Revision: Shorter sentences, better consistency, no extra 'lesson' about how to minimize ratios.\ Formatted for email, plain text.
DAR (display aspect ratio [1]) [noun]: 1, The width-to-height ratio (W:H, e.g. 16:9, 4:3) for the intended display. DAR is 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 (which it turns upside down): "height divided by width"; also H.262 §6.3.3 (i.e. "3 ÷ 4", "9 ÷ 16"). PAR (picture aspect ratio [1]) [noun]: 1, The horizontal-to-vertical size [3] ratio (H:V, e.g. 5:4, 3:2) for pictures. PAR is distingushed by metadata: 'horizontal_size' & 'vertical_size', [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 by 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 (which it turns upside down), and by implying that metadata: 'aspect_ratio_information', is also such a quotient (which it also turns upside down), H.262 causes much confusion that helps explain why so many Internet sites get DAR, PAR, and SAR wrong. SAR (sample aspect ratio [1]) [noun]: 1, The physical horizontal-to- vertical spacing ratio (H:V) for samples [2][3]. SAR is distinguished as a computed value: DAR/SAR. 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 [4] [3] Ideally, SAR would also be the width-to-height ratio of the sampling aperture, but that is not mandatory. [4] 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) [5] and sample spacing should be 32 µm horizontally by 36 µm vertically [6]. [5] (345,600 samples)/(480 rows). [6] (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. [7] Criticism: H.262 claims that SAR is a "ratio", then, as it does with DAR, wrongly defines it as a quotient (which it turns upside down). -- What if you woke up and found yourself in a police state? African-Americans wake up in a police state every day. _______________________________________________ ffmpeg-user mailing list ffmpeg-user@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-user To unsubscribe, visit link above, or email ffmpeg-user-requ...@ffmpeg.org with subject "unsubscribe".