Internet Engineering Task Force J. Zern Internet-Draft Google LLC Intended status: Standards Track 31 August 2021 Expires: 4 March 2022 WebP Image Format Media Type Registration draft-zern-webp-02 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. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 4 March 2022. Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Zern Expires 4 March 2022 [Page 1] Internet-Draft WebP Image Format Media Type Registratio August 2021 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. The 'image/webp' Media Type . . . . . . . . . . . . . . . . . 2 2.1. Registration Details . . . . . . . . . . . . . . . . . . 2 3. Security Considerations . . . . . . . . . . . . . . . . . . . 3 4. Interoperability Considerations . . . . . . . . . . . . . . . 3 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 6.1. Normative References . . . . . . . . . . . . . . . . . . 4 6.2. Informative References . . . . . . . . . . . . . . . . . 4 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5 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 Media type name: image Media subtype name: webp Required parameters: N/A Zern Expires 4 March 2022 [Page 2] Internet-Draft WebP Image Format Media Type Registratio August 2021 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. Applications which use this media type: Anywhere image size is an issue. Fragment identifier considerations: N/A Restrictions on usage: 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 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. Zern Expires 4 March 2022 [Page 3] Internet-Draft WebP Image Format Media Type Registratio August 2021 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, . [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, . [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, . [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013, . [webp-lossless] Alakuijala, J., "WebP Lossless Bitstream Specification", September 2014, . [webp-riff] Google LLC, "WebP RIFF Container", April 2018, . 6.2. Informative References [crbug-security] "libwebp Security Issues", . [cve.mitre.org-libwebp] "libwebp CVE List", . Zern Expires 4 March 2022 [Page 4] Internet-Draft WebP Image Format Media Type Registratio August 2021 [gif-spec] "GIF89a Specification", . [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)", . [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, . [riff-spec] "Multimedia Programming Interface and Data Specifications 1.0", . 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 Zern Expires 4 March 2022 [Page 5]