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Compute Ellipse Width

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Hi, guys, When I am reading a piece of code that computations the width of an Ellipse, I can't understand it. The Ellipse (2D) with Theta degree rotation and L1 and L2 for major and minor axias length. The half width of Ellipse is computed from an equation: sqrt( L1^2*sin(th)^2+L2^2*cos(th)^2 ); (1) Is this right? I know the matrix representation of Ellipse is: Maitrx: transpose(ellipseRotation) . ellipseScaling . ellipseRotation; (2) The expression (1) is actually : L1*L2*sqrt(Matrix[1][1]) If expression (1) is right, so why? Thanks.

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