[x3d-public] TriangleStripSet and ccw

Don Brutzman brutzman at nps.edu
Fri Oct 30 07:41:41 PDT 2015


wow what a great group effort, looks like we are converging on excellent clarifications.

very glad to see the references... curiously OpenGL Programming/Basics/2DObjects is silent about vertex-order reversals for even-numbered triangles.
https://en.wikibooks.org/wiki/OpenGL_Programming/Basics/2DObjects

alternatively the wikipedia reference quotes the OpenGL Programming Guide (red book) in a helpful manner, and the example list of vertices is numbered from zero, but the illustrating figure is numbered from 1.

https://en.wikipedia.org/wiki/Triangle_strip
https://en.wikipedia.org/wiki/Triangle_strip#/media/File:Triangle_Strip_In_OpenGL.svg
========================================================================================
	Quoted directly from the OpenGL Programming Guide:

	GL_TRIANGLE_STRIP

	Draws a series of triangles (three-sided polygons) using vertices v0, v1, v2, then v2, v1, v3 (note the order), then v2, v3, v4, and so on. The ordering is to ensure that the triangles are all drawn with the same orientation so that the strip can correctly form part of a surface.
========================================================================================

We might do well to quote or paraphrase the above. To be totally clear we should also include a closeup diagram showing four TSS triangles and order/normals defined by v0..v6.

The Mantis bug should explain how similar corresponding clarifications are needed for X3D IndexedTriangleStripSet, that also emphasize similar triangle order definition, with normal directions alternating, since this is further confusable when the indirection of indices is included.

Interesting OpenGl also includes GL_QUAD_STRIP. Perhaps we should define a pair of new X3D nodes, QuadStripSet and IndexedQuadStripSet, corresponding to QuadSet and IndexedQuadSet? Defining a Mantis bug for this is probably good, the CAD Working Group can then explore whether there is much content provided out in the wild (i.e. in other formats) that would benefit from this. We would also want feedback from the SRC team in this respect.

https://en.wikibooks.org/wiki/OpenGL_Programming/Basics/2DObjects#Using_glBegin_with_GL_TRIANGLE_STRIP_or_GL_QUAD_STRIP

Double check: wondering if anyone is familiar with, or can at least locate, similar references for DirectX/Direct3D?



On 10/29/2015 4:16 AM, Roy Walmsley wrote:
> Hi all,
>
> I have reviewed the implementation of triangle strip sets with OpenGL. They are programmed as shown in the following example (from https://en.wikibooks.org/wiki/OpenGL_Programming/Basics/2DObjects):
>
> glBegin(GL_TRIANGLE_STRIP); // draw in triangle strips
>                 glVertex2f(0.0f, 0.75f); // top of the roof
>                 glVertex2f(-0.5f, 0.25f); // left corner of the roof
>                 glVertex2f(0.5f, 0.25f); // right corner of the roof
>                 glVertex2f(-0.5f, -0.5f); // bottom left corner of the house
>                 glVertex2f(0.5f, -0.5f); //bottom right corner of the house
> glEnd();
>
> A triangle strip set, when discussed from the perspective of a user looking at a 2D image, does indeed have vertex ordering alternating. The ccw flag specifies the ordering for the first, and subsequent odd numbered, triangles. The even numbered triangles have the opposite ordering.
>
> Within OpenGL, if normal are automatically calculated, then this will be taken care of internally. If normal are provided by the user, would they have to incorporate the alternating ordering? I don't know the answer to that.
>
> If an implementation does not pass the triangle strip directly to OpenGL as above, but uses, say GL_TRIANGLES, then it must internally correct for the alternating ordering before sending each set of three vertices to OpenGL.
>
> In my opinion the specification 19775-1 needs a sentence or two to clarify the meaning of the ccw field for both TriangleStripSet and IndexedTriangleStripSet.
>
> As noted during our meeting yesterday, the issue does not arise with triangle fan sets, as all triangles are oriented the same way.
>
> Regards,
>
> Roy
>
> -----Original Message-----
> From: x3d-public [mailto:x3d-public-bounces at web3d.org] On Behalf Of simon place
> Sent: 27 August 2015 16:23
> To: x3d-public at web3d.org
> Subject: Re: [x3d-public] TriangleStripSet and ccw
>
> my thinking is that you could think of list-order and drawing-order, as not necessarily the same thing, then;
>
> the definition of a Triangle Strip (see wikipedia) is telling you the drawing-order of even numbered triangles is reversed from their list-order, in which case the CCW is correct for the drawing-order.
>
> of course this should be explicit, or refer to an external definition of triangle strip.
>
>
>
> On 27 August 2015 at 15:08, Alekseyev, Vsevolod (NIH/NIAID) [E] <VAlekseyev at niaid.nih.gov> wrote:
>> Any trivial one with 3 triangles will do.
>>
>>
>>
>>         <Scene>
>>
>>                <Transform>
>>
>>                       <Shape>
>>
>>                             <Appearance><Material diffuseColor="0 0.5
>> 0.5"/></Appearance>
>>
>>                             <IndexedTriangleStripSet index="0 1 2 3 4">
>>
>>                                    <Coordinate point="0 0 0 1 0 1 1 0 0
>> 2 0 1
>> 2 0 0"/>
>>
>>                             </IndexedTriangleStripSet>
>>
>>                       </Shape>
>>
>>                </Transform>
>>
>>         </Scene>
>>
>>
>>
>>
>>
>> From: Roy Walmsley [mailto:roy.walmsley at ntlworld.com]
>> Sent: Wednesday, August 26, 2015 6:49 PM
>> To: Alekseyev, Vsevolod (NIH/NIAID) [E] <VAlekseyev at niaid.nih.gov>;
>> x3d-public at web3d.org
>> Cc: x3d at web3d.org
>> Subject: RE: [x3d-public] TriangleStripSet and ccw
>>
>>
>>
>> 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/Tria
>> ngleStripSet.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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>>
>
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all the best, Don
-- 
Don Brutzman  Naval Postgraduate School, Code USW/Br       brutzman at nps.edu
Watkins 270,  MOVES Institute, Monterey CA 93943-5000 USA   +1.831.656.2149
X3D graphics, virtual worlds, navy robotics http://faculty.nps.edu/brutzman



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