Re: [Mjpeg-users] Compare yuvscaler and y4mscaler

[Matto M. <ma...@mi...>]

Hiya,

 >How does its speed compare to yuvscaler when encoding to
 >similar quality (it that is possible anyway)? I have been
 >using yuvscaler with the command line call:
 >
 >  yuvscaler -v 0 -O SVCD -n n
 >
 >as a filter. What would be the scaling method whose resulting
 >quality best aproximates to the yuvscaler call above?

I believe that for downscaling, yuvscaler defaults to its "RESAMPLE" mode,
 which is similar to y4mscaler's "box" kernel.  The other yuvscaler mode
 is "BICUBIC", which should/would be similar to y4mscaler's "cubic" kernel.
 However, last time I checked, it still wasn't implemented quite correctly.
 (I need to re-check again, and record the version numbers when I do it.)

I haven't run any speed benchmarks lately.

So, to answer your second question:

 This is probably equivalent quality to the above yuvscaler command:
   y4mscaler -v 0 -I norm=ntsc -O preset=svcd -S option=box

 And this is definitely equivalent, if not much better, quality:
   y4mscaler -v 0 -I norm=ntsc -O preset=svcd -S option=cubic

 >Do the presets set a good scaling method? Or should I worry
 >about learning them to choose one?

No (and not really)... the current default is 'linear'; a better would be
 'cubic', and even better is something like 'sinc:8'.  I haven't had the
 time/attention yet to do more research into this question, so I haven't
 changed the default yet.

 >I am trying it with the following commands:
 >
 >$ mkfifo -m 660 stream.yuv
 >$ mplayer -noframedrop -vo yuv4mpeg -nosound -v -osdlevel 0 \
 >    the.two.towers.avi -sub the.two.towers.srt -subpos 78 \
 >    -vf expand=:504
 >$ cat stream.yuv |
 >  y4mscaler -O preset=SVCD |
 >  mpeg2enc -v 0 -I 0 -s -f 5 -V 230 -S 800 -a 2 -F 1 -n n -4 2 -2 1 \
 >    -b 2800 -B 284 -q 6 -K FILE=matrix.txt -R 0 -E -11 \
 >    -o the.two.towers.m2v
 >
 >But y4mscaler gives the messages:
 >
 >   INFO: [y4mscaler] Input Stream Header:
 >   INFO: [y4mscaler] <<<   frame size:  672x504 pixels (508032 bytes)
 >   INFO: [y4mscaler] <<<   frame rate:  239759/10000 fps (~23.975900)
 >   INFO: [y4mscaler] <<<    interlace:  none/progressive
 >   INFO: [y4mscaler] <<< sample aspect ratio:  ?:?
 >**ERROR: [y4mscaler] Unknown norm; cannot determine SVCD format.
 >
 >Adding the option "-I norm=NTSC" to the y4mscaler command line does
 >not help (the error message is the same from y4mscaler).

You need to add "-I norm=NTSC" *before* the "-O preset=SVCD"; options are
 evaluated in order.  (This is not clear in the manpage.  I'll fix that.
 In fact, it might make more sense to evaluate all -I options before any
 -O options...)

The 'norm' is inferred from the frame rate (not the frame size).  mplayer
 has provided a lame approximation to "24000/1001", and y4mscaler is looking
 for an exact match.  (Hmm... y4mscaler could be more lenient on the norm
 heuristic; that also goes on the TODO list.)


After that, y4mscaler will probably complain that it doesn't know the
 sample aspect ratio (SAR) of the input --- and, really, it can't do much
 without that.  mplayer should have provided that info; but maybe even
 mplayer doesn't know.  Or, maybe mplayer is rescaling everything, and
 pumping out plain old 1:1 pixels?  Or, mabye not, and the source still
 has a standard NTSC (10:11), or PAL (59:54) SAR.

If this is just some bootleg movie warez, it's probably in pretty sad shape
 as it is, and no one will notice anyway; just say "-I sar=1:1".


Remember:  Digital video is like heroin.  If you want the best quality,
           you must "Know your Source."  Otherwise you get what you get.
-matt m.