search

wpoa / open-access-media-importer

A tool for harvesting media files from Open Access articles for upload into Wikimedia Commons
http://commons.wikimedia.org/wiki/User:Open_Access_Media_Importer_Bot
23 stars 8 forks source link

Limitation/bug of libav/ffmpeg regarding ogg theora videos with large gaps between keyframes #105

Open Daniel-Mietchen opened 11 years ago

Daniel-Mietchen commented 11 years ago

I guess this bug report will have to be propagated to libav/ffmpeg: https://bugzilla.wikimedia.org/show_bug.cgi?id=47465 .

erlehmann commented 11 years ago

I also could work around that by messing with the framerate, making our files bigger. But a) I have no idea what the “large” in “large gaps between keyframes” means and b) I think this is a genuine MediaWiki failure and nothing that should concern us in particular.

http://www.hackerfactor.com/blog/index.php?/archives/307-Picture-Go-Back.html According to this post ffmpeg is unable to accurately seek to frames – meaning that it is unable to reliably find previous frames. This becomes a problem if previous frames are needed to decode the current video frame, as most video frames only hold differences to previous frames.

erlehmann commented 11 years ago

I just generated a lot of thumbnails from https://commons.wikimedia.org/wiki/File:Functional-Impairment-of-Microglia-Coincides-with-Beta-Amyloid-Deposition-in-Mice-with-Alzheimer-pone.0060921.s004.ogv using the following command line:

oggThumb -f $(seq 30 80 | tr '\n' ',') Functional-Impairment-of-Microglia-Coincides-with-Beta-Amyloid-Deposition-in-Mice-with-Alzheimer-pone.0060921.s004.ogv

Not a single one of those shows compression artifacts. The MediaWiki developers could just use the Ogg Video Tools to fix their problem: http://www.streamnik.de/oggvideotools.html

Daniel-Mietchen commented 11 years ago

Could you post that at the MediaWiki bugzilla entry?

erlehmann commented 11 years ago

I don't have an account there. Will make one, then link them to this bug report.