[x3d-public] TriangleStripSet and ccw

[Roy Walmsley]

Vsevolod,

 

Great catch!

 

Tomorrow I'll raise a Mantis issue covering this and let you know the
details.

 

Do you have a reference to an X3D example file that can be used to test
implementations?

 

Roy

 

From: x3d-public [mailto:x3d-public-bounces at web3d.org] On Behalf Of
Alekseyev, Vsevolod (NIH/NIAID) [E]
Sent: 24 August 2015 20:58
To: x3d-public at web3d.org
Subject: [x3d-public] TriangleStripSet and ccw

 

While we're on the topic of standard ambiguities, here's another issue.

 

Imagine a TriangleStripSet. It has a sequence of points with a sliding
window logic to them - the first triangle is points 0, 1, 2, the second is
1,2,3, then 2, 3, 4, etc. It's illustrated perfectly at
http://www.web3d.org/documents/specifications/19775-1/V3.3/Images/TriangleSt
ripSet.png

 

The element also has a ccw flag that tells us which side of the triangle is
the outer side.

 

Now, the issue. If the ccw logic is to be followed *for every triangle*,
every second triangle would look the other way from the previous one.

 

Look at the image. For the first triangle, the logic for determining the
outer side is - if you look at the triangle from the outside, vertices 0, 1,
2 should go counterclockwise. But if you look from the same vantage point at
the second triangle, vertices 1, 2, 3 would go *clockwise* instead. Were the
algorithm to go by the strict letter of the ccw rule, one would have to
conclude that odd numbered triangles in the strip look one way, even
numbered ones look the other way.

 

Fortunately, few implementors think so. Neither X3DOM nor X3D-Edit read the
standard that way. My Blender X3D importer flips the vertex order for every
odd triangle, too. The only implementation that does not is meshlab. Upon
rendering, it gives a very characteristic "checkerboard" pattern - triangles
that are seen from the inside are dark.

 

The same issue would obviously plague IndexedTriangleStripSet, but not
[Indexed]TriangleFanSet.

 

Maybe a clarification in the standard would be in order, that ccw only
applies to the *first* triangle in the strip, for the subsequent ones one
should go by consistent orientation of adjacent triangles.

 

Vsevolod "Seva" Alekseyev

 

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