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Old 6th June 2018, 03:38 PM   #1
dvdl
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Default how to use the knife tool ?

Hi

I'm not new to 3D, however, the AC3D docs mention the Knife Tool but I can't get it to work.
A tutorial would be great.

cheers

Last edited by dvdl; 6th June 2018 at 04:29 PM.
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Old 7th June 2018, 10:14 AM   #2
Andy
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Default Re: how to use the knife tool ?

The Knife tool is best used with a rectangle (think like a knife blade) - it's the surfaces that do the slicing.

Perhaps you want the boolean knife. You'll need to ensure the "knife" object is closed and the normals face outwards. On the model you show, Object->Fill-hole should do it. Press 'n' to show normals and check they are all outwards.


Then you can select the sphere, followed by the knife shape and use Object->Boolean->knife, of any of the other boolean functions.
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Old 7th June 2018, 03:38 PM   #3
dvdl
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Default Re: how to use the knife tool ?

Hi Andy

I tried that and still no luck - any chance to send you the file to have a look ?

cheers

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Old 8th June 2018, 07:09 AM   #4
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Default Re: how to use the knife tool ?

There were a few problems in the model. Both surfaces on the end of the block were not joined to the the other surfaces. I deleted those and then tried Object->Fill-Hole. This failed because there were other holes in the block. Using Object->Optimize-vertices removed lots of duplicated vertices and then Fill-hole worked fine to close the ends of the shape.


The sphere had artifacts (split surfaces), probably left over from other experimenting, so I made a new sphere, and copied the dimensions.


After that, a boolean subtract seemed to work fine.


Here's the file with your original model plus the fixed parts and the subtraction result:


http://www.inivis.com/external/forum..._ac3d_001c.zip
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Old 8th June 2018, 04:46 PM   #5
dvdl
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Default Re: how to use the knife tool ?

Hi thanks Andy this helped

The boolean knife definitely does the better job however I get on the opposite side of the mesh a black polygon, first I thought this might be a reversed normal but that wasn't the case. After exporting to OBJ there seems no issue but inside AC3D it is visible.

Could I ask you to have another look ?
I tried to optimise but there were no vertices to fix.
Not sure what I done wrong again this time (I'm new to AC3D but getting used to quickly)

And another unrelated question if I may
how do I select edges ?
Polygons and vertices yes, but I don't know how to select edges.

cheers

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Old 12th June 2018, 10:01 AM   #6
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Default Re: how to use the knife tool ?

That surface is black because the first three vertices of the complicated polygon form an obtuse angle and that is causing a problematic surface normal calculation (surface-normals are used for lighting). It's best to break up the surface - triangulate will do this. You can also use Surface->change vertex order (one or more times) on polygons like that.



There's no edge selection in AC3D, since edges are essentially pairs of vertices. There are some functions which work on edges (selected vertices). The most useful is - after selecting two vertices, pressing 'l' to select a loop - which is just like a loop select in other software.



Loop select also works with surfaces - select some adjacent surfaces and press l to select a loop of surfaces - very handy.
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Old 12th June 2018, 01:15 PM   #7
dvdl
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Default Re: how to use the knife tool ?

Hi Andy

will watch out next time - that was an issue in another software too where triangulating helped.

with edge selection that will do, just needed to know

thanks for the help
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