Len Granowetter (lengrano++at++mak.mak.com)
Wed, 25 Jan 95 18:00:49 -0500
We have been having a bit of confusion lately about textures that
contain alpha.
We recently obtained a MultiGen .flt database of a cloud polygon
(well, actually two back-to-back polygons with cloud textures). The
cloud texture is an rgba texture. Certain areas are completely
transparent, certain areas are completely opaque, and most areas have
alpha values somewhere in between.
When we used the cloud database with our application (the MaK DIS
Stealth, which is based on perfly) on a reality engine at the I/ITSEC
show, the clouds looked great. We brought the database back here, and
tried to run it on our VGXT, and now all areas of the clouds look
either completely transparent or completely opaque. (This is true in
perfly as well.)
We went and added the following line at initalization time:
pfTransparency(PFTR_ON);
and things looked much better, but not great. Specifically, near the
edges of the clouds, the alpha value gradually goes to zero, making it
hard to tell exactly where the cloud ends and the zero-alpha area
begins. This is the way it appeared on the reality engine, and this
is the way it appears on the VGXT under MultiGen. However, in
Performer on the VGXT, the cloud has a much sharper edge.
Several questions:
1) If PFTR_OFF is the default pfTransparency value, then why did
transparencies work fine on the reality engine without explicitly
setting the mode to PFTR_ON? Is the default different on different
machines?
One thought is that the clouds' pfGeoSet has its transparency state
set to PFTR_ON. But then it would have worked on the VGXT too.
Another thought is that the clouds' transparency state is
PFTR_MS_ALPHA, which does not work on a VGXT. But then it wouldn't
work on the VGXT even if the global state was PFTR_ON, which it does.
Is blindly setting the global state to PFTR_ON a bad idea, i.e. is it
a better idea to just set the clouds' state to PFTR_ON?
2) Why do we get sharper cloud edges (as described above) on the VGXT?
Someone suggested that the VGXT is capable of only 16, rather than 256
levels of alpha. Is this true?
Any help with any of these questions would be greatly appreciated.
Feel free to write or call me for more information about the problems.
-Len Granowetter
----
|\ /| . . | / Len Granowetter
| \/ | _ | ( MaK Technologies
| | /-\ | \ (617) 876-8085 Ext. 121
T E C H N O L O G I E S lengrano++at++mak.com
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