ZBrushCentral

Helper program to convert obj to heightmap?

Hello,
I am looking for a program that can open obj-Files and export them as BMPs (heightmap), first priority.
Additionally, it should be able to import heightmaps and convert them to obj-files (preferred) or a different 3D file format, i.e. STL.
I am aware that ZBrush can handle these tasks, but it would be good to have a free or commercial tool for other users.

Any help is appreciated!