix::GetMaxBasisLengthXY() const {
+  Scalar max = 0;
+  for (int i = 0; i < 3; i++) {
+    max = std::max(max, e[i][0] * e[i][0] + e[i][1] * e[i][1]);
+  }
+  return std::sqrt(max);
+}
+
 /*
  *  Adapted for Impeller from Graphics Gems:
  *  http://www.realtimerendering.com/resources/GraphicsGems/gemsii/unmatrix.c
diff --git a/impeller/geometry/matrix.h b/impeller/geometry/matrix.h
index d30d803491288..6938cf930fffa 100644
--- a/impeller/geometry/matrix.h
+++ b/impeller/geometry/matrix.h
@@ -290,6 +290,8 @@ struct Matrix {
 
   Scalar GetMaxBasisLength() const;
 
+  Scalar GetMaxBasisLengthXY() const;
+
   constexpr Vector3 GetBasisX() const { return Vector3(m[0], m[1], m[2]); }
 
   constexpr Vector3 GetBasisY() const { return Vector3(m[4], m[5], m[6]); }