[Bf-docboard] Bf-docboard Digest, Vol 124, Issue 10

Sat Jun 20 20:33:29 CEST 2015

On 06/20/15 07:12, Ivan Paulos Tomé wrote:
> Hi friends !
>
> The question about double sided and single sided faces, and manifold
> or non-manifold meshes is related to it's normals.
>
> The normal of the face is the face that is shaded.
>
> Normals, are generally pointed to a single side of the objects. And
> normally, because of the shading, normals are pointed (when we have
> closed meshes) externally to the world.
>
> The standard in the majority of the softwares, is to have a single
> sided face (only one normal per face, which means that the materials
> assigned will only be detected and rendered at the normal direction),
> but for compatibility with game engines, we can make Blender have
> double sided faces too.
>
> A manifold mesh is a mesh which faces are completely pointing to the
> outside or to the inside of the mesh, if you recalculate those
> normals, they will always point at a single direction (outside or
> inside).
>
> A non-manifold mesh is a mesh in which we can't determine the outside
> and the inside of the mesh, because the normals will *always" point at
> diferent directions, and it doesn't matter if we recalculate them
> using a tool or flip them manually, due to the way that the structure
> of the mesh is conected.
Well, it is possible to have non-manifold meshes with consistent normals 
(e.g. a 2D plane)
And it's possible to have manifold geometry which cannot have consistent 
normals (e.g. a klien bottle <https://en.wikipedia.org/wiki/Klein_bottle>)

So this definition doesn't hold up under all circumstances.

The manual doesn't really attempt to define it beyond saying 
"non-manifold geometry cannot exist in the real world", but this too is 
slightly misleading (klien bottles are manifold, but they cannot exist 
in the real world because they intersect themselves).
Perhaps it would be nice to write up a detailed explanation of all this 
on the manual somewhere (like this 
<http://blender.stackexchange.com/q/7910/599>, but better).

>
> Cheers !
>
> 2015-06-20 7:00 GMT-03:00, bf-docboard-request at blender.org
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>> Today's Topics:
>>
>>     1. Some weak points in the documentation. (brasshat at core.com)
>>     2. Re: Some weak points in the documentation. (gandalf3)
>>     3. Re: Some weak points in the documentation. (brasshat at core.com)
>>     4. Re: Some weak points in the documentation. (brasshat at core.com)
>>
>>
>> ----------------------------------------------------------------------
>>
>> Message: 1
>> Date: Sat, 20 Jun 2015 00:21:12 -0400 (EDT)
>> From: brasshat at core.com
>> Subject: [Bf-docboard] Some weak points in the documentation.
>> To: "Blender Documentation Project" <bf-docboard at blender.org>
>> Message-ID: <65486.99.58.2.94.1434774072.squirrel at webmail.core.com>
>> Content-Type: text/plain; charset="iso-8859-1"
>>
>>
>>
>> Friends,
>>
>> While answering a question on Blender Stack Exchange,
>> I went looking for some things in the manual that I thought should be
>> there, but were not.
>>
>> 1) Any information on what Blender
>> considers to be an interior face. There was no entry in the glossary, and
>> doing a search using the parameter "interior faces" did not
>> provide any hits. The only explanation I located was in the tool tip,
>> where a hidden face is defines as a face where all of the edges of the
>> face have more than 2 face users.
>>
>> 2) I could also not find any
>> information on removing non-manifold bits of a mesh when I attempted to
>> search on "non manifold".
>>
>> Both interior faces
>> and? non-manifold are items in the select menu of the 3D viewport
>> header.
>>
>> I'd add them myself, but haven't been able to rustle up
>> enough time to learn to make the edits.
>>
>> Noel Stoutenburg
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>>
>> ------------------------------
>>
>> Message: 2
>> Date: Fri, 19 Jun 2015 23:44:55 -0700
>> From: gandalf3 <zzyxpaw at gmail.com>
>> Subject: Re: [Bf-docboard] Some weak points in the documentation.
>> To: Blender Documentation Project <bf-docboard at blender.org>
>> Message-ID: <55850BE7.4040103 at gmail.com>
>> Content-Type: text/plain; charset="windows-1252"
>>
>> On 06/19/15 21:21, brasshat at core.com wrote:
>>> Friends,
>>>
>>> While answering a question on Blender Stack Exchange, I went looking
>>> for some things in the manual that I thought should be there, but were
>>> not.
>>>
>>> 1) Any information on what Blender considers to be an interior face.
>>> There was no entry in the glossary, and doing a search using the
>>> parameter "interior faces" did not provide any hits. The only
>>> explanation I located was in the tool tip, where a hidden face is
>>> defines as a face where all of the edges of the face have more than 2
>>> face users.
>>>
>>> 2) I could also not find any information on removing non-manifold bits
>>> of a mesh when I attempted to search on "non manifold".
>>>
>>> Both interior faces and  non-manifold are items in the select menu of
>>> the 3D viewport header.
>>>
>>> I'd add them myself, but haven't been able to rustle up enough time to
>>> learn to make the edits.
>>>
>>> Noel Stoutenburg
>>>
>>>
>>> _______________________________________________
>>> Bf-docboard mailing list
>>> Bf-docboard at blender.org
>>> http://lists.blender.org/mailman/listinfo/bf-docboard
>> They are here:
>> http://blender.org/manual/modeling/meshes/selecting/advanced.html
>>
>> Not sure why 'non manifold' and 'non-manifold' don't turn up any
>> results, but 'manifold' gives some.
>>
>> --
>> -gandalf3
>>
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>>
>> ------------------------------
>>
>> Message: 3
>> Date: Sat, 20 Jun 2015 04:08:51 -0400 (EDT)
>> From: brasshat at core.com
>> Subject: Re: [Bf-docboard] Some weak points in the documentation.
>> To: bf-docboard at blender.org
>> Message-ID: <51458.99.58.2.94.1434787731.squirrel at webmail.core.com>
>> Content-Type: text/plain; charset="iso-8859-1"
>>
>>
>>
>> Gandalf3,
>>
>> Regarding your question:
>>
>>> ? Not
>> sure why 'non manifold' and 'non-manifold' don't turn up any     results,
>> but 'manifold' gives some.
>>
>> I can't explain "non
>> manifold", but "non-manifold" does not appear, because in
>> every instance there is a space between the hyphen and the word
>> "manifold", , and if the search facilities are case sensitive,
>> both the initial letters N and M are capitalized, so "Non-
>> Manifold.
>>
>> Noel Stoutenburg
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>>
>> ------------------------------
>>
>> Message: 4
>> Date: Sat, 20 Jun 2015 04:27:38 -0400 (EDT)
>> From: brasshat at core.com
>> Subject: Re: [Bf-docboard] Some weak points in the documentation.
>> To: "Blender Documentation Project" <bf-docboard at blender.org>
>> Message-ID: <51715.99.58.2.94.1434788858.squirrel at webmail.core.com>
>> Content-Type: text/plain; charset="iso-8859-1"
>>
>>
>>
>> Ooops.?
>>
>> I didn't look closely enough. and found after
>> hitting the "send" button that this statement,
>>
>> both the initial letters N and M are capitalized, so "Non-
>> Manifold.
>>
>> is not accurate.
>>
>> Noel Stoutenburg
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>>
>> End of Bf-docboard Digest, Vol 124, Issue 10
>> ********************************************
>>
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-- 
-gandalf3

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