This is a story - this is not literally true, but is a roughly similar shape of things.
My experience is that I am a little thing piloting a giant thing (my body) - I sit in a seat and watch a screen. That screen shows me what my eyes see, and to the edges and beyond are empty. In front of me - between the screen and me - is an empty space. In that space I can draw lines, color things, move things, rotate things.
If I concentrate hard enough I can overlay (by seeing the space while I have my eyes open) an imagined thing in real space (The imagined item is still originating from that space - not the screen - but the screen and that space...mix). The screen from my eyes is an order of magnitude brighter - so I have to focus in order to do that. I can imagine things with my eyes open... but when I do, I am not facing the 'eye-screen' inside my head, I'm looking down and away from it and only at that space of imagination.
I recently starting drawing: It seems to solidify this space/ability more than any other mental exercise I've tried. I have noticed a distinct improvement when I started deconstructing what I was seeing in real live down to basic shapes (In that imagined space and overlaying it on what I was seeing).