Aspect Ratios (1.33 -> 1.5, 1.78 -> 1.85)

Discussion in 'Archived Threads 2001-2004' started by Brian Dobbs, Feb 3, 2003.

  1. Brian Dobbs

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    Ok something has been bothering me for quite some time now. When a DVD is released with a pan & scan aspect ratio and it's labeled 1.33:1, isn't it really 1.5:1? The resolution of the digital picture stored on the DVD is 720 x 480 which = 1.5:1.

    And another weird resolution quirk I've found is that when a film is released in 1.85:1 it's actually only 1.78:1. For instance, both Blade 2 and The Princess Bride are technically considered 1.85:1, but a couple of screen grabs shows that they each have a resolution of 852 x 480 which = 1.78:1. Check for yourself.

    Blade 2
    http://userpages.umbc.edu/~bdobbs1/blade2.jpg

    The Princess Bride
    http://userpages.umbc.edu/~bdobbs1/princessbride.jpg

    Is there something I'm missing?
     
  2. Vince Maskeeper

    Vince Maskeeper Producer

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    You're assuming square pixels, which TV doesn't use. The 720x480 resolution can be arranged into whatever intended shape- resulting in a non-square application of pixels.

    Many 1.85:1 discs are slightly cropped to 1.78:1, many have a little bit of black at top and bottom to box the 1.85:1 inside the 1.78:1 taget of the format.

    Even anamorphic 1.78:1 stuuf is natively 720x480 on DVD.

    -vince
     
  3. Brian Dobbs

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    What type of pixels does a TV use? And wouldn't what you are saying slightly distort the image? I've always thought that it would be a 1.5:1 image being displayed on a 1.33:1 display unit, like a standard TV. I thought it was done this way because of overscanning problems.

    I see what you're saying about 1.85:1, and the little area of black pixels that sometimes surround the picture. Thanks for clearing that up.
     
  4. Dan Rudolph

    Dan Rudolph Producer

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    Brian, tvs don't use pixels. They use scanlines (at least standard definition ones do). The DVD player converts the pixels into an analog stream for display on the tv. It doesn't distort the image because the image isn't designed to be shown at 1.5:1 in the first place. The 1.5:1 is squashed/stretched from the correct ar and is converted back to the correct shape by the player.
     
  5. Brian Dobbs

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    thanks for the reply Dan, but somehow I'm not fully understanding...

    I know TVs use pixels, but I thought they also use pixels. I think computer monitors also use scan lines. Since I'm watching the disc on my computer monitor, which is analog, how is that any different from a TV besides the fact that it looks better.

    1.33:1 is all a TV can physically display, but the picture being sent to it is slightly wider. This is where overscan becomes a factor. My only conclusion is the source image on a DVD is wider than 1.33:1, possibly 1.5:1, but only the central 90-95% is actually shown on the TV.

     
  6. Vince Maskeeper

    Vince Maskeeper Producer

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    But that's the issue- it isn't "wider".

    Think of it this way:
    If you have 720 pixels of information in a digital form, it can be converted to an analog signal to take up 1 inch wide, or 100 inches wide. As I'm sure you've seen, the pixel number doesn't tie you to a specific screen size. You can buy a 50 inch screen or a 5 inch screen- and it still distributes those same pixels over the space in question.

    So, the same is true for aspect ratio (shape of how these pixels are arranged). If you can cram all 720 pixels across into the width of your set, you're all set. It doesn't care how tall the picture is in comparison- it can arrange the pixels as needed into an analog signal that simply displays in the given shape, full width and full height.

    Just like how you can use the adjustments on your analog monitor to stretch or compress the picture when needed- you're not changing the number of pixels- you're simply adjusting how they're arranged...

    You could use those controls to squash the picture in a 1:2 ratio (tall rectangle)-- and all 720x480 pixels are still there- just crammed into less space. (Heck, if the picture you;re displaying had been created to be displayed as 2:1, it would look perfect in that manner.) It is only because the material is designed to be displayed at 1.33:1 that it would look odd at all.

    The same thing top to bottom. You could use those controls on the monitor to squash the picture in a 1.78:1 ratio (widescreen rectangle)-- and all 720x480 pixels are still there- just again crammed into less space. (Again, if the picture you're displaying had been created to be displayed as 1.78:1, it would look perfect in that manner.) It is only when the material is designed to be displayed at 1.33:1 that such a squishing on your part would look odd at all.

    Either way- it's 720x480 pixels arranged into different shapes as needed!

    So, the material on the disc, despite taking up 720x480 in the digital world- is intended to be shown at a specific shape (again, 1.33:1 dvd and 1.78:1 dvd use the exact same 720x480 pixels)-- it's simply how the display screen arranges those pixels that matters.

    The 720 pixels are simply converted to an analog signal that fills the screen, side to side. Those pixels don't care what size that relates to... it could be a 30 inch wide or 300 inch wide grouping of pixels...

    Hope that makes more sense.
     
  7. Jeff Gatie

    Jeff Gatie Lead Actor

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    Brian,

    Vince is correct. The "pixels" of a tv are not square. Look at it this way - A 4:3 DVD is 720 X 480 pixels displayed in a 4:3 aspect ratio. An anamorphic DVD on a 16:9 screen is 720 x 480 pixels displayed in a 16:9 ratio. Both are the same amount of pixels, they are just displayed differently and neither distort the picture if they are displayed in the correct aspect ratio. Similarly, if you display an anamorphic on a 4:3 (after setting your player to 16:9), it *does* distort everything to be tall and skinny. Play a 4:3 DVD on a 16:9 set in the same mode as anamophic widescreen and everyone is short and fat.

    The 'Y' by 'X' number of pixels has nothing to do with the aspect ratio that is displayed. In fact, it has nothing to do with the perceived resolution without knowing the aspect ration the DVD is designed for. Afterall, displaying 720 x 480 with the pixel 100 times taller that it is wide could not be called "optimum vertical resolution", but it is still 720 x 480.

    Edit - Ooops, Vince got to you first. Hope I helped a little.
     
  8. Brian Dobbs

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    This is an interesting discussion and I'm eager to understand this fully, but still the information at hand doesn't quite add up. If you are saying that a 1.78:1 image is stored on a DVD in a 720 x 480 resolution, than why are my screen captures of Blade 2 and The Princess Bride in a 852 x 480 resolution?

    I should have clarified that i already understood that pixels do not have a fixed physical dimension.

    On a side note...

    I've made VCDs before that contain a lot of my video work that I've done while at college. A standard VCD requires an video file with an MPEG 1 compression codec applied, as well as a resolution of 352 x 240 (a ratio of 1.46:1). When I play the disc at home on my rear projection TV, the image plays fullscreen, and is slightly wider (and slightly taller) because of overscanning.

    my point is that the DVD player nor the TV doesn't seem to be squeezing the picture in any way to form a perfect 4:3 AR. Thus the picture remains at it's original pixel resolution, and is a slightly wider picture than the display unit can display.

    Does that make sense?
     
  9. Alex Spindler

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  10. Brian Dobbs

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  11. Alex Spindler

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    I guess the best way to describe it is that normal televisions have defined vertical ranges (in this case 480 lines) but not so well defined horizontal steps. In that sense, 4:3 material uses the 720 pixel data stream to figure out how to approximate the information. In that sense, each pixel represents 1/720th of the horizontal line. In your computer's case, it is using that to build a 640 pixel image. In your television's case, it is using it to create a horizontal analog line.

    Of course, if it is anamorphic, your computer will use that same 720 data stream to create the 852 pixel wide image.

    I think that's correct.

    *Edit : Yes, computer DVD software successfully detects and converts anamorphic images if you choose to have them displayed that way*
     
  12. MarkHastings

    MarkHastings Executive Producer

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    I'm sorry if I don't explain this 100% correctly (I understand it in my head and it's hard to translate into words)

    The 4x3 relates to rectangular pixels and not square pixels. A pixel on the computer is completely square (i.e. 1x1) whereas the pixels on a tv are rectangular (i.e. 0.9x1)

    Example, if you divide 720 by 480 you get 1.5...why don't you get 1.33? That's because you can not divide two different units like that. (i.e. The 720 is in a width unit of 1 and not 0.9). When I say a video pixel is 1 unit by 1 unit, I'm really saying it's 1 pixel by 0.9 pixels.

    In order to divide 720/480, we must first convert the 720 into a 0.9 width... so 720 times 0.9 = 648 - In all actuallity the number is 640 (hence, the monitor resolution 640x480)

    Now, when you divide 640/480 you get 1.33

    --

    p.s. Think of it this way...A Yard ruffly translates into 3 feet. If I have a box that is 1 yard by 1 foot, the aspect ratio is 1x1. That doesn't make it a square!

    In the computer world (i.e. computer pixels), the box is 3x1 because your computer uses only 1 unit of measure for the height and width).

    Again, 4x3 and 1.33:1 refer to rectangular pixels...and 720x480 refers to square pixels and they can not be comparred directly without conversion.

    Hopefully that made sense [​IMG]
     
  13. Nick_Scott

    Nick_Scott Second Unit

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    Brian-
    Another way to look at it:

    You said you've made VCDs before in 352x240.
    Other standard NTSC resolutions for CDs are:
    352x480 (CVD)
    480x480 (SVCD)
    704x480 (DVD)

    All 3 of those "resolutions" are valid for showing a 4x3 NTSC image.
    Note- The 2nd number has to be 480 because a TV has 480 scanlines (basically).
    The 1st number is 'how much detail' is going on that scan line.
    The "704x480" resolution has twice as much information as the "352x480" for each scanline.
    So, the "pixel" for 352x480 would be twice as wide as 704x480.

    Likewise, your computer will detect the proper ratio and display is properly. (Though my computer won't resample 352x480 and 480x480 on the fly).

    Hope this helps.

    Nick
     
  14. Alex Spindler

    Alex Spindler Producer

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    Good example Nick. I hadn't thought of that angle before.
     
  15. Brian Dobbs

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    Interesting, is there a site someone can provide me that explains the difference between a computer monitor and a TV display? I'm starting to understand a little better now.

    But one thing....

    If a Pan & Scan release is played on a computer at 720 x 480, but converted to 640 x 480 when played on a TV, which is showing the correct aspect ratio?

    Also, there is a DV setting in Adobe Premiere to capture video at 720 x 480. I don't understand why this resolution is selected by default. Can anyone explain this to me?

    sorry one last question...
    What resolution to they broadcast TV programs at? Isn't it something like 704 x 480?
     
  16. Vince Maskeeper

    Vince Maskeeper Producer

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    Digital or analog? Analog signals will vary in "resolution" because it's not always as cut and dry as PC digital signals.
     
  17. Dan Rudolph

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  18. Nick_Scott

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    Brian-
    Here is a link about NTSC video, aspect rations, and resoluitions

    Broadcast TV is usually around 450 pixles per line, VHS is 300, Laserdisc is 550, and DVD is 720. They all have 480 lines.

    This might help- Consider that a PC can not display a DVD image perfectly because the aspect ratio of a PC is 1:1 (meaning it has square pixels).
    So, for a PC to display DVD video, it has to convert the 720x480 image to a 640x480 by removing 80 pixels from each "line". (1 pixel for every 10)

    So, when you watch a DVD (or VCD) you are not seeing the actual image, but a down-converted one. Much like watching an anamorphic DVD on a 4x3 TV. Your not seeing all the information.

    Nick
     
  19. MarkHastings

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    ok, I think I can explain this better. Remember, the 4x3 ratio has nothing to do with the actual number of pixels, but rather, the way those pixels dsiplay on a TV screen.

    First let's look at a comparison of TV (rectangular 1:0.9) pixels and Computer (square 1:1) pixels
    Note: To make things easier, I've multiplied the numbers by 100
    [​IMG]
    After multiplying them by 100, we see that the TV pixels is 90x100 and the computer pixel is 100x100...

    Now let's create a 720x480 picture using these enlarged pixels...Also remember, sometimes we use 720x486 (to create a better 4x3 image even though 6 of those lines will be ignored)

    Anyways, if we create a 720x486 picture using the Computer pixels, we end up with an image that is 72,000 x 48,600:
    720 pixels x the width of 100 = 72,000
    486 pixels x the height of 100 = 48,600
    [​IMG]
    hence, 72,000 divided by 48,600 = 1.48 (roughly 1.5), but remember, this image is in the computer world and that 1.5:1 ratio can not be used as a correct ratio.


    Now if we create a 720x486 picture using the TV pixels, we end up with an image that is 64,800 x 48,600:
    720 pixels x the width of 90 = 64,800
    486 pixels x the height of 100 = 48,600
    [​IMG]
    hence, 64,800 divided by 48,600 = 1.33. Hence, the 4x3 aspect ratio is achieved.

    Remember...The width of both images contain 720 units of color, but since the computer can not display rectangular pixels, it stretches the picture horizontally and can not be used as an effective way to determine aspect ratio...

    Did this help?
     
  20. Alex Spindler

    Alex Spindler Producer

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    A big [​IMG] to this thread. That example was great Mark.
     

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