Need help with simple VEX function

[BlackPariah]

Hello!

New Houdini user here with barely 3 weeks in... so please bare with me  

I'm trying to write a small VEX function/expression to translate an object along it's Y axis by an exponential function... like gravity.

The math behind is simple but I'm having trouble expressing it in VEX...

I tried the following line with a VEX attribute wrangle node on a default sphere and it's giving me errors and the sphere just disappears on the 1st frame:    t.y = t.y + (9.8 * ($T ^ 2));

If someone could steer me in the right direction I would appreciate it thank you :-)

 

 

[fsimerey]

Use the good attributes see here http://www.sidefx.com/docs/houdini15.5/vex/snippets#attributes

For your case the good expression is this:

@P.y = @P.y + 9.8 * pow(@Time, 2);

and you can simplify with:

@P.y += 9.8 * pow(@Time, 2);

But if you want a gravity effect, subtract is the better way

Edited October 28, 2016 by fsimerey

[Sepu]

I think this is what you are looking for

float gravity = 9.8; 
@P.y += gravity * (pow(@Time, 2));

I believe the "$" expression isn't valid in Vex, however you can use it in hscript ... check out the help docs to see what is the correct syntax for Vex.

http://www.sidefx.com/docs/houdini15.5/vex/snippets

 

 

Edited October 28, 2016 by Sepu

[BlackPariah]

Wow great stuff thank you guys I ran into one small issue now... & the object in question will keep accelerating forever with this method.

My guess is I would need some kind of a loop or if / then structure to define a maximum limit of the result... I'm not numerically inclined so it's difficult for me to put it in numbers... more so with VEX.

Maybe I'm way over my head with this but it would help a lot if I could see an example of how to structure the code... if it's not asking too much I'd appreciate it thank you again

Edited October 29, 2016 by BlackPariah

[fsimerey]

float gravity = pow(@Time, 2);
if (@Time > ch("limit")) {
    float increment = pow(ch("limit"), 2) - pow(ch("limit") - @TimeInc, 2);
    gravity =  pow(ch("limit"), 2) + increment * ((@Frame - ch("limit") / @TimeInc) - 1);
    }
@P.y += -9.8 * gravity;

Here an example.

The limit is a time value in seconds. Don't forget to click on the white button on the right of the editor area to spare parameter.

[BlackPariah]

I didn't notice that button on the side... my head is spinning just reading the code here but I'll figure it out from the example... thanks a lot Francois