I want to render just the shadow of this object here, how do I do that? (Cycles)

[Ian_SAfc (Ian)]

I have this dial on a brushed metal background.

I want to just render the shadow of the dial. I am currently trying “holdout” but its cutting a hole in the whole render, including the brushed metal plate under it. This is how it looks when rendered:

How do I get the brushed metal background to still appear, and keep the dial shadow ?

(As well as “holdout” I also tried these shader nodes)

[G_De]

Is this kind of what you’re looking for?

[Ian_SAfc (Ian)]

The object itself transparent, but its shadow still rendered, yes.

[G_De]

Then unchecking the two boxes in the yellow box in the picture and then rendering should do what you want. Good luck.

[Ian_SAfc (Ian)]

Its not doing it for me, am I doing something wrong?

[G_De]

It looks like it is working properly, (Is that red bit a separate object from the rest of the dial?) just that there’s still a deep shadow where the dial is, because there’s still technically no light there. If you want light in that area, maybe a proxy? Instead of this:

Try something like this:

That’s just half a cylinder that mimics the original but lets light in on the back side, and then hide the dial in the render as normal?

[G_De]

When I added a similiar metal material also turning off glossy got me better results.

There’s also composting after the fact as an option you might want to consider:

Brushed steel rendered only, shadow object turned off

Brushed steel plane set to shadow catcher, film set to transparent, shadow object rendered with previous settings of diffuse and camera turned off.

Shadow overlayed onto steel in editing software of your choice.

[Ian_SAfc (Ian)]

I might look at compositing as you suggest.
This is close to my final aim:

[G_De]

Hope it works out for you.