The Expression Plugin "Ground" simplifies the positioning of objects on uneven surfaces. You can e.g. position the trees of a forest on the XZ-plane using Cinema 4D's random function without to care for their vertical positions. "Ground" will set the correct Y-position on the basis of the assigned ground objects. All polygon objects with correctly aligned normals can be used as ground objects.

The handling is really easy: As seen in the figure right, group all objects, whose Y-positions are to be controlled. Assign a ground expression tag to this object group. Then, arrange all ground objects to a further group and drag it to the object field in the attribute manager of the ground expression.

"Ground" works not only in the editor, but also in animations. For that simple clip which can be seen above, the object group "balls" was only moved in X-direction.

Any object outside the area of the ground objects, will be set on the zero level.