86 What is the MPEG color space and sample precision? (MPEG-2)
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
86 What is the MPEG color space and sample precision? (MPEG-2)
MPEG strictly specifies the YCbCr color space, not YUV or YIQ or YPbPr
or YDrDb or any other many fine varieties of color difference spaces.
Regardless of any bitstream parameters, MPEG-1 and MPEG-2 Video Main
Profile specify the 4:2:0 chroma_format, where the color difference
channels (Cb, Cr) have half the "resolution" or sample grid density in
both the horizontal and vertical direction with respect to luminance.
MPEG-2 High Profile includes an option for 4:2:2 chroma_format, as does
the MPEG 4:2:2 Profile (a.k.a. Studio Profile) naturally. Applications
for the 4:2:2 format can be found in professional broadcasting,
editing, and contribution-quality distribution environments. The
drawback of the 4:2:2 format is simply that it increases the size of
the macroblock from six 8x8 blocks (4:2:0) to eight, while increasing
the frame buffer size and decoding bandwidth by the same amount (33
%). This increase places the buffering memories well past the magic
16-Mbit limit for semiconductor DRAM devices, assuming the pictures are
stored with a maximum of 414,720 pixels (720 pixels/line x 576
lines/frame). The maximum allowable pixel resolution could be reduced
by 1/3 to compensate (e.g. 544 x 576). However, if a hardware decoders
operate on a macroblock basis in the pipeline, on-chip static memories
(SRAM) will increase by 1/3. The benefits offered by 1/3 more pixels
generally outweighs full vertical chrominance resolution. Other
arguments favoring 4:2:0 over 4:2:2 include:
Vertical decimation increases compression efficiency by reducing
syntax overhead posed in an 8 block (4:2:2) macroblock structure.
You're compressing the hell out of the video signal, so what possible
difference can the 0:0:2 chromiance high-pass make?
Is 4:2:0 the same as 4:1:1 ?
No, no, definitely no. The following table illustrates the nuances
between the different chroma formats for a frame with pixel dimensions
of 720 pixels/line x 480 lines/frame.
CCIR 601 (60 Hz) image Chroma sub-sampling factors
format Y Cb, Cr Vertical Horizontal
chroma
format
pixels/
line
Y
lines/
frame
Y
pixels/
line
Cb, Cr
lines/
frame
Cb, Cr
horizontal
subsampling
factor
vertical
subsampling
factor
4:4:4
720
480
720
480
none
none
4:2:2
720
480
360
480
2:1
none
4:2:0
720
480
360
240
2:1
2:1
4:1:1
720
480
180
480
4:1
none
4:1:0
720
480
180
120
4:1
4:1
3:2:2, 3:1:1, and 3:1:0 are less common variations, but have been
documented. As shocking as it may seem, the 4:1:0 ratio was used by
Intels DVI for several years.
The 130 microsecond gap between successive 4:2:0 lines in progressive
frames, and 260 microsecond gap in interlaced frames, can introduce
some difficult vertical frequencies, but most can be alleviated through
pre- processing.
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