Keystone...How does it work and why all the distortion? - AVS Forum
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post #1 of 13 Old 10-09-2003, 03:47 PM - Thread Starter
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How does digital keystone correction work? I have been playing with mine on my G-150 DILA and am trying to understand why it distorts the image so.

My PJ is higher (over the screen) than recommended. Hence, a slightly trapazoidal picture.

As I played with it, I wanted to know more as to the means behind how it works.

Thanks!


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post #2 of 13 Old 10-09-2003, 03:54 PM
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Keyston correction is evil.

Digital keystone correction works by reproducing the number of pixels used to produce the picture on the panel. In essence, putting that trapezoid onto the panel and making the other pixels black. So if you have been using "pixel perfection" scaling for the D-ILA you have just defeated the purity of it.

I think there is other types of keystone correction like optical etc but I'm not that familiar as I wouldn't ever consider it. I am a purist ;)

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post #3 of 13 Old 10-09-2003, 03:57 PM
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Keystone correction degrades the picture because by adjusting it, you end up giving part of the LCD or DLP panel. The PJ uses fewer pixels to draw a perfectly rectangular image. Think of it as drawing a rectangle inside a trapezoid. Before keystone, each pixel is perfectly alighned to each other and thus the image is crisp. With keystone, you pixel will be used at an angle to produce a perfectly vertical line and it is not possible. Think of it as taking a grid and tilting it slightly and wondering why the vertical lines are not perfect.

I had a real hard time explaining this in writing so forgive me if it is confusing.
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post #4 of 13 Old 10-09-2003, 05:27 PM
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like it was said above digital keystone produces a trapazoid picture on the display itself, so the onboard scaler must make a trapezoid image, Also the trapezoid is inverted from what your keystone is (i.e. if the pic is /_\\ then the scaler produces a pic that is \\_/ )so that the smaller side that travels farther becomes the same size as the larger side.

Like Tryg said, there is also (or at least used to be) optical keystone correction. Basically in an optical system a lens is used to change the shape of the image. the benefit of optical is that you start with a rectangular image and finish with a rectangular image, but you do need a good lens and that costs a lot.
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post #5 of 13 Old 10-09-2003, 05:44 PM - Thread Starter
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I also noticed that a horrible amount of light spill occurs with the most minute keystone correction. I am using a Panamorph and it light up my Ve.lux very nicely.


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post #6 of 13 Old 10-09-2003, 08:34 PM
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I have never seen a projector with optical keystone correction. I wonder if Tryg is thinking of scheimpflug focus, which is used to tilt the image plane to match the screen. This does not fix keystone errors.

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post #7 of 13 Old 10-09-2003, 08:57 PM
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Three clicks of keystone correction on the HT1000, PJ tilted about 5 degrees, image is still outstanding, borders sharp. Just my experience.
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post #8 of 13 Old 10-09-2003, 09:14 PM
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I believe it was a sharp (sharpvision?) LCD projector I saw with optical keystone correction. What you did was you mounted the projector PERFECTLY LEVEL, then something in the lens assembly moved, and the image shifted up and down. Worked pretty well, and didn't have the scaling artifacts that digital keystone correction does.

But that's all I know, because us CRT folks don't have those problems. :D

- David

... when I bought my first pair of CRT projectors, nobody ever told me they worked like rabbits!
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post #9 of 13 Old 10-10-2003, 07:54 PM
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I did a google search for optical keystone (just to mention some models for the sake of wm's scepticism) and found this http://htrgroup.com/?tab=projector-d...ction=keystone
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post #10 of 13 Old 10-11-2003, 07:41 AM
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Lens shift (Sharp Z10000 for instance) is a kind of optical keystone correction ;)
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post #11 of 13 Old 10-11-2003, 08:27 AM
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I have gone back and forth with the digital keystone on the SX21 and I can't see much difference. You guy's have me thinking though and I am going to the hardware store this morning to get materials to build a new projector box so I can mount my projecto upside down and eliminate the need for keystone.

I just have to see if this will make a significant difference.

Phil
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post #12 of 13 Old 10-12-2003, 07:35 PM
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Well, after spending all weekend building a new projector box and re-installing my SX21 so that I would not need keystone, I am quite pleased that I did it. I do notice an improvement in the quality of the picture. it appears sharper to me. I thought it looked good before, but now it really looks great.

Phil
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post #13 of 13 Old 10-13-2003, 01:54 AM
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Jeff, a much smarter member than me (Thumper) showed me how I could eliminate keystone and have my pj lower than recommended. My basement room is only 7' high so I needed to get my screen as high as possible. I got my pj about 5 or 6 inches lower than required. (Actually got my Screen 5 or 6 inches higher than required) You could reverse the process for a pj that is higher than recommended. For my set up and his, we tilt the screen out from the wall and tilt the pj up. You can graph it out ( I did ) the geometry works. It depends on how much you lower or higher you are trying to steal but mine is out about 2" from the wall. You cannot notice it at all. pm me and I will give you my phone number if I have not explained this clearly. Mr. Wigggles first put me in touch with Thumper.
Dan
p.s. you have a great HT

Dan
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