physical scaling differences between video formats

[stravy]

I was not sure where to post this.

I am wanting to take some physical measurements from a video and I was wondering if there are any differences in the vertical and horizontal scale between the mpeg1, mpeg2, mpeg4 and avi?

Thanks

[Guest]

I was not sure where to post this.

I am wanting to take some physical measurements from a video and I was wondering if there are any differences in the vertical and horizontal scale between the mpeg1, mpeg2, mpeg4 and avi? Specifically, what do you mean by "the vertical and horizontal scale"?

[stravy]

basically, if you had a ruler in the horizontal and it measured 12 inches in the real world, and you measured the pixels between 0 and 12 and the number of pixels was 100. Then if you rotated that ruler so it appeared vertical, would it still read 100 pixels?

[Ghitulescu]

Wellcome to the world of Aspect Ratios.

[CWR03]

To get the physical scale of a video, you just check its properties. The file type has nothing to do with it at all. A "real world" comparison will only matter on one specific display and has no relevance to anyone else.

It might help if you describe your purpose to know this information, since it makes little sense the way you've asked it.

[setarip_old]

@stravy

Hi!if you had a ruler in the horizontal and it measured 12 inches in the real world, and you measured the pixels between 0 and 12 and the number of pixels was 100. Then if you rotated that ruler so it appeared vertical, would it still read 100 pixels?Only if the pixels are square (They don't have to be and, more often than not, they're not)

[stravy]

@stravy

Hi!Only if the pixels are square (They don't have to be and, more often than not, they're not)

Ok then, how does that get determined. Does the video file format (avi, mpeg-1,2,4) make the difference? Can you provide a little more detail or point me in a direction where I can find out more.

The most i have been able to find out is that it depends on the video being ntsc or pal, and 4:3 or 16:9.


Thanks

[Ghitulescu]

An inch is a convention (so is the meter) and the pixel is another convention. Unlike the inch/foot/mile/meter, which have the same length all over the Universe (if Einstein was right), the pixel can change its shape and size. For instance an inj-jet-printer having 4800x1200dpi would always print circles and squares, despite we measure 4800 pixels wide and only 1200 length.

I suggest you to read about aspect ratios (if you prefer the engineers' approach), or maybe some neo-kantian philosophy (in case you tend to think that everything we sense around us is just an illusion).

[Wilbert]

Ok then, how does that get determined. Does the video file format (avi, mpeg-1,2,4) make the difference? Can you provide a little more detail or point me in a direction where I can find out more.
What setarip_old means that the answer is affirmative when the pixel aspect ratio is equal to one. The pixel aspect ratio is just a flag in your video stream telling how your picture should be displayed (that is whether it should be stretched upon display or not). If the pixel aspect ratio is not equal to one it means that the image itself is shrunk or stretched (which is always the case for dvd's) and that this should be undone upon display to view it correctly. It is supported by most video streams and also containers, but of course it can be set incorrectly.

@Ghitulescu,
An inch is a convention (so is the meter) and the pixel is another convention. Unlike the inch/foot/mile/meter, which have the same length all over the Universe (if Einstein was right), the pixel can change its shape and size.
Of course you can see it that way, but a pixel is just a dot. It has no shape or size.

For instance an inj-jet-printer having 4800x1200dpi would always print circles and squares, despite we measure 4800 pixels wide and only 1200 length.
There is no such thing as 4800x1200 dpi ( http://www.rideau-info.com/photos/mythdpi.html ). The dpi tells you how it should be printed. So when printing a 4800x1200 pixels image at 300 dpi, means it will be printed in a rectangle of 16 x 4 inch. When printing it at 1200 dpi, means it will be printed in a rectangle of 4 x 1 inch.

[Didée]

Wilbert, Ghitulescu was referring to the physical resolution of an inkjet printer. 4800 dpi horizontal / 1200 dpi vertical is common technical data. It does exist. ;)

[Ghitulescu]

Of course you can see it that way, but a pixel is just a dot. It has no shape or size.
Guess what, any FullHD TV is exactly 1920 pixels (x3 as 3 colours) wide and 1080 pixels height. So divide the physical measurements of the screen by 1080 or 1920 and you'll end with pixels having a certain height or wide. Isn't it this what less educated people call size?
There is no such thing as 4800x1200 dpi ( http://www.rideau-info.com/photos/mythdpi.html ). The dpi tells you how it should be printed. So when printing a 4800x1200 pixels image at 300 dpi, means it will be printed in a rectangle of 16 x 4 inch. When printing it at 1200 dpi, means it will be printed in a rectangle of 4 x 1 inch.
You have me completely misundestand. I was talking about printing a square or a circle with a printer that has, as usual, different scaling factors for XoY. I said that one needs to print 4800 pixels wide but just 1200 pixels height in order to have a perfect square or circle. Because Wpixels are not the same as Hpixels.

Welcome again to the world of aspect ratios.

[Wilbert]

Guess what, any FullHD TV is exactly 1920 pixels (x3 as 3 colours) wide and 1080 pixels height. So divide the physical measurements of the screen by 1080 or 1920 and you'll end with pixels having a certain height or wide. Isn't it this what less educated people call size?
That's different for different displays. It would be a bit strange to say that the size of a pixel on my monitor is different from the size of a pixel on your monitor. You don't see pixels directly on your screen: http://www.alvyray.com/memos/6_pixel.pdf .

You have me completely misundestand.
You are right, i'm sorry.

[Ghitulescu]

I understand myself what you are trying to say, too.
There's a certain truth in what both papers you linked to: but this is only for pixel-mapping (I don't know if this is the correct term) for the first link, and in analog - digital interfaces for the second (or, generally speaking, the discrete mapping of non-discrete entities, also called rendering).
Both links support my assertion that a pixel is a convention and further detail how this convention was made and for what purposes.
However, the discussion, although fruitful (thank you again for both links), won't help the OP, being too technical. I think the OP should understand first the difference between a container (AVI) and a codec (MPEG-1, MPEG-2 and MPEG-4) then to arrive in the world of pixel mapping.