simple questions for vex

[wes]

1. For light and shadow context, is L the vector from Ps to P, or P to Ps?

2. Is the vex function frontface() the same as prman's faceforward()?

I get a little bit confused...

[anakin78z]

from the docs:

L: The vector from the point on the surface to the light

source. The length of this vector represents the distance

to the light source.

Initialization of the L Variable

In the Light and Shadow contexts, the P variable represents the

position of the light source and the Ps variable represents the

position of the surface point being shaded.

In Houdini, lights can have either perspective or orthographic

projections. An orthographic light in Houdini is used to represent an

infinite (or very distant) light source which has all the light rays

parallel with each other. A perspective light acts more as a point

light source.

When a perspective light shader runs, the L variable is

initialized as follows:

L = P - Ps; Orthographic lights, on the other hand, are initialized so that the

direction of L is the same for each light ray eminating from the light

source.

L = Lz * dot(Lz, P - Ps); Where Lz represents the normalized "z-axis" of the light source

(i.e. a unit vector pointing down the z-axis in the space of the

light). Thus, the scale of the L variable will be the orthographic

distance from the plane of the light source and the surface point

being shaded.

In a light shader, it is possible to change the L variable to any

value you choose. However, this is how the L variable is initialized.

[wes]

thanks, anakin78z...

L is from Ps to P, it's true for surface, displacement, fog,...I get confused because for photon context, L is from light position to the surface point, and in renderman, L is from P to Ps for light shaders. So, I wanna make sure if L is from Ps to P or P to Ps in vex light & shadow contexts.

Can someone give an direct answer?

Thanks in advance.

[anakin78z]

err... L = P - Ps ... If you try a simple test in houdini, you'll find that it means that the vector is from Ps to P.

bye,

Jens

[wes]

oh, my question sounds stupid to u, sorry for that. I really appreciate u took the time to answer me. Thanks! :>