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Fix Matrix3(Matrix4) copy producing transposed linear transform #29

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Fix Matrix3(Matrix4) copy producing transposed linear transform #29

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21 changes: 14 additions & 7 deletions CSMath/Matrix3.cs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
using CSMath.Extensions;
using CSMath.Extensions;
using System;
using System.Text;
using System.Threading;
Expand Down Expand Up @@ -91,14 +91,21 @@ public Matrix3(

public Matrix3(Matrix4 matrix)
{
// [PATCH] Matrix4 stores Mij with the FIRST index as the column (its own remarks show
// row 0 = M00, M10, M20, M30 and its vector operator computes X' = M00*x + M10*y + ...),
// while Matrix3's operators treat the FIRST index as the row (X' = M00*x + M01*y + ...).
// A plain copy therefore made the 3x3 linear part the TRANSPOSE of the real transform:
// every rotation applied through getWorldMatrix() ran with the wrong sign (arc start/end
// angles, text rotation, ellipse major-axis, ...). Transpose on copy so that
// Matrix3 * XYZ applies exactly the same linear map as Matrix4 * XYZ.
this.M00 = matrix.M00;
this.M01 = matrix.M01;
this.M02 = matrix.M02;
this.M10 = matrix.M10;
this.M01 = matrix.M10;
this.M02 = matrix.M20;
this.M10 = matrix.M01;
this.M11 = matrix.M11;
this.M12 = matrix.M12;
this.M20 = matrix.M20;
this.M21 = matrix.M21;
this.M12 = matrix.M21;
this.M20 = matrix.M02;
this.M21 = matrix.M12;
this.M22 = matrix.M22;
}

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