Partial rendering in a small box

Hi,

I have a big volume/mesh and I only want to render part of it.

The part is defined in a small 3D box and only volume/mesh inside this box is rendered.

There are two situations: one is the data is too big to load into video memory and the other one is the data can be loaded into video memory.

For both cases, I just want to render the volume/mesh inside the box.

How to do this?

Thanks in advance.

YL