From: Scott Herod (herod++at++rtset.com)
Date: 03/01/2000 12:43:13
Hello Angus,
Thanks for the response.
After thinking about your comments about billboards, I agree that in
case of billboarding to the local view vector it is reasonable to
depth sort using Euclidean distances. ( I have an image of a mirror
ball. )
I have two problems with the Euclidean technique. ( Computational
complexity is the same in both cases. )
First, if I have two rectangular geosets on planes parallel to the eye
space planes and I move them backwards and forwards w.r.t each other,
there is a region where they depth sort one way, and alpha blend
another. This region is the volume between the plane corresponding to
the depth of the center of the center most plane and sphere centered
at the eye point which passes through the center of that plane. For
a plane centered in the viewing frustum the distance between where
depth buffering switches as you move and where alpha-blending starts
to work correctly is significant.
Second, with a perspective viewing frustum, the difference between
a Euclidean distance and projected distance is a function of the
field of view. As the FOV decreases, the discrepency diminishes with
limit 0 as the FOV becomes 0. However, switching to an orthographic
channel does not use projective distance. ( That might be considered
a bug. )
Thank you,
Scott Herod
herod++at++rtset.com
Angus Dorbie wrote:
>
> The Performer method is more accurate for the general case.
> It seems that you have a contrived example in mind where your test
> produces better results, (planes parallel to eye space planes of equal
> z) but for the general case of flying through a complex world the range
> method is better.
>
> For instance your method would be desirable for sorting trees which were
> billboarded to the channel forward viewing vector, the Performer method
> is better for sorting billboarded trees which billboard to a local
> viewer vector to each individual tree.
>
> Unfortunately it is not an option for you to change this, the sort is an
> internal Performer algorithm.
>
> Cheers,ANgus.
>
> Scott Herod wrote:
> >
> > Hello,
> >
> > I have another question about drawing order and the relationship
> > to the depth buffer. If I have a symmetric, prospective pfChannel
> > and draw a rectangular geoset that is perpindicular to the frustum's
> > axis, the depth buffer is filled with a rectangle of constant
> > values. ( It's easy to check and is essentially similar triangles. )
> >
> > On the other hand, depth sorting of transparent geosets seems to
> > be done by computing the Euclidean distance between the centers of
> > the geosets' bounding spheres and the eye-point.
> >
> > Why isn't depth sorting of geosets computed so that iso-surfaces
> > are planes parallel to the front and rear clipping planes as is the
> > case for filling the depth buffer rather than concentric spheres?
> > Is is easy to change the sorting algorithm?
> >
> > Thank you.
> >
> > Scott Herod
> > herod++at++rtset.com
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