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In Blender (in other 3d editors too) any raster image can be used as a displacement map. Displacement map has an effect on a mesh: the values for displacement are taken from pixels' brightness. For working with this map you should subdivide your mesh many times (this method requires the large amount of additional geometry to be present and therefore a powerful computer).
In Blender displacement maps can be applied by two ways:
1) By means of mesh modifier (to find out how it works see tutorial "Blender-2.45: Creation of Fantastic Landscape Using Modifier and Bump-map" http://www.assistcg.com/index.php/component/content/article/49.html )
2) Activating the "Disp" option in texture channel. To make this, download any image you want to use as a Displacement Map to a texture channel (fig. 1), activate the "Disp" option and set the degree of influence on a mesh by means of "Disp" and "Nor" sliders (fig. 2). It is necessary to note that this option doesn't modify a mesh, unlike "Displace" modifier; you can see the result only when your image is rendered.
Pay attention, that the "Nor" option is working even without being activated.
fig.1 Adding procedural texture for making Displacement Map (F6)
fig.2 Using procedural texture "Clouds" for Displacement Map
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