How to Convert Images Privately Between Formats

Why image conversion often silently leaks or modifies data

A converter that runs on a server has to receive the file to convert it, and what happens to it after that is governed by that service’s own terms rather than by anything you can see. Retention periods, logging and what a provider may do with uploaded content vary, and are worth reading before uploading something you would not publish. We make no claim about any particular service.

Furthermore, server-side conversion utilities often strip or mutate metadata unpredictably, sometimes embedding third-party branding or conversion software identifiers into the output file.

Format choices: JPEG, PNG, WebP, and AVIF compared

Choosing the right destination format requires understanding the technical tradeoffs between compression ratio, fidelity, and browser compatibility:

• JPEG — the ubiquitous standard for photographic imagery. Uses lossy discrete cosine transform (DCT) compression. Does not support transparency (alpha channels).
• PNG — the gold standard for lossless graphics, screenshots, diagrams, and artwork requiring clean transparent backgrounds via a dedicated 8-bit alpha channel.
• WebP — Google’s modern web format providing both lossy and lossless compression. WebP files are typically 25% to 34% smaller than equivalent JPEGs while retaining transparency support.
• AVIF — the next-generation format based on the AV1 video codec. Delivers outstanding compression efficiency at low bitrates with full HDR and alpha support.

Client-side conversion using HTML5 Canvas and OffscreenCanvas

Modern web browsers are capable graphics rendering runtimes. By taking advantage of HTML5 Canvas, modern browser APIs can decode source files natively, render them into an offscreen frame buffer, and re-encode the pixel data into target MIME types.

In NoMoreData, conversion occurs entirely within your local browser session. The source file bytes are read through the File API, loaded into an ImageBitmap, and encoded into the selected format with customizable quality compression parameters.

Your file is never sent anywhere. The conversion itself does make one request — the first time it needs a codec it fetches it from this site, between about 54 KB for MP3 and 1.1 MB for AVIF, and then it is cached. That is a request for our code, never a transfer of your file, and it is why the first conversion to a format is slower than the ones after it.

Handling alpha transparency and compression artifacts

A frequent pitfall when converting from PNG or WebP to JPEG is transparency loss. Because JPEG lacks an alpha channel, transparent pixels must be composited against a background color (typically solid white or black).

NoMoreData handles background compositing cleanly to prevent black halos or jagged fringe artifacts around semi-transparent edges. When converting from lossy formats to lossless formats (e.g., JPEG to PNG), remember that while quality is preserved from further degradation, existing compression artifacts cannot be undone.

Metadata behavior across different image containers

Different container formats organize metadata using completely different standards. JPEG stores EXIF inside APP1 segments, PNG uses tEXt/zTXt/eXIf chunks, and WebP wraps EXIF in a RIFF chunk named "EXIF".

A canvas re-encode writes a fresh container rather than copying the old one, so camera headers and maker notes are not carried across by the encoder itself. What this tool then writes back is what you asked it to: the carry is on by default. If what you want is a file with as little as this build can leave in it, use the remove tool and read the verification report — which lists what was taken out and what survived, because “completely clean” is not something any build can promise about a format it did not define.

Questions