WebP MIME Type

What does image/webp mean?

The image/webp media type identifies a WebP raster image. A WebP file can contain a still image or an animation, use lossy or lossless compression, and carry transparency, an ICC color profile, EXIF metadata, or XMP metadata. The current IANA registration is defined by RFC 9649 and has no required or optional media-type parameters.¹

The registered filename extension is .webp. The file starts with the ASCII bytes RIFF, followed by a four-byte little-endian size, then WEBPVP8. The next character distinguishes chunks such as VP8 for a simple lossy image and VP8L for a simple lossless image.¹

How WebP became a web image format

Google announced WebP as a developer preview on September 30, 2010. The first release applied VP8 intra-frame coding to still images and placed the result in a lightweight RIFF container. Lossless coding and alpha transparency followed in 2011.²³

The format existed for years before its present standards registration. RFC 9649, published in November 2024, brought the container and lossless bitstream descriptions together and registered image/webp with IANA. The RFC is Informational rather than an Internet Standards Track specification.¹

How WebP compression works

Lossy WebP uses VP8 intra-frame encoding. It predicts pixel blocks from nearby data and stores the remaining difference. Lossless WebP combines transforms with LZ77 backward references, prefix coding, and a color cache, allowing the decoder to reconstruct the ARGB pixel values exactly.¹

The RIFF container adds the features that make WebP more than a single compressed frame:

• VP8X advertises extended features and canvas dimensions.
• ALPH stores transparency for a lossy frame.
• ANIM and ANMF describe animation settings and frames.
• ICCP, EXIF, and XMP carry color and metadata information.¹

WebP is an 8-bit format. Lossy images use 8-bit Y'CbCr 4:2:0 color, while lossless images use 8-bit ARGB. The maximum canvas is 16,383 by 16,383 pixels.⁴

Is WebP smaller than JPEG or PNG?

It often is, but the answer depends on the source and encoder settings. In Google's reproducible lossy study, WebP 0.1.2 and JPEG outputs were created from PNG sources and matched by Structural Similarity Index (SSIM). Across the Kodak set, the Tecnick set, and roughly 11,000 web images, WebP was 25% to 34% smaller than JPEG at equivalent or slightly better SSIM.⁵

Google's separate lossless study compared WebP against size-optimized versions of 12,000 web PNGs, with and without translucency. At default settings, WebP's reported compression density was 23% better than ZopfliPNG and 42% better than the study's libpng-based output. These figures come from different studies and should not be combined into a universal ranking.⁶

How to open a .webp file

Chrome, Edge, Firefox, Opera, and Safari can display WebP directly. WebP also works in ordinary HTML <img> elements and can be selected in a <picture> source.⁴

Adobe Photoshop 23.2 and later can open, create, edit, and save WebP without a plug-in. Photoshop exposes lossy and lossless modes as well as options for retaining EXIF and XMP metadata.⁷ Google's dwebp command-line utility can decode a WebP, while cwebp creates one from formats such as PNG and JPEG.⁸

If an application rejects a .webp file, confirm the signature before converting it. Renaming the extension does not change the encoded data.

Support in Poly

Poly recognizes both image/webp and .webp filenames. Its image loader decodes the file, applies readable orientation information, and generates lossless WebP thumbnails at several sizes. The metadata path records dimensions and parses an EXIF chunk when one is present.⁹

Poly then creates a visual embedding from a generated thumbnail, enabling visual-similarity and color search. Filename and readable EXIF fields are also searchable. Plain images are not OCR'd, so words drawn into a WebP are not indexed as text.

The image viewer loads the original WebP directly in browsers that support it, which preserves browser animation playback. Thumbnail generation, visual indexing, and agent reading use a single rendered image. The agent can interpret that visible frame, but it does not receive every animation frame, RIFF chunk, ICC profile, or original metadata field.⁹

WebP compared with JPEG, PNG, GIF, and AVIF

WebP is useful when one web-oriented format needs to cover photographs, transparent graphics, and short animations. JPEG remains easier to exchange with old software. PNG supports 16-bit channels and is a dependable lossless working format. AVIF supports higher bit depths and HDR workflows that WebP does not.⁴

Converting WebP without losing the important parts

Choose the destination according to what the file contains:

• Convert a lossy still image to JPEG for older software, understanding that another lossy encode may introduce more artifacts.
• Convert a lossless still image or a frame needing transparency to PNG.
• Convert an animation to GIF only when legacy compatibility outweighs full-color and alpha quality. A video format may be better for long animation.
• Convert to AVIF when higher compression efficiency, HDR, or higher bit depth matters and the target software supports it.

Conversion tools do not always preserve animation timing, ICC profiles, EXIF, XMP, or unknown RIFF chunks. Keep the original until you have compared the pixels, frame sequence, transparency, color, and metadata.