CODEWITHSUNDEEP
html5-videomp4
looselycoupled
looselycoupled

Reputation: 629

MP4 with single video track and multiple resolutions

I have an MP4 which concatenates video from two separate camera streams of different resolutions. This plays back appropriately in VLC, but Chrome and Windows Media Player fails to handle the change in resolution - the second half of the video is totally distorted.

Tools which read an MP4 file and provide technical data all report a single AVC1 video track of resolution #1.

ffprobe shows: Duration: 00:00:34.93, start: 0.000000, bitrate: 825 kb/s Stream #0:0(und): Video: h264 High avc1, yuv420p, 2688x1520 (Resolution #1) , 823 kb/s, SAR 189:190 DAR 15876:9025, 13.42 fps, 15 tbr, 90k tbn, 30 tbc

Deconstructing the MP4 (using onlinemp4parser.com and gpac.github.io/mp4box.js/test), it shows one track atom (trak) which contains one media atom (mdia). further down, inside the sample table description atom (stsd), there are two AVC1 items. The two AVC1 items describe the two resolutions correctly.

e.g. trak -> mdia -> minf -> stbl -> stsd

These tools also show resolution #1 under the track header (tkhd).

Can someone comment on how the final playback resolution is determined? Or why VLC is able to correctly read the second AVC1 block out of the sample description? Is there a different way to express the change in resolution between the samples that browsers will interpret correctly?

Upvotes: 2

Views: 876

Answers (1)

Steve
Steve

Reputation: 11

If anyone knows of a better way please feel free to share. The best way I've found to do this is by ensuring the SPS/PPS (and VPS for H.265) NAL units precede every I frame. This still isn't always perfect because I assume the players just don't expect to handle the the video parameters changing mid-stream.

Here's a quick reference of how some players handle this as of 12/5/2022: Chrome: Regular playback good, seeking initially shows corruption then resolves.

Firefox Regular playback does not resize the player on the transition and makes the right and bottom pixels of video fill the remainder. Seeking shows corruption but will eventually resolve itself, but player still doesn't resize.

Edge Regular playback shows only the top left corner on transition, but will eventually correct itself. Seeking shows corruption, size corrects itself, then just jumps to the end, but remains corrupted.

Media Player Regular playback good, seeking shows corruption (note you must click the spot you want to jump rather than sliding it).

VLC is good in pretty much all cases. It's just much more resilient than most other players.

Upvotes: 1

Related Questions