Why are rectified Uv's always warped?

JACKJAKE

First of all, Apologies for the nooby question, maybe to some, this is trivial info, but I can't for the life of me no matter the research, figure out why this happens.
I rectify the UV it looks perfect. Afterward, when I test it on the model it's always warped.
How do you explain this? any solutions for fixing it? 

teodar23

The question is, which surfaces should be straightened and which should be as close to the 3d counterpart as possible.
When you flatten a surface and dont keep the "shape" of the surface in the uv, stretching/warping is inevitable. Only rectify surfaces that you dont mind having some warping.
I dont have a tutorial at hand but looking at a few will reveal that only certain shapes like pipes, barrels, almost straight shapes are prime for straightening, and for other shapes there are other ways of mapping, like unwrap, angle-based flattening, pelt, etc.

JACKJAKE

teodar23 said:

The question is, which surfaces should be straightened and which should be as close to the 3d counterpart as possible.
When you flatten a surface and dont keep the "shape" of the surface in the uv, stretching/warping is inevitable. Only rectify surfaces that you dont mind having some warping.
I dont have a tutorial at hand but looking at a few will reveal that only certain shapes like pipes, barrels, almost straight shapes are prime for straightening, and for other shapes there are other ways of mapping, like unwrap, angle-based flattening, pelt, etc.

thank you! I don't know where I have this habit of obsessing over straitening UV's but I guess only go for it for certain shapes, got it

Fabi_G

If you triangulate the mesh, the patterns lines are aligned with the right angle UV edges of each triangle:

Aligning UV borders with pixel grid will help to reduce visible seams. If you are mainly projecting or painting in 3d space when texturing, the distortion will be in the textures (important to have consistent triangulation).

okidoki

If you really want to know: Have a look a this .. (attention math ahead ) https://mtrebi.github.io/2017/03/15/texture-mapping-affine-perspective.html ..or just simply: because they aren't rectangular 

ZacD

Heavily straightened UVs can work great for stylized assets and textures, but really starts to fall apart with more realistic models if you're not being careful.