Raster of light and dark lines for coded picture information incorporates lines of varying width with phase changes to code for additional light and dark areas

摘要

The picture is made up of alternate white and black lines set at an equal spacing and typically sloping at 45 degrees to the vertical. Gray tone variations are produced in the manner of a normal half-tone printed picture by variations in the widths of the alternate white and black lines. Coded picture information is added by introducing a phase-change in the lines at the boundaries of the coded areas. At the phase change the lines are shifted sideways by half a step so that black lines but onto white lines and vice versa. With a fine line pattern this discontinuity is almost impossible to see. A line raster is placed over the picture to bring up the coded pattern. In some areas the opaque black and transparent white lines of the raster coincide with the black and white raster lines of the picture and shades of gray are seen as in the naked picture. The phase shift matches black lines with white lines and vice versa so that the coded areas show up dark.