Digital Video - November 2007 - (Page 45) > THE umentary Speed and Angels converted to the new SMPTE standard format, JPEG2000, with the help of Dolby Laboratories. In early 2007, I decided it was time to learn about these specialized codecs. I took a trip to the Novato, Calif., headquarters of Sonic Solutions, maker of the CineVision encoding system for HD optical discs. Over the next two days there, I learned how things have radically changed from MPEG-2 encoding for standard-def DVD. The following week I went to the Microsoft R&D lab in Redmond, Wash., to get an in-depth view of the VC-1 codec. JOY OF JPEG-2000 Dolby’s Digital Cinema Encoder. ENCODING REDEFINED Sam Orton-Jay, the senior product manager for Sonic CineVision, outlined the new HD encoding workflow succinctly. First of all, I learned that even the best originating picture formats—such as HDCAM, film, and CGI—can reveal glaring artifacts when viewed on a large screen. The most common of these artifacts are video noise, film grain, and color banding. For example, in SD, the size of an individual film grain is typically smaller than a pixel. In HD, you might use as many as three pixels just to describe a single grain. Maybe you thought you were seeing film grain in your fancy home theater setup, but in fact, you were seeing more of an approximation than the real thing. Luckily, preprocessing tools have evolved to tackle this and other picture problems. Sonic has incorporated preprocessing into its products using DVO plug-ins from Digital Vision, as well as X-Scaler from Microsoft. Now let’s say you were able to get rid of unwanted picture artifacts. Keeping what’s left (the desired image detail in a scene) may not be easy when you need to pack a twohour feature, multiple tracks of uncompressed surround sound and 90 minutes of extras on a single- or even dual-layer HD DVD or Blu-ray disc. That’s right: sometimes 50 GB ain’t enough! This is when expertise in operating a compression system becomes paramount. On most features, you’ll need to go through each scene with a fine-tooth comb, determining if it passes muster with the base encode or whether you need to “reencode.” Back in the days of standard DVD, continued on 48 PEG-2000 has been available as a still image format since Adobe Photoshop CS. Only since 2006 has it been implemented for digital cinema. The goal is to exceed the viewing experience of 35mm film. Some of the tests involved side-by -ide playback of digital projection versus film. Not so long ago, you could only compress JPEG-2000 in software. It took up to 20 hours per 20-minutes of film. Now, with hardware-based systems such as Dolby’s Cinema Encoder, among others, you can do real-time encoding—and it looks great on the big screen. A plethora of different codecs, including MPEG-2 and VC-1, have been used (and are still being used) in non-standard digital cinema applications. JPEG-2000, however, was agreed upon as the new digital video codec standard by the seven major Hollywood studios, through a joint organization called Digital Cinema Initiatives. Here’s some basic info on the JPEG-2000 motion picture codec: > It uses intra-frame only coding. > It’s based on wavelet transform; HD optical disc codecs use discrete cosine transform. > It does not divide the frame into macroblocks. > It has an ultra-wide color space known as X’ Y’ Z’. No more reduced color sampling here. The color space is wider than RGB, and is sampled at full bandwidth with 12 bits per color channel. > It supports resolutions of 2K and 4K. > It is not related to JPEG, the still-image format, except by name and development history. We interviewed Dolby Digital Cinema’s Dave Schnuelle and Kevin Stec to learn more about the newest codec on the block. DV: Is Dolby using mathematical or perceptual lossless compression? It seems that either would be suitable for digital cinema applications. Dolby: The JPEG-2000 profile that is defined by DCI uses a perceptual lossless model. Mathematical lossless coding would require a very high bit rate and would result in unacceptably large files for distribution. What is the usual bit rate employed for feature films? Is it a variable or a constant bit rate? We offer both constant bit rate and variable bit rate modes. The VBR mode is Dolby’s proprietary technology. DCI specifies a maximum of 250 Mb/s, which is typically used for CBR coding. VBR coding is content dependent. We have coded features at less than one fourth of this rate which have been judged to be visually lossless. Results with animated content are even better. Is frame accuracy critical? Is the lack of frame accuracy in MPEG-2 (vs. JPEG2000) a problem for digital cinema? In general, this is an editing issue, which is not what you expect to do on a release file. In reality, we encounter problems such as the need for multiple versions of a release for different regions. Since JPEG-2000 is frame-based, changing sections of the movie is not a problem. Is JPEG-2000 the only codec that supports 12-bit X’ Y’ Z’ color space? The AVC/H.264 codec also has a high profile that can support this color space. Are there unique challenges for the compressionist when using this particular codec vs. the HD DVD and Blu-ray codecs? How much of the compression work requires technical and/or artistic expertise, and how much is done automatically by the encoder system? We do not have the same low-bit-rate constraints that the HD optical discs have. Our coding rate control is based on a constant quality metric, so there is very little interaction with the operator. So there is really not a compressionist in the sense we see in consumer DVD product. How has your experience using this codec changed over time? The codec itself has not changed, but we continue to add input features such as DPX and RGB capability, with 3D lookup tables to be available soon. This gives the encoder product a great deal of flexibility in the postproduction environment. —David O. Weissman J www.dv.com dv november 2007 45 http://www.dv.com
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