# Normal Mapping

The newest feature implemented in the game is normal mapping (a form of bump mapping). This is a method for creating more detail in a rendered mesh without actually creating more geometry.
In the early days of computer graphics, lighting wasn’t calculated at all, later, lighting was calculated on a per vertex basis, today, graphics cards are capable of calculating lighting on a per pixel basis, and normal mapping is one form of doing that.

Lighting can be computing using the normal vector of a surface, in combination with the direction that the light is coming in. On a per vertex basis, this is a fairly easy computation since vertices typically have a normal attached to them. On a per pixel basis this is a little more difficult, since each pixel doesn’t have a vertex attached to it, and that is where the normal map comes in. The normal map is loaded as if it were a texture, but each pixel in the texture is treated as if it were a normal.

From there is seems like it would be easy to calculate lighting based on that. Wrong. Consider that a texture is actually “wrapped around” a mesh. So each normal in the texture isn’t actually the normal of the pixel we are rendering. It’s a normal that doesn’t take any regard to the orientation of the vertex.

Therein is the problem with normal mapping. It is necessary to transform the per-pixel normal into the space of the vertex. Or, alternatively, transform the direction of the light into the space of the vertex, which is a more useful computation.

Transforming a light vector requires an orthogonal transformation into the vector’s tangent space. And for this to be accomplished a tangent vector must be computed for each vertex. The tangent vector depends upon the normal of the vertex, in combination with the orientation of the texture about that vertex. It’s a lot of linear algebra, and theory that I don’t want to explain, but after programming a complex algorithm, the tangent vector is now computed in the Emergence engine, which in turn can be used for per pixel lighting in a pixel shader. The result is thus,