utility

Practice Mirror

A delayed camera mirror for reviewing sports or dance movements with slow playback and frame-by-frame inspection.

Local processing

About your filesFiles and content selected in this tool are processed in your browser and are not uploaded to a Browser Kitty backend.

Operation demo See the basic workflow in a short video.

About this tool

Practice Mirror shows your camera feed after a chosen delay. Finish a dance or sports movement, look at the screen, and watch what you did moments earlier without repeatedly stopping a recording and switching to a gallery app.

Freeze recent video in Review to slow it down, seek to a point or step through individual frames. Use alignment guides or mirror the live view. Camera frames and Review data stay on the device; save only the footage you want as a silent WebM file.

Good for

Check your posture just after practicing a dance sequence or turn
Look back at a golf swing, practice swing or exercise movement using a short delayed view
Place guides over the screen to inspect how body or equipment position changes
Inspect only a relevant moment in slow motion or frame by frame, saving footage when needed

What it can do

Key capabilities available in this tool.

3, 5, 10 and 15-second delay presets, plus a custom delay from 1 to 30 seconds
Set Review history to 10–180 seconds; review becomes available after about 10 seconds
Review at 0.25×, 0.5×, 1× or 2×, seek, and step one frame backward or forward
Add and move vertical or horizontal guides, flip the view, and switch supported cameras
Fullscreen, optional screen wake lock, and mobile-friendly controls in landscape
Save only a selected frozen Review as a silent WebM video on the device

How to use

Open Practice Mirror on a phone or computer and choose a 3, 5, 10 or 15-second delay, or enter a custom 1–30 second value.
If needed, set a 10–180 second history limit in Review settings before starting the camera.
Choose Start camera and allow browser camera access. After warm-up, the delayed image appears.
Practice while watching the screen. Use guides, mirror mode, camera switching or fullscreen when useful.
After about 10 seconds of history, open Review and inspect the movement with 0.25×–2× playback, seeking or frame stepping.
Use Save clip to download the frozen Review as WebM. Choose Back to practice to discard that Review and begin collecting a new history.

Supported

Delay: 3 / 5 / 10 / 15-second presets or a custom 1–30 second value.
Review history: 10 seconds by default, 10 / 30 / 60 / 180-second presets or any whole number from 10 to 180 seconds. Available after about 10 seconds.
Playback: 0.25× / 0.5× / 1× / 2×, seek and one-frame stepping in both directions.
View: vertical/horizontal guides, mirrored display, supported camera switching, fullscreen and optional screen wake lock.
Export: download the frozen Review as a seekable WebM. No audio, guides or mirrored-view effect is written into the clip.
Japanese/English interface for desktop and mobile. Camera frames are processed in the browser.

Limitations and notes

Camera access requires a secure context such as HTTPS and camera permission. Camera behavior on local file:// pages varies by browser and OS.
The delayed mirror requires WebCodecs (VideoEncoder / VideoDecoder) and requestVideoFrameCallback. It is unavailable in browsers lacking those features.
Review history is limited to 180 seconds. Longer history uses more memory, and sustained load can lead to reduced resolution or processing frame rate.
Review export is WebM only; there is no MP4 export or audio recording. Guides and mirrored-view effects are not embedded in the saved clip.
Back to practice discards the frozen Review and rebuilds the delayed history. Saving a long clip can take roughly as long as its runtime.
The app does not provide AI form analysis or automated scoring. Long camera sessions use CPU and battery power.

Frequently asked questions

How is this different from an ordinary camera preview?

Instead of showing only the live feed, it displays the camera image from 1–30 seconds earlier so you can review a movement right after doing it.

Can I use this on a phone?

Yes, in a mobile browser supporting the required APIs, with camera permission. Camera switching, fullscreen and screen wake lock depend on your phone and browser.

How far back can I review?

The delayed live view supports 1–30 seconds. The separate Review history can retain 10–180 seconds and becomes available after about 10 seconds of footage.

Can I save a Review clip?

Yes. Save the frozen Review as a seekable WebM. It is not MP4 and does not contain audio, overlays or the mirrored-view effect.

Does AI score my technique or posture?

No. There is no pose estimation or automatic scoring. The app helps you inspect your own movements with delay, slow playback, frame stepping and guides.

Are camera images sent to a server?

No. The hosted HTML is downloaded initially, but camera capture, Review and clip creation run in the browser. The app does not request microphone access.

Offline version and source code

The downloadable practice-mirror.html is self-contained and needs no runtime CDN or conversion server; its CSP uses connect-src 'none'. Camera access from file:// pages varies across browsers and operating systems, so use the HTTPS-hosted version if the local file cannot access your camera.

View source on GitHub