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.
--
What would you do if you woke up in a police state?
African-Americans wake up in a police state every day.
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