What is a normal map?
A normal map is a type of texture that allows you to fake extra detail onto models while keeping the same number of polygons of the model, meaning less render time. That’s the short definition.
In Unity3d, you can auto-generate normal maps from textures. Of course, this will only look decent on generic textures, such as stone, metal, wood, dirt, cloth, etc.
If you want to make nice looking normal maps, you would need to generate a normal map from higher detail meshes. For instance, you might have a brick wall which has each individual brick set into the wall. Each brick is fully 3d; made up of polygons. However, you can create a simple cube, 6 quads, and stretch it to the size of your detailed brick wall. Then simply transfer the detail from the brick wall to your cube using a normal map. The two meshes will look almost exactly the same, yet one will be significantly higher poly count (meaning more render time) than the other. The lower polygon mesh will be the one with the normal map. And yet they will both look pretty much identical.
You can make nice normal maps in Mudbox and ZBrush easily, but you can also do it in Maya and 3ds Max or whatever.
Some of you might be wondering what is the difference between a normal map and a bump map. Well, let me just answer this real quick. They are basically the same. They are both ways of creating fake detail on geometry; they just do it slightly differently. But for all we care, they are the same.
One great program that can automatically generate normal maps, ambient occlusion maps, and more is CrazyBump. Google it.
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