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From: oster@dewey.soe.berkeley.edu (David Phillip Oster)
Newsgroups: comp.sys.mac.programmer
Subject: Re: turning on a pixel
Message-ID: <30894@ucbvax.BERKELEY.EDU>
Date: 25 Aug 89 04:23:45 GMT
References: <6294@hubcap.clemson.edu> <227700031@uxa.cso.uiuc.edu>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: oster@dewey.soe.berkeley.edu.UUCP (David Phillip Oster)
Organization: School of Education, UC-Berkeley
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Many people think they can optimise single pixel drawing by just drawing
directly into screen memory, and you can, bu tlook at the problems:

(i) your drawing isn't clipped against screen boundaries, the boundaries of
windows above the one you are trying to draw into, or drawing over the
image of the cursor. (circa 1985)

(ii) If the user has color QuickDraw, the screen could be an unusual size,
shape, and depth. In addition, since a window may span multiple screens,
you can't depend on the basAddr of the pixmap pointing anywhere in
particular, you have to work your way down the list of gDevices, looking
at screen rectangles in global coordinates. (circa 1987)

(iii) If the user has 32-bit Color Quickdraw, the screen may be in another
world entirely: when you call QuickDraw with 32-bit color quickdraw
installed, it flips the system into 32-bit mode, thereby radically
changing the memory map. If you try direct memory access through the
basAddr of a  gDevice directly, from an ordinary program, the system will
probably crash with a bus error. (circa 1989)


Notice, that Apple adds a new,  unanticipatible reason not to do this
every two years or so. Do everything right today to directly twiddle
screen memory, and it is likely that Apple will still kill your program
within two years from now.

Conclusion: directly bit-twiddle an off-screen bitmap, then call CopyBits
to display it for you. It will save you much prsent pain, and still more
future pain.
--- According to the Constitution, the Constitution is unconstitutional:
--- David Phillip Oster            --U.S.Constitution I.10.1: "No State shall
Arpa: oster@dewey.soe.berkeley.edu --enter into any treaty, alliance, or
Uucp: {uwvax,decvax}!ucbvax!oster%dewey.soe.berkeley.edu -- confederation..."