[Steely Tom]

Because take any ol' random particle: you bounce light off it, you have no idea of knowing what reflection angle that light is gonna take. Though maybe that's part of the "modification"... Hmmm..

My reasoning is a lot less elegant than this. Based on first principles: you can't form an image in free space because free space is so linear. Any image-forming means has to involve some non-linearity distributed in space.

Particles are non-linear (with respect to ray angle). Ionization is non-linear (with respect to electric field).

Remember the "air spark" phenomenon seen when certain high-power lasers were discharged through a converging lens?

In any event, this technology is probably "smelly."

(steely)

[maxwell]

What is an image? The convergence of light rays, either real or virtual, according to the transmission functions of the projector optics. I don't understand what you mean by the nonlinearity requirement: that there must be a ray-distorting agent, or in the case of particle ionization, that there must be an ionizing agent?