Keystone correction or trapeze correction is an aid for balancing out or at least reducing trapeze shaped distortions in a projected image. By stretching the image artificially, keystone correction balances out a distortion that has arisen through projection and – in the ideal case- creates an image for the user that is no longer distorted. The effect arises in video or slideshow projections which are not located at a right angle to the projection surface and can be recognised from the fact that one side of the rectangle projected onto the wall is higher or broader than the others.
The distortion is amended either using optical correction, i.e. moving the lens, or through the electronic correction mentioned above, which artificially stretches the image. In the latter variant, image information is lost, since the side of the image that seems longer is compressed without increasing the resolution. If a projector has a maximum of 1024 pixels horizontally, the information contained in 124 pixels is lost if the image width is reduced to 900 pixels. For example, if these contain horizontal lines, these will in all probability no longer be displayed.
In practice, a projector is generally either too steeply or too flatly positioned, which means that correction of the upper and lower edges is necessary in most cases. Most modern projectors have automatic keystone correction: a length sensor measures the positioning angle of the projector and from this angle calculates the ideal image stretching needed for a horizontal wall. The correction does not kick in if the image is projected onto a tilted surface.
Originally, the term keystone was used in construction to denote the keystone or capstone of a doorway. In order to complete the archway of a door, it is necessary to place the last stone in the centre in such a way that the arch can support itself. The keystone is thus the key to success and is wedge-shaped or trapeze-shaped because of statics.