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Does digital keystone correction compromise the image?

Discussion in 'Projectors, Screens & Video Processors' started by alanfcross, Jan 28, 2005.

  1. alanfcross

    alanfcross
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    I see most projectors with keystone correction state that it is 'digital'. This sounds to me like fudging a rectangular image into a trapesium shape so it looks square on a crooked screen. Is this a resampling of the image, and does it therefore compromise image quality? Is optical correction possible? Is it better? This would involve some sort of tilt/shift lens, but would be preferable, if more expensive. Anyone know what really happens?
     
  2. Oakleyspatz

    Oakleyspatz
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    you are right to think that keystone correction is not an ideal solution if you're after the best picture quality. It's a bit like using a graphic equalizer to adjust your sound, it has an effect, but it's not quite 'hi-fi'.
    It is better to either position the projector central to the screen ( from left to right mainly) or buy a projector with lens shift which physically moves the lens left to right and up and down. Many of the newer LCD models such as the AE700, the Z3 and the Sony HS50 have lens shift so it doesn't have to cost the earth.
     
  3. PJTX100

    PJTX100
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    To emphasise the importance of lens shift, my TX100 is a fixed ceiling installation and relies on both vertical and horizontal lens shift to fine tune the image position - it works brilliantly on the TX100 and doesn't compromise the picture quality (as far as I can see anyway).

    Oakley, like the graphic equalizer analogy, spot on...PJ
     
  4. KraGorn

    KraGorn
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    Just to explain what's going on ...

    As you surmised Alan, the difference between Keystone Adjustment and Lens Shift is that the former is done by the projector's scaler 'massaging' the image, distorting pixel information to force the image back to a rectangle. Since most projector's scalers are at best 'average' this results in a noticeaby poorer picture, the effect being immediately obvious even if you only 'nudge' the keystone adjustment by a single setting, ie. as soon as the X or Y values are changed from 0.

    Lens Shift OTOH is done purely optically and so the projected image is indistinguishable from what it would be normally .. the only caveat is that on most projectors you wouldn't use the extreme ends of the shift range since that often starts to reveal small imperfections in the lens system which result in minor but noticeably focusing problems.


    Using Keystone isn't the end of the world but is best avoided if at all possible.
     
  5. MikeRJ

    MikeRJ
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    Lens shift is fundamentaly different to keystone correct though, whether done digitaly or opticaly. Lens shift simply moves the picture up/down/left/right and cannot compensate for the picture distortion caused by the projector lens not being at 90 degrees to the screen surface in either plane.
     

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