Pixel Aspect Ratio (PAR)
A pixel is a pixel, it is a small block of color information. However, some playback devices have different shaped pixels than others. Computer monitors have square pixels and hence everything that is designed for display on a monitor should have a PAR of 1. TVs however have ‘rectangular pixels’ which have a different PAR depending on the format (NTSC or PAL).
Display Aspect Ratio (DAR)
In mpeg video this is often stored as a flag that basically says “this is the aspect ratio that you should display me at”. There are two possible Display Aspect Ratios (DARs) on a DVD - 4:3 and 16:9. TV shows will generally be 4:3 and movies will generally be 16:9 unless they are pan&scan or letterbox (see below). Movies with a 16:9 DAR are what we refer to as Anamorphic (or sometimes listed as “optimised for widescreen TV” etc).
Sample Aspect Ratios(SAR)
Now, as if Pixel Aspect Ratios and Display Aspect Ratios weren’t enough, now we have another one to throw into the mix. Sample Aspect Ratios were introduced with the MPEG4 container. As such, they have absolutely nothing to do with DVDs! Since you might want to distribute your AMV as an MPEG4 though, it is worth learning what they are. Sample Aspect Ratios are actually very simple to understand - they are just a little hard to calculate. Fortunately, you probably wont ever need to calculate an SAR by hand!
The SAR basically just says “stretch the video by this ratio”. So lets say for example, we have a 704x480 video (we took a dvd, and simply cropped off those annoying bits on the sides). If we want this video to be displayed at an aspect ratio of 4:3, then it would need an SAR of 10:11.
Why 10:11? Just look:
(70410)/(48011) = 4/3
See, it is easy to understand, right? If we wanted that same 704x480 video to be displayed at an aspect ratio of 16:9, then we would use an SAR of 40:33, as (70440)/(48033) = 16/9.
But like I said, you wont need to be calculating these manually, so all you really need to worry about is just understanding how the SAR differs from the PAR and DAR.
ffprobe output
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Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'c434ae5e-4cfb-4a21-a558-627b166ebcb4.mp4':
Metadata:
major_brand : mp42
minor_version : 0
compatible_brands: isommp42
creation_time : 2021-12-30T07:52:52.000000Z
com.android.version: 10
Duration: 00:00:05.80, start: 0.000000, bitrate: 2293 kb/s
Stream #0:0(eng): Video: h264 (Constrained Baseline) (avc1 / 0x31637661), yuv420p(tv), 1280x720, 1541 kb/s, SAR 1:1 DAR 16:9, 29.33 fps, 29.42 tbr, 90k tbn, 180k tbc (default)
Metadata:
rotate : 90
creation_time : 2021-12-30T07:52:52.000000Z
handler_name : VideoHandle
vendor_id : [0][0][0][0]
Side data:
displaymatrix: rotation of -90.00 degrees
Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 193 kb/s (default)
Metadata:
creation_time : 2021-12-30T07:52:52.000000Z
handler_name : SoundHandle
vendor_id : [0][0][0][0]