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UV Mapping Tips And Tricks

March 25, 2004 Article Start Page 1 of 2 Next
 

The movie and game industries have made giant leaps in visual realism over the last 10 years. We're able to create fairly realistic creatures now, featuring everything from muscle dynamics to cloth simulation and even accurate liquid dynamics. Visual effects work has become a science and an art rolled into one.

But one subject that gets little attention is texture mapping. It's always the last part that the 3D modeler wants to deal with, yet it's something that can make texture artists go insane.

Imagine you've just finished your latest creation: a fantastic, multi-limbed creature with layers of internal mesh details. Now it's time to create the UVs. But despite its beauty, you've created a nightmare: none of the basic modifiers will work. It's an all-out custom job to complete the texture mapping. You have to give each surface its own unique mapping to eliminate any distortions. That means no stretching or shearing, and keeping a consistent pixel density over the entire character. While you're doing this, you also have to come up with a texture map that's understandable to the texture artist. And of course, you need to finish all of this in time to hit your deadline. Welcome to the world of texture mapping.

My primary goal with this paper is to arm you with every trick in the book when it comes to UV mapping, and provide tips and methods to use in every situation. My second goal is to share some tricks in achieving more texture detail in real-time games by the use of blending techniques using small modular detail textures.

The Basics

Mapping Types And Their Uses

Planar

Planar mapping is the most basic of the mapping modifiers to apply to objects. It works by projecting the texture onto a model from one direction. So it's useful for objects like walls and basic terrain. But it can't effectively be used on complex objects with many overlapping surfaces, since it often stretches the polygons that don't face the projected map directly. There several ways to fix these problems, which we will be cover in a moment.

 

 

Box

Box mapping works by projecting the desired texture onto the model from six sides. This is very handy when working on technical or architectural objects, and when you just need quick mapping for less important parts of an object. Unfortunately, it's not very useful for organic objects where you need accurate mapping. An example of a situation where box mapping is useful is for tiny screws on a cargo container, or some metal bars or grates in a factory. It becomes less useful when you start working with more complex objects that require specific mapping.

 

 

Spherical

Spherical mapping projects the texture in a spherical pattern onto an object. Space junkies love using this technique to map asteroids and planets, but one side effect that it causes is very high pixel density at the poles of the spheres mapping. This causes a pinching effect that's hard to counter when painting the texture. It's helpful in blocking out mapping on human heads, but it still requires significant tweaking afterwards. It starts to lose its usefulness on models with many overlapping parts.

 

 

Cylindrical

Cylindrical mapping is used more widely then any other type of mapping. Projecting the texture in a radial pattern inwards makes it very useful for mapping objects like tree trunks, arms, torso and legs. It's very handy for blocking out mapping on various types of meshes. But it still requires a lot of tweaking afterwards in the UV editor. Like the other mapping options, it doesn't work in every situation.

 

 

Shrink Wrap

I have yet to see anyone use this modifier to map objects. I have yet to figure out its purpose or how it works exactly, because it seems to create some of the most useless mapping solutions of any technique. I don't recommend using this technique for any object, and if somebody does find a use for it, let me know. I am curious to see how it could be put to practical use.

 

 

Pixel Density and Stretching

It's important to make sure that your mapping keeps a consistent aspect ratio for the pixel size in the texture map -- nothing annoys a texture artist more than warped mapping. Keep a lookout for areas where the texture gets stretched or skewed. The last thing the texture artist needs to worry about is correcting his texture to counter any warped mapping. It's also considered wasteful when you use more pixel area than required.

Mapping Seams

Seams are an artifact produced by cuts in the mapping because of the shape of a mesh. You can't avoid seams, but you can minimize their effects and the amount of effort it will take the texture artist to fix them. Simply align the vertices of the seam with the corresponding connection in the mapping on either the horizontal or vertical plane of the texture coordinates. This way the pixels align on one of the axes when the texture artist fixes and hides the seams. For technical objects, it's easier to get away with seams since they tend to be quite fragmented, and the nature of the object allows it. But for organic meshes, you should minimize the amount of seams as much as possible by using accurate, continuous mapping (which I'll cover a later in this article). The other solution is to use a good 3D paint program like DeepPaint 3D.

 

 

Symmetry Mapping and Overlaps

Sometimes you encounter models - character models, usually -- that are the same on both sides (also called bilateral symmetry). When this is the case, you can cut your mapping time in half by mapping one side of the model and then mirroring it onto the other half.

Another optimization that you can use is overlapping mapping coordinates. For instance, say you have a character with lots off spikes protruding from his back. You can map all those spikes using the same texture by overlapping them all onto the same area. For a real-time character, this saves you a lot of unneeded texture resolution that could be better used elsewhere, instead of mapping each spike individually.

 

 

Tiling Textures

Tiling graphics has long been one of the quickest ways to save memory and mapping space. For example, a repeating pattern could be used on a rubber hose or the treads on a tank. A tiled texture allows you to use a small texture area and repeat it, conseving pixel density while achieving a realistic effect. Terrain artists use tiled textures extensively, but they use blend maps to hide the tiling pattern by mixing other tiling textures over each other. We will cover this more in depth later.

Advanced

Optimized UV Layouts

Optimized UV layouts are particularly relevant for real-time characters. In a nutshell, don't waste any space in your texture mapping. Since the entire texture gets loaded into memory, you should use as much of the texture area as possible. To do so, you should scale, rotate and move those UV-mapped vertex bits until you can't save another ounce of space. Think of it like a giant puzzle with no picture to work with. Your job is to maximize pixel resolution and leave as little unused space as possible. A good UV-mapped model can use up to 90 percent of the final picture. If there are parts of a model that rarely get seen, scale the UV mapped space down to 50 percent of its original size and cram it in somewhere.

 

 

Continuous Mapping

Continuous texture mapping usually only used for organic creatures. With this method, you try to link all the relevant body parts' mapping so as to reduce the number of seams. This typically involves clever warping and arranging the UV-mapped bits for the mesh you're attempting to map. In the figure below, you can see that the body, legs and head consist of a single mapped area. I also used symmetry to save texture space. This gives the texture artist fewer seams to work around and makes his life easier. In short, the more parts you can link together, the better. Trying to do this with mechanical parts is almost impossible, however: it usually warps the texture.

 

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