Distorted UV of a symmetrical object - why?

[Honey Icechant]

Hi everyone! Can you please tell me why do i get a distorted uv of a symmetrical object, please? It's not happening for the first time and i can't find the answer, why, what am i doing wrong, what to check. Maybe i want to apply a symmetrical patterned texture, but i wont be able with this uv. It's an island selection (top view) that needs a separated uv map. Please help. 
My second question is about the moving tool, it shouldn't be positioned at the center of the object? - the origin was set to the center of mass right after importing the object. and it is at the center of the object, in object mode, but when one island selected as in the pic, in edit mode is not at the center. Maybe it has to do something with the distorted uv?
Thank you in advance.



 



sorry for the 2 images, i didnt find where to delete the first.

 

[Drongle McMahon]

The position of the manipulator is easy. With default settings, the pivot/origin is the median point of the selected vertices. That is a point where there are equal numbers of vertices on each side of it in each dimension. Because there are a lot of vertices in the hemispheres, thisis dragged upwards and rightywards, in x and y directions, because the missing fifth hemisphere means there are more vertices in thoise directions from the centre. You can change the pivot.origin location in one of the icons at the bottom of the 3D view.

The squeezing of the mapping in one corner is less obvious. I made something as near as possible to your geometry (but see below) and this didn't happen (with default mapping). However, there is something strange going on because I can see only three rings of faces around the edges in your 3D view, while there are six in the UV map. This suggests there are some faces so narrow (even zero?) that we can't see them in this view. In that case. there may be some differences in the geometry in the 'hidden' rings. The visible result in the UV map appears to be that the outer edge of the distorted corner has only two segments around the curve, while the others have three. Thus there may be some differences in the geometry that we can't see in the 3D view, because they are too small, that affect the unwrapping differently at the distorted corner.

[Chic Aeon]

If you use PROJECT FROM VIEW when you unwrap you should be able to get something close to what you need. If there is height in those outside edges that needs to be adjusted for, you can stretch out those edgeloops in the UV mapping plane as needed.  

 

You can check your scale buy using the GRID as your texture. The squares sizes will let you know what areas need to be changed, if any.

 

In general NOW (for some time actually) when you are marking seams and using the UNWRAP method to unwrap, you need to go Object > Apply > Rotation and Scale  BEFORE you unwrap or things will not work as you expect. You need to be in Object mode to get to that  Object > Apply menu.

 

To get the origin to the center of the object  (be in object mode again) use Origin > Transform > Origin to Geometry.

 

Hope that helps some.

 

PS. Drongle responded while I was typing and his answer makes me think I misunderstood the issue but leaving this here just in case that is not the case .  

[Drongle McMahon]

Since the problem may have arisen from the way you constructed the geometry, here is one way I might have done it (making some assumptions about the aim). This gives a minimally distorted UV map, with or without the added hemispheres...

1. Add cube; edit mode; stretch to desired dimensions.
2. Select and delete bottom face.
3. Select all edges except bottom rim.
4. Mesh>Edges>Bevel, segments=3.
5. Alt select bottom edgesand stretch as required.
6. Select top face and Mesh>Faces>Inset (I); pull to right place.
7. Select middle top face and raise as required.
8. Default UV mapping.



[Honey Icechant]

Thank you for your reply Chic. Project from view will not always give me what i need, that works better with flat objects i guess, in this case i see overlapping faces. I will post an image to see why. 



"In general NOW (for some time actually) when you are marking seams and using the UNWRAP method to unwrap, you need to go Object > Apply > Rotation and Scale  BEFORE you unwrap or things will not work as you expect. You need to be in Object mode to get to that  Object > Apply menu."

I do not know what is that good for, but i will make some research and find out.

[Honey Icechant]

Thank you Drongle. I am amazed by your professionality and detail of your replies..
After checking 1,2,3,4... times my object, and back, I have come to the conclusion that a distorted uv is always caused by a bad geometry. Even if you check it x times and dont see anything wrong, the problem is there.
I tweaked some verts, then cut the object in half, deleted te wrong (left) one, then mirrored the right one.
Ok, i got 2 corners that i have no idea (yet) where did they come from, but the uv is correct this time. 

 


That border caused the problem. The cross-section shows why i cant use project from view. The border turns to inside at the bottom.

 



About the manipulator.. of course its easy, i should have known. Thank you!