How to convert images to WebP or AVIF without uploading

1. Put the photos on the device you will convert on

Shift never sends the file to a backend, so the bytes have to exist in this browser. On a laptop, drag from a folder. On iPhone, tap Choose files and pick from Files or Camera Roll. Do not email yourself a HEIC and expect this page to open it — HEIC is out of scope; convert that with HEIC Local to JPG first. A 40 MB panorama may warn about memory. That warning is the device talking, not an upsell.

If the photos live on a camera card, copy them to the computer first. Network drives and iCloud placeholders that have not finished downloading will fail as empty or truncated files. Shift reads the first 64 bytes for magic: JPEG FF D8, PNG signature, RIFF/WEBP, BMP BM, GIF, or an AVIF ftyp brand. A PDF renamed to .jpg is rejected here, not “converted.”

2. Read the capability badges before you pick AVIF

Two badges sit above the dropzone: WebP encode and AVIF encode. WebP should say yes on any current Chrome, Safari, Firefox or Edge. AVIF is the unreliable one. Shift probes native Canvas, then a WASM encoder. If the AVIF badge is no, pick WebP. Forcing AVIF on a phone that cannot encode it is how other tools produce files that say .avif and are not AVIF. The WebP vs AVIF page explains the trade-off in more depth.

The probe is a 1×1 encode whose bytes are checked for real AVIF brands, not a feature-detect string. Native toBlob('image/avif') on several mobile browsers returns an empty blob, a JPEG, or a WebP with the wrong type. Shift treats that as a fail and tries jSquash (libavif compiled to WASM, single-thread). If WASM is blocked or times out, the AVIF button is disabled. That is the product. It is not a paywall.

3. Choose output, quality, and optional max width

Output is global for the batch: WebP, AVIF, JPG or PNG. Quality defaults to 85%. That is a still-photo setting, not a thumbnail setting. Drop to 70% for Open Graph images and product-grid thumbs. Raise to 95% only if you are printing or archiving a lossless-looking WebP and can afford the size. PNG ignores quality. Max width 1920 is a sane cap for web heroes; 1280 is enough for blog inline images. Height is optional and keeps aspect ratio. Both fields empty means “keep the pixel grid,” which is rarely what you want for a 48-megapixel export.

Quality on AVIF is mapped from the 0.1–1.0 slider onto the encoder’s 10–100 scale. Do not compare a 70% WebP to a 70% JPEG by filename alone — look at a 100% crop of skin, type, or a logo edge. If the destination is email or a printer, output JPG at 90–95 instead of fighting AVIF support.

4. Convert, then actually look at the result

Tap Convert. Progress is per file. A failure on one image (corrupt JPEG, empty file, HEIC) does not abort the others. The result row shows original size → new size, dimensions, and a download button. If the WebP is larger than the JPEG, you probably converted an already-small screenshot at 95% — drop quality or use PNG. If AVIF is tiny but looks smeared on skin, raise quality or stay on WebP.

Encoding is sequential on purpose. A worker pool would finish a 12-file batch faster on a desktop and freeze a 3 GB Android phone. One image at a time is the conservative path. Orientation is applied when createImageBitmap supports it, so a phone JPEG shot in portrait should not land on its side.

5. Download one file or a ZIP

Individual download uses a blob URL. On iPhone, if the browser can share files, you get a Share sheet (Save to Files, AirDrop). Desktop browsers get a normal download named like street-shift.webp. Two or more successes unlock Download ZIP. The archive is built in the tab. Nothing is stored against your email because there is no email in the converter.

Blob URLs are revoked after the download starts so the tab does not leak object URLs for every convert you ran this session. If you cancel the iOS Share sheet, nothing was uploaded — there is no server waiting for the file. Allow a second before the URL is revoked if the sheet is slow to open.

6. Reverse path: WebP or AVIF back to JPG

Designers send AVIF; your printer wants JPEG. Set output to JPG, quality 90–95, convert. Transparency becomes white. If you need the alpha channel, use PNG. Native decode is tried first; if this browser cannot open the AVIF, Shift falls back to the same local WASM decoder. See AVIF to JPG for that specific job.

7. If something fails

“Not a supported image” means magic bytes did not look like a raster Shift handles. “Too large for this device’s memory” means lower max width or convert fewer files. AVIF encode timed out means the WASM path did not finish — use WebP. A white or blank download is almost always a failed encode; Shift should have shown an error on the row. If the whole page crashed, reload: no server-side job exists to resume.

If WebP encode itself fails, you are on a very old browser. Update Chrome, Safari, Firefox or Edge. Shift does not polyfill WebP encode. BMP and still GIF are accepted as input; they are not interesting outputs. Animated GIF/WebP/AVIF is not a v1 pipeline.

8. What not to use Shift for

Do not use it as a GPS stripper (Strip), a compressor-first tool (Crush), a HEIC pipeline (HEIC Local), or a PDF toolkit (Folio). Do not feed it copyrighted assets you are not allowed to process. Do not expect animated WebP/AVIF to survive as animation. Those are different products and different codecs.

9. Quality recipes for common jobs

• Article hero, unknown browsers: WebP, 85%, max width 1920.
• Product grid or card: WebP, 75–80%, max width 1280.
• Open Graph / social crop: WebP, 70–75%, max width 1200.
• Your own site with a picture tag: AVIF at 80–85% plus a WebP fallback at the same size.
• Client review or print shop: JPG, 90–95%, no max width unless the file is huge.
• UI screenshot with type: PNG, or WebP at 90% if you accept a little softness.

These are starting points, not laws. Convert twice when the picture matters. Look at the face, the sky, and a hard edge. Then pick. Dedicated notes for the most common direction live on JPG to WebP.

10. How the encoder actually runs

Decode: createImageBitmap from the File. Optional downscale: the canvas is sized so the long edge respects max width or max height, aspect ratio kept. Draw. Encode: canvas.toBlob for WebP, JPG and PNG; jSquash for AVIF when the badge is yes. JPG output fills white first so transparent pixels do not become a black hole. PNG keeps alpha. WebP keeps alpha. There is no EXIF rewrite, no GPS strip, no color profile conversion beyond what the browser’s decoder already did.

That last sentence is load-bearing. If you needed GPS gone, that is Strip. If you needed a smaller JPEG at all costs, that is Crush. Shift will change the codec and optionally the pixel grid. It will not pretend to be a DAM.

11. Checking the file is really WebP or AVIF

On a computer, the first bytes of a WebP are RIFF....WEBP. AVIF has an ISO-BMFF ftyp box with an avif or avis brand. If a site gave you an “AVIF” that starts with JPEG SOI (FF D8), they lied. Shift’s probe exists because that lie is common on phones. After you download, you can also drop the result back into Shift and convert the other way — if decode fails, the file was not a real AVIF.

12. iPhone, in one paragraph

Use Choose files, not drag-and-drop. HEIC is refused. If Camera is set to Most Compatible you already have JPEG. Cap max width on old phones. Download tries the Share sheet. Full notes: iPhone photos. Questions: FAQ.

When the convert worked and the other person still cannot open it

That is a delivery job, not a slider job. Chat recodes frames: WhatsApp. Mail clients that will not paint AVIF, plus Gmail’s 25 MB cap: email. Chrome on a phone is not the shop WebView: Android.

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