On Sat, 19 Mar 2005, Steven Boswell II wrote: > >This makes me wonder: There is a difference > >between a clean, high quality stream from a human > >perspective and from the perspective of an encoder ... > > Yes, there's a difference. A human may not be > able to tell the difference between luminosity > levels 128 and 129, but the computer can, and if
Thanks for reminding me about that section - I was going to respond to that question but was in a hurry and cleaned up the mailbox too soon. Yes indeed there are data sets that to the eye look fine but will not look good when encoded. In fact the 'blocks in dark scenes' problem (which has been the subject of frequent discussion) is a perfect example. The slight variations get made more visible as the luma value becomes lower. We might not see the difference between 125 and 129 but we can see the difference between 16 and 20 I believe. > you have a screen that's all 128-level except for > lots of 129-level dots spread throughout randomly, Or as I've seen in the chroma you'll have values like 125, 127, 129, etc - close to 128 but not quite. The encoder does it's job and the result is a set of blocks that become noticeable as the scene gets dark. A similar thing happens with the luma - in bright scenes a minor variation isn't noticeable but as the brightness decreases those minor variations become visible. The raw DV file looks fine to the eye but the data needs to be processed, sometimes quite heavily, to get the encoded result to look good. > But it'll all look the same to a human > being. From this simple example, one can easily > infer that something that looks the same to a human being does not > necessarily look the same to the computer. In a similar way it should be pointed out that using a computer monitor for making adjustments to video streams is not a good thing at all. Most computer monitors aren't calibrated for one thing. Something that looks good on a computer can be way out of spec and/or look terrible on a TV set. A good first step is to calibrate the computer monitor to 6500K, a second step would be to adjust the response to be NTSC (or PAL)-like but that might cause normal computer activities to look a bit different. The best way of all is to have a production/broadcast monitor but those become very expensive in a hurry - a more economical way would be to use a regular TV and get it close to calibrated using color bars. > y4mdenoise run at very low tolerances ("-z 1 -t 1" > or "-z 1 -t 2") qualifies as such processing. It's also quite a consumer of cpu cycles :-) -t 3 seems to run more quickly and does a nice job. I'm not sure how Canopus is doing the denoising in the ADVC300 but it is quite effective without softening/blurring the image. Still needs to be run thru y4mdenoise BUT since the data is a little cleaner the y4mdenoise program runs more quickly. Quite the winning combination :) Cheers, Steven Schultz ------------------------------------------------------- SF email is sponsored by - The IT Product Guide Read honest & candid reviews on hundreds of IT Products from real users. Discover which products truly live up to the hype. Start reading now. http://ads.osdn.com/?ad_id=6595&alloc_id=14396&op=click _______________________________________________ Mjpeg-users mailing list Mjpeg-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/mjpeg-users