The large-screen projector with Keystone correction
To allow any member of the audience to
see the full size of the picture, you have to
project it at an extremely steep angle which
leads to a trapezium-like distortion of the
overall dimensions.
The special overhead projector with
Keystone correction in upward or sideward
projection excludes such a distortion up to a
projection angle of 13°. Parallel lines remain
parallel, a rectangle remains a rectangle.
