Understanding Pixels On Pixels

Discussion in 'Displays' started by Ken Burkstrum, Sep 15, 2005.

  1. Ken Burkstrum

    Ken Burkstrum Stunt Coordinator

    Joined:
    Dec 19, 2003
    Messages:
    149
    Likes Received:
    0
    I just realized im not really sure how this works. If I take a DVD that is 720x480p and watch it on a 1280x720p screen, whats really happening? Am I watching 720x480 pixels or am I watching 1280x720 pixels that were stretched or created/duplicated by a 720x480 source? Would watching 720x480 on a 720x480 display loko better then watching on it on a 1280x720 display? LCDs and DLPs and Plasmas have a fixed pixel number right, so what determines the pixel limit a Tube TV or rear projection TV with the same technology. What makes one a Tube TV without a high definition tuner not capable of HD while another Tube TV without a tuner is capable of HD if you get one?
     
  2. John S

    John S Producer

    Joined:
    Nov 4, 2003
    Messages:
    5,460
    Likes Received:
    0
    With analog displays it is pretty much wide open. HDTV CRT based displays will actually only give you about 960 scan lines on average, some less some more, but they can still display 1080i HD and do a darn good job of it too.
    With digital displays it called scaling, everything is scaled to the panel res for display.

    I'm not sure if I helped answer the question though.
     
  3. Leo Kerr

    Leo Kerr Screenwriter

    Joined:
    May 10, 1999
    Messages:
    1,698
    Likes Received:
    0
    All digital displays have some sort of scaler - as you noted yourself, they have a fixed number of pixels of a fixed size.

    Note also that NTSC video is 720x480 with a 4:3 aspect ratio. That gives you a pixel aspect ratio of 0.9:1. Your DLP / LCD / Plasma display has a pixel aspect ratio of 1:1 - square pixels, which vaguely translates to 640x480 for the same image.

    Yes, pixel math gets really screwy - and we're not even talking about anamorphic video.

    Direct view CRTs are limited by the shadow mask, unless they're a trinitron, in which case they're limited horizontally by their aperature grill, and vertically by their minimum electron-spot size and steering magnets.

    Projection (FP or RP) 3-tube CRT sets use three monochrome displays. There is no shadow mask. Therefore, maximum resolution is determined by the spot size, steering magnets, power supply, beam control, and all sorts of things. All else being equal, a larger tube will give you a better picture (able to display finer detail.)

    As for what determines the limits of a 3-tube display - it's the electronics. If the sweep generators and steering systems can't scan faster than 60Hz vertically, and 15kHz horizontally, you're stuck with SD.

    Pixel math is strange; repeat that.

    Leo Kerr
     
  4. ChrisWiggles

    ChrisWiggles Producer

    Joined:
    Aug 19, 2002
    Messages:
    4,791
    Likes Received:
    1

    Also remember that with bigger tubes in a projection CRT, you need to feed an appropriately high resolution. A 7-inch CRT may look great with 480p, but put 480p on a 9-inch well setup CRT, and it will resolve so well that it will look like garbage because you should really be feeding a display like that something like 1080p.

    Leo has covered it pretty well.

    Remember this as well: in theory the best way to view any video is scaled up to infinite resolution, however there is diminishing returns. Viewing SD at resolutions like 720p or even 1080p will look much better than at its native 480 resolution. The end quality does depend a lot on the quality of the scaling though, so not all video displayed at upscaled resolutions is equal. And upscaled video played back at HD resolutions is not going to be as detailed as HD sources.
     
  5. Allan Jayne

    Allan Jayne Cinematographer

    Joined:
    Nov 1, 1998
    Messages:
    2,404
    Likes Received:
    0
    If you watch a regular DVD (720 x 480) on a 1280 x 720 HDTV, the very best you can see will be 720 x 480 worth of picture detail spread out over 1280 x 720 pixels. The amount of picture detail you see can be less for the reasons stated earlier.

    In order to construct the 1280 x 720 video frame, some scan lines must be duplicated or must be interpolated from neighboring scan lines (same for pixels within a line). Interpolation usually gives a smoother picture especially where there are diagonal lines or edges.

    The nature of the display will also affect the picture quality. The gaps between pixels of a 720 x 480 LCD display will probably be more noticeable than the gaps between pixels of a 1280 x 720 LCD display showing the same subject matter.
     
  6. Ken Burkstrum

    Ken Burkstrum Stunt Coordinator

    Joined:
    Dec 19, 2003
    Messages:
    149
    Likes Received:
    0
    So your saying, at the very best watching a DVD on an HD LCD will look just as good as watching it on an SD LCD?

    Lets try the reverse, what about watching 720p on a 480p TV, what's happening and what is the result? Will 720p source viewe don a 480p TV look better then 480p on a 480p TV. If it does, how far does that extend to, i.e. 1080p, 1200p, 1600p, 1920p etc.

    Does any of this disapprove of the idea that video viewed on a smaller screen will look better then video viewed on a bigger screen, as far as distance? Or to be better describe it, im under the impression that 720p will look better if I shrink the video down instead of watching it full screen, is that true or is it absolutely the same until a certain extent where a large screen shows 720p's imperfections?
     
  7. Allan Jayne

    Allan Jayne Cinematographer

    Joined:
    Nov 1, 1998
    Messages:
    2,404
    Likes Received:
    0
    Watching 720p on a 480p TV. The best quality you can get out of the 720p show will be slightly less than the same show televised as 480p.

    To convert 720p to 480p some scan lines have to be dropped. Better results are usually had by blending or commingling or interpolating the content of a to-be-dropped scan line with its neighbors rather than dropping the scan line outright.

    Because 480 does not divide evenly into 720, there will be some scaling artifacts when the 720p show is digested down to 480p. What were once evenly spaced identical horizontal or vertical lines will not longer be evenly spaced, or some of the lines may no longer be as crisp as the others. This is not always noticed because live visual material rarely contains very crisp uniformly spaced lines.

    **************

    If something needs doing, get it done. The done well part can come later.
     
  8. ChrisWiggles

    ChrisWiggles Producer

    Joined:
    Aug 19, 2002
    Messages:
    4,791
    Likes Received:
    1

    This is unrelated to the question of scaling, however the quality and resolution of the source, together with the quality of the display, will lead to a determination of an appropriate viewing angle. A high-quality display and source will support a much larger viewing angle than a lower quality one. I don't understand what you mean when you say "shrink the video down."
     
  9. Ken Burkstrum

    Ken Burkstrum Stunt Coordinator

    Joined:
    Dec 19, 2003
    Messages:
    149
    Likes Received:
    0


    I mean on a computer screen, watching it in a window.
     
  10. ChrisWiggles

    ChrisWiggles Producer

    Joined:
    Aug 19, 2002
    Messages:
    4,791
    Likes Received:
    1
    No, that is definitely not preferred, because you are essentially reducing the resolution of the display by only using part of it. If you repeated the same viewing angle of the original full image by moving yourself closer to the screen, the image would get worse because you are reducing the resolution. Conversely, if you stay where you are and shrink the image, you may not be impacted by the loss in resolution, because by staying put you are decreasing your viewing angle; all you are doing is making the image smaller.

    Further, a windowed image will usually ahve very odd resolutions that are difficult to scale to, almost guaranteeing scaling artifacts will be more severe than a full-screen video playback with a properly chosen resolution that is an easily-scaled resolution related to the source.
     

Share This Page