In terms of what you can visually resolve this is overkill; you only need ~300dpi at 18" to be beyond the resolving power of the average human eye. But if you

In terms of what you can visually resolve this is overkill; you only need ~300dpi at 18" to be beyond the resolving power of the average human eye. But if you want to make a window you have to do something else entirely; your display has to show a holographic interference pattern at which point when it is illuminated correctly you can achieve a full parallax, full color, glasses free 3D display. In both theory and practice you can do this with an LCD but to start getting RGB capabilities you would need some incredible pixel densities on the order of 30,000dpi.