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Author
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Topic: Race to witch mountain hard drive capacity label
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Julio Roberto
Jedi Master Film Handler
Posts: 938
From: Madrid, Madrid, Spain
Registered: Oct 2008
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posted 03-13-2009 11:37 AM
What I think is that that's potentially far worse quality due to compression than a (theoretical) 50GB blu-ray disc which normally uses far, far, far, far more efficient codecs for video (h264 vs m-jpeg2k) and audio, and has to do so in a smaller colorspace (4:2:2 8bits).
In that sense, a well done blu-disc could easily have some 30 times less loss of detail DUE TO COMPRESSION than this DCI. It will have more loss of detail due to the format differences between DCI and blu-ray, but the compression loss itself would be orders of magnitude better in the domestic format than in the "professional" format.
A 90m movie "done right" in DCI should take at least 150GB. If it takes much less than that, somebody is throwing away detail unnecessarily.
Even if the detail wasn't there to begin with (i.e. the movie was shot in 16mm or something), it would still not make sense to compress it much more than what's limited by the bandwidth.
It only has an advantage in the amount of time it takes to replicate the hard drives and to ingest the content in the servers, of course.
It would be not possible to notice the quality difference for us mere mortals, so perhaps they figured they could just save the replication time. A replication farm could put out a batch of hard drives with a verified 100GB "print" in about half and hour or less.
But it makes you wonder about how much "superior" a movie in theater at say 1998x1080 4:4:4 12bits at 99GB is compared to some home systems capable of up to 1920x1080 4:2:2 8bits at 50GB, considering the home system codecs of superior efficiency. That's if the aspect ratios were the best possible 1.85 at the theater and the best possible 1.78 at home. If we compare a Scope DCI movie with a HD blu-ray program at home, the difference could be even more in favor of blu-ray.
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Julio Roberto
Jedi Master Film Handler
Posts: 938
From: Madrid, Madrid, Spain
Registered: Oct 2008
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posted 03-13-2009 02:56 PM
My motto is: if the bandwidth is available, use it. If you target constant Q and it gives it to you at under your bitrate ... just increase the Q.
But why would you want to do constant Q instead of automatic bit rate? Bitrate is your limit and not Q for this application. Analyze 24 frames, obtain max Q under 250Mbs and under max peak bitrate (audio etc taken in acccount) and spit it out.
For something like Dcinema, where the bandwidth and the storage is available, it would make sense. Slightly Q variations in consequtive frames at such high quality levels shouldn't be apparent to the naked eye anyway, so no need for constant Q.
Even if you decide to do constant Q, if you see your bitrate 50% less than the limit and no frame hit anywhere near the peak limit, just ran another freaking pass with a little higher Q, for pits sake.
American Gangter etc was probably MPEG2 encoded instead of M-JPEG2K, I would guess.
With more efficient compression codecs, like just about any interframe one, you can get smaller file sizes at same quality levels as M-JPEG2K (if they are developed for the same bit depth and colorspace).
But for Dcinema, file size is not such an important parameter. It should be quality of image, as the storage space and the bitrates are available by definition and the only difference is, say, 30 minutes more or less to ingest a movie and a few minutes more or less to replicate the package.
This is just your usual lazyness and saving $1, like when they put out blu-ray transfers that are only (say) 15GB (including extras or whatever), when they have at least 25GB available, i.e.
Of course nobody is going to notice the difference in quality, but since it basically doesn't cost you anything, why compress more than necessary? More compression always equals throwing away more detail. The detail may just be noise (grain) or not visually important, but it was there in the master and you are throwing it away for no good reason.
I can see them heavily DNR'ing the DCI video stream next to save even more space in file size after compression ![[Wink]](wink.gif)
I don't mind that they don't hit the maximun bitrate, but a reduction of more than 30% like we are talking about here for no apparent reason, seems a bit worrying to me.
If we get into conspiracy mode, perhaps some studios are giving Imax a break by lowering the quality of regular DCI and allowing Imax to max out bitrates etc in their own masters to claim better quality than mom-and-pap multiplexes. ![[evil]](graemlins/evil.gif) ![[Cool]](cool.gif)
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