In the second generation of 3D games (Quake, Unreal) a light map was combined with the color map to create a more realistic sense of shading and soft shadowing. The trouble with this technique is that the shading on a particular texture would often be at variance with the perceived sources of light in the scene in which they were used. For example, a texture might be drawn as if it were lit from the upper left, but in a particular scene, the actual light source may be from the right, or below the texture.

The color map and light map combined on a surface in a game engine. Although the surface is realistically shaded by the light map, highlights and shadows on the texture ignore which direction the light is actually coming from.

A new technique was developed, generically called "bump mapping". In its simplest form, a texture on a surface contains two textures - a color map and a second map that contains the height information for the surface. A height map would be a gray-scale image that encodes height information as shades from black (the lowest) to white (the highest). The game engine is able to use this height map to render highlights and shadows on the surface according to where the lights in the scene are located, resulting in a vastly improved surface realism.

The color map and height map conbined in a game engine. The height map allows the game engine to render the surface as though it has been illuminated by the light source in the game. Highlights and shading appear coming from the correct direction. Because of the limitations of this technique, the resulting surface appears flattened and lacking in richness.

There is a limitation to simple height mapping techniques - the game engine is restricted to portraying transitions in elevation only, resulting in a flattened appearance of the actual height information the designer is trying to convey. Fine details get blurred out of the bump map render, because a single pixel contains no real data of its own - it is only in relation to its neighbors that it has any meaningful information. A 100% white height map renders exactly the same as a 100% black height map. A game engine looking at a height map has to take a sample of a certain radius around each pixel to determine if this pixel is the same relative to its neighbors, or sloping up, down, to the left or to the right, before it can rendering the shading on that surface. This sampling slightly degrades the detail that can be portrayed with the height map.

Enter the "normal map". While a height map only contains 1 plane of information - the "Z" (height) plane, a normal map contains 3 vectors of information - "X" direction, "Y" direction and "Z" direction. Thus, each pixel in a normal map encodes which direction that particular point is facing - the "normal vector" of the surface. Each pixel in a normal map has meaningful information, so details can be rendered more crisply than with bump maps alone. This allows modern game engines (Doom III) to more realistically portray the lighting on a surface. A properly constructed normal map can fool the eye into perceiving much more complex 3D geometry on a simple surface. Theoretically, normal maps on a cube can make it appear spherical, at least in terms of shading properties (the outline remains unchanged).

A combination of color map and normal map on a surface in a game engine. Note how the surface appears much deeper and more geometrically complex. The shading is much more accurate now, appearing as rich as the original flat shaded texture, but with the addition of realisitic highlights and shadows that react to the lighting around them.

Normal maps use three channels of information to encode their information. This can be conveniently mapped to a standard RGB image. The red channel is used to encode normal vectors in the X direction. 100% red indicates a vector facing right - an X normal direction of +1. 0% red indicates a vector facing left - an X normal direction of -1. A 50% value in the red channel indicates an X normal component of 0. Similarly, the green channel encodes normal vectors in the Y direction. 100% green indicates a vector facing up - a Y normal direction of +1. 0% green indicates a vector facing down - a Y normal direction of -1. 50% value in the green channel indicates a Y normal component of 0. The blue channel encodes normal vectors in the Z direction. 100% blue points straight out of the surface. 0% blue points straight behind the surface. A value of 50% in the blue channel indicates a Z normal component of 0. Normal maps don't contain values below 50% in the blue channel since these would be pointing behind the surface.

Where Can I Work With These Normal Maps?
As of the date of this writing, DOOM III is a future glimmer. But there is a game engine that can be used as a testing ground for developing familiarity with these texturing methods in advance - Tenebrae. Tenebrae is an open source modification of the old Quake 1 engine, adding modern techniques such as real time lighting, and normal mapping of surfaces. These two techniques combined can give a budding game designer a very rich and realistic game world in which to play.

How Can I Use My Current 3D Program To Make These Normal Maps?
Texture designers used to use painting programs to create textures for games. More and more, textures are being created from 3D geometry in rendering programs such as Lightwave, 3D Studio Max, or Maya. The big advantage of using a 3D program to create textures, aside from the powerful texturing algorithms these programs enjoy, is that you can use the geometry to directly create the normal map to go along with the color map. Plug-ins are available to developers, created by video card manufacturers to enable the production of normal maps from a certain select group of 3D applications. But what if your application of choice has no plug-in available for it? This tutorial will show you how to use your current 3D application to create normal maps. There is a bit of work involved, but the results are well worth the extra effort. Please read the next page to find out the details.