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ISCC-UNIT Content-Code Mixed

IEP: 0007
Title: ISCC-UNIT Condent-Code Mixed
Author: Titusz Pan tp@iscc.foundation
Comments: #12
Status: DRAFT
Type: Core
License: CC-BY-4.0
Created: {{ git_creation_date_localized }}
Updated: {{ git_revision_date_localized }}

!!! note

This document is a **DRAFT** contributed as input to 
[ISO TC 46/SC 9/WG 18](https://www.iso.org/committee/48836.html). The final version is 
developed at the International Organization for Standardization as
[ISO/DIS 24138](https://www.iso.org/standard/77899.html)

General

  1. The Content-Code Subtype Mixed (Mixed-Code) shall be a similarity preserving hash of a collection of assets of the same or different media types combined into a single multimedia file.
  2. An ISCC processor that supports the creation of Mixed-Codes shall publicly document the supported file formats and the rules by which it divides the different parts of a multimedia file.
  3. The Mixed-Code shall be robust against format conversions, scaling, compression, and minor edits of the individual parts of the multimedia file.

Format

The Mixed-Code shall have the data format illustrated in Figure 9:

![Figure 9 - Data format of the Mixed-Code](images/iscc-iep-0007-f09-mixed-code.png) Figure 8 - Data format of the Mixed-Code

!!! example "EXAMPLE 1: 64-bit Mixed-Code in its canonical form:"

ISCC:EQASD57JXX7U73P7

!!! example "EXAMPLE 2: 256-bit Mixed-Code in its canonical form:"

ISCC:EQDSD57JXX7U73P7HPPH2P3U5OXZM7PL65T3HZ5JZ76H577P77NO5ZY

Inputs

  1. The input for calculating the Mixed-Code shall be the Content-Codes of the individual parts of the multimedia file.
  2. At least two Content-Codes shall be required as input to calculate a Mixed-Code.

Outputs

Mixed-Code processing shall generate the following ISCC metadata output elements:

  1. iscc: the Mixed-Code in its canonical form (required);
  2. parts: the list of Content–Codes used for calculating the Mixed-Code (recommended);
  3. Additional metadata extracted from the multimedia file (optional).

Processing

An ISCC processor shall pre-process the multimedia file as follows:

  1. Generate individual Content-Codes for each part of the multimedia file according to the specifications in IEP-0003, IEP-0004, IEP-0005 and IEP-0006.

An ISCC processor shall calculate the Mixed-Code as follows:

  1. Create a byte sequence from each Content-Code retaining the first byte of the ISCC-HEADER concatenated with the bytes of the ISCC-BODY.
  2. Apply the similarity hash to the list of byte sequences from step 1 to calculate the ISCC-BODY of the Mixed-Code.

Conformance

The normative behaviour of an ISCC processor in generating a Mixed Code is specified only for Content-Code inputs. An implementation of the Mixed-Code algorithm shall be regarded as conforming to the standard if it creates the same Mixed-Code as the reference implementation for the same Content-Code inputs.

!!! note "NOTE"

For further technical details see source-code in modules 
[code_content_mixed.py](https://github.com/iscc/iscc-core/blob/main/iscc_core/code_content_mixed.py)
and [simhash.py](https://github.com/iscc/iscc-core/blob/main/iscc_core/simhash.py) of the 
[reference implementation](https://github.com/iscc/iscc-core).