# Keystone effect - Wikipedia, the free encyclopedia

```Keystone effect - Wikipedia, the free encyclopedia
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Keystone effect
The Keystone effect is caused by attempting to project an
image onto a surface at an angle, as with a projector not
quite centered onto the screen it is projecting on. It is a
distortion of the image dimensions, making it look like a
trapezoid. In the typical case of a projector sitting on a
table, and looking upwards to the screen, the image is larger
at the top than on the bottom. Some areas of the screen may
not be focused correctly as the projector lens is focused at
the average distance only.
Illustration of the Keystone Effect
Contents
1 Theory behind Keystone
2 Keystone Effect in Stereo Imaging
3 Solving the problem
4 Correction
4.1 Functionality
Theory behind Keystone
The distortion suffered by the image depends on the angle of the projector to the screen, and the beam angle.
The distortion (on a two-dimensional model, and for small focus angles) is given by:
where • is the width of the focus, and
• is the angle between the screen axis and the central ray from the projector
From the formula is clear that there will be no distortion when • is zero, or perpendicular to the screen. See a
derivation of this formula at Derivation of the equation to combat Keystone
(http://freespace.virgin.net/tom.baldwin/keyst_deriv.html)
Keystone Effect in Stereo Imaging
See [1] (http://www.microscopyu.com/articles/stereomicroscopy/stereointro.html)
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Keystone effect - Wikipedia, the free encyclopedia
In stereo imaging, two lenses are used to view the same subject image, each from a slightly different
perspective, allowing a three dimensional view of the subject. If the two images are not exactly parallel, this
causes a keystone effect. This is particularly noticeable when the lenses are close to the subject, as with a stereo
microscope, but is also a common problem with many 3d stereo camera lenses.
Solving the problem
The problem arises for screen projectors that don't have the depth of focus necessary to keep all lines (from top
to bottom) focused at the same time. The problem can be solved by:
Move the projector more to the center of the screen, if this does not interrupt vision
Tilt the screen in a small angle
Use some type of software on the projector (or computer controlling the projector). Well known video
card producer NVidia delivers a package called NVKeystone with most of their software solutions.
Correction
Keystone correction, colloquially also called keystoning, is a function that allows multimedia projectors that
are not placed perpendicular to the horizontal centerline of the screen (too high or too low) to skew the output
image, thereby making it rectangular. Without such correction the image will appear as a horizontal trapezoid,
the shape of an architectural keystone; hence the name of the feature.
It is often necessary for a projector to be placed in a position outside the line perpendicular to the screen and
going through the screen's center, for example, when the projector is mounted to a ceiling or placed on a table
top that is lower than the projection screen. Keystone correction is a feature included with many projectors that
provides the ability to intentionally "distort" the output image to recreate the original rectangular image
provided by the video or computer source, thus eliminating the skewed output that would otherwise result due
to angled projection.
It is rare for a projector to have the ability to correct horizontal keystoning distortion (too far left or right of the
vertical centerline of the screen), although Sony has had capable projectors in its line-up with the feature dubbed
Side Shot. Typically this is easily corrected by moving the projector left or right as necessary, or less often by
lens shifting.
Functionality
In modern projectors keystone correction technology is performed digitally (rather than optically) via the
internal (LCD) panels or (DLP) mirrors of the projector, depending on the technology used. Thus, when
applying keystone correction to an image, the number of individual pixels used is reduced, lowering the
resolution and thus degrading the quality of the image projected. Home theater enthusiasts would argue that
keystoning should not be used because of the impact it has on image quality. However, it is a useful technology
in cases where the projector cannot be mounted directly in front of the screen, or on projectors utilizing lens
shift technology where the projector must be mounted outside the frame of the screen.
Homography
Perspective correction
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Keystone effect - Wikipedia, the free encyclopedia
Keystone Correction: Digital and Manual
(http://www.presentationmart.com/resources/keystonecorrection.asp)
Categories: Projectors
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