Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!infonode!genius!matt
From: matt@genius.Berkeley.EDU (new user)
Newsgroups: comp.graphics
Subject: Re: Ray Tracing Acceleration ... Is non-uniform s
Message-ID: <1991Jun10.164429.14398@infonode.ingr.com>
Date: 10 Jun 91 16:44:29 GMT
References: <1991May14.070444.18261@images.cs.und.ac.za>  <1991Jun8.231208.498@otago.ac.nz>
Sender: usenet@infonode.ingr.com (Usenet Administrator)
Reply-To: matt@genius.Berkeley.EDU (new user)
Organization: Intergraph Corp. Huntsville, AL
Lines: 55


|> > 
|> > Graham Furniss, PhD student, Graphics Research Lab, University of Otago, writes

|> > |> 2). Octree flattening. This takes the adaptive division from 1) and converts it
|> > |> into a regular space division. This allows the use of a fast voxel skipping
|> > |> algorithm. ( John Cleary and Geoff Wyvill )
|> > |> 
|> > This sounds like an interesting idea.  How do turn an octree into uniform space
|> > division without losing the advantages the octree gave in the first place.
|> 
|> If you have a voxel which is not divided to the bottom level yet, generate the
|> number of bottom level voxels it contains and put the same sub-model in each.
|> No calculation required, just a flood fill (sort of). This is a gross
|> simplification but gives the flavour.
|> 
|> > 
How is this different fom regular uniform division?  It seems like USS is the same
as an octree method that has been divided to the lowest level.  I'm not sure what
is meant by sub-model.  Are the same object that intersect the higher level voxel
associated with all the lower level voxels?

|> > |> 3). Edge following. This uses the fact that in any picture each colour will
|> > |> cover more than a few pixels. This means that rays are cast only on the edges
|> > |> of areas of colour; the inside areas can be flood filled without casting extra
|> > |> rays. This has resulted in casting less than %10 of the rays for a full ray
|> > |> trace in some cases. Note that the resulting picture is identical to the fully
|> > |> traced version. ( Geoff Wyvill, Alistair Ward and Tim Brown )
|> > |> 
|> > How do you determine where the color edges are without casting rays?
|> 
|> Dare I say it? "By casting rays". To begin with we cast a sparse grid of rays
|> over the image space. If two adjacent rays have different colour values we
|> search for the point or points of change, then use these as seeds for the edge
|> follower. This grid represents a small over head and there is the chance that
|> if the grid is too coarse it will miss small objects but this proves to be
|> relatively rare. Typicaly we use a 50 x 50 grid for a picture 512 x 512. 
|> 
|> >
This sounds very much like adapative anti-aliasing.  How do you known when to
do the flood fill?  How is a boundary generated and saved?  Is this method
effective with textured surfaces?  Could this method be used to generate shadows?
 
 
|> > |> All the techniques used in our system are published, mostly 
|> > |> in CGI proceedings and "The Visual Computer". 
|> > 
|> > Can you post the specific month and year?
|> 
|> You asked for it...
|> 
Are these papers available at any ftp sites?

take it easy
Matthew