Dan Lee


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Still no alpha channel for transparent material May 13, 2024, 10:29 a.m.

Can you post a visual example of this?

Of course, here's a screencap of the parameter window showing the LPE set up and its output. Note that karma does not support 'negate' operation in LPE, so you'd negate it in comp should you need it.

Still no alpha channel for transparent material May 11, 2024, 9:55 a.m.

Hi. As of Houdini 20.0 karma supports 'B' (background) event in LPE so you can describe any light path to uses as custom alpha (which then you would invert in comp).
For example, if you want to consider any background hits with 0 or more transmissions, use 'CT*B'.

Karma LPE Problem With Treat As Light Source April 24, 2024, 10 a.m.

Hi, if you enable "Treat as Light Source" the emissive material is treated as light and not emission, so it will only contribute to LPE AOVs that accumulate direct/indirect lighting (i.e. expressions that end with 'L' not 'O').

As for the render being darker without light source: It's because diffuse indirect ray hitting transmissive surface (which then hits emissive surface or anything for that matter) is a caustic path and true caustics is disabled by default. Fake caustic is automatically enabled on MaterialX-based transmissive materials, so the colour of shadow rays for the light source is adjusted and thus appears brighter. Otherwise you'd have to enable true caustics (via rendergeometrysettings LOP on the glass object, or globally).