The scan-line conversions of the drawing functions are very much host-dependent. In other words, you cannot assume that, for example
(draw-point port x y)
has exactly the same effect on all machines. Some machines might put pixels down and to the right of integer coordinates
while others may center the pixel at
X11/Motif note: The stipple, pattern , mask-x, and mask-y slots of the graphics state are also used.
See also the LispWorks
CAPI Reference Manual
(which draws a circle) and
(which draws an ellipse) and other drawing functions.