You can search for the same information I can find. There's a LOT of articles out there talking to 100GB 4K Blu Ray discs dating all the way back to 2010... some appearing to be wishful thinking, some proclaiming it's arrival, some proclaiming the arrival of writable 100GB, some proclaiming 128GB variant and a few talking to 200GB via 8-layers and/or double-sided discs.
Besides, I'm under a long-standing impression that a read-only (movie) disc is easier to manufacture than a (data) writable one and "data is data." So I'm not quite grasping the implication that just because there's a Blu Ray data disc that can store 100GB, one shouldn't assume that 4K movies can't come on 100GB Blu Ray discs too. If it has become easier to make writable data discs using cheap home equipment than read-only movie discs using commercial equipment, that would be (completely new) news to me.
Or maybe you have knowledge that the first batch of movies will happen to NOT be longer movies that need more than 50GB of Blu Ray space. Again, there's plenty of 1080p movies that would fit into a hypothetical 18GB Blu Ray disc (if that existed) without having to compress them so much that someone would notice. 4K with H265 is not going to be the disc space hog that means that any 50GB 4K disc is automatically "over-compressed."
While I don't know this to be the case, I would guess that in the as is, whoever is packaging existing blu ray discs puts whatever must be on the same disc as the movie and then targets the remaining space for the movie. In other words, if the disc will have a net of 44GB after extras, trailers, etc, target the movie file size at <=44GB. That doesn't mean the movie needs 44GB to be free of visual artifacts- it's just how much space is available so they use it (if they want. Some definitely do not).
One more round of H264 compression with HB and 1080p file sizes can drop to 10%-50% of their original size without appearing to lose ANY visual quality. I assume (naively perhaps) that there's a lot of fluff in the BD file that could easily be compressed out without sacrificing anything (visible) if those doing the disc mastering were mastering for the balance of quality vs. file size instead of being able to just allocate up to all 50GB to the movie.
Another way to think about it is with the iTunes > Blu Ray crowd posts here. Apparently many (most?) of the big brains that frequent this site will swear in writing (every chance they get) that the iTunes versions of 1080p movies looks just as good as the BD version of the same movie. Compression of the streamed versions seem to run at about 10%-20% of the BD movie file size. So whether they are right or wrong (probably the latter biased by the "Apple is always right" RDF), iTunes 1080p does look very good (good enough to fool them) at significant relative compression. Thus, that too makes me believe that BD file sizes have a fair amount of visual fluff because the disc simply provides a huge amount of free space to potentially use (and why not use it).
If there's anything to that idea, pairing:
- larger BD capacities (66GB and 100GB, maybe 200GB) with
- a better-than-H264, cutting-edge compression in H265, and
- optimizing the compression to fit the (larger) space (or not larger space in some cases)
...should still yield spectacular quality 4K on BD discs... even 50GB discs if that's all that possible in some "first batch." If you've ever worked with HB, sliding the "constant quality" slider just a notch in either direction (for slightly less or slightly more compression) can dramatically alter the resulting file size without our eyes being able to see the difference. 50GB, 66GB and 100GB+ is a LOT of space in which to "fit" a typical movie even at 4K. If some movie is slightly too large with some default setting, slide the commercial-grade HB software equivalent's slider .25 or .50 (for slightly more compression) and get it to fit. Or shift all of the extras to a second disc so that you have the full 50-100+GB of space for the movie (as is often done on longer BD movies now; I think LOTR Return of the King Extended at 4 hours, 23 minutes came on- if I recall right- FOUR BD discs- 2 for the movie in 2 parts and 2 more for all of the extras).
I believe we're worrying(?) about nothing here. They're not going to roll out 4K movies over-compressed and thus suffer public rejection of the new format: way too much money at stake for that. If we want to worry about over-compression, shift the thinking to how the streamers are going to deal with 4K. That's where too much compression is much more likely to be applied in trying to address broadband bandwidth & caps, average broadband speeds, phones with only 16GB of total storage, etc. Whatever compression is used on the discs is going to always be far less than the streaming version of the same movie, and yet the streaming version is going to be good enough to be watchable (even argued by some around here to be "just as good" as the 4K BD version). 4K video that is already streaming can look very, very good. Just imagine when the file size (streaming target) caps are lifted in the form of 50-100+GB discs!
