Identification of two-tone images; some implications for high- and low-spatial-frequency processes in human vision.

Unlike most multitone images, two-tone images such as print, geometric figures, and line drawings are as easy to interpret in photographic negative as in positive form. However, images derived from a multitone original in which intensity values are quantised to two levels are not. Bi-level quantised images, distinct from most other two-tone images, are shown to contain picture related components in their low spatial frequencies. Since it is the low-spatial-frequency components alone of negative images that present difficulties for vision, it is proposed that images which are as easy to interpret in negative as in positive form are those which are readily identified using only their high spatial frequencies.