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From Unreal Wiki, The Unreal Engine Documentation Site
Sculptris is a free and powerful 3D-modeling program that makes it easy to create organic looking models by using a sculpting system like the commercial program ZBrush. The resulting mesh can be exported in OBJ format and the UV colour and bump map in PNG or JPG.
It is a one-man project by Thomas Pettersson, who started the development back in November 2009.
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The easiest way to learn Sculptris is learning by doing. It is recommended to start modeling without the intention to make a serious model at the beginning and to just try out how the tools work. By default will Sculptris mirror your actions on one side of the mesh to the other side, but that feature can be turned off. It is not recommended though to enable the feature again once you disabled it because all asymmetric details will either be removed or mirrored on the other side again.
Sculptris is superior to Blender's sculpting mode as it adds more polygons to the model to describe the shape instead of just moving existing polygons around. Usually you work with hundred-thousands of polygons when creating your mesh and afterwards use Sculptris' "Reduce Brush" tool to bring the polycount down to a number that is reasonable for your targeted use.
The program is able to run even with many polygons uninterrupted on older hardware, but if it should ever crash for some reason it will recover the last session at the next start.
During the modeling process can textures be used as brush shape to draw detailed patterns with real polygons onto your model. By using the OBJ format is it possible to exchange the mesh between Blender and Sculptris in both directions, as both can import and export the format.
 Mesh Painting
After the shape of the mesh is defined, you can start painting it to create the UV map. Once the paint mode is entered, it's not possible to change the shape of the mesh any more except by loading a previous saved file.
Sculptris will effectively divide the mesh's UV coordinates so that the whole UV map is equally shared on the whole mesh, but it's also possible to distribute the UV space according to the number of vertices in an area rather than the size of the faces. Finally the modeler can define the areas that shall get more UV space by weighting those areas with a special tool in the model creation phase.
The user can specify the pixel size of the UV map, but keep in mind that the width will be doubled if you leave the symmetry mode during painting as the program won't use the same UV coordinates for both sides any more then.
You can paint with monotonous colours, load a texture and paint it at the places you want and also create bump maps to add further detail to your mesh without increasing the polygon count.
Once you finished your work in paint mode, you can export the mesh in OBJ format and it will keep the UV coordinates. Sculptris uses a pretty rough method to split up the UV space and a human selection would put make more use of the texture space, but the result is nonetheless impressive.