For more advanced Blender users, you probably have at one time or another wanted to create something very detailed and realistic, only to realize that one modeled head or one cool building has taken an enormous amount of memory, and rendering (or worse, animating!) something like that would probably take days! Well there is an awesome trick you can use to create intricate details and design shading, with a relatively low amount of faces. You can use blender to create and apply normal maps, which are RGB colored maps that can be used as a texture to calculate how light will bounce of an objects surface. All you need is blender and photo-editing software to tweak the picture a bit.
Creating a Normal MapEdit
You can very easily create a normal map once you figure out all the necessary settings. You can start by either making a high-poly count version of your object, or making the material that will be applied to the object to make the normal map.
We will first start by making a simple object with a high-poly count. Start up blender, and let's delete everything there first, then go to top-view, orthographic, and start by adding a camera.
Next, start with a circle, with a radius of 2 and 32 vertices in circumference. Keep it filled. Select the edges of the circle, keeping the inner vertex unselected, and extrude edges only. Scale the new vertices by 1.1. Repeat this with only the outermost vertices 6 more times, until you have a ring of 7 vertices and a center vertex. Now, we will refer to the center vertex as center, the innermost ring as Ring 1, and the outermost ring as Ring 7. Deselect all vertices, then select all vertices of ring 2, 4, and 6 (This can easily be done by holding the Alt and Shift keys, then selecting an edge adjoining two vertices of the same ring for each ring). With these rings selected, move it 0.4 in the Z direction. Now, let's smooth this bowl out. In the editing index, under the modifiers tab, add a subsurf modifier of 2, and then apply the modifier. Then under the links and materials tab, select Set Smooth. Next after the object looks all nice and smooth, select your Camera and move it positive in the Z direction so it's positioned directly above the object looking straight down at it, and edit the camera Lens properties so that it is set to Orthographic, and scale the lens so that it includes all the bowl and fills up the camera space as much as possible.
Creating the Color-Map MaterialEdit
Now, to be able for the Render to apply the colors on the right areas of the object. For this, we will start by applying a material the bowl. Make this material 0.0 on the R, G, and B spectra, and depress the shadeless button. Now, go to the textures index and add 3 textures to the material, and set them as "Blend" textures with "Lin" depressed (a linear calculation). For these 3 textures, name them something to do with either "Red"/"Green"/"Blue" or "X"/"Y"/"Z" in that order, because doing this will ensure you keep the channels in order. The following image will show all settings that need to be set to make the material work.
Now before your Rendering you want to make the World texture to be completely black, so this way it won't interfere when you apply it as a normal map. Now Render your image, and hit F3 to save. It should come out to something like this.
As a final note in this subsection, what I suggest doing is deleting the bowl and saving the file as something like "BlenderForNormals.blend" so if you ever want to make another normal, the area and materials are already set up and all you need to do is import or create the object. Just a suggestion.
Applying as a Normal MapEdit
Well we got the image, now let's apply it and make it come out as a low poly-count texture. So, let's continue by deleting the high poly bowl and instead add a plane in the middle of the camera. Now, apply a material to the object and make the material hard, soft, rocky, wet, or however you want it to look. After you've made the plane how you want, add a texture to the material, and make it an image texture. Load the image as the texture, depress the normal map button, and set the normal spacing to object. Now, on the materials index under the Map To tab, make sure the Col button is relieved and instead depress the Nor button. Just add a lamp above the plane, and Render! You should now have the same shading effect as the original bowl, with about 0.01% of the original faces--one.
If your Render comes out odd, it may be because the shading is being applied too heavily. You can alter this by adjusting the Nor slider directly below the "Mix" drop-down menu. O used about 0.2 and it came out just fine, like this picture.
Congratulations, that's all there is to normal mapping. It's a simple technique with many great uses!