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From: pj@pnet51.orb.mn.org (Paul Jacoby)
Newsgroups: comp.sys.mac.programmer
Subject: transfer mode that doesn't clobber black bits?
Message-ID: <3666@orbit.cts.com>
Date: 20 Dec 90 09:45:55 GMT
Sender: news@orbit.cts.com
Organization: People-Net [pnet51], Minneapolis, MN.
Lines: 24

I am in the process of writing some simple graphics routines for a modeling
program.  I have a screen which displays a grid filled with arrows.  On each
arrow, I draw an arrowhead (how appropriate!).  I then want to turn control
over the user, giving them the ability to vary parameters which make the
arrows swing like little VU meters (whooopee!).

Problem: I draw the arrows (based on an array of endpoints) using
PenMode(patXor), and then go in an add the heads.  If I leave the PenMode as
patXor, I end up knocking a few pixels out of the arrowshaft, at the points
where the head and shaft meet.  However, if I change the PenMode to srcCopy, I
then have to figure out how to ERASE the arrowhead...using patXor seems so
convenient for the shaft, I'd love to use it for the head too.  But those
dropped pixels look awful.

Question: Is there a transfer mode that will draw on TOP of those
already-existing black pixels in such a way that RE-drawing will erase (as per
XOR)?  Or am I better off to read farther into IM and other references, learn
how to use copyBits, and give up drawing the individual arrowhead lines one at
a time?
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