转个东西：
http://forum.doom9.org/showthread.php?s=&postid=562048====================================================================================
Assuming
DAR = display aspect ratio - x/y ratio of physical dimensions (e.g inches or cm - NOT pixels) of display screen
PAR = pixel aspect ratio - x/y ratio of the physical dimensions of a single pixel
SAR = storage aspect ratio - x/y ratio of the #of pixels being encoded, always known from the file
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MPEG-4
DAR = PAR x SAR, where
PAR is based on "aspect_ratio_info" in the Video Object Layer
For aspect_ratio_info values 1 - 5,
PAR = 1:1, 12:11, 10:11, 16:11, and 40:33 respectively.
If aspect_ratio_info = 15, PAR is the custom value:
"par_width" / "par_height"
where par_width & par_height are any two relatively prime integers
between 1 and 255. Those two values are stored in the VOL as well.
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MPEG-2
Based on "aspect_ratio_information", stored in the Sequence Header
If aspect_ratio_information is "1", PAR = 1:1, and therefore DAR = SAR
If aspect_ratio_information is "2", "3", or "4", DAR = 4:3, 16:9 or 2.21:1 respectively,
in which case PAR = DAR/SAR
However (this is relatively rare):
If Sequence Display Extension is present and display_horizontal_size
and display_vertical_size are present and differ from the SAR,
then their ratio will replace SAR in the above equation.
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MPEG-1
Based on "pel_aspect_ratio" located in the Sequence Header
DAR = PAR x SAR
where PAR = the reciprocal of 1.0000, 0.6735, 0.7031, 0.7615, 0.8055,
0.8437, 0.8935, 0.9157, 0.9815, 1.0255, 1.0695, 1.0950, 1.1575 or
1.2015 for pel_aspect_ratio values of "1" thru "14" respectively.
However:
0.9157 is commonly accepted to mean 11/12 or .9166, the reciprocal of 12/11 (1.091) and,
1.0950 is commonly accepted to mean 11/10 or 1.1, the reciprocal of 10/11 (0.909)
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