Scroll Video Stutter Diagnosis
Scroll-scrubbed video — where the playback position follows the scroll position — stutters easily. Enter how you built it (method, codec, keyframe interval, scroll handling) and get the likely causes with a fix for each.
About this tool
Scroll-scrubbed video is the effect where a clip or a run of frames advances as the visitor scrolls. How smooth it feels depends on how the file was encoded and how the page reacts to scrolling. This tool applies a set of common rules to the values you enter and tells you which ones are likely to cause stutter.
It is a rule-based check, not a measurement. Real performance depends on the browser and the device, so always test on real hardware too. Everything runs in your browser and nothing is sent to a server.
How it works
- MP4 scrubbing: a keyframe interval over 1 second is flagged as a problem (each seek must decode from the previous keyframe), 0.5 to 1 second as a caution, and 0.5 second or less as fine. H.265 and AV1 are flagged because they may fall back to slow software decoding on older devices.
- Image sequence: the total size is images × size per image. Preloading everything is flagged as a problem above 20 MB; above 10 MB (any loading style) it is a caution. More than 200 images is flagged for memory use.
- Both methods: a width of 3840 px or more is flagged. An unthrottled scroll listener is flagged as a problem, requestAnimationFrame throttling is fine, and IntersectionObserver comes with a note that it does not give you an exact scroll position.
- A keyframe interval of 0 means every frame is a keyframe, which is the smoothest for scrubbing but makes the file much larger.
FAQ
Q. What keyframe interval should I use for scrubbing?
A. As short as the file size allows: every frame (an intra-only encode) is smoothest, and one second or less is a practical target. With ffmpeg the -g option sets the keyframe interval in frames, for example -g 15 at 30 fps.
Q. Why is H.264 usually the safe choice?
A. Almost every device decodes it in hardware. H.265 and AV1 compress better but can fall back to software decoding on older phones and laptops, which makes seeking slow.
Q. Should I use MP4 or an image sequence?
A. Image sequences seek instantly but cost a lot of bandwidth and memory. MP4 is far smaller but needs a short keyframe interval to scrub well. Enter both and compare the results.