Implement Holdout Shader

[MarioPeper]

In the cycles setting you have the possibility to set Outliner > filter > Holdout and the Object Properties > Visibility > Holdout, together with the function Render Properties > Film > Transparent, to make it invisible but still makes indirect light. Very useful in compositing. I think Luxcore should have this.
Edit: For a better understanding https://youtu.be/fJtaboOKxeE?t=95

[Theverat]

Blender manual on holdout shader: https://docs.blender.org/manual/en/latest/render/shader_nodes/shader/holdout.html

Simply put, areas on the film containing a holdout material should be transparent. Otherwise the material behaves normally.
(in Cycles it appears black to camera rays, but I'm not sure if we necessarily need to copy this behaviour)

[MarioPeper]

No I didn't mean the holdout shader. It is already listed in the Cycles scene reader task.I have no global illumination on this. Please have a look in the linked video (it is only short)

[Theverat]

I am pretty sure they are the same thing internally in Cycles.
At least this looks like it to me: https://github.com/blender/blender/blob/c3651adf8914048ecd2668937c3ed4414a6707bd/intern/cycles/kernel/kernel_path.h#L283

[MarioPeper]

But when I use the holdout shader, it no longer casts light on other objects. Whereas the holdout setting in the object properties > visibility does. If there is an easier way to achieve this in Luxcore, why not. But this makes a big difference in compositing.

[Theverat]

From a user perspective, the best way to implement this might be a material flag, so any material can become a "holdout".

[MarioPeper]

Yeah that sounds good.

[Dade916]

I added the support for holdout. It is a material attribute and can be set with property ".holdout.enable". This is a normal rendering:

and this with ".holdout.enable" set to 1:

[MarioPeper]

Wow, that was fast. Thank you so much.

[Theverat]

I added support for it in the Blender addon.

[MarioPeper]

Thank you very very much, you are incredible.

[Theverat]

Note: I didn't yet add support for holdout to the Cycles scene reader.
The view layer holdout setting is also not supported yet.
Until that is done, you have to open the "advanced" options in a Lux material output node and enable "Holdout" there.

[MarioPeper]

Actually I find this solution much better than the scrolling nightmare in cycles. It fits perfectly to the Shadow Catcher and to luxcore :-) . The sense of the Holdout shader (without GI) is not quite clear to me either. Being able to set holdout and indirect only in the outliner would be nice, but it is no longer necessary in my workflow now. Now I also know how to set it in the text editor for LuxCoreRender, so I get the render layers much faster. Thanks again for that feature.