Originally Posted by Kelson
I'll let him explain but his statement seems clear to me. Basically any movie that does not have a 1.78 aspect ratio (16:9) does not fill the screen vertically and has black bars above and below the picture. Therefore the actual picture does not use the full 1080 lines of vertical resolution. For example, a movie with a 2.35 aspect ratio shows a picture of approx. 817x1920.
While there are quite a few movies that are 1.85 (most romantic comedies), there are very few titles that are 1.78. In contrast, all HDTV TV series are 1.78 and fill the full 1080x1920 frame of the display.
The context was converting to 1080 or 720, though. Black bars and aspect ratio don't change the math there: 1080 always has 2.25x the pixels of 720, regardless of aspect ratio.
Now, it could be that many Blu-ray movies don't have 1920x1080 pixels of image detail in every frame (like a checkerboard test pattern would) and the image can be adequately presented at 1280x720, but that depends on a lot of factors. It is similar to the recompression debate: what can the eye see, under what circumstances?
To me, the choice between encoding at 1080 and 720 is easier: the display is 1920x1080 and I want 1:1 pixel mapping with the source so the display doesn't have to scale.