Is there a way to hide/show seams on model?

Question

I have unwrapped character and I want to understand how it was done: Usually, when we unwrapping character, we need to mark seams on it, so seams will have red outline like this: AS you can see, in 2 screenshot UV is distorted compared with original one which don't have any of it. I want to understand, why I can't see any seams in original model? I know that I can hide vertices with H key, so maybe there is similar hotkey for seams?

Unfortunately, can't share blend file.

Answer 1

How to get UV seams from existing UV islands

As 3fingeredfrog mentioned, you can mark the UV seams from your current UV islands. For this, go in the UV editor while in edit mode, select all your mesh in the viewport, all the UVs in the UV editor, and use the menu UV > Seams From Islands:

Why UV islands don't always have seams

As to why some models can have UVs without seams, "seams" or any form of edge-marking for UVs are only useful for the process of making UVs manually. They are useless to storing or even describing the UV data.

Primitive objects in Blender don't have seams. And it is possible in Blender to make UVs without seams. In fact, all the unwrapping methods you see in the viewport's UV Mapping menu (shortcut U) make no use of edge seams, except the "Unwrap" at the top that flattens your mesh and splits it at the edges marked as seams:

Hence, it's possible to have UV-unwrapped models with no seams in Blender.

Furthermore, it is almost systematic to have no edge seams on models that were imported from outside of Blender.
I say almost, because some formats do support explicit UV seams {FBX, Collada (.dae)}, and as you can see above you can always make seams from existing islands. But even if a data can be sometimes exported by some specific formats, it's also not a very frequent one to export, not every import/export tools exposes it as an option, so it's safe to assume you won't have UV seams even from meshes stored in formats that does support it.

How UV data are stored

UVs are simply coordinates stored per vertex. Here's an example with Blender's default cube in OBJ format:

# Blender 4.2.1 LTS
# www.blender.org
o Cube
v 1.000000 1.000000 -1.000000
v 1.000000 -1.000000 -1.000000
v 1.000000 1.000000 1.000000
v 1.000000 -1.000000 1.000000
v -1.000000 1.000000 -1.000000
v -1.000000 -1.000000 -1.000000
v -1.000000 1.000000 1.000000
v -1.000000 -1.000000 1.000000
vt 0.625000 0.500000
vt 0.875000 0.500000
vt 0.875000 0.750000
vt 0.625000 0.750000
vt 0.375000 0.750000
vt 0.625000 1.000000
vt 0.375000 1.000000
vt 0.375000 0.000000
vt 0.625000 0.000000
vt 0.625000 0.250000
vt 0.375000 0.250000
vt 0.125000 0.500000
vt 0.375000 0.500000
vt 0.125000 0.750000
s 0
f 1/1 5/2 7/3 3/4
f 4/5 3/4 7/6 8/7
f 8/8 7/9 5/10 6/11
f 6/12 2/13 4/5 8/14
f 2/13 1/1 3/4 4/5
f 6/11 5/10 1/1 2/13

In an OBJ file, UVs are described using the vt (vertex texture) lines, and they correspond to texture coordinates in 2D space. Here's how it works:

• o defines the name of the object that follows
• v contains three 32-bit floats x y and z corresponding to the coordinates of the mesh's vertices.
• vt contains two 32-bit floats u and v that defines a UV coordinate, where u is the horizontal axis (0 to 1) and v is the vertical axis (0 to 1).
• For example, vt 0.625000 0.500000 means that one of the vertices maps to the UV coordinates (0.625, 0.5).
• s defines the shade smoothing state. 0 makes the whole mesh flat shaded, 1 Would make the whole mesh smooth shaded. If you exported a smooth shaded object with some sharp edges, you would also have vn lines after the v lines, describing vertex normals in the form of 32-bit floats x y and z to make up the vector of the vertex orientation.
• f defines faces, by associating vertices to UV coordinates. It contains one A/B pairing for each vertex making up the face. Where A and B are respectively the indexes of vertices (v lines) and UV coordinates (vt lines) as they are described in the file.
  In our example, we have a cube, which is six faces of 4 vertices, so we have six f lines with four A/B pairing.
  The first line f 1/1 5/2 7/3 3/4 means that:
  • the first vertex of the face associates 1/1, aka the 1st vertex of the list (v 1.000000 1.000000 -1.000000) to the 1st UV of the list (vt 0.625000 0.500000)
  • the second vertex of the face associates 5/2, aka the 5th vertex of the list (v -1.000000 1.000000 -1.000000) to the 2nd UV (vt 0.875000 0.500000)
  • so on

As you can see, no seams represented here, just 2d/3d math coordinates and associations.

Answer 2

Apart from the fantastic answer by @Lauloque which explains quite good how UV mapping works, to answer your question more generally regarding the points "is there a way to hide seams?" and "usually we need to mark seams on it":

First of all one simple way would be, clearing the seams after using them for unwrapping, so that there have been seams but they have been removed.

Another way is, the edges could still be marked as seams, but they are just not showing, because Seams are disabled in the Mesh Edit Mode Overlays. I guess this is closest to your last question, if there is some kind of hotkey to hide seams, although this is probably not what is going on there (in this case, new seams which you create would not show either):

A third option would be: there never were seams. Instead of using seams which are a great help if you want to unwrap everything at once or make changes without having to start completely all over again, you can just as well achieve the same segmented unwrapping by just selecting parts of the mesh and unwrapping them one after another. The boundary of the selection would then basically work like a seam.