Flat Surface Reflection problem

Flat Surface Reflection problem

Post by Samu » Sun, 02 Jun 2002 14:06:26



Hi...

I am a newbie at renderman, and have been primarily been using BMRT
and entropy. I seem to be having a problem with Flat surface
relfections as described in the App note #6 in the Pixar
Documentation.
I manage to reflect the camera through the mirror plane and then
render the image, but when i apply the image as a texture on the
relflective surface the image seems to be flipped around. I used the
shader described in the app note( transforming P into "NDC" space and
then using it as texture access coordinates). I setup the scene in
Maya and have 2 camera's 1 the view cam and the second the reflected
cam. They r positioned correctly but yet when i render using the image
capured from the reflected camera, it gets flipped around.
Any Ideas...???

Sam!!!

 
 
 

Flat Surface Reflection problem

Post by Will » Wed, 05 Jun 2002 14:41:25



Quote:(Samuel) writes:
>I manage to reflect the camera through the mirror plane and then
>render the image, but when i apply the image as a texture on the
>relflective surface the image seems to be flipped around. I used the
>shader described in the app note( transforming P into "NDC" space and
>then using it as texture access coordinates). I setup the scene in
>Maya and have 2 camera's 1 the view cam and the second the reflected
>cam. They r positioned correctly but yet when i render using the image
>capured from the reflected camera, it gets flipped around.
>Any Ideas...???

>Sam!!!

Well, there is probably some silly little bug in your code where you missed a
minus sign, or something.  You can spend an hour looking for it, or just flip
the image back.  I would probably just make a pragmatic fix, and flip the
reflection image, but then...  I'm lazy.
------------------
What good is knowledge if you can't torment others with it?   --Paul W. DeMone

 
 
 

1. PRMan flat surface reflections

I'm trying to simulate reflections in a flat surface based on PRMan Application
Note #6.  The app note is online at
   http://www.pixar.com/products/renderman/toolkit/Toolkit/AppNotes/appnot
e.6.html

As the app note says in the "Reflecting the Camera" section, it's easy to set
up if the reflection bearing surface is aligned on the x, y or z plane.  Then
it says:

     If the reflection plane is not aligned with the coordinate system
     axes, it's a little harder to reflect the camera through the
     reflection plane. Instead of using just an RiTranslate call to move
     the z=0 (or x=0 or y=0) plane to coincide with the reflection plane,
     it is necessary to use both an RiRotate and an RiTranslate. The axis
     of rotation is the cross product of the normal vectors of the
     reflection plane and the z=0 plane. The direction of translation is
     along the normal vector of the reflection plane. The inverse
     RiTranslate and RiRotate must be used to transform the reflection
     plane back to z=0 after the RiScale is applied. If you prefer, all of the
     rotations, translations, and scales can be combined into a single
     transformation matrix that can be applied using the RiConcatTransform
     call.

I've gotten all of that working, except for one thing.  The note told how to
compute the axis of rotation for RiRotate, but not the angle.  Can someone
suggest a good way to do that?

Doug

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