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From: CXT105@psuvm.psu.edu (Christopher Tate)
Newsgroups: comp.sys.mac.programmer
Subject: Direct-to-screen drawing
Message-ID: <91113.105146CXT105@psuvm.psu.edu>
Date: 23 Apr 91 14:51:46 GMT
Organization: Penn State University
Lines: 30

I'm trying to figure out how to do direct-to-screen drawing, in the course
of working on things like antialiasing.  The problems I'm running into are
strange, though....

If I open a CWindow, its baseAddr is the address of the main screen buffer.
Okay; well, I guess I'll just draw directly onto the screen (since I'm not
worried about being friendly to other apps just yet :-).  Unfortunately
I'm getting crashes when I try to write to the screen in certain ways that
*look* like they should work...

Example:  base is an (unsigned char *) that points to the screen buffer.  I
can verify this by saying  *base = 33, and seeing the pretty red dot appear
in the top left corner of the screen.

However, if I try to set up pointer arithmetic of the form
                 *(base + x + rowBytes*y) = 33;
a couple of different things happen.  If x, y, and rowBytes are declared as
type short, then it works -- sort of.  I get strange wraparound to the top
of the screen which I assume is because of overflow problems in the
multiplication.

If x, y, and rowBytes are declared long, it crashes.  Every time.  Anyone
have any ideas why?  Or, alternatively, has anyone already worked out the
nitty-gritty of direct screen bit-twiddling to a color window?

-------
Christopher Tate                  |  "Hear perfect strangers call you by name,
cxt105@psuvm.psu.edu              |   Pay good money to hear "Fire and Rain"
 ..!psuvax1!psuvm.bitnet!cxt105   |   Again and again and again....
cxt105@psuvm.bitnet               |   That's why I'm here!"   -- James Taylor