Considerations for Interoperable Master Formats – Discussion Paper
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Considerations for Interoperable Master Formats – Discussion Paper September 7, 2009 Edition 1 Contents EXECUTIVE SUMMARY 3 INTRODUCTION 4 COMPRESSION AND VISUAL QUALITY / FILE SIZE 4 CODEC PERFORMANCE - SPEED VS. FILE SIZE 7 INDUSTRY COMPATIBILITY 9 MECHANISM FOR PICTURE-DEPENDENT METADATA 10 INDUSTRY ACCESSIBILITY 12 INDUSTRY EXTENSIBILITY 13 SUPPORT FOR SINGLE VS. MULTIPLE CODECS 13 CONCLUSION 15 APPENDIX A - VIDEO COMPRESSION TECHNOLOGY OVERVIEW 16 APPENDIX B - QUALITY VS. COMPLEXITY 19 CONTACT INFORMATION 21 2 EXecutive Summary Traditionally, film provided a standard medium for producing, editing, storing and replicating content. As the film and television industries evolve more and more into the digital domain, the lack of standard mechanisms for content interchange have meant significant inefficiencies, costs and limitations in the content production workflow. Specifying a well considered, standard “Interoperable Master Format” provides a tremendous opportunity to alleviate many of the issues that exist today, extending the robustness and consistency of film to digital production. In order to achieve this goal, the specification of an Interoperable Master Format needs to take into account not only quality, but longevity, flexibility, security and workflow efficiency. The choice of video codec or codecs has a significant bearing on these attributes and as such, this paper focuses on codec choice in relation to the following key areas: • Compression and Visual Quality / File Size • Codec Performance - Speed vs. File Size • Industry Compatibility • Mechanism for picture-dependent metadata • Industry accessibility • Industry extensibility • Support for Single vs. Multiple Codecs There are a number of codecs considered possible options for an IMF including JPEG 2K, Cineform, H.264/AVC-Intra-Pro, JPEG XR and VC2. There are pros and cons for each of these and ultimately, the codec choice (or choices), should be determined by importance of the aforementioned characteristics and how the codecs perform against these criteria. 3 Introduction In a content production workflow the need for efficient mechanisms to encode and decode content into and out of an Interoperable Master Format (IMF), while maintaining high visual fidelity, is generally acknowledged. Equally important however, are workflow considerations including codec implementation requirements, price, industry compatibility, format extensibility, and other characteristics. The choice of an IMF codec (or codecs) and the software infrastructure that surrounds the format have significant bearing on these characteristics; this paper reviews the state of the art of different compression schemes/approaches and provides commentary on the relative affect on the content production workflow. Compression and Visual Quality / File Size The choice of codec for an Interoperable Master format impacts the workflow in several measurable ways. Firstly, the higher the ratio of compression that is applied, the easier and quicker it is to transfer files between equipment, over networks, or between studios and post production facilities. However, the added convenience of smaller file sizes is generally offset with a corresponding reduction in visual quality when higher compression is applied. There is an important balance between the demand to maintain very high quality and the convenience offered by small file sizes. Also, in instances where video may be re-processed several times (not uncommon in a post production environment), the initial format of the video and the processes surrounding re-compression are critical to the visual quality of the final result. Affect of Compression on File Transfer and Storage As stated, one of the effects of choosing a particular compression technology (or ratio of compression) is the impact on the resulting file size which affects the storage requirement and the corresponding time it takes to transfer. In order to understand this and establish a reference point, let’s consider the parameters of a “typical” film. The table below provides some basic statistics for an uncompressed 10bit 1080p24 video file. 4 Table 1 - Uncompressed 10bit 1080p24 Video File Statistics Film Duration (minutes) 90 Film Duration (seconds) 5400 Horizontal Resolution 1920 Vertical Resolution 1080 BitDepth / Chroma 10 / 4:4:4 FPS 24 Total Film Storage (MB) 961,084 Sustained real-time Data Rate (MB/s) 187 Available HDD Transfer Rate (MB/s) 25 Table 2 below illustrates data storage and transfer requirements for different compression ratios. Table 2 - File Sizes and Data Transfer Times for Various Compression Algorithm Types Visually Compression Type: Uncompressed Lossless Lossy Lossless Compression Ratio: 1:1 2:1 6:1 25:1 Total MBytes to Store/Transfer: 961,084 480,542 160,181 38,443 Transfer Time (Minutes): 641 320 107 26 Transfer Time (Hours): 10.7 5.3 1.8 0.4 Transfer ratio: 7.1x 3.6x 1.2x 0.3x Uncompressed: For a 90-minute film stored in an uncompressed (1:1) format, total storage is approximately 1TB in size. Assuming a data transfer rate of 25 Mbytes/sec for an external USB or Firewire hard drive, the total transfer (copy) time would be about 11 hours, or about 7.5 times longer than a “real-time” transfer of 90 minutes. This lengthy data transfer process and its corresponding large storage requirements are one of the motivations for considering use of compression as a format option within IMF. 5 Lossless Compression: An alternative to uncompressed media is to store the video data using an arithmetically lossless compression format. That is, the Entropy encoder of the codec may be applied to the data, resulting in a 2-3x improvement in storage requirements and file transfer time. The corresponding data parameters using a lossless compression ratio of 2:1 on our “typical” 90-minute film are shown in the table above; e.g., the storage requirement is halved and the transfer speed is doubled. From a technical standpoint, for lossless compression, the data that emerges after decompression is arithmetically identical to the original source. The cost of doing so is merely the compression and decompression time and the requirement to support the codec. A number of lossless video codecs are currently available off the shelf (e.g. HuffYuv, Lagarith, Yuls, FFV1 and others, though a Zip or data compressor could also be used.). These are all capable of delivering a compression ratio between 2:1 and 3:1 at 8 bits of pixel depth. However, most lossless codecs have serious limitations in compressibility at bit depths above 8 bits. As bit depths Increase towards 10 bits or 12 bits, compressibility decreases – typically below 2:1. It is worth mentioning that JPEG 2K, CineForm, some professional profiles of AVC/H.264 and 2 emerging standards (JPEG XR and VC2) are capable of reversible coding at higher bit depths with less impact on compressibility. Visually Lossless Compression: Another alternative is to apply “light” video compression techniques that result in extremely high-quality compressed images in the range of 4:1 to 12:1 compression. Such techniques are often known as “visually lossless” because the human visual system is generally unable to discern visual differences between such lightly compressed files and the original sources. The main codecs available that offer visually lossless compression include JPEG 2K, CineForm, JPEG XR, VC2 and H.264/AVC-Intra-Pro. Note from the table above that at 6:1 compression the transfer rate of the compressed IMF takes just longer than real- time (1.2x) to move from a USB/Firewire drive to other media. On a Gigabit Ethernet connection that sustains 90MBytes per second, our 90-minute film from Table 1 can be transferred in about 30 minutes. It is interesting to note that many digital acquisition source formats use visually lossless compression at the acquisition stage. The RED camera, for example uses JPEG 2K with up to 12:1 compression internally, though is called “RAW” and treated as if it is uncompressed. Silicon Imaging cameras use the CineForm RAW codec at 4:1 compression. Similarly, HDCAM SR in its 440Mbps mode uses 4:1 visually lossless compression. 6 There are differences in the design goals of visually lossless codecs that are worth considering that yield different performance and slight variations in file size. These will be explored in more detail in the next section. Lossy Compression: For completeness, it is also worth considering the consequences of a more lossy compression that is usually used for purposes of distribution (Blu-ray, DVD, cable/satellite delivery, Internet streaming, etc). In some instances, for example for a mobile video stream, the source material may not need to be of equivalent quality to the original source. At a compression rate of 25:1 (illustrated in the above table), you end up with a video quality that with the same codec (i.e. H.264/AVC) would result in a quality superior to Blu-ray Disc. It is not suggested that lossy compression in any way represents a viable option for coding in an Interoperable Master file, although it may be useful in some instances. Codec Performance - Speed vs. File Size Observing existing post production workflows reinforces an important point that in some instances, there may be a need to not only perform multiple different conversions from the source file, but also a need to re-create new source files of a different format from the original source. Performing these conversions efficiently in software on affordable workstations that deliver faster-than-real-time performance is recommended