weird multires problem in blender

Hi;

i recently created a thread in the blender.org comunity forums. But i did not get much further there. So i post my question here, hoping to find someone who can give me a hint. The weird thing is, that "sometimes the problem does NOT occur" but i cant figure out, when that happens...

The original thread can be found at

http://blenderartists.org/forum/showthread.php?t=130114

and the following is a verbatim copy (without the images contained in the original post at blender.org)

====

i found some weird thing during learning UV-mapping: i created a plane object.
then i did the following sequence:

1.) go to edit mode
2.) subdivide 3 times. now i have a plane with 8*8 faces
3.) mesh -> UV-unwrap -> unwrap. Now i get an ideal uv map (8*8 squares, see pic 1)
4.) Now warp the mesh into a cylinder. i place 3D cursor below the plane, then
warp 360 degrees (W-360), so that the plane looks like a cylinder now.
5.) i locate the seem, where the left and the right side of the plane are wrapped together.
the seem is highlighted in pic.2 .
i remove double vertices now. and i end up with a perfect cylinder and a perfect UV-map.
everything is intact until here.

now trouble starts.

6.) I add one level of multires (either catmul-clark, or simple subdiv, it doesn't matter for the effect). When i now look at the UV map, i see 16*16 squares as expected. But when i select the same vertex row in the cylinder, as before, i expect to select the vertices along the sides of the UV-map. But there is a sudden shift at the top 2 rows of the UV-map (see pic 3).

i tried everything to avoid this effect, but as soon as i add a level of multires, the effect apears (ok, in very rare cases the effect does NOT occur, but i was not able to tell, when that happens, and whn it does not happen). The effect only appears when i add multires, AFTER i have applied the Warp-tool.

If i apply multires to the simple plaine, the UV-map looks like expected,
but now i can't remove the double vertices from point 5 (multires does not allow to remove vertices)

Anyone got an idea, what could cause this effect and how i could avoid it ? Yes, i know, there are simpler ways to get a perfectly unwrapped cylinder, but id like to understand my error.

thank you in advance.

Pretty much the entire multires system is being revamped as part of Google's Summer of Code. I'd test again with a build from that branch

http://www.graphicall.org/builds/builds/showbuild.php?action=show&id=716

If you still get problems, then report it as a bug. I don't think it's anything you are doing causing it.

Hope you had a good holiday

I just tested blender 2.47. The UV-map problem i have seen with blender-2.46 seems to have disappeared. Now i can rework the blender purists tutorial and show, how to create sculpties out of plane meshes, using multires and without any scripts usage ( @Domino: You know, it is just for learning purposes )

Grids also work and you can set the number of vertices from the very start.