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Old 03-16-2004, 05:25 AM   #1
Simon Wicker
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passes for an ambient occlusion gi fake

here is one recipe that you can use to make a fake gi render using any of the available ambient occlusion plug-ins (posted also to the postforum).

just for starters you have to remember that this technique only makes sense when you are compositing your image together (such as in ae) and with all the extra mucking about you have to do with extra passes and swapping materials and such-like you won't see much efficiency from the technique unless you are rendering sequences. there is little reason to use this for stills.

so create your scene and render out four passes:

1)key pass. this is just a plain render of your image using the original materials with just key lights lighting the scene. you can add extra fill lights if necessary, it all depends on the effect you are after however in general exteriors should just have a single key light that represents the sun.

2) material colour pass - one of the special passes found in the multi-pass tab. this is just the colour values of the textures ignoring any shading.

3)environment pass. take your scene and replace all of the original materials with a material that has an appropriate image in the environment channel. you should use an image that matches the ambient light you are attempting to match in your scene. this is one of the uses for a 'chrome ball' shot and is something that is usually shot on set so that the ambient light recreated matches the lighting that prevailed when the live action was shot. because we are recreating diffuse bounce light you can use a low res image and blur it quite heavily to smooth out the environment reflection.

4)ambient occlusion pass. throw one of the ambient occlusion shaders into the luminance channel of a new material and apply this to your scene. render that out - the result will be a greyscale image where the cracks and crevices of your scene are darkened.

so now that you have rendered the passes you need to blend them together. the base is the material colour. on top of that you place the environment pass and use the 'multiply' mode to blend the two together. next place the ambient occlusion pass on top and set the blend mode to multiply also. you should now have something that looks very much like a standard sky dome render of your scene. finally you place the key pass on top and set the blend mode to screen. this provides the 'direct' lighting.

the beauty is of course that with all the different passes you can easily change the look of your scene by tweaking the relative colour/opacity/levels of the layers.

cheers, simon w.
 
Old 03-16-2004, 06:14 AM   #2
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A double thanks
 
Old 03-16-2004, 06:48 AM   #3
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I recommend this message be placed in the sticky messages for all time and future retrieval.
 
Old 03-16-2004, 07:02 AM   #4
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Thanks very much!
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Old 03-16-2004, 09:22 AM   #5
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Great!!!
Thanks a lot Simon.

Some questions:

For the 3rd part, can we use a render of the actual scene?

For the AO shader, I have the Stefan Werner's one (I'm on Mac, so I can't try the other one). He say that this plugin should be use in the Environment channel.
Is there a difference if we use the Luminance channel?
Also, with this plugin, we can choose the color.
Do you use a pure white?

And last, why do you think that this is not suitable for stills?
Is there a better method for stills?

Thanks again Simon
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Old 03-16-2004, 09:24 AM   #6
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Can I translate this text and put it in the French Cinema 4D forum, with your name for credits of course?

Thanks again.
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Old 03-16-2004, 12:22 PM   #7
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Quote:
Originally posted by Essania
For the AO shader, I have the Stefan Werner's one (I'm on Mac, so I can't try the other one). He say that this plugin should be use in the Environment channel.
Is there a difference if we use the Luminance channel?

No. Both are added in the shading equation, the only difference between these channels is how they do texture lookups.
Quote:
Environment channel to
Also, with this plugin, we can choose the color.
Do you use a pure white?

Yes.
 
Old 03-16-2004, 12:36 PM   #8
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Hi Stefan!

Thanks for the answers
And thanks for your plugin!

I'll try this passes on a still I'm working now.
I'll show you something...
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Old 03-16-2004, 01:31 PM   #9
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as one of those badgering you for this on postforum...i appreciate you taking the time out to write this up for us bud...thanks alot
top job...cheers.
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Old 03-16-2004, 03:21 PM   #10
Simon Wicker
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Quote:
Originally posted by Essania
Great!!!
Thanks a lot Simon.

Some questions:

For the 3rd part, can we use a render of the actual scene?

And last, why do you think that this is not suitable for stills?
Is there a better method for stills?

Thanks again Simon


you should be able to use a render of the scene and it should work fine - as far as i can tell because of the 'fake' aspect of this process all you really need is a suggestion of the correct ambient environment to get this to work and look okay.

historically this seems to have grown out of a need to replicate the lighting that was used on set while filming. the vfx supervisor would take a chrome sphere image photo so that they could capture the environment and see where all the main lights were. you can then easy add in the correct position for your 3d key lights. then someone worked out that if you use the same image in the environment channel (highly blurred) you could also sample the diffuse lighting in the scene and supply your 'fill' effect as well.

the process is quite long winded in cinema if you need all of the separate passes to composite in after effects or photoshop as it involves physically swapping out materials and manually rendering a couple of the different passes. to my mind this makes it less useful for a still - i would probably just use full radiosity and let it render overnight.

i assume that you could cut out some of the passes if you use a blend of the environment image and the ao shader in the environment channel using fusion. however you once again run into the fact that you are best served having all the separate passes available during the comp process as you need to tweak all of the layers relative brightness and colour to get it to sit into the scene correctly.

you can of course translate this all and pass it on. thanks to everyone and if there are any further questions just ask away.

cheers, simon w.
 
Old 03-16-2004, 03:25 PM   #11
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Thank you very much Simon
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Old 03-16-2004, 03:39 PM   #12
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Thanks Simon
I have a question... When reading about ambient occlusion techniques (especially in Maya/PRMan) I have often heard reference to a 'bent normal' pass.

"A further development of this technique is that of the "average light direction vector" usually referred to as a bent normal vector. This is an additional pre-beauty render pass in which the R, G, and B values of the image do not represent physical colour but instead are used together to make a vector which the beauty pass surface shader uses to "bend" the currently shaded point's normal so that it "looks at" the position on the environment map from where the majority of light came from as calculated by the ambient occlusion render pass. "

Does anybody more versed in the inner-workings of Cinema's shading procedures know whether this would be possible with the current toolset? Or whether it would make much of a difference?

http://www.andrew-whitehurst.net/amb_occlude.html
 
Old 03-16-2004, 03:39 PM   #13
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Thank you very much, works very well. Despite the different approach about the same result as my two-pass method .
 
Old 03-16-2004, 08:13 PM   #14
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Quote:
Originally posted by chris_b
Thanks Simon
I have a question... When reading about ambient occlusion techniques (especially in Maya/PRMan) I have often heard reference to a 'bent normal' pass.

Sounds like they're talking about normal mapping, which Cinema doesn't support.
 
Old 03-16-2004, 08:28 PM   #15
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true true...except via plugin (i.e. DiTools).... or Per's NormalMap shader. Also there is Nlin for generating the maps...

So it seems that all the necessary parts are in place and somebody just needs to push the button
 
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