Methods of compression and expansion of dot pattern

A method of compressing a dot pattern having no two printing dots adjacent to each other in a main scanning direction and storing data representative of the dot pattern in compressed form in a memory, from which the data can be subsequently read out in expansion processing to obtain the original dot pattern for use in printing, comprising the steps of:

• inserting dummy printing dots into positions within rows of the dot pattern so that each dummy printing dot immediately precedes and immediately follows original printing dots, wherein columns of dot positions are arranged in a subscanning direction substantially perpendicular to the main scanning direction;
• for each printing dot, inserting a respective provisional dummy printing dot provided that the next row position following the provisional dummy printing dot does not contain a printing dot;
• comparing a first column with a second column that immediately precedes the first column to determine whether the respective dot pattern of the first column being compared is identical to the original dot pattern in the second column and keeping the provisional dummy printing dots in the column when the dot patterns coincide and removing selected ones of the provisional dummy printing dots in the first column when the dot patterns do not coincide;
• and
• compressing the dot pattern based on the comparison of the columns of the printing dots and associated information indicating repetition of columns;
• and
• storing the compressed dot pattern in said memory.