View Full Version : Can I get a separate shadow channel with HDRI?

11 November 2005, 02:59 PM
I've got a scene set up with a car in the middle and the camera targeting it. The camera circles 360 on a path around the car. I just need the HDRI for the reflections and shadow so I blocked the background with a white plane that doesn't cast and receive shadows.

Now what I'd like to do is get the shadow that the car throws on the floor in a separate channel when I render it.

I browsed the forum for this but all the solutions work for scenes without HDRI.
(ShadowCatcher plugin too)

It's a small animation (about 30 frames) so if there's no other option I can always tweak it manually in PS.... BUT I'd rather not of course!

Edit: I only need the shadow of the floor, the rest of it doesn't match the new background I want to place it in.

11 November 2005, 03:44 PM
The problem is that HDRI ligting isn't really generating a shadow, it's just dynamically bouncing the light everywhere. I think you'll probably have to fake the HDRI to get a separate shadow channel.

Neil V
11 November 2005, 04:48 PM
You're going 360 degrees in 30 frames? Wow, that's one fast camera move! ;o)

11 November 2005, 07:43 PM
you could take your scene, add a material that uses just the luminance channel set to 100% white. Add this to your car.

Duplicate the material. In the luminance channel, add the ambient occlusion shader. Add this material to your floor.

Now if you render this pass, you will have a 'shadow' type layer that you can multiply over your previous render. All objects should render pure white, except the floor which will be occluded by the car.


p.s. why did you block the background with a plane? You should use a compositing tag and uncheck 'seen by camera'. It will then render only in reflections/refractions and it will still be used for GI.

11 November 2005, 11:05 PM
Thanks! The ambientocclusion shader is great.

@TimC: I'm a little embarassed to admit I've only just discovered compositing tags.

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11 November 2005, 11:05 PM
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