by Jojon » Sat Jun 18, 2011 8:55 am
It is pretty logical that you get that distortion, when you know what it going on.
Given that you apparently have got a UV offset animation working, I'm assuming you have unwrapped using a sphere or cylinder projection, which are exactly what you have with real world maps and is the reason that the further you get from the equator an a map, the more landmasses are stretched out. (Although misrepresentative of actual distances, this is very useful in navigation)
Each ring on your UV sphere has a different diameter and is even zero at the poles (we call it our point of view - sorry, I digress ;), but they will all be streched in the UV-map, so that they fit the entire width of the UV-mapped picture, things will become incresingly squashed towards the poles (we are going from a plane to a sphere here - opposite direction compared to world maps).
One might suggest this form of projection is wasteful, given the difference in density of information (texture texel per rendered screen pixel) across the skydome, but it has its uses, not least of all the very ability to scroll the image around using a UV-map offset, that you are exploiting.
One way to get rid of the distorsion with your texture fully wrapped onto the uvsphere, is to warp it using something like photoshop. I'm no photoshop user, so I can't say exactly how to do it, but the principles are universal. You "simply" stretch a 2:1 elliptical portion of the picture to fit the entire rectangle that is the picture, discarding every pixel that falls outside the ellipse.
There is the matter of tiling, which you will want to address either before or after the warp - probably both.
When working with triangles there is another distortion problem. On the cylinder-projection UV-map every square is perfectly rectangular, but the actual polygons become more and more trapezoid the farther to the poles you get. This would not have been a problem if it had really been a matter of four-sided polygons, but in actuality the "quads" you see are two triangles, they are just represented and handled like quads for convenience. To see this, you can press ctrl-T to turn every selected "quad" face to triangles - they will be diagonally split.
This is a problem, because that means moving one corner in the UV-map representation of a quad, may mean you are only affecting one of the triangles. Seemingly working with one surface, you'd expect the texture to stretch across the entire face, but there is an ugly delimiter across it; one half streches, the other is unaffected. So there will a degree of distortion every time the shape of a polygon in a UV-map does not exactly match the shape of the same polygon on the actual 3D model and that will become apparent with your sphere; ok at the equator, not so much at the poles.
It's just something one have to live with and compensate for... :7
This is where the "ruins" come in handy. Like if I have too much trouble mapping a given model, I do my best then cover up the area that didn't map correctly with a piece of fallen ceiling or something. Know what i mean?
