Despite of ever-growing number of people working with digital
video formats daily, there is still a great deal of confusion
regarding how their image geometry and aspect ratios actually
work. This document tries to shed some light on these issues.
Feel free to e-mail me
any comments, corrections, suggestions, additions or opinions.
Should you come across a broken link, please let me know so I can
My warm thanks go to Colin Browell, Andy Furniss, Ole Hansen, and Paul
Keinanen, and Olafs who have provided valuable comments and feedback
concerning this page.
You are free to link to this document. If you do so, please
use the URL <http://www.iki.fi/znark/video/conversion/>.
This ensures that the link will always work, regardless of the
actual physical location of this site.
There is a fair number of mind-blowing, scary oddities and secrets in the world of
One of the very first a beginner will usually encounter is the fact
that in digitized video data, pixels are often not considered "square"
in their form. In most real-world digital video applications pixels have a width/height
ratio – or aspect ratio, as it is more conveniently
called – that can be something completely different from 1/1!
The second great revelation usually comes when one runs into
the concept of anamorphic
16:9 video for the very first time. If it was initially hard to grasp the idea of
pixels changing their shape when displayed in different environments, this one is even more
baffling: the very same pixel resolution you have only just learned to
associate with 4:3 displays can now suddenly represent another, totally
different image geometry. In other words, the pixels have changed their shape again!
Unfortunately, these two are often the only things
most ordinary people will ever learn about digital video and
Tutorials and manuals usually tend to keep very quiet and
secretive about the finer technical details of digital video,
particularly when it comes to the topic of (pixel) aspect ratios
and image geometry.
Even if converting (resampling) video clips to other resolutions is discussed, the accompanying explanation is
usually troublingly simplistic and vague – often inaccurate and misleading
– and sometimes the suggested methods are just plain wrong. It is not uncommon that the examples only deal with arbitrarily chosen ("x
pixels by y pixels") frame dimensions and use ideal frame aspect ratios
such as 16:9 or 4:3 as the basis for calculations – not the actual pixel aspect
ratios – which is usually a good indicator that the writer may not
actually take the real image geometry into account at all.
It is almost as if the whole aspect ratio issue was considered some sort of dirty little secret of
the video industry; black magic you
could not even begin to explain to mere mortals in reasonable terms. This is a shame. In this case, there is really more to it
than meets the eye. Confusing people with incomplete and watered-down
explanations does not do any good to the industry.
Now that you
have read this far, it is time to reward your effort with The
Third Big Revelation about aspect ratios and frame sizes - the
one that is usually left unsaid:
Not a single one
of the commonly used digital video resolutions exactly
represents the actual 4:3 or 16:9 image frame.
Shocking, isn't it? 768×576, 720×576, 704×576, 720×480,
704×480, 640×480... none of them is exactly 4:3
or 16:9; not even the ones you may conventionally think
as "square-pixel" resolutions.
So there. Now you finally know the truth. Let's find out what it actually means.
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Edited: Dec 27, 2015 by
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