uv unwrapping tutorial

Maya UV Unwrapping tutorial

uv_tut_bannerOn the previous pages of this tutorial, I explained the basics of planar mapping, and how to use cylindrical mapping. On this page we tackle the rest of the body.

Is the torso cylindrical enough?

I model the torso as a cylinder. Can I unwrap it the same way?

Here’s what I have so far – the front and the back of the body, each separately projected with a z-axis planar mapping projection.

Let’s try a cylindrical instead. It turns out pretty convoluted, and I end up with the mess below.

Cylindrical mapping on the body is problematic, usually because the body connects to itself at the bottom (under the pelvis) and at the top (over the shoulders). In this case the shoulders cause a problem, which is the long skewing at the top of the UV shell.

Instead I’ll reproject the front and the back of the torso as a single planar projection. You may remember from the first tutorial that this will create overlapping UVs, which make for poor textures.
maya uv unwrapping tutorial

This time, I’m going to use the Polygons > Cut UV Edges tool along the sides of the torso and along the shoulders to break the torso into a front and a back UV shell.
maya uv unwrapping tutorial

Now I can lay out the UV shells separately.
maya uv unwrapping tutorial

Depending on how the character will be used, you’ll want to consider where you place your UV seam.

The two common choices are on the side of the body or in the center of the back.

I am going to create the seam along the center of the back, so I have selected the corresponding edges and will cut them apart.
maya uv unwrapping tutorial

The back is split and I’ve arranged it around my front torso shell. Looking good, right?
maya uv unwrapping tutorial

Mirroring

When I select the edges on the sides of the torso, however, I see that things aren’t so straightforward.

The body is flipped – the opposite side of the back is arranged on the wrong side of the torso. Worse, even if I go ahead and use my Move and Sew tool, the shells will overlap.
maya uv unwrapping tutorial

Time to use a new tool. I’ve selected my back UV shells, and on the top toolbar, I am using the ‘Flip selected UVs in U direction’ button.

It’s on the top row, fourth from the right, and it looks like two triangles back to back. This will mirror my UVs – flip them over their horizontal axis.
maya uv unwrapping tutorial

After being mirrored and repositioned, my torso edges line up just fine.
maya uv unwrapping tutorial

Now I can use the Polygons > Move and Sew UV Edges to connect the torso shells together.
maya uv unwrapping tutorial

Once the torso is assembled, I use the Tool > Smooth UV Layout tool to fix any distortions in my UV shell. And my torso is done!
maya uv unwrapping tutorial

Let’s take a look at the feet. I’ve projected them with some planar projections, and I’m going to circle back on the mirroring tools in a moment. But take a look at this. I try using the Smooth UV Layout tool on my foot:
maya uv unwrapping tutorial

And I end up with a shape that looks like this. That is worse than when I started! What’s going on here?
maya uv unwrapping tutorial

Red vs Blue

To find out, I am going to look in the Image menu, and select the ‘Shade UVs’ option.
maya uv unwrapping tutorial

My UV shells become colored in. This makes everything easier to see!

But it also gives us more information than is visible without shading – for example, ‘good’ UVs are blue, and ‘bad’ UVs are red. How does Maya make this distinction?
maya uv unwrapping tutorial

Red UVs are ‘bad’ because they are flipped or mirrored. Usually, all I need to do is mirror the red UV shell using the ‘flip UV’ buttons on the tool bar. When I flip my foot in the U direction, the red disappears.
maya uv unwrapping tutorial


Take note, red UVs aren’t necessarily bad. This is just Maya’s way of showing us that a UV shell is oriented differently than the base object is in the 3D space.

For example, I’ve got a screen of my character with completed UVs. You’ll see that some parts are blue, but now purple is showing up as well, where blue and red overlap. I have two shells laid on top of each other, occupying the same texture space.

This kind of layout works fine if you are really struggling with a texture budget, such as development for mobile platforms. Textures will show up fine in the mirrored areas (with the possible exception of normal maps).
maya uv unwrapping tutorial

When I lay out all my shells separately, you can see where the purple comes from. I duplicated and mirrored my arms and legs in 3D space.

While the geometry is flipped in 3D, the UV shells aren’t changed at all! This is because the objects were simply duplicated, and so they still have the same UVs.

Because of this mismatch, Maya considers the UV shells to be oriented incorrectly, which is why they show up red. Simply using the horizontal flip button on the UV shells will solve this problem.

Pixel density

But take another look at the image below. You’ll note that the checkerboard texture is unevenly scaled across the model. Some squares are bigger, some are smaller.

This means we’re seeing differing pixel densities across the model. The areas with smaller squares have UV bigger shells and so cover more texture real-estate. And vice versa. This can be a problem!

If I have a really high-resolution texture in one area (like the face), but another area is lower resolution (like the neck), the difference in pixel densities can break the viewer’s sense of immersion.
maya uv unwrapping tutorial

In this example, I’ve rescaled all my shells to create a more even distribution of pixels across the surface of the model.

Almost done!

Let’s look at one more tool – the Polygons > Layout tool.
maya uv unwrapping tutorial

Before I activate it, I like to change the options so that Rotate is set to None.
maya uv unwrapping tutorial

What Layout does, is it scales and moves UV shells so that they fit within the zero-to-one texture space (the top right grid in the UV editor).

When a texture is applied, it will fit entirely in the zero-to-one space, so all the UV shells should fit in there, too.
maya uv unwrapping tutorial

The final layout looks like this on the character. The checkerboard texture is more evenly spaced now.

That’s it! All done. But what about the head? I’ll do that next time. But the head is usually a planar projection on the front of the face, then the rest of the head you can improvise.

If you’ve got questions leave a comment or email me.

Wait, what does UV stand for?

Ha! Trick question. It doesn’t stand for anything. My understanding is that U and V are coordinates, just like X and Y. Since they are 2D coordinates, they are in another dimension (not as cool as it sounds) so they get different names for clarity.

3DS Max actually has a UVW Unwrap tool, so that the UV coordinates are the 3 letters directly before XYZ. What the W stands for?

No idea! I’ve heard someone suggest it represents depth – but how could there be depth in a 2D coordinate system? Sounds like there is a sci-fi story in there somewhere. IT CAME FROM THE WTH DIMENSION!

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