[FFmpeg-user] Glossary: DAR, PAR, and SAR

[Mark Filipak (ffmpeg)]

I seek your comments regarding the following glossary entries (that have been reformatted to fit 
email). Kindly take your time.

-Mark.

DAR (display aspect ratio [1]) [noun]: 1, The intended display's
   width-to-height aspect as a minimized, H:V, integer ratio (e.g.
   16:9, 4:3) distingushed by metadata: 'aspect_ratio_information'
   (also see "SAR", note [2]). 2, H.262 §3.44: "The ratio of height
   divided by width (in spatial measurement units such as centimetres)
   of the intended display" [2].
   [1] It's mistakenly asserted by some that "DAR" is an acronym for
       "data aspect ratio" or "disc aspect ratio".
   [2] Criticism: H.262 claims that DAR is a "ratio", then wrongly
       defines it as a quotient: "height divided by width"; also H.262
       §6.3.3 (i.e. "3 ÷ 4", "9 ÷ 16").

PAR (picture aspect ratio [1]) [noun]: 1, The metadata ratio:
   'horizontal_size':'vertical_size' [3], as a minimized, H:V, integer
   ratio (e.g. 5:4, 3:2) [2][4] (also see "SAR", note [2]).
   [1] It's mistakenly asserted by some that "PAR" is an acronym for
       "pixel aspect ratio".
   [2] PAR can also be calculated from DAR & SAR thusly: PAR = DAR/SAR.
   [3] Note that PAR is virtual (i.e. defined by dataset indexes, not
       physical dimensions).
   [4] Criticism: H.262 doesn't define PAR, however, it does define a
       quotent that correlates with PAR, to wit: H.262 §6.3.3,
       aspect_ratio_information:
         "SAR = DAR × horizontal_size/vertical_size".
       The foregoing implies that H.262 would have defined PAR as
       vertical_size/horizontal_size. Opinion: By defining DAR & SAR as
       quotients that turn the standard ratio definitions on their
       heads, and by implying that metadata:
       'aspect_ratio_information', is also such a quotient that also
       turns the standard ratio definition on its head, H.262 causes
       much confusion that helps explain why so many Internet sites get
       DAR, PAR, and SAR wrong.

SAR [noun]: 1, Sample aspect ratio [1][2], the physical horizontal-to-
   vertical sample spacing [6] as a minimized, H:V, integer ratio [3].
   2, H.262 §3.114: "This specifies the relative distance between
   samples. It is defined (for the purposes of Rec. ITU-T H.262 |
   ISO/IEC 13818-2), as the vertical displacement of the lines of
   luminance samples in a frame divided by the horizontal displacement
   of the luminance samples [2]. Thus, its units are (metres per line)
   ÷ (metres per sample)." [7].
   [1] It's mistakenly asserted by some that "SAR" is an acronym for
       "storage aspect ratio".
   [2] A standardized set of picture sizes & aspects has been
       established:
                  display     DAR     picture     PAR    SAR = DAR/PAR
     16:9-2160: 3840 x 2160  16:9 : 3840 x 2160  16:9 :  1:1
      4:3-2160: 2880 x 2160   4:3 : 2880 x 2160   4:3 :  1:1
     16:9-1080: 1920 x 1080  16:9 : 1920 x 1080  16:9 :  1:1
      4:3-1080: 1440 x 1080   4:3 : 1440 x 1080   4:3 :  1:1
      16:9-576: 1024 x 576   16:9 :  720 x 576    5:4 : 64:45
       4:3-576:  768 x 576    4:3 :  720 x 576    5:4 : 16:15
      16:9-480:  853 x 480   16:9 :  720 x 480    3:2 : 32:27
       4:3-480:  640 x 480    4:3 :  720 x 480    3:2 :  8:9  [3]
   [3] Example: If a 35mm film area (0.906 x 0.680 inches) is to
       produce 345,600 samples (visual density) with 480 rows (vertical
       resolution), then each row must have 720 samples (horizontal
       resolution) [4] and sample spacing should be 32 µm horizontally
       by 36 µm vertically [5].
   [4] (345,600 samples)/(480 rows).
   [5] (0.906 in)(25400 µm/in)/720 by (0.680 in)(25400 µm/in)/480 = 32
       by 36 µm = 32:36 = 8:9 aspect ratio.
   [6] Ideally, SAR would also be the width-to-height ratio of the
       sampling aperture, but that is not mandatory.
   [7] Criticism: H.262 claims that SAR is a "ratio", then, as it does
       with DAR, (wrongly, in the opinion of many) defines it as a
       quotient.

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