On Sun, Jan 18, 2015 at 03:50:11PM +0000, Kieran Kunhya wrote: > > the active area is stored in mpeg1/2 and should be used, its in the > > pan scan variables IIRC, that way there is a very close to correct > > mathematical relation between SAR, DAR and the active area in the > > BT601 case > > There should be no problem here with complying to BT601 and the > > video coding specs > > Close isn't good enough. It has to be right.
yes, i think i understand now what you are talking about let me try to summarize after reading some of these specs NTSC does not seem to specify the active area exactly, or i fail to find it in the specification BT470 does though and being more recent i assume that can be used it specifies the Nominal line period (µs) 63.5555 and the Line-blanking interval (µs) as (10.9 ± 0.2) for NTSC-M this leaves an active area of 52.6555 ± 0.2 micro seconds at a 13.5 Mhz sampling rate that is 710.8492 ± 2.7 pixel as the active area and 486 or 485 lines of that apparently if we assume a SAR 10:11 as specified for 4:3 DAR in H264 that results with 486 lines in 712.8 pixels which is within 710.8492 ± 2.7 pixels and thus correct or with 485 lines in 711.3333 which is also within the tolerance, actually just half a pixel off It appears though the archive.org document ignores the tolerancies and assumes the center of the interval rounded to the next integer or multiple of 2 in case of lines is more correct If someone feels the center is better for analog sources, i wont disagree, but i dont think analog cameras had this level of precission nor does it make sense to round these values if one tries to hit the center with religious fanatism also we do not live in the analog era anymore and the digital formats all choose to pick a convenient point in the allowed tolerance intervall that analog provided. Its a bit like with other units, like the second which was defined based on the period of the Earth's orbit around the Sun until atomic clocks where available when it was redefined accordingly. of course a second before and afterwards do not match entirely exactly nor does the active area in integer pixels of digital video match exactly the midpoint of what analog defined as intended it cant as thats not an integer number of pixels at the sampling frequency the world choose to use and this leads to these tiny differences in the aspect ratio If you think we can improve the precission of handling some material aspect ratio wise or otherwise, lets do it! But please lets work toward improving FFmpeg and not to troll each other, thats just not moving anything forward [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB The misfortune of the wise is better than the prosperity of the fool. -- Epicurus
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