On Monday 30 March 2009 01:07:04 pm Krzysztof Duchnowski wrote: > Jesse L. Zamora pisze: > >>> mencoder tv://3 -tv chanlist=us-bcast:norm=NTSC:buffersize=20 -of mpeg > >>> -oac lavc -ovc lavc -lavcopts > >>> acodec=mp2:vcodec=mpeg2video:vbitrate=4100 -o ~/Record.mpg > >>> > >>> When playing the video, it's not freezy or anything, but seeking to a > >>> different time in the video takes several seconds to load. > >> > >> The default keyint is rather large... > > > > Hmm.....setting keyint to 0 seems to make it play like a normal mpeg2 in > > Windows Media Player & VLC, even though at first, there were tons of > > errors or something until I set -mpegopts muxrate=20000! So, it seems to > > work fine now, but I'd like to know, what is the keyint parameter anyway? > > Frames in MPEG based videos are encoded as [I], [P] and [B]. > [I] frames are full encoded frames so they're rather large and needs > alot of bitrate to encode wihtout visible "garbage". > [P] frames are encoded as delta ( difference ) from previous frame and > as such are smaller and needs less bitrate to encode - encoders save > much file size on that. > [B] frames are bi-directional encoded frames ( delta from previous and > next frame ) thus are smallest and saves most bitrate in stream. > > Every stream must start with [I] frame since it have full information > about frame - it's a base; then it follows feew [P] frames or mixture > [P] and [B] frames, then [I], then feew [P] or [P]-[B] mixtures and [I] > and so on. Such pack is caled GOP - GroupOfPictures; and it stores time > code and bit errors beside RAW data. > > So when you seek player cant seek to any other frame than [I] because it > cannot decode any other frame without surounded data. Unless it have > code to use "index" to find nearest [I] frame and decode some data that > will be used as reference to actually seeked frame. So, if you have > "-keyint=250" that means that encoder is allowed to insert this [I] > frames as far as 250 frames from each other ( it could insert it earlier > - it depends on video compexity ) and if you seek to frame other than > [I] and its possition is far away from previous [I] frame then player > needs some time to decode from there to serve you your seek frame. > > MPEG2 DVD compilant stream must have GOP set to one of these value: > 15 - NTSC frame based > 18 - PAL frame based > 30 - NTSC filed based > 36 - PAL field based > Its fixed sequence of [I], [P] and [B] frames. > > As you don't do DVD stream you don't need to use fixed GOP's but if you > don't set "-keyint" to any value it will be used with "250" value ( > default for MPEG4 type video but not for MPEG2 - sane maximum is 300 for > MPEG4 so I don't think 250 is good ) and that means that if you unlucky > and you seek to frame 249 and one and only previous [I] frame is frame > "1" then player must decode every 248 frame in beetwen but as it don't > draw it on screen then it don't need to decode it in sysnc then it will > go little faster but decoding 5s could last 1s before you will see your > seeked position. ( uff ) > > That's why seeking with some large "-keyint" value could be slow. > > So it is a sane value to set "-keyint" to your video FPS value or twice > as much - no more. > > keyint = maximum keyframe interval > "keyint=0" means "keyint=1" means every frame is [I] frame so video will > be laaaarge without any possitve effect from it. > > > PS: It's my understanding of things - any one please correct it Wow, that made perfect sense! Thanks for explaining it. I've tried different keyint values here, and a keyint of 0 seems to work the the fastest (of course). I suppose there's nothing wrong with using 0, right? With lower keyint values, it seems as though the filesize increases, but filesize is not an issue here (since I burn of captured videos to DVD anyway!). Thanks again, Jesse L. Zamora _______________________________________________ MPlayer-users mailing list MPlayer-users@xxxxxxxxxxxx https://lists.mplayerhq.hu/mailman/listinfo/mplayer-users
