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Who uses keystone?

Discussion in 'Projectors, Screens & Video Processors' started by uk_robk, Mar 20, 2003.

  1. uk_robk

    uk_robk
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    Whilst setting up my PTAE100 I've found that I am forced to use keystone correction if I want a true rectangular image, because my PJ is on a low coffee table projecting at a slight upward angle onto the screen.

    However, I've noticed that with my HTPC signal the image is astonishingly crisp with a keystone of 0, but for any other value the image degrades.

    So I was wondering if everyone else puts up with the slight downgrade in picture quality, or do you folks all setup your PJ and screen at the perfect heights so you don't need keystone?

    I have to admit during films the lower picture quality isn't as apparent, except with many continuous lines (ie a fence or a brick wall). It's only with the windows desktop that the difference is really noticable!

    Any thoughts?

    Cheers,
    RobK
     
  2. John_N

    John_N
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    For maximum image quality you should not be using keystone correction at all. I personally have my projector set up at the correct location.

    You will experience image degradation because in order to make the image square, the projector will distort the image into a parallelogram "opposite" in shape to the distorted image on your wall that results if the projector is not square to it.

    In other words, if your projector is on a coffee table pointing upward, the image will be wider at the top than at the bottom. This will be corrected by having an image on the panel that is wider at the bottom than at the top. This means that 100% of the panel area is no longer used, meaning you lose resolution, and also it means that the distortion is non-linear, having various undesirable effects on the image.

    You pays your money and you takes your choice, as they say. You are effectively trading convenience for image quality and there is nothing wrong with that as long as you are happy with it.

    J
     
  3. Comer

    Comer
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    I use keystone on my HS10 and, watching movies, the difference to me is not noticable.

    Conor
     
  4. uk_robk

    uk_robk
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    Thanks for the reply John_N, does this mean that your PJ is setup so that it's exactly level with the center of your screen vertically? Is it a ceiling mount?
     
  5. Comer

    Comer
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    I would assume that if the pj is table mounted it should be level with the bottom of the screen and if it is ceiling mounted it sholld be level with the top of the screen

    Conor
     

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  6. Mr Pink

    Mr Pink
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    I have mine angled down at about 1.5 to 2 degrees with keystone set to zero as I couldn't set it up any other way. The resulting image is wider at the bottom by about 6-7mm each side which is projected on the black border of my screen.

    In effect, I let the screen reframe the trapazoidal image to a 16:9 rectangle as I don't notice the over projection at the bottom. I do loose a little image though!
     
  7. Kramer

    Kramer
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    I avoid keystone correction at all costs.

    It does noticeably degrade the picture. Especially if using a HCPC & 1:1 mapping. Keystone negates much of the benefits as it's "rescaling" an already perfect resolution.

    But for convenience, if you can live with a small amount of correction, then go for it ;)
     
  8. LV426

    LV426
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    To say that using keystone correction is bad is technically true. However, it is going to be more true, the coarser the native resolution of the device. With HTPC, you scale the incoming video signal into an array of pixels and then transmit those pixels to the PJ. In the case of the lower-end LCD PJs, the native resolution of the panel is similar/the same as that of the incoming (scaled) image.

    However, on the higher-end devices (Sony VW10/11/12 for example) the native resolution of the PJ is much higher - there are many more, finer pixels on the LCDs. In these circumstances, the scaler in the PJ (which rescales the image to a trapezoid shape to achieve keystone correction) has more pixels to use, and can therefore perform a better job.

    I use keystone correction. I use the PJ for movies only, from a standard (tabletop) DVD player. And it has no visible negative impact on the clarity of the image.
     
  9. uk_robk

    uk_robk
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    Yes Kramer, that's what I wasn't keen on - the image is so good when using HTPC & 1:1 mapping, it's like I'm spoiling all that by using keystone!

    Comer, that diagram is interesting - I thought that the PJ projected the image out equally from it's center. This diagram indicates that the actual projection is skewed so that one edge stays in-line with the projector.
     
  10. Comer

    Comer
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    The diagram is obviously looking from the side as the pj should be in line with the centre of the screen looking from the top. The diagram is true for the the four projectors and and for all projectors (including the AE100) that I've seen

    EDIT: When I think about it, the bottom of the image is usually very slightly lower than the projectorr when table mounted and the same applies to the top of the image when ceiling mounted

    Conor
     
  11. uk_robk

    uk_robk
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    Comer, yes I knew that it was equal for horizontal (ie looking top down), I just automatically assumed that this was true for the vertical too - learn something new every day! :)
     
  12. calibos

    calibos
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    I'm not 100% sure about other PJ's but the Ae100 should be positioned 15cm above the bottom of the white area of the screen or conversely 15cm below the top of the white area if you want to avoid using vertical keystone. Because most projectors atm (excluding the Sony HS1,HS2, HS10 & Ae300) have no horizontal keystone you have no choice but to align the PJ dead centre in the horizontal axis.
     
  13. Timh

    Timh
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    I use -2 keystone on the AE100 and cannot see any differance to 0 keystone while connected via componant cables.
     
  14. Kramer

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    I could :p

    Seriously, keystoning really negates much of the benefits of 1:1 mapping from a HCPC. ANY extra scaling is bad, but the very nature of the scaling involved with keystone is VERY, VERY bad.

    "Symetrical" scaling, the former, at least does a uniform "squeeze" or "stretch".

    Keystone, a trapezoidal (sp???) squeeze, is non uniform etc...

    Any fine patterning (stripy shirts etc..) will show the detrimental results of keystone correction. At least is does for me with 1:1 mapping with my HS10. Bear in mind also that the HS10 has an excellent scaler, far better than the AE100's.

    IMO, 0 keystone = :clap:



    :smoke:
     

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