User guide

Aspect ratio in digital video refers to the fraction of the height and
width of pixels measured from the video’s resolution. This does
not measure the actual amount of pixels or length, so two videos
with different resolutions could have the same aspect ratio.
The most common aspect ratios are 4:3 (universally used for
standard-definition) and 16:9 (universally used for high definition).
The Pixel Aspect Ratio, or PAR, measures the actual height and
width of the pixels. The Display Aspect Ratio, or DAR, refers more
specifically to the shape of the image frame in a video. Unlike PAR,
this can result in a different shape than the pixels would otherwise
indicate. One common reason this is used is in Anamorphic
encoding, where the pixels of a video are stretched over a larger
frame size when displayed. So a digital video file that has a resolution
of 720x480, but is flagged with a DAR of 16:9, will play back in
widescreen without changing the pixel count. This is often what
causes videos to show with letterbox or pillarbox.
Incorrect DAR
for standard definition
(16:9 video on 4:3 display)
Correct DAR
for widescreen
(16:9 video on 16:9 display)
Letterbox occurs when
DAR is wider than display
Pillarbox occurs when
DAR is narrower than display
DivX 10
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DivX, LLC User Guide