I know from experience that making this in CGI would be very difficult. Modeling the craft and rendering it out to a file with an alpha channel is the easy part.
Its putting it into a composite video that is nearly impossible. Usually you'd have to have a green-screen (with other expensive chorma-keying equipment) to film a foreground (the people and trees), then render the mid-ground elements (in this case the UFO's) and finally film the background and then compile all three together into a final cut.
This video, however, looks as though the foreground (trees and such) and background (the buildings and buildings and landscape) are filmed all as one. Thus, dubbing in a UFO to not only go behind the trees, but also be clipped (be obscured) by the separate strands of the palm fronds, AND light blooming between the fronds from the UFO, would be nearly impossible to accomplish without some extreme time and very expensive equipment and software. This is not even taking into account camera tracking, lighting, and focus/zoom compensations that must be dealt with in CGI, which takes literally forever to complete with such an unsteady camera shot.
Either someone has a LOT, and I mean LOT, of time AND money on their hands or this is authentic. Whether or not the ships are ET is up for debate.
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