What a frame measurement represents
The video frame angle tool reads geometry from pixels in one decoded frame. In joint mode, the first point is the vertex and the other two locate the limbs or segments. Edge mode treats four points as two mechanical line directions. Every saved record keeps its timestamp, angle, label, mode, and optional note, so a linkage position or classroom example can be compared across time without sending the video to a server.
Use the timeline for broad seeking and the step controls for smaller changes. MP4 and WebM still depend on the codecs installed in the browser. MOV is enabled only when the current browser reports a decodable QuickTime media type. The approximate frame buttons move by 1/30 second because the standard video element usually does not expose a trustworthy source frame rate.
Mark a readable instant in the motion
- Choose a local video and wait for its duration and pixel dimensions to load; rejected codecs produce a compatibility message rather than a blank claim of support.
- Pause near the required movement, then use 0.1, 0.5, or 1 second steps—or the explicitly approximate frame step—to isolate a clear image.
- For a joint, place the vertex first and the two landmarks next. For a mechanism, place both endpoints of edge one and then both endpoints of edge two.
- Label and save the timestamp. Seek back to a saved record to review it, hide marks when the frame is crowded, and compare the unsmoothed time chart after two or more readings exist.
- Export the raw frame when pixels alone are needed, annotated PNG for visual review, CSV for timestamp comparison, or JSON for the complete local session.
Motion blur, perspective, and landmark placement
- Fast motion spreads an edge across several pixels. Prefer a sharper neighboring frame and record uncertainty instead of choosing an arbitrary blur boundary.
- Camera perspective shortens segments that point toward or away from the lens, so a two-dimensional video angle is not the same as a three-dimensional biomechanical joint angle.
- Place anatomical landmarks consistently at visible joint centers. Clothing, occlusion, and soft-tissue movement can make that placement subjective.
- For a machine, mark rigid edge centers far apart; short point spacing magnifies pointer error.
- Compare timestamps captured from the same camera position. A camera move can change apparent angles even when the subject does not.
Worked frame examples
- Mechanical linkage: save the included angle at 0.400 s and 0.900 s, then use the straight-segment chart to compare only the marked states.
- Sports lesson: mark hip, knee, and ankle in a paused side view to discuss visible knee flexion while keeping the result separate from clinical or three-dimensional analysis.
- Classroom demonstration: step through a swinging ruler, save its angle against a fixed edge, and export the annotated frame for a motion discussion.
- Technique review: use consistent labels such as setup, contact, and follow-through so CSV rows remain interpretable outside the page.
Why frame stepping is approximate in a browser
HTML video provides current playback time and decoded frames, but ordinary browser APIs do not consistently expose the source frame rate or guarantee frame-accurate seeks across codecs and keyframes. The frame buttons therefore state their 1/30-second approximation. Use the timestamp as the reproducible reference and inspect adjacent steps when the exact decoded frame matters.
- Keyboard Space controls playback only while the tool has focus.
- Left and Right use approximate small steps; Shift uses 0.5 seconds.
- The chart connects saved observations and never invents intermediate measurements.
Not medical diagnosis or professional motion capture
This page measures projected pixel geometry. It does not reconstruct depth, correct lens distortion, detect anatomical landmarks, identify a reliable frame rate, provide medical diagnosis, or replace calibrated professional motion-capture and biomechanics systems. Large videos remain local, but decoding them can still use substantial device memory.
Replacing or removing a video revokes its temporary Object URL, pauses playback, clears the overlay, and releases the media element. The file is not written to a query string or uploaded by the tool.