Hi,
The attached picture takes 26 minutes to render, but with the van removed it is under 2 minutes.
Is there a setting in the materials panel or elsewhere that makes an object exempt from the AO?
ie. AO is computed for the rest of the scene but the van is ignored.
This would save me a ton in render time.
Thanks!
you could use render layers and then composite the van
Hi waylow,
I thought the same thing, but it doesn’t seem to matter if it’s on a different layer or scene, blender still computes AO for everything in the .blend.
Unless I’m missing something?
so when you say “with the van removed” - does that mean you deleted it?
are you rendering everything in the one pass?
meaning do you have the nodes set up to render the 2 scenes and composite at the same time
or
are you rendering the 2 scenes separately > then using the composite nodes to combine the images - making it a 2 step process
(does that make sense?)
I doubt AO of the van is causing the increased render time probably you have raytrace mirror and/or transperency on some materials of the van.
Ahar! Thanks waylow, that fixed it!
So for those wondering, here’s how to do it:
Along the top header, click the dropdown arrow next to ‘Scene’ then choose Add New, then click, ‘Empty’.
Click the drop down arrow again and go back to your previous scene. Find the object that you want to exclude from AO and hit Ctrl-L, choose ‘To Scene’ then select your newly created scene. You will need to do the same with any lights and cameras you have.
Go to your newly created scene via the dropdown menu and click the World Settings button. It should be empty. Click the arrow and choose ‘World’ then click the ‘2’ next to it to make it a single object. Now go to the Ambient Occlusion Panel and turn it off.
Go back your original scene and go to the node compositor. You will need to add two Render layers for each scene (Add>Input>Render Layer) and combine them together using the ‘Mix’ or ‘Alpha Over’ node.
I always wondered what the difference between a scene and a render layer was. Now I know! This explains a lot actually 
Thanks again waylow!
no worries
but was it the AO or the raytrace & ray mirror?
The cause of the long render times?
With AO turned off the scene rendered in under 8 minutes. With AO turned on it rendered in 28 minutes (the scene by itself was 3 minutes).
It was definitely the van. I’ve never had any luck rendering vehicles and AO. It always jumps dramatically.
You could also just turn down the van’s various materials Amb value to zero (in the Shaders tab). This would prevent an object from receiving any ambient light, including ambient occlusion. Although, the objects would still contribute to AO of other objects (cast a ‘shadow’).
nice one 
That is an AMAZING tip, especially for glasses! I’m glad to find it: turning off ambient value (for a material) speeds up glass raytracing A LOT! (I mean: a lot!)
For glasses Blender should set ambient value to zero automatically.
In my case, I didn’t want the object to contribute to AO occlusion on other objects, and found that disabling ‘traceable’ in materials was the ticket. This completely removes the object from any ray trace calculations (including AO) which was exactly what I needed.
Andrew, you make pretty cool tutorials on your website, so, why don’t you make a tutorial about this ?? it will be much appreciated since rendering is a paint when you don’t know what your doing, a lot of people should find tutorials like this useful
Cheers!
