Fix Invert(ref Matrix3x2) (#962)

* Fix Invert(ref Matrix3x2) * erroneous results were given by matrix fields being altered sequentially, some fields would use new state instead of the input * Check for matrix.Determinant() = 0, now returns the identity instead of det becoming infinite" * Add unit test for Matrix3x2.Invert
craftworkgames · Nov 8, 2024 · 8b02498 · 8b02498
1 parent d3b4482
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diff --git a/source/MonoGame.Extended/Math/Matrix3x2.cs b/source/MonoGame.Extended/Math/Matrix3x2.cs
@@ -562,14 +562,21 @@ public static void Invert(ref Matrix3x2 matrix)
     {
         var det = 1.0f / matrix.Determinant();
 
-        matrix.M11 = matrix.M22 * det;
-        matrix.M12 = -matrix.M12 * det;
-
-        matrix.M21 = -matrix.M21 * det;
-        matrix.M22 = matrix.M11 * det;
+        if (float.IsInfinity(det))  // Det(M) = 0
+        {
+            matrix = Identity;
+            return;
+        }
 
-        matrix.M31 = (matrix.M32 * matrix.M21 - matrix.M31 * matrix.M22) * det;
-        matrix.M32 = -(matrix.M32 * matrix.M11 - matrix.M31 * matrix.M12) * det;
+        // The new 3x2 matrix is the first and second column of the inverse of the 3x3 matrix given by adding the third column (0, 0, 1) to the input matrix
+        matrix = new(
+            matrix.M22 * det,
+            -matrix.M12 * det,
+            -matrix.M21 * det,
+            matrix.M11 * det,
+            (matrix.M32 * matrix.M21 - matrix.M31 * matrix.M22) * det,
+            (matrix.M31 * matrix.M12 - matrix.M32 * matrix.M11) * det
+        );
     }
 
     /// <summary>

diff --git a/tests/MonoGame.Extended.Tests/Math/Matrix3x2.cs b/tests/MonoGame.Extended.Tests/Math/Matrix3x2.cs
@@ -17,4 +17,20 @@ public void ConstructorTest()
         Assert.Equal(y, matrix.Y);
         Assert.Equal(z, matrix.Z);
     }
+
+    [Fact]
+    public void InverseTest()
+    {
+        Matrix3x2 posDeterminant = new Matrix3x2(3, -7, 4, 2, 13, -2);
+        Matrix3x2 singular = new Matrix3x2(2, 1, 4, 2, 3, -4);
+        Matrix3x2 negDeterminant = new Matrix3x2(1, -5, 3, 2, 3, -4);
+
+        Matrix3x2 posExpected = new Matrix3x2(1f/17, 7f/34, -2f/17, 3f/34, -1f, -5f/2);
+        Matrix3x2 singularExpected = Matrix3x2.Identity;
+        Matrix3x2 negExpected = new Matrix3x2(2f/17, 5f/17, -3f/17, 1f/17, -18f/17, -11f/17);
+
+        Assert.Equal(Matrix3x2.Invert(posDeterminant), posExpected);
+        Assert.Equal(Matrix3x2.Invert(singular), singularExpected);
+        Assert.Equal(Matrix3x2.Invert(negDeterminant), negExpected);
+    }
 }