Unlike other RenderMan bridge products, PxrLayerSurface is not exposed in the interface because all layerable parameters must be present in order for layering material parameters to work in Houdini.
You can find out all the details of PxrSurface and its layerable parameters by visiting the Pixar Surface Materials.
In H16, vstruct is suppoted in RenderMan for Houdini. So skip reading H15.5 Only below.
Due to Houdini not supporting virtual struct in H15.5, material layering in RenderMan for Houdini is incompatible with other RenderMan bridge products.
In RenderMan for Houdini 15.5, we layer the material parameters when its local parameter values have changed from the default values.
If you want to force the default value of the layer to override the layer underneath, you can lock the parameter. This is a workaround since Houdini 15.5 does not provide an option to force the value to be local.
We can override the parameter in PxrSurface using PxrLayer. Connect the output of PxrLayer to PxrSurface's PxrLayer.
In this example, we set a green diffuse color in PxrSurface. Then we layer a PxrLayer node on top that has a cyan diffuse color.
We can combine layers using PxrLayerMixer. Connect the output of PxrLayerMixer to PxrSurface's PxrLayer.
We can layer specular, iridescence, and other illumination lobes.