How to have transparent (invisible) material

[Tom_P]

there was a topic within the last weeks about how to render shadows only, not the object … i can t find it … sorry.

but there is similar questions.
maybe closest:

Transparent but still cast shadows?

I am trying to create a “ceiling” in a room that is transparent for the purposes of modeling and viewing the room but still is opaque to lights, sun etc. I’ve tried to get a transparent surface to work and monkeyed around with a horizontal clipping plane below an opaque surface but neiter approach seems to work. Seems like this ability would be useful for people modeling interior spaces who want to see the effect of windows etc. Any advice? Nick

also here with less answers

similar topics i ve found:

Rendering Shadows of Hidden Objects onto Visible Objects

Basically here is what I’m trying to achieve: I’d like to get a rendered elevation view of a particular wall WITH the shadows of the object next to it (behind the camera). I only want to see the shadows the object is casting, so I can get an outline in parallel elevation view of the shadow cast on a neighbouring wall. I’ve tried to use a ClippingPlane but as soon as the shadow-casting object disappears from view, so does the shadow it casts. I’ve also tried OrientCameraToSrf but the shaows ar…

Rendering shadows without the object being there?

This is how currently Raytraced/Cycles works in Rhino WIP. Room lit by sun through circular opening. Wall near orange cube is reflective (with slightest hint of roughness). Green pyramid is behind the clipping plane (i.e. clipped), but still shows in the mirror wall. No lighting happening through the clipped side. [kuva]