Image engine could not start (fallback mode)

Flying Pigs does its image work in your browser with a built-in image engine. On most computers it simply runs. When it cannot, the page does not give up: it switches to fallback mode, uploads your originals as they are, and lets WordPress make the sizes on the server, the way its own uploader does. This article explains how to tell, why it happens, and how to fix it.

How to tell which mode you are in

The top right of the Flying Pigs page shows one of these:

• WASM libvips active · ~25MB batches: the engine is running. Everything works.
• Image engine could not start on this device — WordPress makes the sizes on the server: this browser or device refused the engine.
• Server-side fallback — needs HTTPS + isolation: the page itself cannot run the engine, usually because the site is not on HTTPS.
• Browser processing off (Advanced) — WordPress makes the sizes on the server: someone switched it off in the Advanced panel. Switch Browser processing back on.

If the engine fails when you press Start, the run does not start. A notice says why, the header switches, and pressing Start again uploads the fallback way.

Why the engine cannot start

The site is not on HTTPS

Browsers only allow the engine on a secure page: HTTPS, or a local test site on localhost. Move the site to HTTPS, which most hosts offer free.

If the site already shows a padlock but the page still says needs HTTPS, WordPress may not know it is on HTTPS. That happens behind some proxies, CDNs and load balancers. Check that Settings > General shows both addresses starting with https://, and ask your host to pass the HTTPS status through to WordPress.

The page is not isolated

The engine also needs the page to be "cross-origin isolated". Flying Pigs sets this up itself, on its own page only, with no server configuration. If the page still is not isolated, something on the server is removing or overriding those settings: a security plugin, a firewall rule or a proxy. Try again with security and caching plugins switched off, then turn them back on one at a time.

The device will not give the engine memory

Each image worker needs a fixed block of about 1 GB of memory. Desktop browsers grant it. iPads and iPhones, in Safari and in Chrome alike, usually refuse it, and a computer that is short of memory can too. On those devices Flying Pigs works in fallback mode.

Which browsers run the engine

Chrome, Edge, Brave, Firefox, and Safari 16.4 or later, on a desktop or laptop computer. For big runs of thousands of photos, Chrome is the steadiest: Safari keeps each tab on a tight memory budget and may reload the page during a very large run.

What works in fallback mode

• The upload machinery: packed batches, large files sent in pieces, retries, picking up after a dropped connection, and Pro's turbo lanes.
• Your choice of sizes, JPG quality, WebP and AVIF copies (where your host can make them), media folders, file and post dates (Pro), delivery and throttling.
• WebP only (Pro) works in two steps: the JPEG or PNG uploads, the server makes the WebP, and the original is removed.

What you lose in fallback mode

• Everything only the engine can do: optimizing the original (auto rotate, removing metadata, resizing), PNG to JPG, the AI data-mining statement in each file, and the WebP and AVIF effort settings.
• In Pro, the watermark and overwriting in place.
• HEIC and other formats the engine can read but your host's PHP cannot.
• The speed: your host makes every size again, so the run takes longer and uses the server's CPU.

Options that need the engine are greyed out on the page, with the reason beside them, so what you see is what will run.

Trying again

If the engine stopped for a passing reason, such as a slow first download of its files, the next press of Start tries it once more. If the device refused the engine's memory, reloading the page tries again from scratch. On an iPad or iPhone it will usually refuse again, so use a desktop or laptop for full speed.

Image optimizer plugins do not cause fallback

Some optimizer plugins, such as Smush and EWWW, tell WordPress 7.1 to turn off its own in-browser image processing. That does not affect Flying Pigs: it brings its own engine, sets up its own page, and keeps its uploads away from those plugins.