Image Number 11 for United States Patent #4204232.
"Black" runs thinned from a substantially two-tone original pattern and having a pattern stroke width of a substantially single picture element are classified into four states where a black run in a present scanning line is (1) linked with a leading side of black run in a next previous line, (2) linked with a trailing side of the latter, (3) aligned with the latter in both sides, and (4) not linked with the latter. The former is defined to be linked with the latter when they have either a vertex or a side of a picture element alone in common. The former is encoded according to the successively classified states by using, for states (1) and (2), a black run length. For state (4), the black run length is used in combination with a "white" run length measured up or down to the leading side of the former, if any, from the trailing side of that black run in the previous line which either next leads the former or has more common picture element side or sides than specified on defining linkage with leading and/or trailing sides staggered in relation to the corresponding sides of the former. Thinning processes are iterated preferably not transforming a "black" picture element into a "white" one when situated, in addition to known neighborhood conditions, at a vertex of an acute "black" triangle. The encoded signal is decoded and then subjected to iterative thickening processes into a reproduction approximately representative of the original.