Understanding Ray VEX

[logix1390]

Hello,

I am in the middle of doing a procedural modeling tutorial:
http://lesterbanks.com/2017/03/procedural-modeling-houdini/

 

He creates a curve and manually rays it to the terrain using vex .

The code is as follows:  (the curve is in the first input and the terrain mesh is in the second input)

 

vector rayD = {0,-100,0};

vector hittP, hitUVW;

i@hitPrim = intersect(1, @P, rayD, hittP, hitUVW);

@P = hittP;

This works perfectly

Now I am trying to understand what is going on here exactly and how Houdini is solving this .

I searched for the intersect function in the help and found this:

int intersect(int input, vector orig, vector dir, vector p&, vector uvw&)

I guess what is confusing me is the "vector p&", and "vector uvw&" above and how that related to the vectors he created "vector hittP, hitUVW" .

 

I am still a beginner in VEX ... If anyone can help me understand this, I would really appreciate it.

I will also attach my scene file for anyone that wants to take a look.

 

Thank you!

 

 

 

 

Ray_VEX.hip

[acey195]

for the intersect function, the last 2 parameters are output parameter

you initialize them beforehand and the intersect function will overwrite them, in this case with the position, for "vector &p", where the ray hit (or the "orig" position if it did not hit anything)
"vector uvw&" returns the intrinisic uv of the position on the primitive (i@hitPrim) you could use this together with the primuv() function 

basically whenever there is a & in a function param, thats a way the docs try to tell you its an output parameter.

Hope that helps!
 

[konstantin magnus]

https://www.sidefx.com/forum/topic/49550/#post-223800

[logix1390]

Thanks guys, I can tell this is a very deep topic. I definitely have a better perspective on it now.