Digital halftone rendering of a gray scale image with frequency dependent diagonal correlation

Claim 1

A method of halftoning and displaying a gray scale image on a display device by utilizing a point by point thresholding comparison to a predetermined diagonal correlation dither matrix, said display device being controllable to display dots in predetermined dot positions on a medium along horizontal and vertical axes relative to a viewer of said medium, wherein two displayed dots aligned horizontally or vertically and separated by one dot position on said medium partially overlap, said method comprising the steps of: segmenting said gray scale image into pixel areas; thresholding each of said pixel areas against a predetermined diagonal correlation dither matrix to generate a halftone dot pattern for each of said pixel areas, said dither matrix being substantially comprised of a function that, at least intermittently, forces diagonal correlation of adjacent dots in said halftone dot pattern over vertical or horizontal correlation of non-adjacent dots separated by one dot position in said halftone dot pattern, while also forcing a substantially homogeneous dispersion of dots, thereby producing visually unobtrusive output dot patterns; and displaying said halftone dot pattern by said display device on said medium to display a halftone rendering of said gray scale image.

Claim 2

A method of halftoning and displaying a gray scale image on a display device by utilizing a point by point thresholding comparison to a predetermined diagonal correlation dither matrix, said display device being controllable to display dots in predetermined dot positions on a medium along horizontal and vertical axes relative to a viewer of said medium, wherein two displayed dots aligned horizontally or vertically and separated by one dot position on said medium partially overlap, said method comprising the steps of: segmenting said gray scale image into pixel areas; thresholding each of said pixel areas against a predetermined diagonal correlation dither matrix to generate a halftone dot pattern for each of said pixel areas, said dither matrix being substantially comprised of a function that, at least intermittently, forces diagonal correlation of adjacent dots in said halftone dot pattern over vertical or horizontal correlation of non-adjacent dots separated by one dot position in said halftone dot pattern, while also forcing a substantially homogeneous dispersion of dots, thereby producing visually unobtrusive output dot patterns; and displaying said halftone dot pattern by said display device on said medium to display a halftone rendering of said gray scale image.

Claim 5

A method of generating a diagonal correlation dither matrix and of using said dither matrix, said method comprising the steps of: setting a matrix of pixels so that each of said pixels are in an OFF state; initializing the matrix to a predetermined initial state corresponding to a lowest gray scale tone level consisting of only one pixel in the matrix being set to an ON pixel; identifying as candidate pixels all of the OFF pixels in the matrix; determining a cost value sum for each candidate pixel based on respective radial distances and relative angles between a respective candidate pixel and the 0N pixels within a predetermined search area surrounding said respective candidate pixel, where a single cost value is obtained for substantially each candidate pixel based on its radial distance and relative angle with respect to a single ON pixel within said predetermined search area, wherein a single cost value for a candidate pixel diagonally adjacent an ON pixel is less than a single cost value for a candidate pixel vertically or horizontally adjacent an ON pixel; comparing the respective cost value sums of the candidate pixels to determine a lowest cost value sum for the candidate pixels; selecting one of the candidate pixels that has the lowest cost value sum; turning ON the selected candidate pixel in the matrix; repeating said identifying, determining, comparing, selecting and turning ON steps until the number of ON pixels in a resulting matrix corresponds to the desired tone level, said resulting matrix forming a threshold matrix which when thresholded against a corresponding gray scale tone level identifies a halftone dot pattern corresponding to said ON pixels in said resulting matrix; storing each resulting threshold matrix for each tone level in a memory, an accumulation of resulting threshold matrices providing said dither matrix; thresholding said dither matrix against a gray scale image to produce a halftone image for display; and displaying said halftone dot pattern on a medium to display a halftone rendering of said gray scale image.

Claim 6

A method of generating a diagonal correlation dither matrix and of using said dither matrix, said method comprising the steps of: setting a matrix of pixels so that each of said pixels are in an OFF state; initializing the matrix to a predetermined initial state corresponding to a lowest gray scale tone level consisting of only one pixel in the matrix being set to an ON pixel; identifying as candidate pixels all of the OFF pixels in the matrix; determining a cost value sum for each candidate pixel based on respective radial distances and relative angles between a respective candidate pixel and the 0N pixels within a predetermined search area surrounding said respective candidate pixel, where a single cost value is obtained for substantially each candidate pixel based on its radial distance and relative angle with respect to a single ON pixel within said predetermined search area, wherein a single cost value for a candidate pixel diagonally adjacent an ON pixel is less than a single cost value for a candidate pixel vertically or horizontally adjacent an ON pixel; comparing the respective cost value sums of the candidate pixels to determine a lowest cost value sum for the candidate pixels; selecting one of the candidate pixels that has the lowest cost value sum; turning ON the selected candidate pixel in the matrix; repeating said identifying, determining, comparing, selecting and turning ON steps until the number of ON pixels in a resulting matrix corresponds to the desired tone level, said resulting matrix forming a threshold matrix which when thresholded against a corresponding gray scale tone level identifies a halftone dot pattern corresponding to said ON pixels in said resulting matrix; storing each resulting threshold matrix for each tone level in a memory, an accumulation of resulting threshold matrices providing said dither matrix; thresholding said dither matrix against a gray scale image to produce a halftone image for display; and displaying said halftone dot pattern on a medium to display a halftone rendering of said gray scale image.