pixel shape ?

Discussion in 'Amateur Video Production' started by marks542004, Jan 19, 2005.

  1. marks542004

    marks542004 Guest

    I have seen discussions in some computer graphic forums where the
    argument is about the image size to create for use in video for burning
    to DVD.

    The issue seems to be that TV uses a square pixel and computer monitors
    use a rectangular pixel (or vice versa). Therefore a tv image on a
    computer should look stretched on a computer and squashed if computer
    generated to TV.

    Can anyone clarify or point me to a resource ?
     
    marks542004, Jan 19, 2005
    #1
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  2. You've pretty much summed it up in your post here. And, yes a tv image
    looking at it on a monitor will look stretched.

    http://www.lurkertech.com/lg/pixelaspect.html
    http://www.adobe.com/support/techdocs/322300.html
    http://www.larryjordan.biz/articles/lj_pixels.html

    Also.. you can adjust for this in PhotoShop CS too. It has the ability
    to work in non square environment.

    -Richard
     
    Richard Ragon, Jan 19, 2005
    #2
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  3. marks542004

    nap Guest

    Are you sure that's right? Not every video format is rectangular, so..
    And I don't think you can simply state that all computer monitors are
    rectangular
     
    nap, Jan 19, 2005
    #3
  4. That's true.. HD uses square pixels. However, I don't know a single
    monitor that doesn't use square pixels.. Are there?

    -Richard
     
    Richard Ragon, Jan 19, 2005
    #4
  5. marks542004

    FLY135 Guest

    It isn't the monitor, it's the resolution. Given that the monitor is
    usually 4:3, it depends on the resolution as to what the pixel aspect
    ratio ends up being. For example, 1024x768 on a 4:3 monitor results in
    square pixels. However, 1280x1024 on the same monitor does not result
    in square pixels. Many plasma displays that are 16:9 run at various
    resolutions that don't result in square pixels.
     
    FLY135, Jan 19, 2005
    #5
  6. marks542004

    marks542004 Guest

    I have been using a computer for years for preparing computer generated
    lead-in and titling and have never really noticed a problem. I started
    questioning it because of comments that you needed to compensate for
    it. I used to do 800 x 600 avi files and changed by request to 720 x
    480. 720 x 480 seems to be a standard for video editing software but
    is not a 4:3 ratio.

    I will have a look at the sites listed

    thanks .
     
    marks542004, Jan 20, 2005
    #6
  7. marks542004

    nap Guest


    There are many sites which discuss how to generate graphics in square pixel
    apps for rect. pixel displays. google is your friend.
     
    nap, Jan 20, 2005
    #7
  8. The 720 x 480 matrix (or 720 x 576 for PAL) is the standard for putting
    video images on DVD. So, the way you have been working is correct. These
    standard matrices are not 4:3 but are converted by the DVD player to a 4:3
    video signal. Even 16:9 widescreen images are recorded in this 720 x 480/576
    matrix and then converted by the player to a 16:9 signal. In the last case
    you would therefore need to prepare your graphics for NTSC in say 853 x 480
    and shrink them horizontally to 720 x 480 to be included in the video.
    However, you have to make sure that the editing software imports images on a
    pixel by pixel basis and doesn't do any conversion itself.
     
    Lou van Wijhe, Jan 20, 2005
    #8
  9. marks542004

    Rockinsax Guest

    dv video is 720x480, when creating graphics in Photoshop for use in
    that timeline, create them 720x540 as PS uses square pixels versus the
    non-square DV.
     
    Rockinsax, Jan 20, 2005
    #9
  10. I'm not particularly convinced that just because a screen resolution is not
    4:3 the pixel is not also square. I do see the logic in that the true 4:3
    ratio would be 1280x960 rather than 1280x1024. This is true the 1280x1024 is
    not 4:3. I had conversation on another forum with someone having trouble
    with the Edius interface on a single LCD monitor only capable of 1280x1024
    max. It seems due to the natural way the program opens is meant for dual
    monitors and it was frustrating him. I did a reshuffle of the interface
    within 1280x1024 for him and sent this link for hime to see:
    http://digitalvideosolutions.com/photos/Edius.jpg

    After reading this thread on 4:3 ratio and resolutions, I wanted to see what
    the true pixel shape was of this screen cap. I used a generic screen capture
    program and told it to capture the full screen. I made no other changes to
    the natural way it captured the image, other than to tell it to save in the
    JPG format at the highest quality possible. I just went to Photoshop CS,
    which has a pixel ratio management module, to see what it said about the
    pixel ratio of that image. Photoshop CS reports it has 'square pixels' in
    its' natural state. Check it for yourself.

    If the assumption holds true that using ratios other than 4:3 changes the
    pixel ratio it surely would show in this images' properties. My assumption
    is that the pixel stays square regardless of the screen resolution because
    the screen itself has no pixels to be squared or rectangled in the first
    place, it is only the resultant image made from or in those resolutions on
    the computer which are generated in square pixel. Just for grins I would try
    some screen caps in all sorts of odd-ball resolutions to see if the pixel
    every changes.

    With digital video it is the device that creates the native pixel 0.9. On
    the computer most software read and translate to square. What the original
    postee was asking about was obviously DV because analog video is square
    pixel when capture with non-DV ready equipment, and DV video does wide on
    the computer screen due to the native pixel ratio of DV.

    As for what the original person asked, there are seveal ways to combat the
    differences in pixel shape. One I have seen used is this, if you generate a
    graphic which is a true 4:3 at 720x540 then squeeze this into the confines
    of the DV framework 720x480 0.9 pixel it should look absolutely fine when
    output on a television screen. You can test the theory for yourself by
    creating this size image with a circular object in the center of the screen,
    and another using 720x480 square pixel. Place them side by side and create a
    looping video of them, then output them to a miniDV tape or as a squence
    onto a DVD rewritable for playback on a television monitor for comparison.

    Adobe Photoshop CS allows you to change the pixel from square to D1/DV 0.9
    just for reference.
    --
    Larry Johnson
    Digital Video Solutions

    http://www.digitalvideosolutions.com
    877-227-6281 Toll Free Sales Assistance
    386-672-1941 Customer Service
    386-672-1907 Technical Support
    386-676-1515 Fax
     
    Digital Video Solutions, Jan 20, 2005
    #10
  11. marks542004

    FLY135 Guest

    I think your talking apples and oranges here. Just because an image is
    reported as say... 72dpi, that doesn't mean that it is actually 72 pixels
    per inch on the screen.

    The OP was talking about the fact that pixel aspect ratios vary between
    computer oriented images and tv oriented images. This is correct. Square
    pixels on computer displays occur when both the monitor is 4:3 (vs. say wide
    screens that are now available) and the display resolution is set to a 4:3
    aspect (like 800x600). However you can monkey with the sizing controls and
    make this visually incorrect. You can also make it visually incorrect by
    selecting a non 4:3 resolution if you are assuming that the pixels are
    square no matter what.
    This only means that the software does not take into account the screen
    resolution and apparently thinks that the computers pixels are by default
    square.
    If a tree falls in the forest and no one is there to observe it, did it
    really fall?
    It simple math... If your screen display is measurably 4:3 and your
    resolution is not 4:3, then an image that is square pixel will not display
    correctly unless your software knows your display is 4:3 and uses the
    current resolution to rescale the display window to correct for the non 4:3
    resolution.
    Sorry I cannot parse this statement.
    screen,

    Yes, the math would indicate this to be true (480/540 = 0.88888). Which is
    the same as (480 *4)/(720 * 3) = 0.8888.
     
    FLY135, Jan 21, 2005
    #11
  12. marks542004

    Ken Maltby Guest

    Has anyone mentioned the utility called "Aspect", yet?
    www.aspect.fre3.com (turn on your pop-up stopper
    first)

    Luck;
    Ken
     
    Ken Maltby, Jan 21, 2005
    #12
  13. Not once did I bring the dpi of a photo into the discussion. Since a DV
    frame has no dpi then bringing dpi into the mix is a moot point.
    Though it is possible to use programs such as Photoshop CS to alter the
    pixel shape of an image, it still holds true that no matter what the screen
    resolution a capture of that computer screen to a bitmapped image will in
    fact have square pixels regardless of the resolutions of the computer screen
    proper. Take any screen capture program like Solid Capture and set your
    monitor to any resolution you wish. Set the program to capture the entire
    screen in matching resolutions. Now, get any image editing program and look
    at the properties - it says square pixels.

    By using your earlier logic of dpi - "Just because an image is reported as
    72dpi doesnt mean it is actually 72 pixels per inch on the screen" - gee
    whiz! that was a true lightning bolt of knowledge by the way! Anyway, using
    that logic then just because an image editing program report square pixels
    does not mean the image actually has square pixels. Why not take that logic
    a little further? Just because all editing programs say DV is 0.9 pixel
    doesn't mean it is right. The only monkeying around here is your pretzel
    logic in attemtping to convince everyone a monitors' resolution changes the
    pixel ratio of the resultant captured image of that very screen resolution
    no matter what resolution is chosen!
    Look, you aren't some zen master, and yes the tree falls whether anyone is
    there to see it or not. If it didn't fall why is it on the ground to be seen
    in the first place? The true question is not about a falling tree and
    whether someone was there to observe it. Where have you been? The question
    is "If a tree falls and no one is there to hear it does it really make a
    sound?"
    Duh, the math behind the squeeze. You have proven you can use a calculator.
    Thank you for that.
     
    Digital Video Solutions, Jan 23, 2005
    #13
  14. marks542004

    FLY135 Guest

    The 72dpi reference had to do with the fact that the attributes of an
    image don't necessarily match what you see on the screen. The point
    being that you seem to be advocating that if a program such as
    Photoshop says an image is square pixel then it must be true. If
    Photoshop captures an image that is being displayed correctly in a
    screen resolution that isn't square pixel and then it stores an
    attribute with the image that says it is square pixel, then Photoshop
    is not giving you the correct information.

    If my comments seem "zen like" and I seem to be using "pretzel logic",
    it's because I didn't know how to respond to the assertion that screens
    don't have pixels and therefore what you see doesn't have to match what
    Photoshop tells you. The resultant image is solely dependant on the
    screen aspect, resolution aspect, the image's pixel aspect, and the
    ability of the application to take those into account and properly
    rescaling the image for display.

    And yes, I can use a calculator. But knowing how to use a calculator
    doesn't mean you can actually calculate anything. You don't need an
    editing program to tell you the aspect of a DV pixel, you need to know
    how to formulate the equation. And that equation takes into account
    the screen aspect and resolution aspect. If they don't calculate to
    square pixels on a properly displayed image, then the pixel aspect
    isn't square no matter what Photoshop says.
     
    FLY135, Jan 23, 2005
    #14
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