sop

Software:
Houdini 18.5

Separate a mesh to groups by continous parts (elements):

Use the Connectivity SOP node to assign each mesh primitive an integer attribute based on the continuous mesh element in belongs to.
This attribute can be named “element” or “part” for example.
Use the GameDev Group by Attribute* node to assign each mesh primitive to a different group according to its “part” attribute (if you named it “part” that is..).
Set a group name prefix, like “part_” or “element_”. the node will generate numbered groups per each mesh part like so: “element_0”, “element_1”, “element_1″….
Use this group separation as a filter to create whatever part-selective effect you need.

* The GameDev Group by Attribute node ships with the Game Development Toolset

Software:
Houdini 18.0.499

Took me some time to figure out how to set the points color (“Cd”) attribute with data stored initially in custom points attributes.
I kept trying to use the Color SOP node with a “point()” function in its R, G, B fields attempting to refer to the wanted attributes but it didn’t work for me,
I also tried various loop setups iterating the geometry points, couldn’t get that to work either..
* I’m new to Houdini so the fact these approaches didn’t work for me doesn’t mean they can’t be used for this..

I finally managed to do this using a Point Wrangle node with the following VEX expression that sets the Cd (color) attribute’s vector components by referring to the 3 custom attributes “att_a”, “att_b” and “att_c” (see image below):

@Cd = set(@att_a,@att_b,@att_c);

What the Point Wrangler node does that I couldn’t achieve by writing expressions into the RGB fields of the Color node or by using loops is that it iterates all its input SOP’s points, and within its expression the attribute name i.e. “att_a” etc. automatically refers to that named attribute in the same point that is now being iterated over.

Note:
The reason I need such a workflow in the first place is to generate geometric property masks for a Houdini asset, that will be available for the target shading system via vertex color input.
* The Houdini point color attribute propagates to vertex color on output.