Hard-edged uv mapping
Cirstyn Bech-yagher takes you through the fundamentals of the critical task of UV mapping
Cirstyn Bech-yagher explains the fundamentals of the critical task of UV mapping
make sure you build proper Geometry, and keep in mind that your intended end result Can impact your uv map and its layout
Known as the most tedious task in 3D, especially for beginners, UV mapping is also the glue that binds models, bakes and textures together. It’s a necessary task, too, as a bad set of UVS and their resulting output can make a really good model look really bad. Whether you love or hate UVS, there’s no escaping them – they are essential to understand.
Some of the most common problems that novices run into when starting out with UV mapping is finding the whole process an incomprehensible soup of terms and concepts, as well as never quite knowing where to begin, especially when working on hard-edged models. However, if you get into the habit of doing a little prep and find a UV mapping tool you click with, it’s not quite the tedium it used to be.
UV space fundamentals
The reason a UV map is the glue between model and textures is that it’s not only the flattened and mapped topology of your model, it’s also the basis for your map bakes. This means you need to take your mapping into consideration as you model – bad UVS give bad bakes. You can always tweak bad curvature, height or occlusion maps in Photoshop in a pinch, but bad normal or similar map output can be a real pain to fix if you don’t keep them in mind from the get-go.
This means the first thing you need to know about UV mapping is what UV space is. Based on a 0 to 1 grid, with 0.5 as the middle coordinates, a UV map consists of your 3D model’s XYZ coordinates flattened into 2D UVW space – or tile, as it’s called.
Depending on your modeller and mapper, 3D’s horizontal X-axis equals U in 2D space, vertical Y equals V, and the depth coordinate Z equals W. There is no madness to the letters, only method: 2D’s UVW is used solely to avoid confusion with 3D’s X, Y and Z. This equalling of coordinates is – to put it simply – how 3D space translates into flattened 2D space.
You may be wondering why 2D space also has a depth coordinate. This is to make sure that no matter how you choose to map, flip or stack your UV’D bits and pieces – called islands or shells – the depth coordinate will ensure they show correctly in 3D no matter their mapping. It’s also vital that all your model’s flattened polys need to be inside the UV tile in order to provide your baker and renderer with accurate texture information.
Some renderers and texturing applications take this a step further by also providing support for something called UDIM – U-dimension. In simple terms, UDIM makes UV mapping and texturing easier by enabling you to create multiple UV grids for the same model by allowing you to have 10 tiles on the U-axis, and an (almost) infinite amount on the V-axis. As with a normal UV tile, you can’t have polys outside of the UDIM tiles’ boundaries either, but we’ll get to that later.
pre-modelling considerations
Now you know what UV space is, we can move on to the few fundamental concepts that apply to UV mapping no matter your model, UV mapper, or whether you’re using a single-tile UV map or UDIM. First and foremost, a UV map needs to be as distortion-free as possible. This means that a texture won’t
look stretched or pinched when applied onto the model. It also needs to maintain scale. Textures applied onto a model should not show disproportionately to each other. For example, the head of your mech shouldn’t have a much smaller scale and details than its torso, and even worse if the arms have even larger details. You can of course work around this by using triplanar mapping when working in apps such as Substance, for example, but the best thing is to get it right straight out of the gate.
As well as no distortion, a good UV map also has borders and padding. Avoid seam trouble on the tile’s edges by adding a threeto four-pixel border, and get into the habit of creating enough space between your UV islands to handle edge padding and gutters to head off trouble at the pass. Even two to three pixels is often enough to avoid texture bleed and normal mapping artefacts when baking, and almost all UV mappers have functionality for this. Use it.
It also pays to keep your layout tidy and economical, with similar items arranged together, or stacked on top of each other where possible to save space. This will allow more room for the items needing finer detailing or with higher visibility; the larger the item on a UV map, the larger the texel density, meaning the more space it gets for painting and detailing.
When you start modelling, make sure you build proper geometry, and keep in mind your intended end result can impact your UV map and its layout. If you’re creating a game model, for example, you’ll