145. Plotting ellipses
~~~~~~~~~~~~~~~~~
By referring to the list of PLOT commands on page 232 of the Master Welcome
guide, you can easily work out how to draw rectangles etc., using similar
methods to drawing triangles. However, drawing ellipses is a bit more tricky.
You must first 'visit' the dead centre of the ellipse, as you might expect.
You must then visit the point where the ellipse cuts a horizontal line drawn
through the centre. Since the Y-coordinate of this point is by definition the
same as that of the centre, it doesn't matter what Y-coordinate you put in
the MOVE, DRAW or PLOT command you use to visit the point, as the true value
is assumed automatically. The horizontal line cuts the ellipse twice, but it
doesn't matter which of the two points you choose.
Finally, you should use the ellipse PLOT command, eg 197, to specify the
highest point of the ellipse, ie the point with greatest Y-coordinate.
(Alternatively, it can be the lowest point of the ellipse.) Anyone with
access to the Acorn GXR manual should find no difficulty with the Master's
extra PLOT commands, otherwise you need to see the Master Reference Manual
part 1, which is at last available.