How do you render a small cut-out frame of an image?

To avoid having to wait for the render time in complex images, I have seen other Blender uses do something where inside the 3D view window, they placed a small frame over part of the image–and everything inside of that frame was rendered alone.

I’ve never known how to do this, but I want to learn. I’ve searched for tutorials or information all over here, on Google, etc… so hopefully someone can help me.

Thank you for reading this. Please help if you can.

In Object Mode, press “P” to open a preview pane and drag it over the area of interest. Lovely little tool!!

Thank you for your help and for the quick reply, but when I press “P” it just quick renders the whole scene; no preview window is displayed… and the mouse disappears until I press the Escape key…

I just installed Blender on this computer. Do you think that’s the problem?

EDIT: Also, I noticed in the DOS command prompt window, it said “Shaders not supported!” many times over, for each time I pressed P, I think…

Sorry, I forgot it is Shift-P :frowning:
Just P enters Game Engine…

Ahh, I see. lol Thank you again for the quick help. I really appreciate it!