Method and apparatus for gray level signal processing

A gray level signal processing method comprising the steps of:

• dividing a pictorial image into blocks, each block being composed of a plurality of picture elements;
• detecting the maximum optical density level (P.sub.max) and the minimum optical density level (P.sub.min) among optical density levels of picture elements for said each block;
• providing the difference value (P.sub..DELTA.) between said maximum optical density level (P.sub.max) and the minimum optical density level (P.sub.min);
• determining an image area to which each said block belongs according to said difference value (P.sub..DELTA.);
• selecting one of a plurality of threshold matrices is selected according to the result of determination of the image area to which each said block belongs;
• and
• quantizing the optical density level of each picture element in each said block in binary with said selected threshold matrix.

A method of image area determination comprising the steps of:

• dividing a pictorial image into blocks, each block being composed of a plurality of picture elements;
• detecting the maximum optical density level (P.sub.max) and the minimum optical density level (P.sub.min) among optical density levels of picture elements for each block;
• providing the difference value (P.sub..DELTA.) between said maximum optical density level (P.sub.max) and said minimum optical density level (P.sub.min);
• comparing said difference value (P.sub..DELTA.) of said optical density levels with a reference value;
• identifying each said block as a character image area when said difference value (p.sub..DELTA.) is larger than said reference value;
• and
• identifying each said block as a continuous-tone image area when said difference value (P.sub..DELTA.) is smaller than said reference value.

A method of image area determination comprising the steps of:

• dividing a pictorial image into blocks, each block being composed of a plurality of picture elements;
• detecting the maximum optical density level (P.sub.max) and the minumum optical density level (P.sub.min) among optical density levels of picture elements for each block;
• providing the difference value (P.sub..DELTA.) between said maximum optical density level (P.sub.max) and said minmum optical density level (P.sub.min);
• comparing said difference value (P.sub..DELTA.) of the optical density levels with a plurality of reference values (m.sub.1, m.sub.2, . . . );
• and
• based on the result of said comparisons, identifying each said block as one of a character image area, a continuous-tone image area and at least one intermediate area.

A gray level signal processing method comprising the steps of:

• dividing a pictorial image into blocks, each block being composed of a plurality of picture elements;
• detecting the maximum optical density level (P.sub.max) and the minimum optical density level (P.sub.min) among optical density levels of picture elements for said each block;
• providing the difference value (P.sub..DELTA.) between said maximum optical density level (P.sub.max) and the minimum optical density level (P.sub.min);
• determining an image area to which each said block belongs according to said difference value (P.sub..DELTA.);
• selecting a set of threshold matrices from a plurality of threshold matrix sets, each set including a plurality of threshold matrices, according to the result of determination of the image area to which each said block belong;
• and
• quantizing the optical density level of each picture element in each said block in multi-levels by using the selected threshold matrix set.

An apparatus for image area determination comprising:

• memory means for temporarily storing levels of picture elements in one block in a pictorial image;
• arithmetic means for comparing optical density levels of the picture elements stored in said memory means so as to obtain the maximum optical density level (P.sub.max) and the minimum optical density level (P.sub.min);
• arithmetic means for calculating the difference value (P.sub..DELTA.) between said maximum optical density level and said minimum optical density level;
• determination means for comparing said difference value (P.sub..DELTA.) between said maximum and minimum optical density levels with at least one of predetermined reference values (m.sub.1, m.sub.2. . . m.sub.n, where m.sub.1 .ltoreq.m.sub.2 .ltoreq.. . . .ltoreq.m.sub.n), and providing and output signal which represents the image area to which said block belongs in accordance with the result of said comparison,
• said determination means comprising an area determination correcting circuit for initially identifying a block in a pictorial image as an image area in accordance with the difference value between the maximum optical density level and the minimum optical density level, and for thereafter correcting said area determination by referring to the result of area determination for blocks adjacent to the initially identified block.

A gray level signal processing apparatus comprising:

• detecting means for detecting the maximum optical density level and the minimum optical density level among optical density level of picture elements in successive blocks, each block being composed of a plurality of picture elements;
• means for calculating the difference value between said maximum optical density level and said minimum optical density level for each block;
• comparing means for comparing each said difference value between said maximum and minimum optical density level with at least one of a plurality of predetermined reference values;
• threshold matrix memory means for storing a plurality of threshold matrices;
• selecting means for selecting one of said plurality of threshold matrices from said threshold matrix memory means in accordance with the output of said comparing means;
• and
• means for quantizing the optical level of each picture element in said block in binary levels by using the threshold matrix selected by said selecting means.