Tiled Terrain

In this article, I'll examine some of the issues you may want to consider when starting to create a terrain tile set for a game. When done well, the terrain becomes a believable world in which your characters come to life. The user will simply accept your work as part of the natural backdrop. Often, the best compliment for a good environment is that it doesn't call attention to itself.

As the terrain artist, you may not have a ton of texture memory space to deal with, so it's always a good idea to figure out how to stretch your budget as far as possible. Obviously, the larger the number of base texture tiles, the more random (and hence natural) the terrain will look. When only a few terrain tiles are available, a repetitive pattern becomes very obvious. The trick is maximizing the usage of each texture while minimizing the disjoint that occurs when you see the same pattern over and over.

The Basics

Before you begin, you will need to evaluate the needs of your particular project. Let's assume that you are trying to make one large plain of cooled lava stone. In theory, you can accomplish this with just one texture map tile and one large polygon. You can tile the texture map multiple times on the single polygon, which gives the appearance of a higher resolution on the terrain. While this method works conceptually, it can quickly lead to some technical problems, such as the inability to add additional texture variety into the field, and shading issues over a large polygon.

Another method is establishing a grid of polygons similar to a chessboard mesh. You can map each square with a different texture map, and there is a lot more flexibility with what you can do on an individual square basis. This method addresses some of the variety and shading issues as well as making the process of editing and tweaking much easier.
Now, suppose you want to add a lake of molten magma to the middle of the landscape. With the single large polygon method, you'll quickly face some problems. Since you can't interrupt the tiled terrain and insert a different texture map in the middle of the sequence, you have to figure out an alternative method. One such alternative is to include the lake in one large texture map. While this would allow you to create a completely custom terrain, you will quickly encounter texture memory usage and texture resolution challenges.
The other alternative is to make a custom polygon for the magma only, and then perform a Boolean operation on that polygon, combining it with the other terrain polygon. This too presents some particular problems if you are using vertex shading to light your terrain. In addition, making the seam between the two texture types invisible will prove to be difficult, if not impossible.

For this tutorial, I'll focus on a more forgiving method of maximizing the effect of your tiled terrain by using a standard grid and texture-mapping each polygon quad or set of quads as a unique element. This method will allow you to control the layout of the textures much more accurately, and it also provides vertex-shading advantages. In addition, when you start to add vertical information to your mesh, you will find that the additional vertices give you more control of the 3D aspects.

Creating the Basic Flood Fills

When making a texture tile set, you'll need to create a minimum set of tiles. The complexity of the planned environment will determine the number of tiles, how well the tiles appear visually without showing repetitive patterns, and how many different types of random tiles will be required. The amount of available memory for the terrain will impact all these factors. As a good starting point, create the minimum base set, then add to it only when necessary. Reuse is king, so try to stretch your texture budget as far as possible.

The number of different terrain types you will have in your environment will be another big impact on your tile set. This impact can be compounded by which terrain types can touch or transition with one another. Take, for example, a terrain that consists of water, sand, and grass. If water can only touch sand and sand can only touch grass, then you only have two transition sets to make. However, if water touches grass as well, then an additional transition set is required. As you can tell, planning out the environmental requirements ahead of time makes a lot of sense. You may find areas where you need to limit the number of transitions in order to keep the texture budget in check.

For the sake of argument and to keep the number of variations low, I'll assume that the new terrain will consist of two basic ground types: stone (cooled lava) and molten lava. This will require you to make only one transition set. For now, don't concern yourself with the 3D aspects, and assume that the terrain is a flat plain.

First, create the flood-filled texture for each basic ground type. This is the basic tile that is used as the default flood fill of an area. If possible, since it stretches your texture budget, it's a good idea to make the texture able to tile in any direction, even if it's rotated 90 or 180 degrees. This will give you more visual mileage out of a single texture by breaking up the repetitive pattern of the texture when you lay it down next to itself.

	Figure 1. A basic stone texture that is seamless in only one orientation.

In Figure 1, you can see the stone texture in its original orientation. It's easy enough to use the Offset filter and Rubber Stamp tool in Photoshop to make a texture seamless, but you're limited to only one orientation. If you rotate it and try to place it in a grid, the seam will be plainly visible. In order to make a tile seamless when it rotates, the texture needs to have identical edge pixels on each of the four sides. This requires a bit more work than just using the Offset filter, but it also extends the usability of the texture. With a bit of Photoshop magic, you can create the seamless effect pretty easily.