When looking at the circle representation on a graphics device , which distance between the rows of pixles is different from the distance between the columns, it is obvious that the circle on the screen looks like an ellipse.
The construction of a centred ellipse can be
done by the Bresenham's algorithm, similarly as at the circle,
while we have to compute the values as a quadrant of a circle.
Algorithm is used from John Kennedy paper : http://homepage.smc.edu/kennedy_john/