Building UVs without applying the mirror modifier

Question

I'm having a really tough time finding a solution for this (seemingly common) situation. I have a complex model that uses the mirror modifier, that also has an armature attached. Blender allows me to fully build this model to almost completion without applying the mirror modifier, which is great, because it reduces the work by half. My vertex weights, bone relationships, etc. Everything works flawlessly with the mirror modifier active. Until I get to the point where I'm defining the texture UVs.

How in the world is this done?

Here are some more random notes about my quest to keep my mirror modifier:

1. I certainly don't want to apply the mirror modifier. If I do this, then build a complex UV layout, I'm completely stuck with my model as-is, and cannot make changes later without repeating all of the work that happens after I apply it. Not to mention I would have to save two versions of my object so I could "undo" my work.

2. Blender has a mirror UVs toggle in the mirror modifier settings, but it only has one offset. How am I supposed to use this effectively? Doesn't this require every single separately mapped entity to reside in the absolute center of my UV space? How does one map out a complex object with many small symmetrical parts with everything in the absolute center? Is there some clever way to offset specific elements to different places?

3. Blender has a toggle near the options button in the 3D view that enables "mesh symmetry on the X axis", but I cannot get it to function correctly. Even a simple action like joining two vertices with an edge (key J) does not work on the other side. Does this mode only support simple offsetting actions? If it worked correctly, I could simply do away with my modifier, but that doesn't seem possible at the moment. Unless there is an option or add-on somewhere that makes it work the way its supposed to? Some type of aggressive symmetry?

Really appreciate any guidance on this, as I feel stuck between two terrible choices. Either commit to my work so far, which I absolutely know I will regret, or find some way to deal with the extremely limited functionality of the mirror modifier's UV help.

Answer 1

I think I ran into a similar issue with UV's for my projects, and I used UV warps to offset my uv's procedurally as needed. One possibility is:

1. Plan to have all your uv's start on the center line (0.5)
2. Make two vertex groups named left and right
3. Separate geometry that you want to send left or right from that center (but keep them as part of the same object). Assign the geo that you want to send left or right to the corresponding vertex group. If the verts between the different groups aren't separate, it will result in shearing
4. Uncheck merge vertices for your mirror modifier (otherwise you won't get a clean break between the vertices on the different halves, which will result in uv shearing)
5. Check the Mirror U, option for the Mirror modifier, but keep it at 0
6. Make a UVWarp modifier, set it to the proper UV Layer, and use the transform to offset it left or right as needed.
7. Add a weld modifier at the end, with .001 as it's default. Because it's after all your uv stuff, it won't affect any of the information you need to modify vertex groups beforehand

For additional fine control, you can have many different vertex groups applied to each object, and tie a specific UV Warp modifier transform to each vertex group (for additional ease/clarity, name the vertex group by the offset amount [e.g. left.01, right.15, etc]). Then you can place each uv shell in a vertical line with the knowledge that it will be offset in a given direction so that it doesn't overlap. It's kind of cumbersome, and probably won't yield the most efficient UV layout, but if you're intent on keeping your mirror modifier, it's a technical process that may work well for you.

Answer 2

If you don't mind overlapping UVs then you can use the mirror modifier without applying it, each side of the model will simply share the same UVs.

If you want both sides of the mesh's UVs to not overlap, then checking the Mirror U checkbox will mirror the UVs to the other side of the image along the x-axis. That will allow you to use one side of the image for each side of the mesh.

You can also still do more complex UV mapping things like marking seams and selecting small parts of the mesh to unwrap individually. Just have to keep all the UVs on one side of the image, like so:

The UVs will exist on the mirrored part of the mesh even if you don't apply the modifier, but they are hidden. If you find that difficult to work with, there is a trick where you can export the full UVs, then open them as an image and that will give you a bit of a preview to know where everything is:

1. Duplicate your mesh and apply the mirror modifier to it
2. Open the UV Editor
3. Select UV > Export UV Layout
4. Save it as an image
5. Open that same image under Image > Open
6. Go back to the mesh you didn't apply the mirror modifier on, and you'll see where both sides of the UV maps are:

Answer 3

1. Unwrap the object while the mirror modifier is on, do your UV work naturally, fully covering the 1:1 space.

2. Move the Slider of the "U" coordinate to 100%, so all mirrored islands from half of that part are moved 1 quadrant right outside of the UV, once you apply the modifier.

3. Exit out of Edit mode and Apply the mirror modifier

Due to the nature of UV space, textures repeat tiling when crossing the 1:1 coordinates, so moving the sibling/mirrored islands of half of your object to exactly 1 unit, regardless of direction U, or V, will instance the same detail.