Posted by Mike Hayes on Mar 12, 2015; 12:23am
URL: https://forum.librecad.org/Isometric-Circles-tp5711165p5711178.html

An isometric "square" is a four sided quadrilateral. All four sides are of equal length and each opposite pair of sides is parallel. So there should be a unique mathematical solution to an ellipse touching the midpoint of all four sides.

I've gone back and experimented with the method in my original post and it seems the "Draw Ellipse Inscribed in a Quadrilateral" tool is sensitive to the order in which the four lines are selected and or the direction they are drawn in (i.e. left to right, top to bottom, etc).

For example, draw a rectangle using the "Rectangle" tool on a grid set to orthoganal. Then use the "Draw Ellipse Inscribed in a Quadrilateral" tool. If the sides are selected in the order left, bottom, top, right then the tool successfully draws an ellipse. But if the sides are selected in the order bottom, right, top, left then it doesn't work.

The "Rectangle" tool always draws the lines in a specific order with a specific direction so the behaviour described above is at least predictable. But now try creating a rectangle by drawing four separate lines with the "Line" tool in random order and with random up/down left/right direction. Then use the "Draw Ellipse Inscribed in a Quadrilateral" tool and you will find that sometimes it works and sometimes it doesn't with no obvious pattern.

Mike