QUOTE(UNGLAUBLICHUSA @ May 27 2011, 05:39 AM)
how to do seamless mapping on cylinders?
I will assume you only want it seamless in horizontal direction. Easiest is to start with a seamless image and apply that as a cylindrical map. If you apply the decal with the image centered on x axis, cylindrical mapping will automatically create a seamless map.
I will assume you know how to create a seamless image, in one or both directions.
It is not necessary however to start with seamless image, as you can always change the repeat count for the cylindrical mapping from 3 x 1 (the default) to 6 x 1 and set seamless property to ON (have to render to see it work, it will not appear correct in model window).
If you start with a "unseamless image" - It may not have the right proportions, and the "front/back" may not be where you'd like. You can manipulate in A:M uv editor somewhat to move image - but easiest is to start with seamless image.
EDIT: I will assume your cylinder is vertical in the y axis. If it is not, then you should apply the decal in an action, where you have first rotated the cylinder to be vertical. If you have a mass of odd, non-vertical, non-straight pipes, cylinders - then might be best to apply a blank image and paint the sucker in 3D painter.