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Video review was the most requested feature in our first two years. Clients wanted to comment on a moment, not on a file. This is a short look at how we built frame-accurate comments, and what turned out to be harder than expected.
Timestamps aren't frames#
The obvious approach is to store a timestamp with each comment. It almost works. The problem is that browsers report playback position slightly differently, and a timestamp of 12.48 seconds can land on different frames depending on the video's frame rate and the browser.
For "around the middle", that's fine. For an editor trying to fix a single-frame flash, it isn't.
So we store the frame number, not the time. When a video is uploaded, we read its frame rate and keep it with the version. Comments convert the player's position to a frame, and markers on the timeline convert back.
Variable frame rates#
Most professional video has a constant frame rate. Phone footage often doesn't. If you calculate frames from time on a variable-rate file, comments drift as the video goes on.
We handle this by generating a frame index when a video is processed: a list of exact timestamps for every frame. Seeking to a comment uses the index, not a calculation.
Comparing versions#
When a new cut is uploaded, old comments need to stay meaningful. If the editor removed two seconds near the start, a comment at frame 900 in v2 is about a different moment in v3.
We don't try to move comments automatically. Instead, comments stay on the version they were made on, and side-by-side compare lets the client check whether their notes were addressed.
What's next#
We're working on comments on a range of frames, so clients can mark a whole shot rather than a single moment. It's the second most requested video feature, after the one we just shipped.
