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art
05-12-2004, 02:48 PM
I just received UVEditPro from newtek. I played with it for a moment. I have very little experience with UV mapping, but I thought I understood the basic idea of it. I guess I do not. Is this what many of you refer to as "UV distortions"? I did a simple test: a plain quad with a checkboard mapped onto it. It is being mapped in a very unexpected (to me at least) way.

Any insights?

mkiii
05-12-2004, 05:35 PM
Photoshop doesn't use triangles to create a bitmap. It simply distorts each pixel to make it fit. You will have noticed that the distortion is either side of a diagonal line.

You should try to avoid distorting a uv map too much. To do so doesn't really make much sense. If you have a texture that needs distorting - distort the texture - as you have found, it is more accurate.


If you really do have to, then consider subdividing the mesh a few times. This will more or less eliminate the distortion.

I think the distortions that you have been hearing about are more likely to concern the way the UV mapping of a Sub-Division object doesn't match the polygons that are uv mapped, because only the undivided cage is used in the UV map, not the final mesh.

art
05-12-2004, 06:29 PM
Thanks for the reply. I noticed the distorions along the diagonals before and thought I was doing something wrong.

I will keep your tips in mind (image stretching and subdividing)

Now Im off to learn more UV mapping :)

jin choung
05-12-2004, 06:34 PM
right,

it has to do with the TRIANGLES.

and no, this is NOT what most of us are usually talking about when we're talking about uv distortion and SDS.

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a programmer explained this to me once... let me try to remember:

in lw or in your imagination, turn that distorted QUAD into two triangles.

got it?

notice that the AREA of one of the triangles is MUCH larger now than the other.

mkiii is absolutely correct in saying that texture placement in lw is a function of the triangles and that it is NOT so in photoshop... in photoshop, it manipulates the entire image pixel by pixel... photoshop keeps track of where every pixel is at all times during the 'shearing' operation and deletes and filters pixels on the fly.

this is NOT so in 3d and with polygons.

with polygons, the ONLY PLACES that lw EXPLICITLY KNOWS WHERE A TEXTURE GOES IS AT THE UVs!

(and this is closely related to the fact that with polygons, the only way that you know where a model is is by their vertices - which is very unlike a NURBS surface where every possible point on the surface is parameterized and not just the CONTROL POINTS [and this is why you can arbitrarily trim shapes out of NURBS but not on subdivision surfaces])

for the placement of EVERY other texel, lw merely INTERPOLATES.

that being the case, since the area for one triangle is now much larger than the other triangle, the math MUST place the texels differently.

and so you get a distortion, 'cracked' along where lw made the two triangles.

if you explicitly cut the triangles in another manner, the distortion would be along your slice.
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and finally, mkiii is right about avoiding the distortion... the 'aspect ratio' of a poly should be the same in 3d space and in uv space.... this results in no squishing or stretching of texels when everything else will.

but if you wanted to do some kind of unique effect, all you need to do is HEAVILY SUBDIVIDE YOUR QUAD so that there are more 'sampling points' and you will be able get the effect you are looking for... just as mkiii said. :)

if you subdivide it EVEN ONCE, you will see a marked difference in the results.

jin

mkiii
05-13-2004, 07:23 AM
Yeah... that's what I meant to say ;)

art
05-13-2004, 07:23 AM
Jin,

Thanks for the explanation. Somehow I knew you would join in this thread :)

what I got out from mkiii's and jin's explanation is this:

stretch image, not UV points
distort UV points as little as possible
if I need to distort UV points a lot, I might need to subdivide first.

Is that right?

jin choung
05-13-2004, 01:27 PM
jin