Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!ucbvax!hoptoad!tim
From: tim@hoptoad.uucp (Tim Maroney)
Newsgroups: comp.sys.mac.programmer
Subject: Re: 3K "limitation" for CopyBits
Message-ID: <7731@hoptoad.uucp>
Date: 21 Jun 89 19:35:02 GMT
References: <3224@ncsuvx.ncsu.edu>
Reply-To: tim@hoptoad.UUCP (Tim Maroney)
Organization: Eclectic Software, San Francisco
Lines: 24

In article <3224@ncsuvx.ncsu.edu> jnh@ecemwl.UUCP (Joseph N. Hall) writes:
>Does the 3K-byte suggested restriction for CopyBits (mentioned in IM I)
>still apply?  Does any such "restriction" still apply?  Obviously, even
>a large menu requires more than 3K of cached bitmap underneath; is there
>any advice on what the limits on the maximum size of a CopyBits transfer
>ought to be?  The whole screen?  Whatever will fit on the stack?

I have it on fairly good authority that what happens in CopyBits is
that each row gets pushed onto the stack.  therefore, it is not so much
the total size of the bitmap, but the width, that determines the
safety.  If you do a MacPaint file bitmap complete, that's safe,
because the bitmap is narrow despite being over 50K.  On the other
hand, if you do a single row that stretches from here to Cleveland,
then you could be in big trouble from the stack sniffer.  Very wide
bitmaps should be broken up into narrower ones to do CopyBits.  You
should be able to do this by twiddling the source and destination
rectangles.
-- 
Tim Maroney, Mac Software Consultant, sun!hoptoad!tim, tim@toad.com
Postal: 424 Tehama, SF CA 94103; Phone: (415) 495-2934

"I slept with Faith, and found a corpse in my arms on awaking; I drank and
 danced all night with Doubt, and found her a virgin in the morning."
    -- Aleister Crowley, THE BOOK OF LIES