Problem obtaining a continuous dPds and dPdt on polygon / subdivision surfaces

[Olivier Renouard]

Hi people!

I'm having trouble converting normal maps back from tangent space to
world/camera space in a surface or displacement shader in Renderman. What
stops me is I can't seem to compute a continuous dPds or dPdt (the tangents)
on a polygonal or subdivision surface (it's no trouble on a nurbs patch
though). Of course I expect discontinuities where s and t are discontinous,
but I get them also at facet boundaries? I'm not very experienced in
Renderman and I usually manage this without much hassle in Maya or Mental
Ray renderer so I guess there is something I'm overlooking or doing wrong
here. Declaring s and t as "facevarying" on a subdivision surface didn't
help though I expected so.

You can check the general idea of what I'm trying to achieve here :

http://www.drone.org/tutorials/rayDisplace_renderman.html

And here is a sample source rib as well as the images of what I get (note
the torso_dPds.jpg where I'm hoping for continuous tangents and failing to
get them) :

the sample rib file

http://www.drone.org/shared/subdTorsoNormals.rib

the dPds vector expressed as color, showing the discontinuities causing my
trouble

http://www.drone.org/shared/torso_dPds.jpg

the st coordinates

http://www.drone.org/shared/torso_st.jpg

the subdivision surface

http://www.drone.org/shared/torso_subd.jpg

In case someone tackled the problem already or get some insights on this,
I'd be must obliged to see soem light shed for me!

Olivier Renouard

[Dan Maas]

"Olivier Renouard" <oli...@drone.org> wrote in message news:<3e60abcb$0$10448$79c1...@nan-newsreader-03.noos.net>...

> I'm having trouble converting normal maps back from tangent space to
> world/camera space in a surface or displacement shader in Renderman. What
> stops me is I can't seem to compute a continuous dPds or dPdt (the tangents)
> on a polygonal or subdivision surface (it's no trouble on a nurbs patch
> though).

The following post may or may not be relevant:

http://groups.google.com/groups?selm=36AF924D.41C6%40hammerhead.com

Regards,
Dan

[Olivier Renouard]

Yes, it's a possibility, akin to what game engines seem to use, storing
tangents in the mesh definition.

However I would have to update them in the 3D animation package every frame
to account for deformations...

I'd be curious to see wether I can get a continuous tangent information at
render time the way using tangentU and tangentVCamera works in Maya...

Olivier

"Dan Maas" <danie...@yahoo.com> a écrit dans le message de news:
fcd1af44.03030...@posting.google.com...