Determining the Shape of Pixels and Frames

  • Thread starter Thread starter Bob [MVP]
  • Start date Start date
Yes, been to that page several times in my search for DV-AVI pixel aspect
ratio specification.

"... AVI files do not contain pixel aspect ratio information,..."
 
Yes, been to that page several times in my search for DV-AVI pixel aspect
ratio specification.

"... AVI files do not contain pixel aspect ratio information,..."
Click to expand...

You're right. The file itself doesn't contain that
information. But the standard for converting from
analog 4:3 NTSC to 720x480 DV results in a specific
nonsquare pixel aspect ratio.

Sorry, but I just don't understand why this is such
a difficult concept to understand. It's a matter of
simple geometry and simple arithmetic.

The aspect ratio of a pixel is equal to the frame's aspect
ratio divided by the digital resolution (expressed as a
ratio of the number of horizontal pixels divided by the
number of vertical pixels) of the frame.

For NTSC DV-AVI the frame's aspect ratio is 4:3. The
digital resolution of the frame is 720x480. This
results in a pixel aspect ratio of:

(4/3) / (720/480) = 0.8888... = 1:1.125

So each pixel is slightly taller than it is wide.
HTH...

--
-Bob
_______________________________
Microsoft MVP
Windows XP Media Center Edition
http://www.microsoft.com/ehome
 
It is not a difficult concept understand. What is difficult to understand
is that some people think DV-AVI pixel aspect ratio is specified and is
always non-square, when in fact (so far as I have been able to determine)
DV-AVI pixel aspect ratio is neither specified nor required to always be
non-square. They are just pixels.

For example if I have a digital video camera with a field aspect ratio of
1.5 (3:2) and captures 720x480 pixels (3:2) and import that into a DV-AVI
file, what is the pixel aspect ratio going to be? Are they square or
non-square?

--

Al Stu - MVP
If your world is flat, be careful not to step over the edge.

Copyright © 2005, Al Stu (2348410). All Rights Reserved.
 
The DV standard (originally known as DVC for Digital
Video Cassette) specifies the resolution for converting
analog NTSC and PAL television video to digital video.

Using NTSC as an example, that standard specifies a
frame aspect ratio of 4:3. The DV standard specifies
a resolution of 720:480. I'd tell you to do the math,
but I've already done that. ;-)

We have been talking about the *standard* DV format
used by Movie Maker -- not some arbitrary non-standard
digital video camera format, as you described in your
example below. Right?

It seems that while I have been referring to resolutions
and aspect ratios specified in the NTSC and DV standards,
you seem to be referring to DV more generically as a
video encoding algorithm.

I think we both "get it"... :-)

--
-Bob
_______________________________
Microsoft MVP
Windows XP Media Center Edition
http://www.microsoft.com/ehome
 
I'm certainly not going to hold my breath on that. Especially coming from
someone who just in the past few days tried to defend the now debunked
theory of image resampling being the cause of the fuzziness discrepancy
between DV-AVI and WMV. You certainly demonstrated your lack of even basic
understanding there by just blindly following your "leader". My guess is
that if you actually do get it, its only been within the past few hours.

--

Al Stu - MVP
The world may be round, but I'm still living on the edge.

Copyright © 2005, Al Stu (2348410). All Rights Reserved.
 
Lets see now. I spent several years working in the video
industry as a developer, for the worlds largest maker of
professional non-linear video editing hardware & software.

Follow my "leader"????... LOL! But that's exactly the
type of response I'd expect from you when you are proven
to be dead wrong (AGAIN!)...

Do you still think that Movie Maker is NOT a non-linear
editor???? LOL!!! Do you STILL think that DV-AVI is
NOT compressed???? LOL!!! Do you still think that ALL
pixels are square???? LOL!!!

Bye bye John Kelly... Oh, sorry. I meant "Al Stu - MVP"

--
-Bob
_______________________________
Microsoft MVP
Windows XP Media Center Edition
http://www.microsoft.com/ehome
 
LOL,

So if your so video/image smart why would you try to defend such a theory
that even a rank amateur could see through?

I have never claimed any of that. Making such false claims and assumptions
only shows your true colors.

--

Al Stu - MVP
If your world is flat, be careful not to step over the edge.

Copyright © 2005, Al Stu (2348410). All Rights Reserved.
 
Defend what theory? That quality is lost when the resolution
of an image is changed from 640 to 720 pixels wide, then
displayed at 640 pixels wide? That's not a theory. That's a
fact.

At least you correctly identified yourself as a rank amateur.
Bye bye John... er ... I mean "Al Stu - MVP".

--
-Bob
_______________________________
Microsoft MVP
Windows XP Media Center Edition
http://www.microsoft.com/ehome
 
So now you claim to not even understand what the discussion was about.

--

Al Stu - MVP
If your world is flat, be careful not to step over the edge.

Copyright © 2005, Al Stu (2348410). All Rights Reserved.
 
While everybody else told me that Al Stu is JK himself, I thought to give
you some benefit of doubt. But there is only one person who can deny the
existence of non-square pixels altogether as you seem to be claimng now...
and that is the man himself. So I am thinking everybody else was right...

Please do us a favor, give us the link to your MVP public profile... (or the
mvp-only one).

For example if I have a digital video camera with a field aspect ratio of
1.5 (3:2) and captures 720x480 pixels (3:2) and import that into a DV-AVI
file, what is the pixel aspect ratio going to be? Are they square or
non-square?
Click to expand...

Yes the pixels would still be non-square ... (boo!!!)

If you had read the blog entry of Dean Rowe about image sizes (and
understood it) you wouldnt have made the stupid assumption that a 3:2 aspect
input image would come out as exactly 720x480 when saved by Movie Maker in
DV AVI format. When such a DV AVI video is displayed in Media Player the
720x480 input image is mapped to an area of 640x426 within the frame of size
640x480 with black borders on top and bottom. Just try it and capture the
running video in Media Player. Why are there black borders? According to
your assumption it should fit the output frame of 720x480 completely and
exactly. Why is it 640x480?

Frame this on every wall in your room:

*The*displayed*frame*of*DVAVI*video*(NTSC
4:3)*in*Media*Player*would*be*640x480 **** and*not*720x480.

640x480 (boooooo!!!!!)

--
Rehan
MS MVP -- Digital Media
www.rehanfx.org - get transitions and effects for Windows Movie Maker
https://mvp.support.microsoft.com/profile=da9b0032-ad3f-4a96-ace7-66b26095ebac
 
Please do not make false accusations you can not prove. As I have
previously stated I am not John Kelly and for people to say or imply that
shows their true colors. Furthermore, I have never denied the "existence of
non-square pixels altogether". What I have said is that DV-AVI does not
specify pixel aspect ratio, neither square nor non-square.



I never said anything about Media Player. That is twisting what I said.



Bill Birney and David Workman
Microsoft Corporation
May 2003



"The only disadvantage is that AVI files do not contain pixel aspect ratio
information, so many players, including Windows Media Player, render all AVI
files with square pixels."



http://www.microsoft.com/windows/windowsmedia/howto/articles/PixelFrames.aspx



But how Windows Media Player renders it really isn't even part of the
question, I just threw this in as an extra since you mentioned WMP.




--

Al Stu - MVP
The world may be round, but I'm still living on the edge.


Copyright © 2005, Al Stu (2348410). All Rights Reserved.
 
"The only disadvantage is that AVI files do not contain pixel aspect ratio
information, so many players, including Windows Media Player, render all
AVI files with square pixels."
Click to expand...

Please read this statement with open mind. What it is saying is that

* AVI file does not contain info regarding pixel aspect ratio
* This absence of of info makes it difficult to display the contents
correctly
i.e. When rendered with the assumption that pixels are square (by some
players), it may get displayed incorrectly.

This clearly implies that the pixels in DV AVI were *not* encoded as square
pixels to start with. Otherwise what is the problem?

DV AVI files for NTSC would always contain 720x480 pixel frames. Similarly
for PAL the frame would always be 720x576. However these strange frame
ratios can cater for the source content that is 4:3 as well 16:9

Why is it so difficult for you to understand that a 4:3 or 16:9 frame cannot
fit into a 720x480 frame area without stretching of some kind. This
necessary stretching would mean that pixels wont map 1-to-1. This non 1-1
mapping means that each pixel in DV AVI needs to encode color information
from multiple source pixels. There exist many algorithms for resolving this
encoding: point sampling, bilinear sampling, weighted average sampling...
etc. The algorithm used would determine how good the content looks but it
would always be slightly less in quality and sharpness than for 1-to-1 pixel
mapping.

Note that the article you mention was written in 2003 and since then we have
two major vrsion upgrades of Windows Media Player and I beleive that WMP10
is capable to show the DV AVI with correct pixel adjustment adjustment (even
when pixel aspect info is not provided in the file. This is simple maths...
not magic).
 
Like I said Media Player is not even part of the question, just threw that
in since you mentioned WMP. If think the document is out of date then let
Microsoft, and their two authors know so they can updated with current
information or publish new document. As for that specific quote I have
cursorily tested it in WMP 9 and it does appear to still be correct. As for
WMP 10, like a lot of people I didn't like it and rolled back to WMP 9.

Still twisting what I said.

--

Al Stu - MVP
The world may be round, but I'm still living on the edge.

Copyright © 2005, Al Stu (2348410). All Rights Reserved.
 
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