Screen Door Effect

Discussion in 'Displays' started by Dave Koch, Oct 29, 2003.

  1. Dave Koch

    Dave Koch Stunt Coordinator

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    Hi:

    What exactly is the screen door effect? I had thought I knew, but I saw something while TV shopping, and now I am not sure...

    What I had thought SDE was would be seeing the black around a pixel, and every pixel. Very VERY small, but obviously an annoyance. Is this right?

    What I have seen when TV shopping is this (and I don't know what this is called, could this really be the SDE?) What it looks like are fairly large squares of color, maybe 1/10 or 1/20th the screen height. And they are square. Reminds me of a very compressed JPG image. In fact, that was my first thought- a digital signal, WAY too compressed.

    Could it be the source is split into too many TV's? Is it poor or over compression? Or are these TV's handling the signal poorly?

    Thanks for your help.

    BTW, I am figuring on getting the new Sony GW III. Yes, I know ALL the arguements... and I do like the look of the DLP's, but I see the rainbow. I like the blacks on this MUCH better than any other LCD.
     
  2. Anthony F.

    Anthony F. Stunt Coordinator

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    What you describe sound like SDE--I think it's the gaps between the pixels. On DLP displays the gaps are much small so there much less of an effect. When you say "TVs" I assume you mean LCD monitors or projectors, since normal CRT displays (tube TVs) don't have this problem. The SDE is usually minimized on LCD displays by increasing the resolution (less black space) or increasing the viewing distance. Even with good sources, though, SDE can be visible unless you take steps to reduce it.

    Edit: I should have said that you're initial description--the black around pixels that looks like a grid is SDE. What you describe later could in fact be some sort of compression artifact. Others will have to chime in on that.
     
  3. Dave Koch

    Dave Koch Stunt Coordinator

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    Anthony:

    Thanks!

    >>> When you say "TVs" I assume you mean LCD monitors or projectors, since normal CRT displays (tube TVs) don't have this problem

    Actially, I see something akin to this on my CRT based RPTV- more lines than than boxes, but there is definite space between the "light" to my eyes in all formats, CRT, Plasma, LCD and DLP.
     
  4. Anthony F.

    Anthony F. Stunt Coordinator

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    For a CRT-based RPTV it could be scanlines you're seeing. This would be minimized by a progressive-scan source or a source with line-doubler (if I undestand your description correctly).
     
  5. ChrisWiggles

    ChrisWiggles Producer

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    Screen door is not that large. Screen door is the space between EACH pixel, i.e. there are hundreds of thousands of little pixels. Big blocks of color are not SD, that's probably digital compression it sounds like. Screen door is ALWAYS there, and totally non-changing. Go right up to a digital display, like a plasma, or a DLP or LCD projector, and you will see the grid of pixels.

    RPTVs (crt-based) won't have this, but you will see what you see if you get really close, that is scanlines. Ideally, with CRT projection, the goal is to get rid of scan-line visibility by using a high enough resolution, and you end up with a seamless image.
     
  6. Dave Koch

    Dave Koch Stunt Coordinator

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    Cool- thanks for the SDE 101 class!

    Got the GWIII home this morning. Very pretty but I do seem to have some convergence problems. So I guess I will be getting a new one.
     
  7. Bob Maged

    Bob Maged Stunt Coordinator

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    Another possibility for your CRT based RPTV is that you are seeing the structure of the lenticular lens. I have a 43" Toshiba HD ready RPTV. It has a built in line doubler and even with my eyes a few inches from the screen there are no visible scan lines. However, the structure of the lenticular lens becomes obvious at a too-close viewing distance. Screendoor effect does look as you described it in your original post -- it is like looking through a window screen. For front projectors like my Panasonic PT-L200U, so long as you stay at least 1.5x screenwidth away from the screen SDE is not visible. The much larger blockiness you describe is not SDE, but probably some sort of compression artifacts.
     
  8. ChrisWiggles

    ChrisWiggles Producer

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    ?? I had to go look up what a lenticular lens was, but unless I'm mistaken at thinking that lenticular lenses are sort of like fresnel lenses except with parallel running lens parts. If so I don't know of any RPTVs that have such a device, they use normal, big round lenses. (again, I'm not really an optics/lens knowledgeable person, just that they use "normal" lenses.) These lenses shouldn't have any visible structure, they should be ground and polished well. You are probably seeing scanlines.
     
  9. Bob Maged

    Bob Maged Stunt Coordinator

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    I am referring to the screen, not the CRT lens. One of the layers in the screen stack is a lenticular lens, whose purpose is to maximize image brightness by directing more light straight out the front of the screen. I thought all RPTVs used one.
     

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