If you want a Particle Source to follow a Null, you can just stick
(etc) into the Position boxes.

But that'll only work if the Null itself is animated/translated. If it's parented to something else, you'll just get out. So how do I get the Null object's position in world space? I've found optransform(), but it doesn't seem very stuff-it-in-a-parameter-as-an-expression-friendly, if you get me.

(And whenever you're controlling something's location/rotation with expressions, you always end up with three separate copies of your expression as Vectors are always broken out into separate axes in the UI, when often it would be easier to be able to just use a Vector. Is there any way to have Houdini kinda munge translation or rotation parameters into a nice single expression-friendly Vector type box?)

Hi howiem,

Have you checked out the RMB menu on a parameter? Try going to Reference->Scene Data and then in the selection dialog find your target obj and look under Transforms->World. This will populate the parameter with a bunch of expressions (that you probably wouldn't want to type by hand )

Oh my goodness. That's wonderful. Thanks a million

And no, I wouldn't have liked to type this by hand (let alone copy and paste and tweak for Y and Z):


Crikey - I didn't know you could stuff that much code in one of those tiny little boxes either

Why not create a constraint? No funky cryptic expressions, just nodes.

Oh, if it was going to an obj node's transforms, constraints totally the way to go.

I was thinking that we were looking at sticking an objects world space translate into a vector parm buried in a dopnet or some such… In which case you could possibly export the t channels from a Get World Space chop node, but the reference->scene data was fewer clicks

… and right now less clicks = more likely to get home on time. But I promise to learn the constraint approach. Thanks!

This….. Just changed my life.
Many thanks!

friedasparagus
Hi howiem,

Have you checked out the RMB menu on a parameter? Try going to Reference->Scene Data and then in the selection dialog find your target obj and look under Transforms->World. This will populate the parameter with a bunch of expressions (that you probably wouldn't want to type by hand )

friedasparagus
Hi howiem,

Have you checked out the RMB menu on a parameter? Try going to Reference->Scene Data and then in the selection dialog find your target obj and look under Transforms->World. This will populate the parameter with a bunch of expressions (that you probably wouldn't want to type by hand )

This is the greatest thing in the world. Thank you so much!

howiem
And no, I wouldn't have liked to type this by hand (let alone copy and paste and tweak for Y and Z):

{

matrix src_xform = optransform(“../MY_NULL”);
matrix target_xform_inverted = invert(optransform(opinputpath(“.”, 0)));
matrix final_xform = src_xform * target_xform_inverted;


matrix rest_xform = identity(4);
matrix self_xform = rest_xform * final_xform;

float result = explodematrixpr(
self_xform,
vector3(ch(“./px”), ch(“./py”), ch(“./pz”)),
vector3(ch(“./prx”), ch(“./pry”), ch(“./prz”)),
chs(“./xOrd”), chs(“./rOrd”), “TX”);
return result;

}

Why all this code when it could just be done with the origin() vtorigin() vrorigin() expression functions?