WebP

WebP's lossy compression algorithm is based on the intra-frame coding of the VP8 video format[19] and the Resource Interchange File Format (RIFF) as a container format.[2] As such, it is a block-based transformation scheme with eight bits of color depth and a luminance-chrominance model with chroma subsampling by a ratio of 1:2 (YCbCr 4:2:0).[20] Without further content, the mandatory RIFF container has an overhead of only twenty bytes, though it can also hold additional metadata.[2] The side length of WebP images is limited to 16,383 pixels.[4]

WebP is based on block prediction. Each block is predicted on the values from the three blocks above it and from one block to the left of it (block decoding is done in raster-scan order: left to right and top to bottom). There are four basic modes of block prediction: horizontal, vertical, DC (one color), and TrueMotion. Mispredicted data and non-predicted blocks are compressed in a 4×4 pixel sub-block with a discrete cosine transform or a Walsh–Hadamard transform. Both transforms are done with fixed-point arithmetic to avoid rounding errors. The output is compressed with entropy encoding.[20] WebP also has explicit support for parallel decoding.[20]

The reference implementation consists of converter software in the form of a command-line program for Linux (cwebp) and a programming library for the decoding, the same as for WebM. The open source community quickly managed to port the converter to other platforms, such as Windows.[21]

WebP's lossless compression, a new format unrelated to VP8, was designed by Google software engineer Jyrki Alakuijala. It uses advanced techniques such as dedicated entropy codes for different color channels, exploiting 2D locality of backward reference distances and a color cache of recently used colors. This complements basic techniques such as dictionary coding, Huffman coding and color indexing transform.[13] This format uses recursive definition: all of the control images such as the local entropy code selection are encoded the same way as the whole image itself.[22]

Like VP8 on which it is based, former lossy WebP only supports 8-bit YUV 4:2:0 format,[55] which may cause color loss on images with thin contrast elements (such as in pixel art and computer graphics) and ghosting in anaglyph. To overcome this restriction, new lossless WebP supports VP8L encoding that works exclusively with 8-bit ARGB color space.[56][57]

Google actively promotes WebP. The proprietary PageSpeed Insights tool suggests that webmasters switch from JPEG and PNG to WebP in order to improve their website speed score.[58] However WebP is not included in the Google Merchant Feed specification for images.

In September 2010, Fiona Glaser, a developer of the x264 encoder, wrote a very early critique of WebP.[19] Comparing different encodings (JPEG, x264, and WebP) of a reference image, she stated that the quality of the WebP-encoded result was the worst of the three, mostly because of blurriness on the image. Her main remark was that "libvpx, a much more powerful encoder than ffmpeg's jpeg encoder, loses because it tries too hard to optimize for PSNR" (peak signal-to-noise ratio), arguing instead that "good psycho-visual optimizations are more important than anything else for compression."[19]

In October 2013, Josh Aas from Mozilla Research published a comprehensive study of current lossy encoding techniques[59] and was not able to conclude WebP outperformed JPEG by any significant margin.[60]

• FLIF, a work-in-progress lossless image format which claims to outperform PNG, lossless WebP, lossless BPG and lossless JPEG2000 in terms of compression ratio, introduced in 2015
• BPG, an image format intended to be a more compression-efficient replacement for the JPEG image format, based on the intra-frame encoding of the High Efficiency Video Coding (HEVC) video compression standard, introduced in 2014
• HEIF, another image format based on HEVC
• JPEG XR, an alternative to JPEG 2000 supporting HDR and wide gamut color spaces, introduced in 2009
• JPEG 2000, an improvement intended to replace the older JPEG by the JPEG committee, introduced in 2000
• MNG and APNG, PNG-based animated image formats, supporting lossless 24-bit RGB color and 8-bit alpha channel
• AV1 Still Image File Format, a container format based on the AV1 video codec[62]

Wikimedia Commons has media related to WebP file format.

• Official website