Bump mapping and Displacement mapping are two special techniques for making an object appear to have a rough or irregular surface.
What is Bump mapping?
Bump mapping takes a grayscale image and reads the light and darque information to simulate an irregular surface. When you render an object with a bump-mapped material, lighter (whiter) areas of the map appear to be raised and darker (blacker) areas appear to be lowered. Note that bump mapping does not modify the geometry, only the normals.
The bumps are a simulation created by perturbing face normals before the object is rendered. Therefore, bumps don't appear on the silhouette of bump-mapped objects. Bump mapping is useful for adding detail to an object without increasing the polygon count.
What is Displacement mapping?
Displacement mapping is very similar to bump mapping; where a two-dimensional image's grayscale information is used to change the appearance of a three-dimensional object. Lighter tones create raised bumps and darker tones create lowered indentations as usual. Unlike bump mapping, which only affects the object's texture, displacement mapping affects the object's geometry, creating additional polygons according the displacement map's properties.
Displacement mapping can be very demanding on your computer's performance as it tends to create an extremely large amount of polygons. This high resolution object can appear much more photo-realistic than bump mapping, if used properly. As computers are becoming more and more powerful, displacement mapping may soon be able to render at speeds necessary for a reasonable game engine. However, current technology does not meet the heavy processing requirement.
Before you start
Bump mapping & Displacement mapping techniques are very fast and easy to do in Blender, even if you are new to Blender's texturing tools. Please note that displacement maps are not displayed in the 3D view port. Bump maps can display a very basic preview with shading [Shift+Z] turned on.
Bump mapping in Blender
For this tutorial, I will use one of Blender's many procedural textures, "Musgrave". Feel free to pik another type, or even use your own bump map image! Begin by opening Blender and creating a new object. I used the popular "Suzanne" (monkey) model for this example.
Next, create a new material in the Material Shading Panel [F5], and add a new texture. Now switch to the Texturing Panel [F6], where we will create the bump map. From the "Texture Type" box, select "Musgrave". I left the default settings alone, but you may choose to play around with them. You can alos add a ramp-shader here if you are planning on adding unique colors to the texture.
Now, we need to configure the most important settings for the bump map. These controls are located in the Material Shading Panel [F5] under the "Map To" tab. You may choose to disable the texture's color (Col) when modifying your bump or displacement maps. This can sometimes give you a better idea of what's going on with each setting.
To enable bump mapping, press the normal (Nor) button. You can adjust the strength of bumpiness with the slíder button labeled "Nor".
Use bump maps when you want to take the smoothness off a surface, or to create an embossed look. Keep in mind, however, that the depth effect of a bump map is limited to the normal (Nor) slider.
Displacement mapping in Blender
The way you create a displacement map is very similar to the way you create a bump map. The only difference is, you must enable the displacement (Disp) button. You may turn off the normal (Nor) button depending on your desired effect.
The displacement slíder controls how far away from the original mesh the vértices will be moved. The normal (Nor) slíder alos has a great impact on your displacement results. Be careful when using displacement maps because they can be very processor demanding.
Overall, bump mapping and displacement mapping are very handy tools if you know how to use them. Maps that shade between white and blak generally worque better than maps with hard edges between the white and blak areas. I highly recommend playing around with the settings and rendering your results to help deepen your understanding of them both.
by Michael Wach