"Subtraction models"?

# 1 17-02-2008 , 12:36 AM

Registered User
Join Date: Feb 2008

Posts: 22

Hi everybody. Total newbie here. I just joined and this is a great site!

Anyway, what I want to do is create a polygon or NURB model that will make any other model it intersects or passes in front of invisible.

I'm trying to make Maya footage to composite with live action footage, and I want to make a model of a spear/arrow/etc and have it stick in a filmed person. So I thought if I could make a rough approximation of the person's chest in Maya in 3D, then I could match the movement of the 3D chest with the live action footage of the person moving, and just link the spear model to that model. But I don't want the segments of the spear that should be inside the chest or on the other side of the chest to show.

Does anyone understand what I'm trying to say? My apologies if this has been asked a thousand times before. I've tried searching this forum and the internet, but I really don't know how I should phrase my searches, so I haven't had any luck.

Thanks.

Edit: The chest model itself also needs to be invisible. I feel stupid for forgetting to say this up front.

Last edited by InverseTelecine; 17-02-2008 at 03:51 AM.

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# 2 17-02-2008 , 12:42 AM

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Join Date: Feb 2004

Location: Chicago

Posts: 1,712

I would think the easiest way to achieve that effect would be to model and animate the spear and just mask out the spear/arrow tip in the compositing package when you comp the live action.

2 x Modeling Challenge Winner

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# 3 17-02-2008 , 01:10 AM

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Join Date: May 2005

Location: Manchester Uk

Posts: 6,300

Think that Johns suggestions probably the easiest, or you can make the chest and set the matte opacity to black hole and that should do it for you.

"No pressure, no diamonds" Thomas Carlyle

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# 4 17-02-2008 , 07:40 AM

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Join Date: Feb 2008

Posts: 22

Thanks a lot gster123. From what I'm reading in the documentation about the "black hole" setting that should be exactly what I'm looking for. I haven't figured out how to actually do it yet, but you've given me the clue that I need so I should be able to figure it out. Thanks again!

Thanks jsprogg, that is a good idea. You're right that it would probably be easier to use masking in a composition package in some cases, but in other cases (like a really big spear :-) ) the intersection would be far too complicated to "draw in" with a mask. I'd rather leave that for Maya to handle. But thanks for the suggestion.

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