On Oct 24, 2005, at 2:10 AM, Adrian Miles wrote: > i haven't seen michael's latest guide but if it still suggests > forcing keyframes at the rate of every n frmaes then yes, your > results will be poor in terms of final file size. H.264 and 3.ivx > both support natural keyframes. enabling this is much better and > usually leads to smaller file sizes with little or no loss in quality.
Yeah, my latest tutorial has people set the keyframes to every 75 frames as per the 3ivx website: <URL: http://www.3ivx.com/support/mac/encoding/ quicktime.html#force_keyframe > "Force keyframe after ... delta frames In normal operation the encoder will insert keyframes for scene changes or where a keyframe would result in a more efficient encode. But in order to promote random access it is sometimes necessary to insert keyframes more regularly. The codec will insert a keyframe if it has outputted this many non-keyframes in a row. By setting this value to 0 you can produce an all keyframe video, which could be usefull for an editing format, similar to DV. Keyframes are also usefull for streaming broadcasts, as a user can normally only begin watching a stream after receiving a keyframe, if the stream was 25fps, and keyframes were only inserted after 250 frames (worst case) then the user might have to wait 10 seconds before seeing the stream (worst case). 300 is the default value for normal video tasks, for streaming we recommend a keyframe every 5 seconds (fps times 5)." In the past I've recommended forcing keyframes frames for a couple of reasons. If you're using Apple's mpeg4 codec it will prevent the random blocky frame. Videos by JD Lasica come to mind here. If you watch some of his videos you will see a decent image become really blocky for one frame every 10 - 15 sec. By forcing the keyframes to something like every 5 this problem goes away without a large increase in file size when compressing at around 600kbits/sec. Now, I know that's much higher than you like and forcing the keyframe makes a larger difference in terms of file size at lower bit rates. FWIW, a file with keyframes every 5 frames (or 75 frames for that matter) is much more responsive when scrubbing. Also my recommendation for forcing keyframes stems from trying to come up with a "one size fits all" setting. I generally don't vary the compression of my clips on a clip by clip basis. Forcing the keyframes helps ensure that videos that are more difficult to encode - lots of motion, busy backgrounds - look good without any extra effort by me. If I was trying to compress videos in the 200kbps range I'd definitely have to give up forcing keyframes and, in my opinion, give up a lot of quality depending on the complexity of the clip. Here are two files encoded with 3ivx, keyframes set to the default (every 300 frames). This first one is not too bad - 214kbps <URL: http://michaelverdi.com/codec/small1.mov > The second one has lots of motion and a very busy background - 205kbps - looks pretty bad. <URL: http://michaelverdi.com/codec/small2.mov > Both of these video look much better - the second one especially - at 705kbps <URL: http://michaelverdi.com/codec/big.mov > <URL: http://michaelverdi.com/codec/3ivx.mov > Now I think this is the biggest place where you and I differ - what's an acceptable bit rate. In my mind, I'm designing video for boadband and 700kbps is perfectly acceptable. I would like for my video to look even better but I feel this is an adequate compromise. The image is pretty good (I often get complements about this) and will play with fast start within a few seconds on any broadband connection. Certainly others use even higher data rates without problems. For example, Rocketboom uses 3ivx settings that are similar except that Andrew doesn't limit the data rate. He leaves it set to automatic so that it uses whatever is necessary to produce a quality file. So he'll have an episode like the one on Oct 5 that has almost constant motion and busy backgrounds, clock in at 1431kbps and an episode where Amanda sits at the desk, like October 13, clock in at 793kbps. Ultimatly, I think it's a matter of preference and looking forward, I don't see video delivery via the internet heading in the direction of 200kps. I tell people that videoblogging requires broadband for both production and consumption. If we were all still designing things (websites included) for 56k modems, the web would look a lot different than it does now and my 4 min (200kbps) video would have to load up for 3 minutes before it started playing. -- Verdi <URL: http://michaelverdi.com/ > <URL: http://freevlog.org/ > <URL: http://node101.org/ > ------------------------ Yahoo! Groups Sponsor --------------------~--> Get Bzzzy! (real tools to help you find a job). Welcome to the Sweet Life. http://us.click.yahoo.com/A77XvD/vlQLAA/TtwFAA/lBLqlB/TM --------------------------------------------------------------------~-> Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/videoblogging/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
