I've been using object merge with simple animated objects no problem….. Now I need to go near constraints (shudder) and it all falls apart.

I setup a simple test scene that replicates the elements of a setup I'm doing. A simple ground geo and a small grid that's raycast to the ground and drives around. I Then point constrain a cube to the grid, and create a null that scene data references the world space of that cube, so I now have an null that moves and rotates that I can parent a car to… That works, but I want a copy of the car object merged into my ground object so I can do stuff to the ground (after the grids raycast obviously) but no matter what I do the object merge just isn't working as expected.

If the display flag is on the object merge then the object stays at 0.0.0, if the template flag is on the object merge it behaves as expected.

Constraints in Houdini seem very different to the simple tools in other packages and of course in the grand houdini world i'm a newbie. I know I can bake the constraints out but that goes against the whole concept of procedural.

Should it work? Am I missing something fundamental?

Andi Farhall
If the display flag is on the object merge then the object stays at 0.0.0, if the template flag is on the object merge it behaves as expected.

Sounds like the constraint is picking the object path instead of the sop path then. Make sure the surface constraint is a fixed path to the sop with the ground plane, that way the display flag won't change which object is used for the constraint. You also might be setting up a loop because the surface constraint object's transform is being used to affect itself when you put the object merge in there.

A cleaner way would be to make a separate object that merges both the ground and car without any constraints itself for the FX.