Anyone knows how to export object id from Houdini to after effects.
I try using the extra image plane with Op_Id on an exr file but nothing…not so sure if it is a houdini or After effects problem.

The general consensus is that After Effect open EXR is broken. As of CC 2015 Adobe has not updated any of the 3D Channel tools to work with modern file formats. Another case of legacy rot within the After Effects effects system.

So people say use Nuke instead of After Effects.

You can view that the Op_Id is being exported correctly by simply bringing in one of your outputted EXR file into the Houdini COPs system. There you can use a luma key node to isolate the Op_Id for the object. In order to find the actual Op_Id of any object CTRL-Click on it in the render view when it has no material assigned (what a drag, CTRL-Click pops up a Mantra dialog instead Pixel Information if an actual material is assigned.)

So the work around would be to render out your EXR files with Op_Id, bring them back into Houdini and key out the object IDs using the lumakey node. Follow that up with a ROP Output Driver to write the keyed object back out in an After Effects friendly format such as PNG or JPG. Then you can use them as animated mattes over the master render sequence.

Thanks for the reply Enyvob,
I figured it was something of the sort. I ended up exporting a custom channel for each object I wanted to isolate.
I will love to use Nuke, but the price is quite prohibiting. But thank you for the answer…it kinds of confirm my guess about where the problem is.

No need to wait for money anymore to learn Nuke. They have a non-commercial learning edition just like SideFX. Check it out. Also check out Mari for painting your 3D objects.