Compositor, cycles and sky

Hello all,

A quick question from someone new to compositing.

I am using the Cycles render engine. I would like to add a blur effect to an object in my scene using the compositor.

My naiive attempt at achieving this put the object in a separate render layer, and passed it through a blur node. However, I do not know how to mix the object back in to produce the composite. If I try mixing it with the other layer (containing ‘everything else’) then each layer contains the sky in it, so whatever I do the sky is affected by the compositing process.

Is there a way around this? Given that in cycles the light from the sky is a vital part of the render, the answer is not obvious to me!

Thank you in advance.

use an id mask to isolate the object
if you don’t know what an id mask is, its explained at the beginning of this tutorial

Thank you very much, that really helped.

One additional question: in my particular scene, the object that I want to isolate is occluded by, and passes through, an object with a glass material. When creating the id mask only the parts of the object that are not occluded are selected. Is there a way to select all of it?

Cheers!

Im pretty sure that it can’t be done because the glass material will distort whatever is behind it