Base64 to WebP

How to convert Base64 to WebP

1. Paste the Base64 or the full data:image/webp;base64,... URI. Code that contains one, such as a CSS url() or an <img> tag, works too.
2. Check the status line for Detected WebP, then the preview and the dimensions above it. Animated WebP plays in the preview.
3. Press Download file. The extension is set to webp.

Three kinds of WebP

WebP is a RIFF container with one of three chunk types right after the WEBP tag. Because the tag sits at a fixed offset, the type shows up at a fixed place in the Base64 as well:

The six characters between UklGR and XRUJQ encode the file size and change from file to file. XRUJQ (from WEBP) always sits at characters 12 to 16 and VlA4 (from VP8) at 17 to 20. The 21st character is the one you want: I, T or W.

The kind decides what happens when you convert the file. Lossless WebP is a good stand-in for PNG. Lossy WebP has already lost detail, so converting it to PNG makes a large file without bringing quality back. An extended file can hold several frames; converting that to a still format keeps only the first.

Check the WebP type in code

The same test in Python reads the chunk tag and, for extended files, the flag byte:

import base64

def webp_kind(b64: str) -> str:
    data = base64.b64decode(b64.split(",", 1)[-1])
    if data[:4] != b"RIFF" or data[8:12] != b"WEBP":
        return "not WebP"
    chunk = data[12:16]
    if chunk == b"VP8 ":
        return "lossy"
    if chunk == b"VP8L":
        return "lossless"
    flags = data[20]
    parts = ["extended"]
    if flags & 0x10:
        parts.append("alpha")
    if flags & 0x02:
        parts.append("animated")
    return ", ".join(parts)

For WebP files written by Pillow it returns lossy, lossless, extended, alpha for a lossy image with transparency, and extended, animated for an animation. More in Base64 in Python.

RIFF is not always WebP

UklGR only means “RIFF”. WAV audio and AVI video start the same way and differ in the tag at bytes 8 to 11. The decoder checks that tag, so a WAV pasted here is reported as “This looks like WAV, not WebP” with a button to save it as .wav. Base64 to Audio plays it.

Where WebP Base64 turns up

• Canvas exports with toDataURL("image/webp", quality). Check the result: browsers without a WebP encoder quietly return PNG.
• Image APIs and CDNs that serve WebP to any client that accepts it. If a browser fetched the image, even a field named thumbnail_jpg may contain WebP.
• Inline placeholders and icons in HTML and CSS, where WebP gives a shorter string than PNG or JPEG.
• Chat stickers, which are often WebP, sometimes animated.

When an older program refuses the saved file, compare the SHA-256 shown here with the file it received. If they match, the program does not read WebP, and a PNG or JPEG copy is the way out.

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