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I would like to create a ghost material like this:

ghost materail

It's like in Blender Internal, making some complex material and reducing its alpha value.

But since Cycles is a physically correct renderer, I can't simply add a transparency shader – it always shows the inside of the object.

enter image description here

Here's my material setup so far. Simple enough, it's just a standard material where I want to add transparency. I saw some promising nodes, like Light Path with Transmission Depth but couldn't get any useful result so far.

enter image description here

Is it possible to get this result, without rendering in several layers and composing the picture later? If using this approach, I get difficulties with e.g. reflections.

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To produce the 'ghost' result you need to make the 'front' surface semi-transparent and any subsequent surfaces (either the 'back' surface or any geometry obscured by the 'front' surface) fully transparent. This can be achieved by using the Light Path node to detect rays which have already passed through a transparent surface (the 'front') using the following material :

material

Adjust the Multiply maths node to change the overall transparency. This produces the following result :

result

Note that this only works while there are no other (semi-)transparent objects infront of the 'ghost' mesh as otherwise the Transparent Depth will be affected.

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    $\begingroup$ Wonderful. That other semitransparent objects aren't visible behind an object with this material is no drawback. You just can't see the second ghost behind the first one, which is ok in this not physically correct case. $\endgroup$ – Grimm Aug 21 '18 at 10:04
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You probably want to filter out back-facing sides when using transparency. front-facing-only transparency

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