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I just noticed something odd that I don't understand. If you take a screen shot from a DVD on your PC, the resulting image is always 720x480, which is not surprising since this is DVD resolution. This result is the same whether the DVD is full screen or anamorphic widescreen.


However, what's strange about this is that DVD has 2 aspect ratios, 4x3 and 16x9. 720x480 is neither of these.


4x3 = 640x480

16x9 = 854x480


In order for the resulting 720x480 image to match the aspect ratio of the DVD, you must manually adjust the horizontal size of the image downward to 640 for 4x3 or upward to 854 for 16x9.


How can a 720x480 DVD image display as a 4x3 AND 16X9 aspect ratio when it's neither? I'm sure there's a simple answer to this, but I don't know what it is.
 

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Quote:
Originally Posted by Dixie Flatline
Non-square pixels, no?
There are no pixels on a DVD, just color information that decribes the color of the pixel. It's up to the scaler and/or display to handle rectangular pixels if they exist, eg 1280x768.


larry
 

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Standard definition digital video is not based around square samples is the simple answer. The 720x480 ("NTSC") and 720x576 ("PAL") sampling schemes are based on sampling the analogue video signals (whether 4:3 or 16:9 - they are identical electronically) at a standard sampling rate - 13.5MHz. The systems were based heavily on the analogue TV systems - and are digital versions of analogue component video. They include syncs and blanking, to allow real-time conversion easily, even though for most digital gear syncs and blanking are irrelevant.


In PAL terms - a single 625 analogue line lasts 64us including syncs and blanking, with the active video carried in 52us, whether it contains 16:9 or 4:3 video. (The numbers are slightly different for NTSC)


In standard def broadcast digital video systems the samples are not square in either 4:3 or 16:9 - TV gear etc. isn't limited to square "pixels" - unlike most PCs. Another way of thinking about it is to imagine the samples have an aspect ratio of their own (i.e. they aren't 1:1) - and that in 4:3 and 16:9, and PAL and NTSC, the samples (or pixels if you must) have differing aspect ratios. Imagine a 16:9 sample being wider than a 4:3 one!


PCs have to scale the 720x480 or 720x576 to 4:3 or 16:9 square pixel resolutions if they are running in a square pixel mode - which most are - but normal analogue output players don't - they just convert the 720 samples back to an analogue signal...


(** NB the 720x480 and 720x576 sampling schemes are actually based on images slightly wider than 4:3 or 16:9, to allow for a little bit of leeway in multiple analogue/digital converts. In PAL land a 4:3 or 16:9 image is actually 702x576 - not 720x576. There are 9 extra samples each side to allow for timing error. The 702x576 scales to 1024x576 in square pixel 16:9, with the 720x576 scaling to 1050x576 in the same square pixel land)


(** Also NB, 4:2:0 MPEG is not based around RGB pixels, it is based around 720x480/576 luminance samples and 360x240/288 colour difference samples - so the luminance resolution and chrominance resolution are not the same. Easy to cope with if you think in YCrCb and samples, more difficult to cope with if you think in terms of RGB and pixels)
 

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Quote:
Originally Posted by PooperScooper
There are no pixels on a DVD, just color information that decribes the color of the pixel.
Amazing. You contradicted yourself in the same sentence!


Of course DVDs store pixel information (but MPEG compressed).


Ed
 

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Quote:
Originally Posted by ekb
Amazing. You contradicted yourself in the same sentence!


Of course DVDs store pixel information (but MPEG compressed).


Ed
Well, I see how you can say that. I should have added that the color has no bearing on the dimensional representation of the pixel when it is displayed - which is what I meant. A datapoint of color with no shape inferred. Happy now?


larry
 

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Quote:
Originally Posted by PooperScooper
Well, I see how you can say that. I should have added that the color has no bearing on the dimensional representation of the pixel when it is displayed - which is what I meant. A datapoint of color with no shape inferred. Happy now?


larry
Ok, but the original topic had to do with pixels and A/R and nothing to do with color. You seemed to say that there was no concept of a pixel array to represent a frame.


So I think that the simple answer is that a frame is represented by a pixel array, but the A/R of each pixel is NOT square (implicitly assumed by OP) but can be of 2 different values depending on whether the video is anamorphic or not.


Ed
 
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