Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ukma!xanth!lll-winken!uunet!snjsn1!bilbo!greg
From: greg@bilbo (Greg Wageman)
Newsgroups: comp.graphics
Subject: Re: viewing terminology
Message-ID: <756@snjsn1.SJ.ATE.SLB.COM>
Date: 20 Mar 89 18:04:02 GMT
References: <4400032@m.cs.uiuc.edu>
Sender: news@SJ.ATE.SLB.COM
Reply-To: greg@sj.ate.slb.com (Greg Wageman)
Organization: Schlumberger ATE, San Jose, CA
Lines: 56

In article <4400032@m.cs.uiuc.edu> shirley@m.cs.uiuc.edu writes:
>
>I use the following to specify a view in my programs:
>
>lookfrom (x, y, z)
>lookat (x, y, z)
>view-up-vector (x, y, z)
>field-of-view (angle)
>
>This is umambiguous for a square viewport, but when the width and height
>are different, field-of-view could be left/right or up/down.  Is one
>correct, or if not, is one to be prefered?

I think the most intuitive way to implement this is to have the field
of view referenced to the narrower axis (which is typically the
vertical axis for both motion picture and video media).  The
additional viewing area on the wider axis is then "bonus" (compared to
the square viewport with the dimensions of the smaller axis.

This approach has several advantages:

	1) The distortion introduced by forcing both (unequal-size)
	axes to the same field-of-view is avoided.

	2) It is more natural for the user to specify field-of-view
	for the smaller axis, and get "extra" material in the larger,
	than to specify for the larger axis and try to determine how
	much will be lost along the smaller.

	3) When porting code to displays with different aspect
	ratios, the resulting image will be the same for the square
	of the dimensions of the smaller axis, at the center of the
	image.

	This would allow for the use of "cinemascope"-like effects by
	deliberate selection of the aspect ratio of the image.

Alternatively, you could allow specification of field-of-view for each
axis, allowing for additional effects, such as an anamorphic
compression of one axis.


Longish .signature follows.  Skip now, or don't complain!

Greg Wageman			ARPA:  greg@sj.ate.slb.com
Schlumberger Technologies	BIX:   gwage
1601 Technology Drive		CIS:   74016,352
San Jose, CA 95110-1397		UUCP: ...!uunet!sjsca4!greg
(408) 437-5198
------------------
There's nothing I hate more than a Usenet posting which took three
seconds to compose and three minutes to type, glibly dismissing three
years (or three decades) of an author's work in three lines.
------------------
Opinions expressed herein are solely the responsibility of the author.
(And the author wouldn't have it any other way.)