ISO/IEC 18181-3:2025

International
Standard
ISO/IEC 18181-3
Second edition
Information technology — JPEG XL
2025-01
image coding system —
Part 3:
Conformance testing
Technologies de l'information — Systѐme de codage d'images
JPEG XL —
Partie 3: Essai de conformité
Reference number
© ISO/IEC 2025
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© ISO/IEC 2025 – All rights reserved
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Testing procedures . 1
5 Decoder conformance . 2
6 Encoder conformance . 2
Annex A (normative) Core conformance . 3
Annex B (normative) Extended conformance. 5
Annex C (informative) Description of the test corpora . 7
Bibliography . 9

© ISO/IEC 2025 – All rights reserved
iii
Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are
members of ISO or IEC participate in the development of International Standards through technical
committees established by the respective organization to deal with particular fields of technical activity.
ISO and IEC technical committees collaborate in fields of mutual interest. Other international organizations,
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The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of document should be noted. This document was drafted in accordance with the editorial rules of the ISO/
IEC Directives, Part 2 (see www.iso.org/directives or www.iec.ch/members_experts/refdocs).
ISO and IEC draw attention to the possibility that the implementation of this document may involve the
use of (a) patent(s). ISO and IEC take no position concerning the evidence, validity or applicability of any
claimed patent rights in respect thereof. As of the date of publication of this document, ISO and IEC had not
received notice of (a) patent(s) which may be required to implement this document. However, implementers
are cautioned that this may not represent the latest information, which may be obtained from the patent
database available at www.iso.org/patents and https://patents.iec.ch. ISO and IEC shall not be held
responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www.iso.org/iso/foreword.html.
In the IEC, see www.iec.ch/understanding-standards.
This document was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 29, Coding of audio, picture, multimedia and hypermedia information.
This second edition cancels and replaces the first edition (ISO/IEC 18181-3:2022), which has been technically
revised.
The main changes are as follows:
— Decoder conformance was separated into core conformance and extended conformance;
— Test cases were updated to reflect the second editions of ISO/IEC 18181-1 and ISO/IEC 18181-2;
— References were updated accordingly.
A list of all parts in the ISO/IEC 18181 series can be found on the ISO and IEC websites.
Any feedback or questions on this document should be directed to the user’s national standards
body. A complete listing of these bodies can be found at www.iso.org/members.html and
www.iec.ch/national-committees.

© ISO/IEC 2025 – All rights reserved
iv
International Standard ISO/IEC 18181-3:2025(en)
Information technology — JPEG XL image coding system —
Part 3:
Conformance testing
1 Scope
This document specifies the conformance testing of the ISO/IEC 18181 series, also known as JPEG XL.
NOTE Other desirable aspects of implementation (including robustness and performance) are outside the scope
of this document.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO/IEC 18181-1:2024, Information technology — JPEG XL image coding system — Part 1: Core coding system
ISO/IEC 18181-2:2024, Information technology — JPEG XL image coding system — Part 2: File format
1)
ISO 15076-1 , Image technology colour management — Architecture, profile format and data structure — Part
2: Based on ICC.1:2022
ISO/IEC 60559, Information technology — Microprocessor Systems — Floating-Point arithmetic
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
core conformance
producing decoded samples within the specified tolerances
3.2
extended conformance
conforming to all the testing procedures, including those related to metadata and JPEG bitstream
reconstruction
4 Testing procedures
A set of test cases is defined in this document. Each test case consists of a JPEG XL bitstream, a reference
decoded image, and possibly additional tests. For each test case, core conformance tests (as defined in
Annex A) establish precision tolerances on the decoded samples, as compared to a reference image.
1) Under development. Stage at the time of publication: ISO/DIS 15076-1:2024.

© ISO/IEC 2025 – All rights reserved
Extended conformance tests (as defined in Annex B) include additional decoder functionality such as
extraction of metadata and JPEG-1 bitstream reconstruction.
The electronic attachments (https:// standards .iso .org/ iso -iec/ 18181/ -3/ ed -2/ en/ ) of this document consist
of a testcases/ subdirectory, containing multiple subdirectories, each of which contains a single test case
for conformance testing.
5 Decoder conformance
A decoder shall be considered conforming to Level 5 of the Main profile if it is conforming to all the test
cases specified in the testcases/main_level5.txt file in the electronic attachment to this document.
A decoder shall be considered conforming to Level 10 of the Main profile if it is conforming to all the test
cases specified in the testcases/main_level10.txt file in the electronic attachment to this document.
The tests described in this document are necessary, but not sufficient to determine complete decoder
conformance to all aspects of the ISO/IEC 18181-1 and ISO/IEC 18181-2 specifications.
For core conformance, all test cases shall pass the tests described in Annex A.
NOTE 1 Core conformance to Level 5 of the Main profile suffices to implement an application that can correctly and
accurately render RGB or greyscale JPEG XL images intended for end-user image delivery. In order to facilitate testing,
decoders can generate NPY files as described in Annex A.2. If implementations do not directly support this output
format, generating such files from the decoder output in a sample value preserving postprocessing step is sufficient
to pass the conformance tests. For example, if the decoder produces a (possibly animated) PNG file (with a sufficiently
high bit depth), this still suffices to test core conformance. However, for testing core conformance to Level 10 of the
Main profile, PNG output does not suffice since it is limited to 16-bit precision.
For extended conformance, all test cases shall pass both the tests described in Annex A and in Annex B.
NOTE 2 The extended conformance tests assess functionality that is not necessary for displaying an image, but that
is nevertheless useful for authoring or archival purposes.
6 Encoder conformance
As specified in ISO/IEC 18181-1:2024, any encoding process is acceptable so long as it produces a valid
codestream. Thus, an encoder shall be considered conforming if it produces output files which are
successfully decoded by a conforming decoder as described in Clause 5. More precisely, the steps for testing
encoder conformance are as follows:
a) Select a test image that represents the type of imagery that the encoder is designed to compress. The
reference decoded images provided for decoder conformance tests are acceptable but not required.
b) Encode with the encoder under test.
c) Send the codestream to the reference decoder.
d) An encoder is found to be conforming if a conforming decoder can fully decode the image.
e) Repeat steps a) through d) for all parameters for which the encoder is designed. These parameters
should be varied to the extent to which the encoder will be used.
f) Repeat steps a) through e) for several test images, sampling the breadth of imagery types (small image
size, large image size, odd image sizes, number of components, component bit depths, component
sampling) the encoder is designed to compress.
The above procedure provides a necessary, but not sufficient criterion to determine complete encoder
conformance to all aspects of the ISO/IEC 18181-1 and ISO/IEC 18181-2 specifications.

© ISO/IEC 2025 – All rights reserved
Annex A
(normative)
Core conformance
A.1 Test case structure
Each test case subdirectory contains
— a JSON file (ISO/IEC 21778) named test.json
— a JPEG XL file (ISO/IEC 18181-2:2024) named input.jxl
— a NPY file (A.2) named reference_image.npy
— an ICC profile file named reference.icc. The contents of this file shall be conformant to ISO 15076-1.
Collectively, the NPY and ICC file are known as the reference image for that test case.
The JSON file specifies the tests to be performed as part of this test case.
The NPY file contains sample values for the image frames, represented as a four-dimensional array with
dimensions (num_frames, height, width, num_channels) respectively. The data format stored in these arrays
32-bit float and shall be conformant to ISO/IEC 60559.
reference.icc represents the colour space of the data in the NPY file.
A.2 Numpy file format
A NPY file begins with the sequence of bytes 0x93 0x4e 0x55 0x4d 0x50 0x59 (byte 93 followed by NUMPY
in ASCII), followed by the sequence of bytes 0x01 0x00, followed by two bytes representing a 16-bit little-
endian integer LEN that represents the length of the rest of the header.
The rest of the header contains the newline-terminated ASCII (ISO/IEC 646) string {'descr': ' 'fortran_order': False, 'shape': (X, Y, Z, W), }, where X, Y, Z, W represent integers in their usual
base-10 textual representation.
The NPY file represents an array with dimensions (X, Y, Z, W). The rest of the file contains X x Y x Z x W 32-
bit floating point numbers as specified in ISO/IEC 60559, with little endian byte ordering, in raster order
with the W dimension varying faster, i.e. floating point number in position (x, y, z, w) is stored in the 4 bytes
starting at position 4·(x·Y·Z·W + y·Z·W + z·W + w) after the header.
A.3 Image similarity
Samples are compared in the colour space specified by the reference ICC profile reference.icc, with nominal
values in the interval [0, 1]. For core conformance, clamping is to be applied to values outside this range, in
both the decoded and in the reference image. For extended conformance, no such clamping is to be applied.
An image is si
...