If the camera runs for a while without decoding a single frame, a small toast appears above the preview asking exactly that. Help opens the tips; Dismiss snoozes it (it returns later if things are still dead — tapping a button doesn't make frames arrive).
The fixes are on the sender, which is the non-obvious part. In order:
- On the sender, open Transfer settings and drop bytes / frame to 1465. The 2953-byte default is tuned for close-range phone-to-phone and is exactly what fails on an ordinary monitor at arm's length.
- Still nothing? Drop the sender's tx fps to 24.
- Fill this camera's view with the code, and prop the phone against something — autofocus hunting from hand tremor is the usual culprit.
- Turn the sending screen's brightness all the way up.
- Permission denied — tap the browser's permission prompt carefully; if you hit Block by accident, allow camera for the site and tap Start camera again (no reload needed).
- "camera needs a secure context" — the page is being served over plain http. Browsers remove the camera API on insecure origins; serve over https (the dev server already does, self-signed) or use the hosted site.
- Standalone receiver file — opening
decimen-receiver.htmlfromfile://will not get a camera on iOS or Android. See Install & offline.
See the tuning table in Sending — bytes/frame and tx fps are the two levers that matter.