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Old 01-28-2013, 09:15 PM   #1
cai
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Rendering alpha passes oscured by other objects

Hi,
Is there a way to render an alpha pass of an object in a scene that accounts for other objects moving in front of it?
For example, if I had a scene with a cube in it, and a sphere floating past in front of the cube, is there a way I can render an alpha pass that would have a black hole in the cube as the sphere moves past?
I can achieve something similar using black and white surface shaders, but as I am intending to use this technique for several objects per scene, swapping all the shaders around over and over will be very time consuming so I'm hoping there's a way to do it natively, or at least more efficiently?
Thanks!
 
Old 02-04-2013, 11:03 AM   #2
KHing
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Normally in this scenario I will render a RGB pass. Or if you are comfortable with multipass, you may assign IDs to the objects in renderers such as vray.

For simple task you should just use a RGB pass and separate your elements in post.
Found this tutorial on the internet. Hope its useful:
http://ae.tutsplus.com/tutorials/wo...-after-effects/
 
Old 02-07-2013, 01:37 PM   #3
n3al
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Neal Albert Strydom
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I had this same question, but I have something I want to add to it.

Would this method work for AO renders as well? If so, what would have to be done differently?

Currently I have my setup with a surface shader that has an amb mib texture plugged into it fro my AO and then I have a Use Background Shader assigned to everything I wnat masked out, the use background shader's Specular color is black, reflectivity is zero, reflection limit is 0 and the shadow mask is on 1. This way, everything that I don't want visible, is black and acts like a place holder for the objects that do go there, but the AO calculations and fall off is still present.

I'm using Mental Ray and Maya 2013

Last edited by n3al : 02-07-2013 at 01:43 PM.
 
Old 02-08-2013, 06:40 AM   #4
KHing
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Quote:
Originally Posted by n3al
I had this same question, but I have something I want to add to it.

Would this method work for AO renders as well? If so, what would have to be done differently?

Currently I have my setup with a surface shader that has an amb mib texture plugged into it fro my AO and then I have a Use Background Shader assigned to everything I wnat masked out, the use background shader's Specular color is black, reflectivity is zero, reflection limit is 0 and the shadow mask is on 1. This way, everything that I don't want visible, is black and acts like a place holder for the objects that do go there, but the AO calculations and fall off is still present.

I'm using Mental Ray and Maya 2013


Hi, have u seen the tutorial already? You can render out your beauty, AO passes or any passes you like in your software package, and while you are doing compositing, you can use the rgb pass to mask out the object of your choice, at any stage you want. This gives you flexibility, and hassle free, since you don't need to deal with it in your 3D package.

There are advantages and disadvantages of course..
Advantages:
- Flexibility in comp, save time from trying to set up passes in comp.
- Reflections, refraction, Shadow, AO links etc are retained

Disadvantage:
- There will be problem with transparent objects such as glass. (There is a way to do it right though)
- Only 3 objects for each RGB pass. You may want to look into CMY pass as well, or simply create multiple RGB passes
- Sometimes there will be edge artifacts with post effects such as lens blur.

But overall, this is a good way to isolate your objects in post. Very useful to know how to do it.
 
Old 02-08-2013, 06:40 AM   #5
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