Originally Posted by RLBURNSIDE
Assume the container is in 16:9 FHD or UHD (which it always is in Bluray or UHD Bluray), and then scale down vertically.
Which is the same thing as saying explicit anamorphic encoding like the old DVDs had. For some reason the engineers who designed Blurays assumed that people will always be using 16:9 displays, and that it's appropriate to encode black bars in a display signal.
As a counter-argument, I would say that the engineers may have foreseen that cheap blu-ray players might have really crappy scalers in them, and so they didn't want to risk picture quality for the majority of viewers (cheap players) on the majority of movies (2.35) just to benefit a small number of anamorphic viewers.
I've got to say, I don't see why anamorphically encoded media is so appealing. In order to take advantage of more than 810 lines on a 21:9 encode, you either need:
A. A display with non-square pixels, or
B. A display with square pixels, and more than 1920 pixels width. (2520x1080?)
A. The only modern display with non-square pixels that I know of are projectors with anamorphic lens attached. An anamorphically encoded BD could achieve slightly higher vertical picture quality, assuming the lens is of equally high quality. Although 2.35 screens are pretty common in home theaters, many of them use zoom/lens memory to achieve cinemascope without expensive lens. The high-quality 2.35 lens scenario has got to be <0.1% of BD playback, so it doesn't surprise me that engineers thought they could deal with the 25% reduction in lines.
B. I haven't heard of any 1080p display having more than 1920 horizontal pixels (other than a few high-end projectors). And if it did, you'd still have to horizontally scale the 1920 width up to 2520 within the display; it wouldn't be native resolution.
While I readily acknowledge that anamorphically encoded BDs could provide a small PQ increase (33% in one of two dimensions) for lens users, I just don't see the user-base large enough to justify the other 99.9% of BD viewers having to scale (and lose PQ) for their 16:9 TVs or 1920x810 zoomed projectors. Scalers were okay in the DVD days, but back when BD was released, 1080p scalers were expensive, powerhungry, and/or terrible.
Not that this is a good argument, but I'm sure this also passed through the engineers' minds: HDMI doesn't really have a good way to tell the player what the aspect ratio of the display is (as far as I know). Therefore, devices just kinda assume square pixels. In order to support lens users, then, the BD player would have to have a setup menu option to switch between "widescreen" and "cinemascope", aka "scale and add bars" and "direct," respectively. How many ignorant users would select CinemaScope just to "eliminate the black bars"?
That is exactly what too many people do today with DVDs, SDTV, as well as 16:9 letterboxed TV that is sent in a 4:3 SDTV encode. Such improper display of nice BD content would be disastrous.