| Internet-Draft | WebP Image Format Media Type Registratio | September 2021 |
| Zern | Expires 26 March 2022 | [Page] |
WebP Image Format Media Type Registration
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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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.¶
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].¶
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.¶
IANA has updated the "Image Media Types" registry to include 'image/webp' as described in Section 2.¶
- [RFC2781]
- Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO 10646", RFC 2781, DOI 10.17487/RFC2781, , <https://www.rfc-editor.org/info/rfc2781>.
- [RFC4648]
- Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, , <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, , <https://www.rfc-editor.org/info/rfc6386>.
- [RFC6838]
- Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, , <https://www.rfc-editor.org/info/rfc6838>.
- [webp-lossless]
- Alakuijala, J., "WebP Lossless Bitstream Specification", , <https://chromium.googlesource.com/webm/libwebp/+/refs/heads/main/doc/webp-lossless-bitstream-spec.txt>.
- [webp-riff]
- Google LLC, "WebP RIFF Container", , <https://chromium.googlesource.com/webm/libwebp/+/refs/heads/main/doc/webp-container-spec.txt>.
- [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., .
- [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., .
- [RFC2083]
- Boutell, T., "PNG (Portable Network Graphics) Specification Version 1.0", RFC 2083, DOI 10.17487/RFC2083, , <https://www.rfc-editor.org/info/rfc2083>.
- [riff-spec]
- "Multimedia Programming Interface and Data Specifications 1.0", <http://www-mmsp.ece.mcgill.ca/Documents/AudioFormats/WAVE/Docs/riffmci.pdf>.
James Zern
Google LLC
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United States of America