Path: utzoo!attcan!uunet!aplcen!samsung!usc!elroy.jpl.nasa.gov!ames!sgi!tarolli@riva.esd.sgi.com
From: tarolli@riva.esd.sgi.com (Gary Tarolli)
Newsgroups: comp.sys.sgi
Subject: Re: World Coord -> ScreenCoords
Summary: correct screen coord mapping
Message-ID: <53238@sgi.sgi.com>
Date: 12 Mar 90 17:17:02 GMT
References: <8387@pt.cs.cmu.edu>
Sender: tarolli@riva.esd.sgi.com
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 26

In article <8387@pt.cs.cmu.edu>, cycy@isl1.ri.cmu.edu (Scum) writes:
> Can anybody out there tell me how to map 3d world coordinates to screen
> coordinates? I can seem to find anything in the documentation.
> 				Thanks,
> 					-- Chris.

There are 2 things to remember when trying to do this:
	1) viewports on the IRIS go from the left edge of the left most
		pixel to the right edge of the right most pixel
		same for top and bottom, thus viewport(0,1,0,1) is 
		exactly 2 pixels by 2 pixels.
	2) the screen coordinate of a pixel center is at an integer

To map 3-D world coordinates [100,200] to a 100 x 100 viewport you would
use the following ortho2 call:

	ortho2(100-.5, 200+.5, 100-.5, 200+.5);

The reason for the +- 0.5 is to account for the left and right edges of the
pixels (see (1) above). 

Also check the screenspace() call which does world coords to absolute
screen coordinates (without regard for your window location or origin).

--
						Gary Tarolli