I want to create a seam for an ellipse. (Mathematically I think this means that there are constant-length normal lines between the ellipse and the curve that creates the seam, but I'm not 100% sure that definition is accurate.) I know I can't do this by creating another ellipse with the axes reduced by the seam width because the distance between the two ellipses isn't constant. (i.e. in this image the green line is shorter than the two red lines.)
I can convert ellipses into cubic Beziers.
Is there a way to calculate a modification to the control points of the inner Bezier to make the distance between the outer ellipse and inner Bezier constant?