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From: jimc@iscuva.ISCS.COM (Jim Cathey)
Newsgroups: comp.sys.mac.programmer
Subject: Re: turning on a pixel
Message-ID: <2599@iscuva.ISCS.COM>
Date: 21 Aug 89 16:11:10 GMT
References: <6294@hubcap.clemson.edu>
Organization: ISC Systems Corporation, Spokane WA
Lines: 35

In article <6294@hubcap.clemson.edu> mikeoro@hubcap.clemson.edu (Michael K O'Rourke) writes:
>I am writing a graphics program that turns on many pixels one at a time
>by calculating some mathematical eqns. (kind of a fractal).  It presently
>is using :
>
>moveto(x,y);
>lineto(x,y);
>
>to turn on a pixel.  Is there some quicker way to do this from pascal?
>The program is horribly slow right now and it needs to be speeded up.

For fast point plotting probably your best bet is to allocate an offscreen
bitmap the size of the active area of the screen, and then set the bits in
it yourself.  When done (or periodically) just CopyBits this thing onto the
real screen.

The overhead of clipping tests and all of that really hurts the plotting of
a single pixel.  On your own array it's just a matter of *, shift and mask to
find the bit to set.  If you're really in a hurry you can pre-calculate a
table of byte offsets based on Y value and skip the multiply.  Using assembly
means you can probably do it even faster.

Doing it this way means you don't ever run afoul of bit depth on the screen,
and you wont get whanged on the head if/when the video hardware ever gets
protected, or is in some way strange on any particular machine.  All it costs
you is the array and a bit of skull sweat.

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