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Old 6th April 2013, 12:19 AM   #1
Breems
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Default Booleans, Knives, and Overlap. Oh My!

Hello, all.

So, I have this friend who is a complete novice at 3d modeling/AC3D. He's trying to do a simple task - put a simple chimney on top of a slanted roof.

Here's a simple illustration:



The yellow bit is obviously the chimney and the red volume is one side of the roof (slanted).

Now, this next picture isn't for the faint of heart. Please sit down and get your kids into bed before viewing:



OVERLAP MADNESS!!!

So, I'm basically wanting to get rid of the overlap and allow the chimney to sit perfectly on top of the roof. In the end result, the bottom of the chimney should shape itself according to the top of the roof. I've tried every boolean operator and the knife tool, but nothing seems to work.

Boolean --> Union comes the closest, but it merges the two objects together, and I need them to remain separate volumes. I tried using the Knife tool and selecting the chimney as the knife while leaving the "delete knife" option unchecked and "cut away object" checked, but I wind up getting a "no surfaces to slice" error.

Am I simply using the Knife utility wrong? Should it be able to fulfill my needs?
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Old 7th April 2013, 12:33 PM   #2
jentron
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Default Re: Booleans, Knives, and Overlap. Oh My!

You want to cut the chimney, so set the roof as the knife. In this example I manually deleted the 4 chimney surfaces inside the roof.

Also, set your surfaces to single sided so you don't get the bright lines around the edges.

Ron

P.S., real chimneys go through the roof, they aren't sliced to sit on top. And they are sealed to the roof with tin patches and tar.
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Old 7th April 2013, 02:36 PM   #3
Breems
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Default Re: Booleans, Knives, and Overlap. Oh My!

Quote:
Originally Posted by jentron View Post
You want to cut the chimney, so set the roof as the knife. In this example I manually deleted the 4 chimney surfaces inside the roof.

Also, set your surfaces to single sided so you don't get the bright lines around the edges.

Ron

P.S., real chimneys go through the roof, they aren't sliced to sit on top. And they are sealed to the roof with tin patches and tar.
Thanks for the reply. Using the roof as the knife definitely makes sense. What parameters did you leave on/off, i.e. slice surface, cut away object, and delete knife (obviously unchecked)?

And yea, I know the design isn't realistic, but it's an enclosed building for an old video game so I'm keeping it very simple.
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Old 7th April 2013, 03:30 PM   #4
jentron
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Default Re: Booleans, Knives, and Overlap. Oh My!

I usually just use "slice surface." In this case, "Cut away object" should leave the inside roof piece of the chimney selected so you can delete it more easily. As you said, "delete knife" is unchecked.
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Old 8th April 2013, 12:08 PM   #5
Stiglr
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Default Re: Booleans, Knives, and Overlap. Oh My!

Not sure how well the "knife" tool will work on a 3D object (haven't played with it much).

But, with such simple objects as these, the "long hand" method might be worth a look.

1) Select the roof surface (top) in Surface/wireframe mode. Subdivide the surface a few times.
2) In Vertice mode, arrange the vertices nearest the middle of the surface to align with the "square" in the center formed by the chimney object. Use your viewports to make sure your vertices stay "flush" with the slope of the roof in the side views; that is, don't create any bulges or distortions in the roof.
3) Select unused subdivided areas and merge them to "clean up". The idea is to have only "one square surface inside the square" surface of the roof.
4) Now that you have the shape of the chimney, eliminate that separate chimney object, and instead extract it from the square surface you've formed in the roof. If it's necessary for texturing reasons or whatever, you can always select the surfaces that form the new chimney and create a new object from it.
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Old 9th April 2013, 09:44 PM   #6
Terry Capps
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Default Re: Booleans, Knives, and Overlap. Oh My!

There is a fairly simple way to do what you want.

1. Select the top surface of the roof and duplicate it.
2. Hide the roof object underneath that surface.
3. Select the duplicated surface as an OBJECT
4. Activate Tools, Knife, and then click on "Set Knife".
5. Un-select the duplicated surface and then select the chimney.
6. In the Knife Tool, click on "Cut away object", "Delete Knife", and then "Slice".
7. Select the lower portion of the slice chimney (it may already be selected) and delete it.
8. Unhide the roof (that you hid in step 2.
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Old 10th April 2013, 08:35 AM   #7
Breems
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Default Re: Booleans, Knives, and Overlap. Oh My!

Thanks for the help, all.

Jentron's original solution to simply switch out using the roof as the knife worked perfectly. I tested the other solutions provided and they worked as well.
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