What do SAR and DAR mean in FFmpeg video metadata?

Tech QnA
1

I have been working on video transcoding using FFmpeg. When I try to get metadata of transcoded video using ffprobe -show_streams command, I get the following output.
[STREAM]
index=0
codec_name=vp9
codec_long_name=Google VP9
profile=Profile 0
codec_type=video
codec_tag_string=[0][0][0][0]
codec_tag=0x0000
width=1920
height=1080
coded_width=1920
coded_height=1080
closed_captions=0
has_b_frames=0
sample_aspect_ratio=1:1
display_aspect_ratio=16:9
pix_fmt=yuv420p
level=-99
color_range=tv
color_space=bt709
color_transfer=unknown
color_primaries=unknown
chroma_location=unspecified
field_order=unknown
refs=1
id=N/A
r_frame_rate=30/1
avg_frame_rate=30/1
time_base=1/1000
start_pts=0
start_time=0.000000
duration_ts=N/A
duration=N/A
bit_rate=N/A
max_bit_rate=N/A
bits_per_raw_sample=N/A
nb_frames=N/A
nb_read_frames=N/A
nb_read_packets=N/A
DISPOSITION:default=1
DISPOSITION:dub=0
DISPOSITION:original=0
DISPOSITION:comment=0
DISPOSITION:lyrics=0
DISPOSITION:karaoke=0
DISPOSITION:forced=0
DISPOSITION:hearing_impaired=0
DISPOSITION:visual_impaired=0
DISPOSITION:clean_effects=0
DISPOSITION:attached_pic=0
DISPOSITION:timed_thumbnails=0
TAG:language=eng
TAG:DURATION=00:00:47.600000000
[/STREAM]

As you can see in the above metadata, their sample_aspect_ratio and display aspect ratio, i am struggling to understand the difference between those values.

2

Conceptually, there are three terms DAR (display aspect ratio), PAR (pixel aspect ratio), and SAR (storage aspect ratio), these terms are used interchangeably to refer to/denote video resolution creating confusion sometimes.

DAR: Display aspect ratio or aspect ratio as we usually refer to it, is the proportional relationship between the width and the height of the display.

PAR: Pixel aspect ratio is a mathematical ratio that describes how the width of a pixel in a digital image compares to the height of that pixel. Most digital devices display an image as a grid of tiny, square pixels so for that kind of device PAR will be 1:1. But some devices display an image as a grid of rectangular pixels, in which the pixel width and height are different. The pixel aspect ratio describes this difference.

SAR: Storage aspect ratio is the ratio of pixel dimensions when an image/video is displayed on a particular device. If an image/video is displayed with square pixels then PAR and SAR are the same but for rectangular pixels, the PAR and SAR differ.

FFmpeg denotes PAR as SAR in their metadata information, so what you see as the value of SAR (sample_aspect_ratio) is the PAR (pixel aspect ratio) for the video.

3

Got it!
Thanks, Vatsal!