The keystone function is a useful feature for planar projection.

If a screen is positioned perpendicularly to the projection axis of the projector, the projected image is not distorted as seen in the first image.

But as soon as one axis is not orthogonal, the projected image is not rectangular any more and image correction needs to be done. The distortion relates to the shape of the screen and to the way the projector "looks" at it which includes the orientation and distance as well as all lens settings.

If the screen is planar, as seen in the second image, the keystone parameters in Pandoras Box can deform the image in such a way, that it is rectangular again and that its linearity fits.

Each edge of the output image can be moved in or out and rotated according to the projection surface. Hence, the controls allow very individual keystone correction whilst many projectors offer only a symmetrical, horizontal correction.

Once you have set up each edge, you might compensate for the linearity distortion with the "LinX" and "LinY" parameters. To do this, use a raster grid test pattern on a layer and reset the image center with the X&Y channels.

As soon as the projection surface is not a single plane anymore, e.g. it is a bended screen or a more complex object like a cube the keystoning reaches its limit and an object must be assigned to the Mesh parameter of the output.

Please note that it is not possible to keystone an image if an object is used on the Mesh parameter as the object already deforms the camera texture.