What is a WebP file?

The thing most people get wrong about WebP is assuming it is always lossy. A .webp file is a container, and the chunk inside decides what you have. A 'VP8 ' chunk means lossy coding: block-based prediction and transform coding, close in spirit to how a video codec compresses a single key frame. A 'VP8L' chunk means the separate lossless coding, which uses transforms such as predicting pixels from their neighbours, a colour cache and backward references to repeated data, and decodes to exactly the stored pixels.

Transparency works differently in the two. A lossless image simply carries alpha with every pixel. A lossy image keeps its colour in the VP8 data and its transparency in a separate 'ALPH' chunk, announced by the extended 'VP8X' header. That is why a lossy WebP can have a clean transparent background, which JPEG cannot.

You can see which kind a file is without any image software: open it in a hex viewer and look for the four-letter chunk names after the first twelve bytes. The measured example below lists the chunks the browser actually wrote for each sample.

Any current browser opens WebP, so dragging the file into a browser tab is the quickest way to look at it. Trouble usually starts one step later, when the image has to go into software that predates WebP: an older photo editor, a print shop's upload page, a form that accepts only JPG or PNG.

For those, convert it. A WebP with transparency should become a PNG, which keeps the alpha channel; a photo with no transparency can become a JPG. The WebP to PNG / JPG Converter on AltFTool does this in the browser, and it is also the fix when a site saved an image you downloaded as .webp although you expected a .jpg.

Web pages that create WebP files, AltFTool's Image Converter among them, call the browser's canvas encoder with a quality between 0 and 1. In the Chromium build measured here, qualities 0.82 and 0.90 both produced a lossy 'VP8 ' chunk, and quality 1.0 switched to the lossless 'VP8L' coding: the photo sample came back pixel-identical, and the file was many times larger than the lossy versions.

So a "100%" WebP from that browser is not a slightly better lossy file; it is a different codec with a different size. If you want the smallest file that looks right, stay below 1.0. If you need an exact copy, for example a screenshot you will crop again later, 1.0 is the setting that gives you one.