Internet Engineering Task Force J. Zern
Internet-Draft Google LLC
Intended status: Informational 2 October 2021
Expires: 5 April 2022
WebP Image Format Media Type Registration
draft-zern-webp-04
Abstract
WebP is a RIFF-based image file format which supports lossless and
lossy compression as well as alpha (transparency) and animation. It
covers use cases similar to JPEG, PNG and GIF.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The 'image/webp' Media Type . . . . . . . . . . . . . . . . . 2
2.1. Registration Details . . . . . . . . . . . . . . . . . . 2
3. Security Considerations . . . . . . . . . . . . . . . . . . . 4
4. Interoperability Considerations . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1. Normative References . . . . . . . . . . . . . . . . . . 4
6.2. Informative References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
This document provides references for the WebP image format and
considerations for its use across platforms.
WebP is a Resource Interchange File Format (RIFF) [riff-spec] based
image file format [webp-riff] which supports lossless and lossy
compression as well as alpha (transparency) and animation. It covers
use cases similar to JPEG [jpeg-spec], PNG [RFC2083] and the Graphics
Interchange Format (GIF) [gif-spec].
WebP consists of two compression algorithms used to reduce the size
of image pixel data, including alpha (transparency) information.
Lossy compression is achieved using VP8 intra-frame encoding
[RFC6386]. The lossless algorithm [webp-lossless] stores and
restores the pixel values exactly, including the color values for
zero alpha pixels. The format uses subresolution images, recursively
embedded into the format itself, for storing statistical data about
the images, such as the used entropy codes, spatial predictors, color
space conversion, and color table. LZ77 [lz77], Huffman coding
[huffman], and a color cache are used for compression of the bulk
data.
2. The 'image/webp' Media Type
This section contains the media type registration details as per
[RFC6838].
2.1. Registration Details
Type name: image
Subtype name: webp
Required parameters: N/A
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Optional parameters: N/A
Encoding considerations: Binary. The Base64 encoding [RFC4648]
should be used on transports that cannot accommodate binary data
directly.
Security considerations: See Section 3 below.
Interoperability considerations: See Section 4 below.
Published specification: [webp-riff]
Applications that use this media type: Applications that are used to
display and process images, especially when smaller image file sizes
are important.
Fragment identifier considerations: N/A
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): The first 4 bytes are 0x52, 0x49, 0x46, 0x46
('RIFF'), followed by 4 bytes for the RIFF chunk size. The next 7
bytes are 0x57, 0x45, 0x42, 0x50, 0x56, 0x50, 0x38 ('WEBPVP8').
File extension(s): webp
Apple Uniform Type Identifier: org.webmproject.webp conforms to
public.image
Object Identifiers: N/A
Person & email address to contact for further information:
Name: James Zern
Email: jzern@google.com
Intended usage: COMMON
Restrictions on usage: N/A
Author:
Name: James Zern
Email: jzern@google.com
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Change controller:
Name: James Zern
Email: jzern@google.com
Name: Pascal Massimino
Email: pascal.massimino@gmail.com
Provisional registration? (standards tree only): N/A
3. Security Considerations
Security risks are similar to other media content and may include
buffer overruns and uninitialized data usage as part of the demuxing
and decoding process [cve.mitre.org-libwebp] [crbug-security].
4. Interoperability Considerations
The format is defined using little-endian byte ordering (see
Section 3.1 of [RFC2781]), but demuxing and decoding are possible on
platforms using a different ordering with the appropriate conversion.
The container is RIFF-based and allows extension via user defined
chunks, but nothing beyond the chunks defined by the container format
[webp-riff] are required for decoding of the image.
5. IANA Considerations
IANA has updated the "Image Media Types" registry to include 'image/
webp' as described in Section 2.
6. References
6.1. Normative References
[RFC2781] Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO
10646", RFC 2781, DOI 10.17487/RFC2781, February 2000,
<https://www.rfc-editor.org/info/rfc2781>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC6386] Bankoski, J., Koleszar, J., Quillio, L., Salonen, J.,
Wilkins, P., and Y. Xu, "VP8 Data Format and Decoding
Guide", RFC 6386, DOI 10.17487/RFC6386, November 2011,
<https://www.rfc-editor.org/info/rfc6386>.
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[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[webp-lossless]
Alakuijala, J., "WebP Lossless Bitstream Specification",
September 2014, <https://chromium.googlesource.com/webm/li
bwebp/+/refs/heads/main/doc/webp-lossless-bitstream-
spec.txt>.
[webp-riff]
Google LLC, "WebP RIFF Container", April 2018, <https://ch
romium.googlesource.com/webm/libwebp/+/refs/heads/main/
doc/webp-container-spec.txt>.
6.2. Informative References
[crbug-security]
"libwebp Security Issues",
<https://bugs.chromium.org/p/webp/issues/
list?q=label%3ASecurity>.
[cve.mitre.org-libwebp]
"libwebp CVE List", <https://cve.mitre.org/cgi-bin/
cvekey.cgi?keyword=libwebp>.
[gif-spec] "GIF89a Specification",
<https://www.w3.org/Graphics/GIF/spec-gif89a.txt>.
[huffman] Huffman, D. A., "A Method for the Construction of Minimum
Redundancy Codes", Proceedings of the Institute of Radio
Engineers Number 9, pp. 1098-1101., September 1952.
[jpeg-spec]
"JPEG Standard (JPEG ISO/IEC 10918-1 ITU-T Recommendation
T.81)", <https://www.w3.org/Graphics/JPEG/itu-t81.pdf>.
[lz77] Ziv, J. and A. Lempel, "A Universal Algorithm for
Sequential Data Compression", IEEE Transactions on
Information Theory Vol. 23, No. 3, pp. 337-343., May 1977.
[RFC2083] Boutell, T., "PNG (Portable Network Graphics)
Specification Version 1.0", RFC 2083,
DOI 10.17487/RFC2083, March 1997,
<https://www.rfc-editor.org/info/rfc2083>.
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[riff-spec]
"Multimedia Programming Interface and Data Specifications
1.0", <http://www-
mmsp.ece.mcgill.ca/Documents/AudioFormats/WAVE/Docs/
riffmci.pdf>.
Author's Address
James Zern
Google LLC
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Phone: +1 650 253-0000
Email: jzern@google.com
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