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Quickdraw arc oddities

Are there any known problems with the QuickDraw arc routines as 
accessed from LISP? I'm writing a set of view classes for drawing
various kinds of charts, and the 'fill-arc' and 'frame-arc'
routines are behaving extremely strangely.

My code includes a function that is called by 'view-draw-contents'
and steps through a series of values to draw successive segments
of a pie-chart with different colored segments. Each segment drawn
is drawn by a call to 'fill-arc' (inside a 'with-fore-color'

The values passed to 'fill-arc' are always identical, but the
results vary widely. Sometimes the segments are drawn correctly,
at other times nothing is drawn for most of the segments, and
then a complete filled circle (all one color) is drawn. If I
switch out of LISP and do something that forces a redraw (making
the screen-saver kick in is a good way to do this) then the
pie is redrawn correctly; if I simply switch to a different LISP
window (even if the window covers the pie chart completely,
forcing a complete redraw) then I'll get the single-color circle.

I'm running this on a MacIIci with System Z-7.0.1, 20MB of memory,
and an 8*24 graphics card set to 256 colors (same thing happens
when I draw in black and white, btw).

LISP version is MCL 2.0.

This one has me really confused; has anyone else seen anything
similar and, more to the point, does anyone know how to fix
it? Answers direct to me, please - I'm not subscribed to 'info-mcl'.

Thanks in advance,
			Angus McIntyre
			Knowledge Technologies n.v.