yeah I am not sure this is possible either, since the texture is part of a shader evaluation but LPEs deal strictly with rays.
Depends on the Houdini version. In H21 we now have the RaySwitch VOP (finally!), so we can build a shader-based setup like this. If Efficient Emissive Sampling is on, Karma treats this like a light and contributes to direct rays.
Yes it’s hacky but it actually works fairly well. You could expose a bunch of subnet inputs to facilitate later editing on the stage. And also in 21 we can then supply a stencil map via RGS on the card to cut it out. What I have not confirmed is how well Karma does importance sampling with this method (MIS on textured lighting effects has been hit or miss with Karma in my experience)
But this at least gives easy separate controls over camera rays vs scene rays.
@Jonesy you were asking the other day about possible applications of the RaySwitch VOP
on the topc if LPEs, here is a list I have been maintaining of Karma LPEs as they crop up. These syntaxes have been tested in production but if anyone spots a problem please let me know. And if you know of other useful ones, I would love to know what they are. There are all kinds of things you can do.
gist.github.com/tcrowson/5d033817c63cf6cf27f3703e2df9462b
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