How do UV texture seams work?

Question

My maps does not look like what I expect them to. I don't understand how Blender knows what part of the mesh should have it's own separate map in the UV editor and which should be a part of another map? Does the order in which the edges are selected and the seams made matter? Why does Blender want to rotate the map of one side of the wall?

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I've been playing around with the exercise from the tutorial given above and while I understand the way the box has to be cut up with seams to get this box template UV map, I don't really get how Blender translates the 3D rotation of the seams around the object to the 2D plan in UV editor. I've made this example where I made the seams exactly the same way on each side of the box around the Z axis. The UV map also rotates but I can't see the logic to it so I could predict how it will be rotated when except when the X axis is pointing right, Y away from me and Z up. For example why is the UV map rotated up when the Y points to the right and Z towards me? And even more surprising when the Y is pointing down the UV map has the same orientation as when it's pointing towards me. Simple stuff but so hard..

Answer 1

UV unwrapping is an art… Some things that have helped me:

• In “Object Mode” apply the scale Ctrl + A then select “SCALE” from the drop down menu Before you attempt to unwrap the mesh.

• In general seams are placed on object edges. The order of their selection shouldn't matter.

• After unwrapping you can move/rotate individual maps. Easily selectable by selecting one vertice of the UV map then Ctrl + L to select the rest of the linked vertices.

• Try unwrapping using “Smart UV Project” with the “Angle Limit” set to “89” (Doesn’t always help).

• Unwrap the mesh and then export the UV layout. Found under UVs then at the top of the drop down menu “Export UV Layout”. Bring the exported UV layout into Photoshop or Gimp and place your textures there.

• In a pinch assign a different Material to the mesh faces for each Texture.

• This simple tutorial helped me to begin to understand UV unwrapping. https://www.youtube.com/watch?v=fDABx8wWGR4

Answer 2

From the Blender's Manual page (give it a look, it contains tips & explanations):

Just like in sewing, a seam is where the ends of the image/cloth are sewn together. In unwrapping, the mesh is unwrapped at the seams. Think of this method as peeling an orange or skinning an animal. You make a series of cuts in the skin, then peel it off. You could then flatten it out, applying some amount of stretching. These cuts are the same as seams.

Let's consider a sphere. Once marked the edges surrounding a sector as seams, we define two different, closed surfaces. The result of the unwrapping process will consist in two different, separated, UV islands. One for each closed surface.

If the seams doesn't define a closed boundary, as in the image below, you'll have just one surface domain, so there will be only one UV island:

The surface of the mesh is the paper model, the seams are your the scissors.

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At the end of the Unwrap operator, an automatic scaling&rotating&positioning occours in order to fill the Map extention leaving as less empty areas as possible. That's why you'll find some UV island with unexpected rotation.