Since version 1.6 LuxCore materials have the shadowcatcher option. See this forum thread for the original announcement.
It is intended to be used for compositing, e.g. to integrate a rendered 3D object into a photograph.
A shadowcatcher material will be transparent where hit by direct light, while shadowed areas will stay opaque.
The opacity of the shadow can be controlled by the opacity/transparency of the material.
The color of the shadow can be influenced by the material color.
Any material can be a shadowcatcher, but it does not make sense to use a purely specular material without any diffuse reflection, like glass or mirror.
Usually matte is the material best suited for a shadowcatcher.
The shadowcatcher option is supported by the Path and Tiled Path engines (both CPU and OpenCL), but not by the Bidir and BidirVCM engines.
It is possible, however, to use the alpha channel generated by a rendering using the path engine to compose the images.
- When using a sky light as environment, enable the "groundcolor" option and set the groundcolor to black
- When using an imagemap as environment (e.g. a HDRI), enable the "only sample upper hemisphere" option
Doing this prevents the environment to cast lights onto the shadowcatcher from below, leading to better alpha transparency.