The Meaning Of Global Variables In Vops

[shikung444]

Hi,

How can I know the meaning of the abbreviations in vops for some of the parameters? Here is a list, if anyone can fill me in, that would be great!

Thanks!

Displacement Global variables

P - Position

Pz- Position Z?

dPds-

dPdt-

N- Normal

Ng-

dPdz-

Surface Global Variables

What is the difference between Of (opacity) and Af (Alpha)?

I - is this the vector pointing toward the camera?

Eye - what's the difference between this and "I"?

Lighting Model Vop

What's the difference between nN and nI?

[ihab]

Hey,

-------------------your list ------------------------------------------------------------------------------------------------------------------------------------------------

How can I know the meaning of the abbreviations in vops for some of the parameters? Here is a list, if anyone can fill me in, that would be great!

Thanks!

Generally these are the global variables that are available to the shader about the current point being rendered ( or rather shaded). Think of the shading process as a loop that passes through all the points on the surface.

Displacement Global variables

P - Position

This is the point position of the point currently being rendered.

Pz- Position Z?

this is the z-depth from the camera

dPds-

derivative (or rather the slope of the surface at the current point in the s direction (s and t are u and v )

dPdt-

slope in the t direction

N- Normal ---> the normal (shading normal) of the current point

Ng- ----> Geometric normal. usually it's the same as N

dPdz-

Surface Global Variables

What is the difference between Of (opacity) and Af (Alpha)?

I - is this the vector pointing toward the camera?

Eye - what's the difference between this and "I"?

Lighting Model Vop

What's the difference between nN and nI?

nN is the normalized normal (N) it's essentially N with a unit length. Usually you do that if you want the direction of the vector.

nI is the normalized I

[Tamis]

Opasity is the "transparentie" of the object to say it very boldlie.

and the Alpha is the alpha channel that wil be rendered for your render for use in composeting.

[shikung444]

Thanks very much for your responses. There are still some things I'm not clear about, but I'm trying to get up to speed by looking at books on renderman. Are there any other computer graphics resources that anyone can recommend? Because the Houdini manual doesn't seem to address these things very clearly.

Opasity is the "transparentie" of the object to say it very boldlie.

and the Alpha is the alpha channel that wil be rendered for your render for use in composeting.

[ihab]

Thanks very much for your responses. There are still some things I'm not clear about, but I'm trying to get up to speed by looking at books on renderman. Are there any other computer graphics resources that anyone can recommend? Because the Houdini manual doesn't seem to address these things very clearly.

Advanced Renderman Creating CGI for motion pictures. One of my favorite books.

cheers

[Tamis]

Rendering for beginners:image synthesis using renderman

The art and science of composoting <--- cool book to !! maby not completly on topic but its quite similar to shader creation in a way.

[ihab]

check this out

http://odforce.net/wiki/index.php/VEX

cheers

[Tamis]

BTW any one know a good book that explains the works behind volumetric rendering and voxels and stuff ?

[daniel.phillis]

wiki those voxels !

http://en.wikipedia.org/wiki/Voxel

[Tamis]

THNX Daniel but i was thinking more of some more extended information.

for example how to use voxols in production ? how to handel Voxols using renderman or mental ray mantra.

Shading Volumetrics. <-- defenatly because it dosn't realy consist out of surface data, so how to shade it?

so in all looking for some more in detail stuff.