IMF: One Format to Rule Them All or Just One More Format?
The assets don’t need to be only audio or video files, although an IMF can just as easily contain multiple camera angles and multiple language audio files, but can also be subtitles or timed-text files. The beauty of a single format container holding many complementary files is that these assets can be bound together in one of multiple permutations at the time of delivery—called late binding—that are supported by multiple streaming technologies such as MPEG-DASH or Apple’s HTTP Live Streaming (HLS).
Late binding from a grouping of complementary assets within the IMF container means that IMF is as beneficial for delivery as it is for storage. But editing IMF content can be tricky.
Why? Well, it turns out that IMF packages are active document containers similar to the Microsoft Binder concept in early versions of Microsoft Office. While IMF is made up of modern video and web technologies likes MXF and XML, it turns out that the IMF package essentially works the same way as Binder, which housed all the PowerPoint slide decks, Word documents, and Excel sheets for a given project.
Binder allowed Excel sheets that were dynamically linked into a PowerPoint slide deck to be always available when the PowerPoint slides were edited or delivered. It’s true that Binder added to the size of a typical PowerPoint deck, but only by a negligible amount, since the Excel sheet was a much smaller file size, and the convenience of having the Excel and PowerPoint files together in one container format outweighed the show-stopping inconvenience of having a broken link to an Excel table embedded in the PowerPoint presentation.
One problem with the Microsoft Binder approach, and perhaps a major reason that it was discontinued around the time of Office 2007, was the problem of editing. While the Microsoft Office suite of products could easily edit Binder documents, other editing tools that were Excel- or Word-compatible were often unable to understand the same document types integrated into the Binder format. Even those that did understand the Binder format often could not extract or edit either the internal Excel or Word documents without damaging the document’s relationship with the Binder or other files within the Binder.
Editing Within IMF
As mentioned earlier, the idea of using IMF as a container format for acquisition or postproduction processes is not the initial intent of IMF.
It’s true that IMF is based partly on MXF, which itself is a video and audio container format used in acquisition, but IMF’s additional payload options make it more suited for delivery than a production workflow.
“Although IMF uses ‘simple’ MXF variants for the storage/transport of the actual media frames, it does add an extra layer of complexity in the form of XML files,” says Primestream’s Schleifer.
“These XML files are used to describe the relationship between the different media and their versions, and this extra metadata has more value when the files are transported between systems, and less value when production is occurring within a system or workflow that maintains the same and in many cases even more metadata,” says Schleifer.
But what about editing the CPL, in the same way an M3U8 or MPD manifest file for HLS or MPEG-DASH, respectively, would be edited?
In theory, editing limitations should not be a major roadblock, but editing itself will be an important litmus test for IMF-compatible software tool vendors to overcome.
“Not many editing/archive/ingest/playout components or systems in the production workflow currently support IMF,” according to Schleifer.
One step in the editing process would be to properly modify IMF compositions created with one of several available desktop CPL edit solutions. Another would be meeting necessary interoperability standards when it comes to proper ingestion of complete or supplemental IMF packages.
Not only are storage and playback important, but the included metadata in any IMF package must be readable by both human and software parsers. Therefore, the ability to index, search, and retrieve key portions of an IMF package are critical requirements.
Finally, editing also brings into focus the need to properly allow updating of a CPL within an existing IMF package, whether the package is on the local machine where CPL edits are occurring or on a local server or stored in the cloud.
As such, the ability to upload and download complete or partial packages as part of the overall distribution scheme is critical. In some instances, this distribution may also require stripping away the IMF package itself in favor of transcoded output. In other words, the final deliverable may be the audio-video files rather than a total IMF package.
Next Steps for IMF
In all the excitement around IMF, it’s easy to forget that not only must the XML be extensible, but that the IMF packaging structure must be flexible enough to handle new audio and video formats. Steps are underway to address 360° video as well as VR video.
In addition, continued improvement around packaging data XML—the XML within the IMF that allows for asset mapping, packing lists, and even practical constraints such as a volume index for movie and broadcast delivery compliance—will address VR video and other more exotic use cases.
Already, there is an Application 2E, with the E standing for Extended, which offers support for imaging that has wider color depth—High Dynamic Range, or HDR—as well as larger resolutions.
Specifically, Application 2E addresses “colorimetry specified in IEC 61966-2-4 and Recommendation ITU-R BT.2020” as well as “color primaries specified in SMPTE RP 431-2.”
Following the HDR transfer function specified in ST 2084, IMF packages can now contain HDR frames at a “maximum width and height of 4096 and 3112 pixels, respectively,” to handle both ultra-high-definition (UHD) as well as the Digital Cinema Initiatives 4K specifications.
On the vendor front, Prime Focus’ Sankaranarayanan says that the player portion, while critical, is only one step in streamlining overall workflows.
“The IMF Media Player was just the first step in our innovation-driven product roadmap towards IMF readiness,” says Sankaranarayanan. “Our integrated software offerings offer smart possibilities with automation, and are well-positioned to deliver more speed, more control and much lower Total Cost of Operations (TCOP) to M&E companies. We know that IMF will be a core technology in that future, and we are already well underway.”
[This article appears in the October 2017 issue of Streaming Media Magazine as "IMF: One Format to Rule Them All?"]
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