Method of and apparatus for obtaining binary image

An apparatus for obtaining a binary image from an original image in which respective density levels of pixels are represented in the form of gradation levels, comprising:

• (a) means for obtaining pairs of gradation levels representing respective pairs of adjacent pixels in said original image;
• (b) means for counting the number of said pairs of gradation levels which are equal to one another, each count produced by said counting corresponding to a unique pair of values;
• (c) means for providing a tentative threshold level;
• (d) means for obtaining the number of pairs of adjacent pixels each of which is comprised of such two gradation levels that said tentative threshold level is included in a range defined by said two gradation levels, to obtain an index value;
• (e) means for repeating respective operations of the means (c) and (d) while updating said tentative threshold value among a plurality of tentative threshold values, to obtain a plurality of index values;
• (f) means for comparing said plurality of index values with each other to determine a threshold value;
• and
• (g) means for comparing said respective gradation levels of pixels in said original image with said threshold value to obtain a binarized image.

A method of inspecting a pattern on a printed circuit board, comprising the steps of:

• (a) photoelectrically reading an image of said pattern to obtain an original image representing said pattern, in which representative density levels of pixels are represented in the form of gradation levels;
• (b) obtaining pairs of gradation levels representing respective pairs of adjacent pixels in said original image;
• (c) counting the number of said pairs of gradation levels which are equal to one another, each count produced by said counting corresponding to a unique pair of values;
• (d) determining a tentative threshold level;
• (e) obtaining the number of pairs of adjacent pixels each of which is comprised of such two gradation levels that said tentative threshold level is included in a range defined by said two gradation levels, to obtain an index value;
• (f) repeating the steps (d) and (e) while updating said tentative threshold value among a plurality of tentative threshold values, to obtain a plurality of index values;
• (g) comparing said plurality of index values with each other to determine a threshold value;
• (h) comparing said respective density levels of pixels in said original image with said threshold value to obtain a binarized image;
• and
• (i) inspecting said pattern on the basis of said binary image.

An apparatus of inspecting a pattern on a printed circuit board, comprising:

• (a) means for photoelectrically reading an image of said pattern to obtain an original image representing said pattern, in which respective density levels of pixels are represented in the form of gradation levels;
• (b) means for obtaining pairs of gradation levels representing respective pairs of adjacent pixels in said original image;
• (c) means for counting the number of said pairs of gradation levels which are equal to one another, each count produced by said counting corresponding to a unique pair of values;
• (d) means for determining a tentative threshold level;
• (e) means for obtaining the number of pairs of adjacent pixels each of which is comprised of such two gradation levels that said tentative threshold level is included in a range defined by said two gradation levels, to obtain an index value;
• (f) means for repeating respective operations of the means (d) and (e) while updating said tentative threshold value among a plurality of tentative threshold values, to obtain a plurality of index values;
• (g) means for comparing said plurality of index values with each other to determine a threshold value;
• (h) means for comparing said respective gradation levels of pixels in said original image with said threshold value to obtain a binarized image;
• and
• (g) means for inspecting said pattern on the basis of said binary image.

A method of converting an image of a drawing on which lines are drawn into a binary image, comprising the steps of:

• (a) photoelectrically reading an image of said drawing to obtain an original image representing said lines, in which respective density levels of pixels are represented in the form of gradation levels;
• (b) obtaining pairs of gradation levels representing respective pairs of adjacent pixels in said original image;
• (c) counting the number of said pairs of gradation levels which are equal to one another, each count produced by said counting corresponding to a unique pair of values;
• (d) determining a tentative threshold level;
• (e) obtaining the number of pairs of adjacent pixels each of which is comprised of such two gradation levels that said tentative threshold level is included in a range defined by said two gradation levels, to obtain an index value;
• (f) repeating the steps (d) and (e) while updating said tentative threshold value among a plurality of tentative threshold values, to obtain a plurality of index values;
• (g) comparing said plurality of index values with each other to determine a threshold value;
• (h) comparing said respective gradation levels of pixels in said original image with said threshold value to obtain a binarized image;
• and
• (g) outputting said binary image.

An apparatus for converting an image of a drawing on which lines are drawn into a binary image, comprising:

• (a) means for photoelectrically reading an image of said drawing to obtain an original image representing said lines, in which respective density levels of pixels are represented in the form of gradation levels;
• (b) means for obtaining pairs of gradation levels representing respective pairs of adjacent pixels in said original image;
• (c) means for counting the number of said pairs of gradation levels which are equal to one another, each count produced by said counting corresponding to a unique pair of values;
• (d) means for determining a tentative threshold level;
• (e) means for obtaining the number of pairs of adjacent pixels each of which is comprised of such two gradation levels that said tentative threshold level is included in a range defined by said two gradation levels, to obtain an index value;
• (f) means for repeating respective operations of the means (d) and (e) while updating said tentative threshold value among a plurality of tentative threshold values, to obtain a plurality of index values;
• (g) means for comparing said plurality of index values with each other to determine a threshold value;
• (h) means for comparing said respective gradation levels of pixels in said original image with said threshold value to obtain a binarized image;
• and
• (g) means for outputting said binary image.