And, since you can more or less just point the projector straight up, there shouldn’t be too much keystone. Best digital signage software in 2021. XGIMI Z4 Aurora keystone correction demonstration. Vertical +/-30 degree, Horizontal +/-30 degree. There’s not always a perfect place for your.
As an instructor, you may need to present information in different classrooms and in different environments. No matter if your presentation will take place in a large classroom or a small classroom and whether you are using a screen, a blackboard, or an improvised wall, one of the challenges you will frequently come across is keystone.
What is Keystone Correction?
Have you ever projected an image on the wall and noticed that the image is not rectangular? If so, you understand why keystone correction is necessary.
The reason why this happens is because the projector is not sitting perpendicular to the projection surface. As a projector is turned left, turned right, or tilted up or down, you will see that a trapezoidal image is projected. You are seeing keystoning.
This distorted image is what you as an educator want to avoid. The problem is that in some classrooms there is not the perfect place for you to put a projector. This is where keystone correction comes into play.
Vertical Lens Shift
This feature makes it possible to correct an image where the top and bottom of the image are different lengths. The vertical lens shift allows you to address this issue without needing to move the projector.
You are able to raise or lower the image, taking that trapezoid shape and returning it to a rectangle.
Horizontal Lens Shift
If the left and right sides of the image are different lengths, you can use the horizontal lens shift to correct the problem. Again, this feature allows you to move the image to the left or right, correcting the trapezoid shape without needing to physically move the projector.
There are two types of keystone correction available on most projectors. There is a manual correction and a digital one. The manual correction, as its name implies, is a physical adjustment that happens to the lens.
This correction makes it so that the projector points at an angle that is higher or lower than what it would normally point to if it was on a flat surface. Manual keystoning works great in the majority of circumstances.
The projector does need to be an appropriate distance away from the screen to achieve the desired results.
Digital Keystoning
LCD and DLP projectors are both designed to display digital information. Information that is sent to the projector digitally, such as in a PowerPoint presentation, can be digitally converted and scaled.
Some projectors have added a selected algorithm to the scaling, which allows it to alter the image before the image reaches the lens.
This means that the image will be square, even if the projector is at an angle. This offers more flexibility when setting up a projector, especially if the setup is in a unique environment.
Potential Downsides of Digital Keystoning
If a projector comes equipped with digital keystoning, why worry about manual keystoning? While digital keystoning can be effective in some circumstances, it may add a couple of unwanted changes to your image.
Depending on what you are projecting and the application you are using with the projector, this may or may not be an issue for you.
Digital keystoning will use digital scaling to shrink the long side of the image. When this happens, information is removed from the image.
This leads to scaling artifacts that become easier to see the larger the image is.
For example, if there is text in the area that is being scaled, that text will become fuzzy. This problem becomes less evident if you’re using video or photos.
A second challenge is that the image becomes dimmer. Some of the lumen output is removed because the scaled image is smaller than the native resolution of the projector.
This is why we say that if you are moving a projector from place to place, it’s better to use the manual keystone correction.
If you know in advance that you are going to be moving your projector around, then you may want to start by searching for projectors that have horizontal and vertical shift lenses.
This way, you know you will be able to adjust the projector to meet the needs of the environment that you are in.
You should also look at the amount of correction the projector can provide. For vertical keystone correction, some projectors will offer between 12 and 35 degrees of projection. Keystone correction is a great tool if you need to move your projector from classroom to classroom.
But, remember, it is going to impact picture quality. It can lead to a slightly distorted image, especially when it comes to the sides of the screen.
See related - best HD projectors and best 4K projectors
The keystone effect is the apparent distortion of an image caused by projecting it onto an angled surface. It is the distortion of the image dimensions, such as making a square look like a trapezoid, the shape of an architectural keystone, hence the name of the feature. 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.
In photography, the term is used to describe the apparent leaning of buildings towards the vertical centerline of the photo when shooting upwards, a common effect in Architectural photography. Likewise, when taking photos looking down, e.g., from a skyscraper, buildings appear to get broader towards the top. The effect is usually corrected for by either using special lenses in Tilt–shift photography or in post-processing using modern image editing software.
Theory[edit]
The distortion suffered by the image depends on the angle of the projector to the screen, and the beam angle.
The distortion[clarification needed] (on a two-dimensional model[clarification needed], and for small focus angles) is best approximated by:
where is the angle between the screen axis[clarification needed] and the central ray from the projector, and is the width of the focus[clarification needed].
From the formula, it is clear[clarification needed] that there will be no distortion when is zero, or perpendicular to the screen.[citation needed]
Keystone effect in stereo imaging[edit]
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,[1] but is also a common problem with many 3D stereo camera lenses.
Solving the problem[edit]
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. Common solutions to this problem are:
- moving the projector more to the center of the screen,
- tilting the screen in a small angle,
- the use of special software on the projector
- and computer controlling the projector.
Vertical Keystone Correction Projector
Correction[edit]
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.
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 or higher than the projection screen. Most ceiling-mounted projectors have to be mounted upside down to accommodate for the throw of the image from the lens, with the image rotated right-side-up with software. 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.
The ability to correct horizontal keystone distortion is generally only available on larger or professional level projectors. In most consumer units, this is easily corrected by moving the projector left or right as necessary, or less often by lens shifting, with similar principles as Tilt–shift photography.
Functionality[edit]
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.
See also[edit]
References[edit]
- ^Introduction to stereo microscopy