For example, I have this Attribute Wrangle:



Obviously @P is a point attribute, not a primitive attribute. However, it still has value when I iterate over primitives. How is this value calculated? And does it always work the same way for any attribute, or @P is a special case?

@P is normally a point attribute, but in a Primitive Wrangle, Houdini binds it to the centroid of the primitive — the average of its points’ positions. It's a special case. Other attributes like @Cd or @N won’t behave this way.

animatrix_
@P is normally a point attribute, but in a Primitive Wrangle, Houdini binds it to the centroid of the primitive — the average of its points’ positions. It's a special case. Other attributes like @Cd or @N won’t behave this way.

@N seems to be a special case as well? I run a Primitive Wrangle and access @N. Surprisingly it has value, even the geometry doesn't have a primitive attribute called N.

Yeah, @N is special in its own way, but in a different way. It gives you the geometric normal of the primitive, not the average of any point normals. So even if there's no N attribute, Houdini still provides it based on the primitive's shape.

That's what I meant by @Cd, @N and other attributes won't give you the same result, i.e. the average of the same attribute for all the points of the primitive.