Cameron’s high frame rate 3-D demo at CinemaCon, coupled with Peter Jackson’s announcement that he was shooting The Hobbit at 48fps, puts a spotlight on the future of digital cinema. High decoder bit rates, and more light, are needed to pull this off. But as stated before in this journal, DCI is probably not the one to bring this about.
DCI limits the minimum decoder bandwidth at 250Mb/s. It was the limit one could count on when the DCI spec was first released. Since then, media blocks have been introduced that are capable of 500Mb/s. However, not all media blocks can decode at this rate. One can liken decoder bit rate to the width of a film print. Some projectors might be able to play 70mm, but all projectors can play 35mm. You wouldn’t ship a 70mm print everywhere, nor would you ship a high bit rate DCP worldwide.
Then there’s the question as to whether 500Mb/s is enough? Some have suggested that the compression will be more efficient, as the differences in image on a frame-by-frame basis will be less. But JPEG2000 compresses each frame individually, without knowledge of the history of the images, so no such efficiency is possible. When the limit was introduced, many thought it too low for ensuring detail in 4K images. 3-D images requires about half the data of a 4K image, essentially occupying two frames of 2K images in one 24fps interval. With 48fps 3-D, four frames of 2K images are present, equaling the picture quality of the 4K frame. The difference, however, is that we’ll be watching only one-quarter of the 4K frame because of the 2K nature of the image. The implication is that we probably won’t be happy with the detail. With 60fps 3-D, we’re over the top.
Media block designers have pointed out that the 500Mb/s rate of newer media blocks is applied in other ways not originally conceived by DCI. RealD ghost-busting, or Dolby 3-D filtering, will eat up some of this bandwidth. Subtitles, particularly the forthcoming 3-D subtitles, will consume even more. So what we’re left with will be less available bandwidth for image decompression, say, 400Mb/s.
This discussion indicates that even the best of media blocks available today aren’t going to be enough for tomorrow’s high frame rate cinemas.
Light is another problem. The blanking time between frames, whether using a polarizing filter or shutter glasses, occupies a larger percentage of projection time as the frame rate increases. If a single projector is to do the job, a stronger light source is needed. Obviously, there is a high expectation that laser light sources will fill such needs.
Even if the right technology is not installed everywhere, but selectively installed, it seems we’re counting on technology for high frame rate projection that does not yet exist.
This wouldn’t be the first time for Peter Jackson where his movie technology was ahead of the curve. King Kong, released in 2005, was intended to be the first digital 3-D release. The movie was being converted as it was shot. But there wasn’t enough budget to convert the whole movie, and without 3-D technology rolled out, there was no incentive to complete the job. Three-quarters of King Kong, in 3-D, remains in the can. We’ll see what happens with The Hobbit.
