Michael T. Jones (mtj++at++babar.asd.sgi.com)
Mon, 21 Oct 1996 12:26:25 -0700
> Some things to think about when using subfaces.
>
> 1. They are very slow on the SGI. I would recommend acvoiding them.
I'm not defensive, but I should clarify this. Subfaces per se are
neither fast nor slow. Subface rendering methods (of which there
are several) *do* range from fast to slow as a function of style,
situation, and system.
Here goes:
cut-out-subfaces
+less fill (less than *any* of the other methods)
-more polys
don't-cut-out-subfaces
a) use stencil
1. on RE, RE2 - 50% fill penalty for base and layer geom
2. on iR, only a 5% fill penalty
either way, though, the draw order *must* be base, then each
layer from bottom to top, which reduces our chances to sort
the database by texture and lighting model (probably not a
big deal for the typical decals)
b) use displace ("glPolygonOffset")
1. in IRIS GL, simplistic implementation
2. in OpenGL, fancy/robust implementation
either way, this should be fast.
c) use reference plane
-only on iR.
+full speed
+our idea, so we're proud if you use it. ;-)
d) static manual offset ("stipes hover off runway")
-no "proper" place if eye-to-layer distance can vary greatly.
e) dynamic manual offset ("displace a DCS as a function of range")
-never better than opengl-style polygon offset
-DCS prevents geoset bbox culling :-(
-just say no.
Quality:
Perfect: cut-out-subfaces, and layer styles a and c
Seemingly Perfect (can be made indetectable from Perfect): b, e
Expensive and not-perfect: d
So, my vote is:
1. cut-outs if you can afford the extra polygons (you can on iR)
2. reference plane (only available on iR)
3. displace (OpenGL method only)
4. stencil (fast on iR only)
Michael
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