Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!cornell!uw-beaver!fluke!ssc-vax!uvicctr!bcorrie
From: bcorrie@uvicctr.UUCP (Brian Corrie)
Newsgroups: comp.sys.amiga
Subject: Re:  Sculpt-4D Upgrade
Summary: Why not just triangles
Message-ID: <526@uvicctr.UUCP>
Date: 20 Oct 88 06:46:47 GMT
References: <4919@louie.udel.EDU>
Reply-To: bcorrie@uvicctr.UUCP (Brian Corrie)
Followup-To: comp.sys.amiga
Organization: University of Victoria, Victoria B.C. Canada
Lines: 89

My 2-bits on the subject.....

Disclaimer - I know little about the details of either of these
  raytracers, so my comments are on ray tracing in general.

In article <4919@louie.udel.EDU> kvancamp@ardec.arpa (Ken Van Camp) writes:

Whole buncha stuff deleted about Image File Formats

>Meanwhile, Peter da Silva (I spelled it right!) writes:
>
> >Sculpt is not a good example of a ray-tracer.
> >
> >Why not? Simple... it has only one object: a triangle.
>
>If you want a ray tracer of the type you say Sculpt should be, check 
>out C-Light.  It is limited to a small set of primitive solids, such 
>as you are suggesting.  The results I have seen out of C-Light (based 
>only on the 2 demo disks) are not nearly as impressive to me as 
>Sculpt's results.  Why?  Because most real-world objects can simply 
>not be broken down into cones, spheres, cylinders, cubes and derived 
>solids. C-Light's pictures look boxy and unrealistic, as the original 
>Juggler did.

True enough. I don't know much about C-Light, but I assume it has polygons
( read triangles = 3 sided polygon ) so the same objects constructed in
Sculpt can be constructed using C-Light ( unless my assumption is wrong ).

>If you're going to write a solids modeler, you also have 
>to include some kind of arbitrary polyhedron primitive to take care of 
>things that just can't be rendered otherwise.  And then to make it 
>powerful enough to handle complex objects, you have to offer boolean 
>operations to handle all the special cases.

See above comment. Almost all ray tracers have a triangle of sorts as
a primitive to build up complex objects. Of course Constructive Solid
Geometry is nice, but it ain't a peice of cake.

> And in the end, your user 
>interface is twice as complex (which doubles your code size, since 
>let's face it user interface is 90% of the code anyway) and the 
>results aren't any better.

Interfaces are everything in a user application.

> As you say, though, rendering times can be 
>reduced dramatically for simple objects like the peacock.  But the 
>trend nowadays is toward greater realism in rendering -- what if 
>somebody wanted to ray trace a real peacock?  (See this month's issue 
>of IEEE CG&A for some surprisingly realistic ray tracings of natural 
>objects.) For many complex objects that require arbitrary polyhedra, 
>rendering times would increase, not decrease.  That's why I think Eric 
>Graham went with triangles, and I think it was a valid tradeoff.  

I think the main complaint Peter is trying to make is Why just triangles.
Peter will correct me if I am wrong, I'm sure 8^). Since ray tracing
is so CPU intensive, and gets nastier as more primitives are added,
If I want a sphere in my scene, I would much prefer 1 sphere primitive to
a whole bunch ( Thousands maybe ) of triangles that are an approximation.
A sphere is the simplest object of all to intersesct a ray with, so to
me using just triangles isn't justified. It is the same with quadric
surfaces, bicubic patches etc. They need too many triangles to
approximate them. Adding a few simple primitives like spheres, quadrics
etc. doesn't increase the comlexity of the code that much, but look at
the time savings invloved.

On the other hand, I agree that a triangle primitive is necessary to build
realistic scenes, but most ray tracers provide both. This allows the
flexibility of using both the Sculpt method or the C-Light method to
create complex scenes. Of course if C-Light does not provide a
triangle or polygon primitive then I agree, it is not feasible to
create images of current complexity with it.

>
>OK, let the flames begin.  
>

See ma, no flames, just some thoughts......

>                            --Ken Van Camp 

-- 
  Brian Corrie, University of Victoria, Victoria, B.C., Canada

  Under the most rigourously controlled conditions of pressure,
  temperature, volume, humidity and other variables, the organism
  will do as it damn well pleases.

  Don't worry, I know nothing of what I babble about.......