From ee87ce65172d6764ae4792d777178bfc4573e0b8 Mon Sep 17 00:00:00 2001
From: Brandon DeRosier <bdero@google.com>
Date: Tue, 6 Sep 2022 16:58:53 -0700
Subject: [PATCH] Add utility to check if a Matrix is aligned
 (flutter/engine#35951)

---
 .../impeller/geometry/geometry_unittests.cc   | 14 +++++++++++
 engine/src/flutter/impeller/geometry/matrix.h | 25 +++++++++++++++++++
 engine/src/flutter/impeller/geometry/scalar.h |  7 +++++-
 3 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/engine/src/flutter/impeller/geometry/geometry_unittests.cc b/engine/src/flutter/impeller/geometry/geometry_unittests.cc
index db027c3c341..324e607daf0 100644
--- a/engine/src/flutter/impeller/geometry/geometry_unittests.cc
+++ b/engine/src/flutter/impeller/geometry/geometry_unittests.cc
@@ -351,6 +351,20 @@ TEST(GeometryTest, MatrixGetDirectionScale) {
   }
 }
 
+TEST(GeometryTest, MatrixIsAligned) {
+  {
+    auto m = Matrix::MakeTranslation({1, 2, 3});
+    bool result = m.IsAligned();
+    ASSERT_TRUE(result);
+  }
+
+  {
+    auto m = Matrix::MakeRotationZ(Degrees{123});
+    bool result = m.IsAligned();
+    ASSERT_FALSE(result);
+  }
+}
+
 TEST(GeometryTest, QuaternionLerp) {
   auto q1 = Quaternion{{0.0, 0.0, 1.0}, 0.0};
   auto q2 = Quaternion{{0.0, 0.0, 1.0}, kPiOver4};
diff --git a/engine/src/flutter/impeller/geometry/matrix.h b/engine/src/flutter/impeller/geometry/matrix.h
index 9be6707f085..b299d1ad9fe 100644
--- a/engine/src/flutter/impeller/geometry/matrix.h
+++ b/engine/src/flutter/impeller/geometry/matrix.h
@@ -258,6 +258,31 @@ struct Matrix {
             m[9] == 0 && m[10] == 1 && m[11] == 0 && m[14] == 0 && m[15] == 1);
   }
 
+  constexpr bool IsAligned(Scalar tolerance = 0) const {
+    int v[] = {!ScalarNearlyZero(m[0], tolerance),  //
+               !ScalarNearlyZero(m[1], tolerance),  //
+               !ScalarNearlyZero(m[2], tolerance),  //
+               !ScalarNearlyZero(m[4], tolerance),  //
+               !ScalarNearlyZero(m[5], tolerance),  //
+               !ScalarNearlyZero(m[6], tolerance),  //
+               !ScalarNearlyZero(m[8], tolerance),  //
+               !ScalarNearlyZero(m[9], tolerance),  //
+               !ScalarNearlyZero(m[10], tolerance)};
+    // Check if all three basis vectors are aligned to an axis.
+    if (v[0] + v[1] + v[2] != 1 ||  //
+        v[3] + v[4] + v[5] != 1 ||  //
+        v[6] + v[7] + v[8] != 1) {
+      return false;
+    }
+    // Ensure that none of the basis vectors overlap.
+    if (v[0] + v[3] + v[6] != 1 ||  //
+        v[1] + v[4] + v[7] != 1 ||  //
+        v[2] + v[5] + v[8] != 1) {
+      return false;
+    }
+    return true;
+  }
+
   constexpr bool IsIdentity() const {
     return (
         // clang-format off
diff --git a/engine/src/flutter/impeller/geometry/scalar.h b/engine/src/flutter/impeller/geometry/scalar.h
index 3ca2f1dab66..8a080e18b52 100644
--- a/engine/src/flutter/impeller/geometry/scalar.h
+++ b/engine/src/flutter/impeller/geometry/scalar.h
@@ -19,10 +19,15 @@ constexpr T Absolute(const T& val) {
   return val >= T{} ? val : -val;
 }
 
+constexpr inline bool ScalarNearlyZero(Scalar x,
+                                       Scalar tolerance = kEhCloseEnough) {
+  return Absolute(x) <= tolerance;
+}
+
 constexpr inline bool ScalarNearlyEqual(Scalar x,
                                         Scalar y,
                                         Scalar tolerance = kEhCloseEnough) {
-  return Absolute(x - y) <= tolerance;
+  return ScalarNearlyZero(x - y, tolerance);
 }
 
 struct Degrees;