IMF Virtual Track Fingerprint

• Status
• Introduction
• Normative References
• Virtual Track Fingerprint
  • Timeline Digest Algorithm
  • Text Encoding
  • URI Encoding
    • URN Example
• IMF-VTRACK-FP URN
  • Purpose
  • IMF-VTRACK-FP URN NID
  • IMF-VTRACK-FP URN NSS
• Matching
  • Lexical Equivalence
  • Abbreviated Thumbprint Values
    • Abbreviated URN Example
• Reference Implementation
• Test Material
• Bibliography

Status

This DRAFT proposal describes a method for use by IMF applications. At this time the proposal is open for comment. If the proposal is adopted by a sufficiently large subset of the IMF community this proposal will then be submitted to SMPTE for publication as standard (ST) engineering document. Substantial changes to this proposal might occur during this process, and implementors are cautioned against making permanent implementation or deployment decisions based on its current contents.

This document and associated reference implementation are hosted on github.

Introduction

It is frequently useful to compare the timelines of virtual tracks selected from two IMF Composition Playlists (CPL, SMPTE ST 2067-3), such as a comparison between the MainImage virtual track in an original version CPL and its counterpart in a derivative CPL version. In cases where the comparison is made to determine equality (or the lack thereof), the comparison can be made by proxy, using computed values that are a function of the respective timeline contents. These values can be designed to uniquely identify a particular timeline by its access to edit units in the referenced IMF track file(s). Such values are therefore useful as an identifier, or fingerprint, of a given timeline among sets of CPLs, which can be useful metadata in the media manufacturing process.

Note that in some IMF applications it will be necessary to take additional steps, following the use of the virtual track fingerprint method described below, to determine the equivalency of two virtual tracks, i.e., that the reproduced essence will be identical. An example of such a case can be found in ST 2067-203 ("Audio with Frame-based S-ADM Metadata Plug-in",) where the MGASADMVirtualTrackParameterSet element is used to select soundfield groups from the track files referenced by the respective virtual track. In such a case, it is necessary to compare MGASADMVirtualTrackParameterSet elements – using an application-specific algorithm – to fully determine the equivalence of the virtual tracks.

This proposal defines such a fingerprint in two parts. The first part is a message digest over a canonical iteration of the timeline elements comprising a virtual track. The second part is a URI encoding of that digest. Used together, the parts allow exchange of fingerprint values between disparate implementations with no loss of specificity or accuracy.

The proposed method of canonical timeline iteration is designed to produce a consistent output for equivalent timelines that may be encoded in a CPL using markup and features that do not affect the timeline contents, e.g., the use of Segment elements in the CPL syntax.

The URI encoding is restricted to single-octet UTF-8 characters, and uses only alphanumeric characters, '-' (dash), and ':' (colon).

To promote utility the URN encoding allows the values to be truncated for brevity in cases where the probability of collision is small or otherwise not significant. Provision is made for comparison of two values having unequal length.

Normative References

SMPTE ST 2029:2009 — Uniform Resource Names for SMPTE Resources

SMPTE ST 2067-3:2020 — Interoperable Master Format - Composition Playlist

ISO/IEC 10118-3:2018 IT Security techniques — Hash-functions — Part 3: Dedicated hash-functions

IETF RFC 4122 — A Universally Unique IDentifier (UUID) URN Namespace

IETF RFC 5234 — Augmented BNF for Syntax Specifications: ABNF

Unicode® 14.0.0

Virtual Track Fingerprint

Defined Terms

The following symbols are defined by SMPTE ST 2067-3 Composition Playlist: Resource, TrackFileId, EntryPoint, SourceDuration, and RepeatCount.

Timeline Digest Algorithm

Description

A virtual track timeline is prepared for message digest by canonicalization of the CPL Resources that comprise the timeline. This procedure joins contiguous Resources and consolidates adjacent Resources that repeat the same range of edit units. The canonicalization procedure makes the algorithm tolerant of non-substantive differences in the CPL syntax used to represent a given timeline, i.e., variations in the use of CPL features that result in the same sequence of edit units. This property will produce a consistent output for equivalent timelines without regard to, e.g., the use of Segment elements in the CPL syntax.

The implementation shall construct a list of Resource items by iteration of the Resource elements comprising the virtual track to be fingerprinted.

Procedure

IMF CPL resources are predominately based on the SequenceType element. There is however an exception in the StereoImageTrackFileResourceType, which requires a modified procedure. The SequenceType procedure is presented first.

SequenceType

Having

• an IMF Composition Playlist (CPL) and
• a UUID value identifying a virtual track in that CPL (the track-id) and
• an intermediate list to contain canonical timeline metadata (initially empty)

The set of Resource elements in the subject Composition Playlist, where the element is a descendant of a sub-class of SequenceType having a TrackId value equal to track-id, shall be iterated in order.

For each Resource element so considered, a canonical timeline metadata item shall be created having values for the properties TrackFileId, EntryPoint, SourceDuration, and RepeatCount that are identical to those in the respective Resource element.

If the Resource element is the first in the timeline, then the respective canonical timeline metadata item shall be appended to the intermediate list. Otherwise one of three actions shall occur, as follows.

Given:

• Two Resource items shall be determined to be Congruent if they contain the same TrackFileId, EntryPoint, and SourceDuration properties. Congruency determination shall not consider the value of RepeatCount; and

• A Resource item shall be determined to be a Continuation of the previous Resource item when all the following conditions are true:

  • The given Resource is not the first in the virtual track;
  • The given Resource and the previous Resource have identical values of the TrackFileId property;
  • The given Resource and the previous Resource have a RepeatCount value of 1;
  • The index of the first edit unit of the given Resource is exactly one (1) greater than that of the last edit unit of the previous Resource (i.e., the regions of the track file identified by the given Resource and the previous Resource are contiguous.)

Then:

• If the canonical timeline metadata item (the current item) is Congruent with the item most recently appended to the intermediate list (the previous item,) then the RepeatCount property of the current item shall be added to the RepeatCount property of the previous item and the current item shall be discarded.

• Else if the current item is a Continuation of the previous item, then the SourceDuration property of the current item shall be added to the SourceDuration property of the previous item and the current item shall be discarded.

• Else the current item shall be appended to the intermediate list, thus becoming the "previous item" for the next iteration.

Once constructed, the intermediate list shall be iterated to produce the input to the message digest context that shall determine the fingerprint value. Each item in the list shall be encoded as follows to produce the stream of octets that shall comprise the canonical encoding of that item:

1. The encoder shall produce sixteen (16) octets comprising the binary encoding (per RFC 4122) of the TrackFileId property;
2. the encoder shall produce eight (8) octets comprising the big-endian encoding of the EntryPoint property;
3. the encoder shall produce eight (8) octets comprising the big-endian encoding of the SourceDuration property;
4. the encoder shall produce eight (8) octets comprising the big-endian encoding of the RepeatCount property.

StereoImageTrackFileResourceType

In the case where the resource items comprising the intermediate list are of the type StereoImageTrackFileResourceType, the following procedure shall instead be performed:

1. The encoder shall produce eight (8) octets comprising the big-endian encoding of the SourceDuration property;
2. the encoder shall produce eight (8) octets comprising the big-endian encoding of the RepeatCount property:
3. For each of the child elements LeftEye and RightEye, in that order, steps (1) and (2) of the SequenceType procedure shall be performed.

For clarity, the progression of serialized items submited to the digest shall be:

• StereoImageTrackFileResourceType::SourceDuration
• StereoImageTrackFileResourceType::RepeatCount
• StereoImageTrackFileResourceType::LeftEye::TrackFileId
• StereoImageTrackFileResourceType::LeftEye::EntryPoint
• StereoImageTrackFileResourceType::RightEye::TrackFileId
• StereoImageTrackFileResourceType::RightEye::EntryPoint

Text Encoding

The fingerprint of an IMF virtual track shall be the SHA-1 ( ISO/IEC 10118-3 ) message digest computed over the canonical encoding of each successive item in the list of Resource items comprising the virtual track timeline.

The digest context shall be finalized (closed to further input) immediately following the contribution to the digest context of the last octet of the canonical encoding of the last Resource item in the list of Resource items.

The resulting finalized digest value shall be the Virtual Track Fingerprint, in binary form, of the respective virtual track. Due to the use of the SHA-1 digest algorithm the length of this value shall be twenty (20) octets.

Text Encoding

In some applications it will be useful to use the digest value directly, without the proposed URI prefix. To produce this encoding, the binary Virtual Track Fingerprint value shall be encoded as a string of forty (40) hexadecimal characters from the UTF-8 set (per Unicode) 0 - 9, a - f.

URI Encoding

For best portability it is useful to encode a Virtual Track Fingerprint as a string value that is readily recognizable as conforming to this proposal. This encoding shall be referred to as IMF-VTRACK-FP.

The IMF-VTRACK-FP value is created by encoding the Virtual Track Fingerprint digest value as a URN item of the form urn:smpte:imf-vtfp:<hex-digits>, where imf-vtfp is a registered NSS as defined in this document, and <hex-digits> is the hexadecimal encoding described above in Text Encoding.

URN Example

urn:smpte:imf-vtfp:190fb72cddad682227cb3f075db9dfb558447a03.

IMF-VTRACK-FP URN

IMF-VTRACK-FP URN NID

The NID of an IMF-VTRACK-FP URN shall be smpte, as defined in SMPTE ST 2029.

IMF-VTRACK-FP URN NSS

The NSS of an IMF-VTRACK-FP URN shall begin with imf-vtfp:. The identifier structure for the IMF-VTRACK-FP subnamespace (IMF-VTRACK-FP-NSS), described using IETF RFC 5234 (EBNF), shall be:

IMF-VTRACK-FP-NSS  = "smpte: imf-vtfp:" IMF-VTRACK-FP
IMF-VTRACK-FP = 4*40HEX-DIGIT
HEX-DIGIT = %x30-39 / ; 0-9
            %x61-66 / ; a-f

The hexadecimal digits in the URN representation of an IMF-VTRACK-FP shall be the hexadecimal representation of the Virtual Track Fingerprint.

Matching

Lexical Equivalence

The lexical equivalence of IMF-VTRACK-FP URN values shall be determined by an exact string match that is case-sensitive.

Abbreviated Thumbprint Values

Implementations may truncate IMF-VTRACK-FP URN values by removing hexadecimal digits from the end of the string. The minimum permissible number of hexadecimal digits is four (4).

In cases where the hexadecimal digits have been truncated, the length of the matched segment shall be determined by the length of the shorter string.

Abbreviated URN Example

The following is equivalent to URN Example above: urn:smpte:imf-vtfp:190fb72cdd.

Reference Implementation

This proposal is supplemented by an implementation of the algorithm in Python. See the attached element imf_vtfp.py.

Test Material

A set of IMF Composition Playlist files has been created to accompany this proposal. The first four files (vt-fp-test-1.cpl.xml - vt-fp-test-4.cpl.xml) all represent the same timeline over the same track file clips, but do so using various arrangements of Resource elements to illustrate the variety of formulations the proposed algorithm is intended to accommodate.

These four files contain a single MainImage virtual track, having the Virtual Track Fingerprint value urn:smpte:imf-vtfp:11cbefc227319bf4708a6f0cc228a968ecf7c65b.

vt-fp-test-1.cpl.xml

Contains five Resource elements in one Segment element. Three of the Resource elements have identical contents.

vt-fp-test-2.cpl.xml

Contains three Resource elements in one Segment element. The middle Resource element has a RepeatCount value of 3 (three.)

vt-fp-test-3.cpl.xml

Contains four Resource elements in one Segment element. The second Resource is a Continuation of the first. The third Resource element has a RepeatCount value of 3 (three.)

vt-fp-test-4.cpl.xml

Contains five Resource elements in two Segment elements. The second Resource is a Continuation of the first. The fourth Resource element has a RepeatCount value of 2 (two.)

vt-fp-test-5.cpl.xml

An additional file is provided, identical in structure to vt-fp-test-4.cpl.xml above, but using StereoImageTrackFileResourceType (and thus having a different VTFP value than the other exaples.) The Virtual Track Fingerprint value of this track is urn:smpte:imf-vtfp:e0daeb6f761042adad0e84ad4a85f753523270ed.

Bibliography

Python — The Python Programming Language — Project Home

SMPTE ST 2067-203:2023 – Interoperable Master Format (IMF) — Audio with Frame-based S-ADM Metadata Plug-in