A specialised view of the video
Ordinary encoded video can depend on neighbouring frames. Independently coded frames provide useful access points because they do not require the same chain of picture prediction. HLS can advertise an I-frame stream separately from the normal playback variants.
The I-frame media playlist uses EXT-X-I-FRAMES-ONLY. Its EXTINF durations describe the time to the next I-frame in the relevant presentation, or to the end for the last entry, as defined in the specification. They are not a promise that a still image will remain on screen for that duration during normal viewing.
Preview cadence is not source frame rate
Suppose a hypothetical 25-frame-per-second programme provides one selected I-frame every two seconds for trick play. A ten-second span can then have five such preview positions while the full video contains about 250 frames. Counting previews would badly understate the programme's frame rate.
Similarly, sparse previews do not prove dropped frames in ordinary playback. The preview track serves another purpose and may use different dimensions or packaging. Assess its responsiveness separately from the main stream's motion and audiovisual continuity. Keep the selected preview position and normal playback position separate in a report.
What a preview failure tells you
If normal playback works but scrubbing previews fail, the associated I-frame playlist and its media dependencies are relevant checks. Range delivery and initialisation information can matter for these resources even when full-motion segments are healthy.
The interface might also simply lack a preview feature, despite a valid advertised track. Record the player, chosen action and whether seeking itself succeeds. Distinguish an unavailable thumbnail from an inability to jump to another time, and avoid promising a trick-play feature solely because a manifest contains an I-frame declaration.
Questions about this guide
Are preview images the same as all programme frames?
No. An I-frame trick-play track selects access points rather than representing every displayed frame.
Does a valid I-frame playlist guarantee thumbnails?
No. The player must implement and expose the relevant feature.