[MPlayer-dev-eng] Re: ccache
Billy Biggs
vektor at dumbterm.net
Mon Jan 6 05:22:27 CET 2003
Arpi (arpi at thot.banki.hu):
> and anyway, what does the whole chroma subsamplin means ?
> there are 4 pixels:
> P1 P2
> P3 P4
>
> is U the average of P1..P4's U values, is the average of P1 & P3's
> average, is P1's U or what?
Chroma downsampling in video is downsampling in the signal processing
sense. So, when you have 4:2:2 video:
P1 P2 P3 P4 P5 P6
C1 C2 C3
Chroma value C2 is from the 2x downsampled version of the full pixel
mesh. If you go by the ITU-R BT.601 digital standard for 4:2:2, then
you need a _really_ wide filter to meet the standard. I have one here
which meets it, and C2 depends on the adjacent 35 pixels in a weighted
average.
If you look at the reference implementation for MPEG2, they have a
filter to convert from 4:2:0 to 4:2:2 that is pretty wide:
/* New FIR filter coefficients (*256): 3 -16 67 227 -32 7 */
dst[w*j2] = Clip[(int)( 3*src[w*jm3]
-16*src[w*jm2]
+67*src[w*jm1]
+227*src[w*j]
-32*src[w*jp1]
+7*src[w*jp2]+128)>>8];
So, each output chroma sample depends on the 6 adjacent chroma samples
vertically. I don't know if this meets the spec for MPEG2 or not
though.
--
Billy Biggs
vektor at dumbterm.net
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