Keystone correction

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Keystone correction
Keystone correction

The keystone correction (or keystone correction by which often trapezoidal keystone , engl. Key stone ) refers to the possibility of a trapezoidal distortion of a projected image to remove or minimize. For this purpose, an artificial distortion must be created in the image, which compensates for the original distortion and creates a normal image for the viewer.

The effect only occurs with video or slide projectors that are not set up exactly at right angles to the projection surface . The image then appears wider (or higher) on one side than on the opposite side.

The correction can be made by

  • Software correction by pre-distorting the projected image.
  • Right-angled alignment of the projector to the projection surface and shifting the image by moving the lens .

If the image is pre-distorted, the image area and light output (only a rectangular part of the trapezoid is used) as well as image information (through interpolation) are lost, as the supposedly "longer" side of the image is compressed without increasing the resolution .

Has a video projector z. B. a maximum of 800 pixels horizontal resolution, which are normally all occupied with image information, the information of 50 points is "suppressed" by reducing the image width to 750 points. If, for example , there are table lines in these tables , some of them are no longer displayed. Incidentally, this means that straight lines are displayed as fine stairs and part of the available light output is lost because the pixels at the edges of the image are converted into black points (shown in light gray in the graphic below). At steep angles, this can mean that the available number of pixels and light output are halved.

Since in most cases a correction has to be made to the upper or lower edges (the projector is too far down or too high), some video projectors have an automatic keystone correction: a position sensor inside the device detects the angle of installation and calculates the optimal image distortion for a vertical wall. However, this does not work when projecting onto a sloping surface.