Ok, so I haven't come up with a 'why' the artifact occurs (although it's likely some vestiges of the micro-polygon smoothing not fully being by-passed when smoothing is disabled.. more on this later), but since the problem DOES exist, it doesn't really matter 'why' for existing owners of the software - only how to avoid or work around it.

With the above in mind, here are my findings so far...

To start with, I created a conical-shaped cylinder (pinched at the top), just to help visualize the 'flat' uv-mapping type (and in the following images, the cone #2 has this mapping... mapped looking down from the top, as if draping a table-cloth over the cone).  I was also checking / concerned with highlights and shadows and (of course) texture-mapping results (a 'fix' is not much of a fix if it ruins your ability to texture the object effectively).

In the following images, the cones are labeled 1-4 and have the following properties:

1. Cylindrical-Cap type mapping. This cone shows the artifact.  I moved the uv-seam around to the front-right quadrant, to make it easier to see and not interact so much with the texture used in later images.
2. Flat mapping (from the top).  This is the one mentioned above.  It 'fixes' the artifact, but we'll compare this solution to others, under different circumstances...
3. Cylindrical-Cap type mapping.  This one 'fixes' the artifact by adding a slight bevel to the top edge (an extra physical row of polygons).
4. Cylindrical-Cap type mapping.  This one 'fixes' the artifact by physically disconnecting the top from the sides ('split vertices' fix).  Otherwise, it's identical to #1 (mapped the same, and no bevel added).

...ok, with the above legend in mind, let's move on to the images.

In each image below, the lighting is exactly the same with a shadow-casting infinite light from the above-rear-right and another infinite light (casting no shadow) from the front-mid-left direction.

The first one is a simple render, with no texture, so we can see the artifact and the fixes.  It also shows how each handle the particular lighting setup...

Notes: - note the artifact on cone #1, but 'fixed' on the others

• note the shadow being cast on #1 and #2 is quite different from #3 and #4
• note that the specular highlights of the rear two are also quite different from the front two
• on cone #3, you can see the (tiny) bevel that was added around the top lip

...ok, so the shadow and shading/specular made me curious if this was due to camera angle / cone position-relative, so I swapped the front and rear cones and rendered another...

...hmm, the specular changed a bit with the new positions, but the differences are still there and the shadow differences are definately not due to the positions.  [for the rest of the images, I swapped the positions back to the original configuration].

The next image adds a texture, so we can see what we're working with...

Notes: - this image gives you a good visualization of the 'flat' mapping (cone #2), but keep in mind that the sides are sloped... if they were vertical, it would look much worse (with similar issues when mapping flat from one of the sides).

• note that beveling the top edge (cone #3) retains some 'localized' specular lighting on the top (adding some tothe sides as well), whereas physically disconnecting the top (cone #4) flattens out the top specular completely, as expected.
• Unfortunately, the way I mapped the cones and with this particular texture, the top rim hit right on a 'black' bar across the bitmap, so you can't really see details in that area, but you can still see that the beveled one has a slightly wider rim.
• I noticed that whoever made this texture (I grabbed it off the net somewhere) labeled the U/V axis incorrectly :).  U goes from left->right and V goes from top->bottom

...ok, so far, all of these renders are with Polygon Smoothing disabled.  Let's enable that and let the fun begin...

Notes: - Cone #1 - the overall resulting 'shape' of the cone (as rendered) is exactly the same as #2, but notice that where the 'artifact' is, ALSO effects the smoothing (the texture is now being distorted by the smoothing).

• Cone #2 - aside from the texturing issues mentioned earlier, this method of fixing the artifact also suffers from the smoothing process (the same as cone #1).  ie. not a good general solution.
• Cone #3 - less overall distortion from the smoothing than #1 and #2, but still an issue.
• Cone #4 - we have a winner.  At the expense of the extra vertices and any specular/shadow preferences you may have to the contrary.

Conclusions:

Again, 'why' the artifact is happening is really only of concern by the programmers (who need to figure it out and fix it), so the question for the rest of us is knowing 'when' it's likely to occur (with capped cylindrical type objects, every place there is a gap/seam in the uvs of the sides) and how to work around it.

Except for cone #2, the rest of them use the exact same uv-mapping (a cylindrical-cap style mapping).  The uv-mapping style is only important because of the number of resulting seams, so you can make the issue worse with other mapping projections (adding additional seams), but you can only make it better if there are no seams - a flat type mapping - but that just leaves you with texturing issues and does nothing for polygon smoothing issues. 

Obviously you can enable/disable polygon smoothing to eliminate that particular problem, but that's not always convenient or desired.

That leaves us with the "bevel solution" and the "disconnect solution" (cones 3 & 4, respectively)... - They each add additional vertices, but the bevel solution also adds additional polygons.

• You may prefer a beveled edge over a sharp edge, or vise-versa.
• The beveled edge helps, but does not eliminate polygon smoothing issues.

...the "disconnect solution" seems to cover all the bases, but may or may not give you the shading and/or style you are looking for.

Cheers,

Keith

Cinema4D Plugins (Home of Riptide, Riptide Pro, Undertow, Morph Mill, KyamaSlide and I/Ogre plugins) Poser products Freelance Modelling, Poser Rigging, UV-mapping work for hire.