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Fix Invert(ref Matrix3x2) #962

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Nov 8, 2024
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Fix Invert(ref Matrix3x2) #962

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c04d9c9
Fix Invert(ref Matrix3x2)
Eb3yr Nov 8, 2024
f91e141
Add unit test for Matrix3x2.Invert
Eb3yr Nov 8, 2024
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21 changes: 14 additions & 7 deletions source/MonoGame.Extended/Math/Matrix3x2.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -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>
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16 changes: 16 additions & 0 deletions tests/MonoGame.Extended.Tests/Math/Matrix3x2.cs
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Expand Up @@ -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);
}
}
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