Translating points procedurally

[964racer]

Hello---

I'm relatively new to Houdini. 

I have a task where I have points which I have extracted from geometry

and I would like to translate those points to a location determined by a mathematical function. Let's assume that

I can access the points I need by using the "add" node and I have the points from my geometric structure.  What is the best way to

"loop" through that array of points and translate them to a new location.  For example, ignoring Z for the moment, say I just want to translate them in a parabolic

shape in 2D using the function y = X^2.  (ignore "z" coordinate).

What is the easiest way to do this ?  I would need to "loop" through various ranges of points in this list and perhaps move them differently, so the function may

apply to the first 100 points, but I may want to use a different function for the next 100 etc.  so I do need a way to parse through the list.

Would you use a point VOB for this ?  - or perhaps wrangle it directly in VEX ?  - or is there a way to do this in an expression within the node ?

Please point me in the right direction,  Cheers,

 

 

Edited August 10, 2016 by 964racer

[mestela]

A point wrangle sounds the best way to do this.

@P.y = @P.x * @P.x;

or

@P.y = pow(@P.x, 2);

re splitting, each point gets an id, @ptnum, so you could do something like

if (@ptnum < 100) {
   @P.y = pow(@P.x, 2);
} else {
  @P.y = @P.x * 32 + @P.z + sin(@Time + @ptnum);
}

etc.

 

 

[964racer]

Thanks, I will try that with a simple example to see if I can learn how to do that and then apply it to my larger geometry network..thanks

[964racer]

This is working for me.. Thank you.. but how do I find out what built-in attributes are available from within a pointwrangle node that I can use within the vex expression ?

For example, I know from the code  examples (shown in the arrow menu) in the code editor box, that certain attributes like @ptnum exist but how do I find out what others are available ?

The docs for VEX have a table of some of them, but suppose for example, I would like to know if there is an attribute/variable that contains  the total number of points in the group , so in my calculation

for the current point, I may want to know how many other points are in the group ?

Also, it seems there is a vex function for "ch()", so I would assume that I could get information form other nodes from within a vex script ?  That would seem costly in terms of computation for certain operations,

but if a @nPoints doesn't exist, I know what the dimensions of the grid is feeding the pointwrangle, so I could derive nPoints from the dimension of the grid (doesn't sound elegant though.)

 

Thanks !!

 

 

 

 

 

[Sean-R]

Check out the VEX functions page in the Help, it lists all the functions available to you.

It sounds like the function you need is npointsgroup()

[964racer]

Thanks you ! ...the function, npoints(int) looks like it does it and will help for this value I need , but still looking for where I can find the list of internal attributes I can use like @ptnum etc...

 

 

[mestela]

Laziest way is to put down a point vop, dive inside, and look at the global inputs node. That gives you a list of the more common attributes you'll use. 

[964racer]

15 hours ago, mestela said:

Laziest way is to put down a point vop, dive inside, and look at the global inputs node. That gives you a list of the more common attributes you'll use. 

Thanks and "hovering" over the labels in the input node, shows the variable name and type.  perfect....

I was able to complete my project using some vex code in a point wrangle.   Are there any others ways to enter VEX code in Houdini other than in a "wrangle" node or graphically using a VOP node ?   Is there a way to use your own editor and create a VEX source file to be referenced by a node ?  I watched a recent tutorial on VEX given by the SideFX folks, but it seem to mainly focus on wrangle type nodes for VEX.

Edited August 12, 2016 by 964racer

[bonsak]

Hi

You can use the VEX SOP and point "Script" to a .vfl or precompiled .vex file on disk.
You can also put down an empty Attribute VOP (or any of the vop's) and point "VEX Source" to a file on disk.
It's also possible to set an external editor for the Expressions Editor.

Personally i tend to edit in my code in Sublime and just copy/paste into a wrangle.

-b