IMF Unpacked: How it can save you money, space and time

IMF is the acronym of the moment. Like HDR, the idea of Interoperable Master Format has been around for a while, but has recently moved to the forefront of industry conversation, cropping up at tech talks, dominating expos and inspiring untold panel discussions. But what is it, who is it for and what does it actually mean for your workflow? Our team tackle the basics, and explain how taking the trouble to learn all those new acronyms can save you time, money and bandwidth.

 

Liz Sunter

Since the demise of tape, vast numbers of digital file formats, containers and codecs have sprung up, each offering a different way to store and deliver digital video data, and each preferred by different post facilities, content owners, distributors and broadcasters. The number of versions of a project that are required to satisfy every possible route to market (theatrical, broadcast, DVD, Blu-ray, various VOD providers all with their own specifications,) is huge, and creating, managing, storing and delivering so many versions is impossible to do efficiently.

SMPTE’s goal is for IMF to function as a digital mezzanine that’s as simple to use as HDCAM SR but much more flexible, and which allows organisations at various stages of the post pipeline to create and deliver versions of a project in different resolutions, aspect ratios, languages and output formats. IMF has its roots in the Digital Cinema Package (DCP) standard, an established and hugely popular standard for theatrical delivery, and AS-02 MXF, to which it seeks to add extra granularity.

 

Core constraints and applications

Work on the standard started in 2011, but its core constraints (the rules covering a project’s structure, and the formatting of its media and data) were ratified and published in 2015. As well as the core constraints, there are a number of modular ‘applications’ that plug into the standard to enable specific functionality. For example, Application 2 and Application 2 Extended (the current ‘default’ version of IMF, which adds support for image frames, further colourimetry specs and support for resolutions up to UHD) specify how to handle footage in the JPEG 2000 codec at various resolutions. Other applications cover SStP and Cinema Mezzanine Format.

The parameters of these application layers are strictly defined as shown in the tables below.

 
How it works

First up, two terms you’re going to hear a lot are ‘essences’ and ‘compositions’. The essences are the raw material of your project: audio, video, subtitles, captions and their metadata. A composition is a version of the finished project, made up of combinations of these essence files. So your finished film is one composition, the French dub of it another, the aeroplane edit a third, and so on. IMF uses an EDL-style .xml file called a composition playlist (CPL) to tell systems how to combine the different essences in an IMF package to create the different compositions you need.

What’s the difference between IMF and an IMF package? The first refers to the format itself. An IMF package (referred to, variously, as an IMP, Interoperable Mastering Package, or just a package) is a collection of all the assets required for one or more compositions, plus instructions for how to put them together, all wrapped in MXF.

Rather than sending multiple versions of a project to the client, you just send this one master package, and include multiple CPLs telling them how to combine essences to get the compositions they need.

Here, a CPL is used to add an English audio dub to a German film, change the titles and credits to English, and add credits for the voice artists.

 

What’s in an IMF package?

There are up to seven elements that make up your package:

1. ‘Essences’ – Your original media files. These include:

  • a. Video tracks. Any SMPTE/ISO, non-proprietary codec is fine, and IMF is frame rate/resolution/size agnostic as long as long as those things are consistent throughout the project.
  • b. Audio tracks. These need to be labelled as per the 337-4 MXF standard, and should contain one sound field per track (so your 5.1 mix would be one track, your stereo mix another). Audio in an IMF package should be 24-bit and 48 or 96kHz (but not a mixture of both).
  • c. Captions and subtitles in TTML/IMSC format.
  • d. Dynamic metadata

All of your essences should be in an MXF OP1a/AS-02 wrapper. You receive one .xml file per output format (video and/or audio) per reel.

2. Composition playlist (CPL) – A human-readable XML that functions like a playlist. It tells downstream systems the order in which to assemble various versions of the project. Different versions will have their own CPLs, so there will be one CPL for the French dub, one for the aeroplane edit, etc. The CPL contains hash values for track files, certificates and a verification signature.

3. Output profile list (OPL) – This tells automation systems how to transcode and mix a specific composition so that it can be delivered according to a client’s specifications, or in such a way that it displays correctly in a new environment. For example, Pixar are very particular about the way their 10-bit footage is downgraded to 8-bit, and use an OPL to ensure this downgrade is always done their way.

4. Packing list – A human-readable XML list containing information and IDs about the files in the package, including hash values for most of the files in the IMP, and security certificates and signatures.

5. ID metadata (UUIDs) – IMF doesn’t rely on file names, but instead references unique IDs of each asset in the package. This makes it easier to fit IMF packages into an automated workflow, and means you can move and rename files without any automation breaking down, so long as you don’t change the files’ UUIDs.

6. Asset map – A map of the unique IDs of every file in the package, which is used to map and verify assets on your storage. Also an XML.

7. Volume Index – If your IMP is stored across several storage volumes, the Volume Index maps the assets in the package across those volumes. However, this is an upcoming feature and is not currently supported.

IMF allows you to bundle up your essences (the original project, plus any additional dub tracks, titles, or other additions) and then use CPLs to recombine them without using up lots of storage for each version.

 

What are the benefits? 

Easier to store and move Storage is expensive, both in the cost of the storage itself, and the fact that you need to power, cool and rack it. Transferring multiple versions of a project requires a huge amount of bandwidth and specialist infrastructure. IMF packages are far smaller, as you are storing a record of the differences between versions, not multiple flattened versions of the project.

Faster, more flexible versioning A new CPL is all that’s required to create a new version of a film. IMF uses UUIDs to identify and track files automatically, so it’s very difficult to lose material even if it’s moved or renamed.

Support for automation IMF is designed to work closely with asset management, transcoding systems and more to ensure new compositions can be created automatically.

Interoperability Because IMF is a SMPTE standard rather than a proprietary one, it should be interoperable between different platforms.

Proven, reliable standard IMF is based on established standards including DCP and AS-02, meaning there is plenty of proof it can work in the field. However, if something were to go wrong, the human-readable elements of an IMF package allow you to spot and fix issues.

 

Who's using this? 

SMPTE’s goal is for content owners and post facilities to use IMF as an output format, while broadcasters and distributors receive IMF packages as an input format, and extract their required deliverables from it using automation.

Currently, demand is being driven by SVOD providers like Netflix, who understand the enormous time savings IMF offers and are taking advantage of the cost and workflow savings it makes possible. While IMF is based on DCP, demand for IMF versions of feature projects is currently low, though some studios are said to be using the format internally for archiving.

 

Your IMF vocab list

Applications Modular ‘applications’ plug in to the core constraints of IMF in order to handle different formats, resolutions and types of compression.

Asset Map XML Document detailing the paths of all the files in a package, which can be used to resolve the file locations in a received package.

Core constraints The rules which dictate how your essences should be formatted in order to make an effective IMF package. These constraints form the core framework of the IMF.

Composition Any version of your media files – video, audio and data essences.

CPL Composition Playlist. The instructions as to how various audio, video and data essences are combined and synced to create and play out a given version. You will have different CPLs for different language versions, for example, or to ensure certain scenes are left out of a clean edit.

Essence The files that make up your composition. That’s video, audio and the ‘data essence’ (subtitles and captions), plus dynamic metadata (any metadata that changes over time).

DCP Digital Cinema Package. A predecessor to IMF, in which 24-bit audio, JPEG 2000 video and data streams are packaged in MXF wrappers for delivery.

IMF Interoperable Master Format. A SMPTE standard designed to create a single, master file format and structure for distributing media between different businesses within the post-production pipeline.

IMP Interoperable Master Package, which contains your essences and the various CPLs that dictate how different variations should be made. A package contains the assets needed to create multiple versions of your deliverable, so is sometimes referred to as a ‘master package’.

Package Short for IMP. Your composition, plus its CPLs, a packing list and an asset map.

Packing List An inventory of all the files in your Interoperable Master Package, presented as a human-readable XML.

OPL Output Profile List. A profile for how your essences should be transcoded to meet a specific delivery standard. Dictates things like how images are cropped and scaled, colour transforms, and audio mapping. For example, Pixar have a specific OPL dictating how their 444 10-bit footage should be downgraded to 422 8-bit if needed.

TTML/IMSC Time Text Markup Language format, Internet Media Subtitles and Captions standard. The format your caption and subtitle essences need to be in to form part of your composition.

UUID Universally Unique Identifier. Because filenames are subject to change, IMF relies on the unique identifier for each file to find and identify it. You can edit file names as much as you want and, as long as the UUIDs don’t change, MAMs and similar systems will still be able to find and utilise the files.

 

Where do I find out more?

We’ve been working with post facilities to implement IMF workflows for several years now, and can provide workflow advice, core hardware and support for anyone who is interested in developing an IMF workflow, or has been asked to deliver in IMF and needs to get cracking.

 

If you want to know more, give us a call on 03332 409 306 or email broadcast@Jigsaw24.com. For all the latest news, follow @Jigsaw24Video on Twitter, or ‘Like’ us on Facebook.

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