How does P attribute works for primitives?

[magneto]

I know points have P attribute by default and you can see it in the Details View, but you can also access P for primitives even though they aren't visible in the Details View.

I thought an attribute has to be there for it to be accessed.

Is this a special case, kind of like an on-demand attribute? Also are there other attributes like this that aren't visible by default, but accessible?

Thanks

[Annon]

Jump into the help of any node and look at the available attributes to you:

http://www.sidefx.com/docs/houdini11.1/nodes/sop/primitive#locals

[magneto]

Thanks, I am talking about accessing it via the prim expression that can be used anywhere.

How is it possible to access the "P" attribute of a primitive when I can't see it listed in the details view?

[Annon]

Well you're getting the centroid of a primitive. It shows an example in the help, I can't see what issue you're having with it.

Usage

prim(surface_node, prim_num, attrib_name, attrib_index)

When given the “P” or “Pw” attribute, returns the centroid of the primitive.

Examples

prim("/obj/geo1/facet1", 3, "P", 0)

Evaluates the X component of the centroid of primitive 3 in the specified surface node.

http://www.sidefx.com/docs/houdini11.1/expressions/prim

[magneto]

I know it's the centroid. I am just looking to know how this "special case" works behind the scenes.

[eetu]

A variable is not necessarily an attribute. There are a lot of other variables, $CEX, $BBX, $NPT etc.

Nor is an attribute necessarily available as a variable, eg custom attributes need to have a varmap mapping for that.

[graham]

Most likely if the expression code detects it is looking for "P" instead of an actual attribute it calls GEO_Primitive::baryCenter() and returns the selected index component for the axis you want.

[magneto]

Thanks guys.

@Graham, that's what I was wondering. So P is handled specially in this case. Otherwise I was thinking an attribute has to be present before it's accessed, and Details View shows all attributes available (given they are selected) and P doesn't show up for primitives by default, but yet accessible.

[rade]

Thanks guys.

@Graham, that's what I was wondering. So P is handled specially in this case. Otherwise I was thinking an attribute has to be present before it's accessed, and Details View shows all attributes available (given they are selected) and P doesn't show up for primitives by default, but yet accessible.

If you read eetu post , there is statement that ATTRIBUTE is not equal(!=) VARIABLE .

So there is no special case here.

[magneto]

Yes but the Details View shows attributes not variables. Also you supply the attribute name to the prim expression. I never said anything about variables.