On 8/17/2017 4:46 AM, Kenneth wrote:
> I have never used Blender, but the analogy in POV-Ray would be this:
>
> A mesh object-- like a simple sphere, for example-- should have ALL of the
> triangles' normals facing OUTWARD into the surrounding space, for the sphere to
> render correctly. Those would be "consistent normals." Any mesh-creation tool
> should do that automatically-- if it's working correctly;-)But if a triangle
> happens to be made 'incorrectly' (that is, the three vector-location-points of a
> particular triangle are simply written in the 'reverse order' to what they
> should be, looking face-on at the triangle), its normal will actually face in
> the opposite direction(!), and will not render correctly.

Yes, this fits in with my experience.
When you manually edit a mesh you can do it in a way that is ambiguous 
to the modeller. Hence the need to make the normals consistent.


> (Or rather, it will
> show the INSIDE texture, if the object has one.) BTW, this is a typical mistake
> when trying to write triangle vectors*manually*; it's hard to keep track of the
> necessary and consistent 'orientation' from triangle to triangle.
>
> Making all the normals "face inwards" would be the opposite of the above. I'm
> not clear as to why that would be necessary, but I guess it can be done.

If you wanted to make a solid looking pipe. You could create a cylinder, 
copy the outside faces and scale them down to create the inside of the 
pipe. Those normals will still be pointing outwards. To get the inside 
of the pipe to render properly you reverse them to point inwards. For 
example.

> (I don't know if POV-Ray's 'inverse' keyword-- when applied to an object to change
> its 'outside' to its 'inside') switches the normals as well; I never thought to
> test it.)


-- 

Regards
     Stephen

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