Path: utzoo!attcan!uunet!husc6!rutgers!dayton!joe
From: joe@dayton.UUCP (Joseph P. Larson)
Newsgroups: comp.sys.amiga
Subject: Non-distruction graphics
Message-ID: <6287@dayton.UUCP>
Date: 25 Nov 88 20:37:12 GMT
Reply-To: joe@dayton.UUCP (Joseph P. Larson)
Organization: Dayton-Hudson Dept. Store Co.
Lines: 31


Well, in my never-ending search for Amiga Knowledge, I'm working on
a little draw program.  I've implemented the ever-famous rubber-band
line (hold down mouse, move around and line follows, let go and line
solidifies).  However, as I move around, I'm destroying what was on
the screen before.  My algorithm works something like this:

	a. Set startX and startY
	b. While mousebuttondown at every IntuitionTick
		erase old line drawn on last iteration
		draw new line between startX,startY and currX,currY

What I need instead of draw and erase (erase is basically redraw old
line with background color) is draw and recover old.  The best method
I have discovered for this would be to write a DrawXOR routine to replace
my use of Draw().  It would XOR the bits in the line with what they are
now instead.  Then to erase I can XOR them again.  But this sounds rather
slow.

So -- anyone got a better solution?  Maybe I should add another bit plane
and just fiddle with the bits in that plane?  Ick.

Maybe I have to manipulate everything myself instead of calling Intuition
text and draw routines?

-Joe
-- 
          When you fall on your head do you land on your feet?
UUCP: rutgers!dayton!joe   (Feed my      Dayton Hudson Department Store Company
ATT : (612) 375-3537       picture       Joe Larson/MIS 1060
(standard disclaimer...)   collection)   700 on the Mall      Mpls, Mn. 55402