how to convert 3d object to its height field

A height field? Something like a distance from a point or a plane recorded in a bitmap?

I came accross this from a paper which said a height field could be obtained by reading the z-buffer after an orthographic or a perspective projection.

I see. Then it seems you already know the answer to your question. You get the height (or depth?) field by reading the z-buffer. Some more advanced 3D editors may support this. I would check Blender's documentation to see if they have this feature.

ok, Thank u.