When a projector is projected upward or sideways, the picture is distorted as a keystoning. There are two types of keystoning; vertical keystoning where the projector is above or below the centreline of the screen and horizontal keystoning where the projector is to the left or right of the screen.
In general, keystoning occurs when the distance to the top
Some video processors allow you to select a particular colour from one source input and superimpose another input source to replace than colour. So, for example, if you record a presentation with a blue background ("bluescreen" in the movies) you can then select this colour with the processor and have another source replace it.
The principal
It allows the edges of the projection are completely vertical when the picture was distorted by projecting upward or sideways. Most projectors handle the current correction and vertical models with more features will also handle the horizontal correction.
The "keystone distortion" is a form of video image distortion that occurs with front