According to the Wikipedia article about Outcast: Although Outcast is often cited as a forerunner of voxel technology, this is somewhat misleading. The game do

According to the Wikipedia article about Outcast: Although Outcast is often cited as a forerunner of voxel technology, this is somewhat misleading. The game does not actually model three-dimensional volumes of voxels. Instead, it models the ground as a surface, which may be seen as being made up of voxels. The ground is decorated with objects that are modeled using texturemapped polygons. From what I see it was not a true point voxel system, but rather a ray tracing algorithm that was only used for the ground. Animated objects were still rendered using polygons.