Method and apparatus for forming high resolution halftone images

• US 4,547,812 A
• Filed: 12/22/1981
• Issued: 10/15/1985
• Est. Priority Date: 06/20/1980
• Status: Expired due to Fees

• Current Assignee: Information International, Inc. (Agfa-Gevaert N.V.)
• Original Assignee: Information International, Inc. (Agfa-Gevaert N.V.)
• Inventors: Anschultz, Dean W., Waller, Robert W.
• Primary Examiner(s): Britton, Howard W.
• Application Number:
  US06/333,385
• Time in Patent Office: 1,393 Days
• Field of Search: 358/283, 358/256, 358/298, 364/523
• US Class Current: 358/3.02
• CPC Class Codes: H04N 1/4055 producing a clustered dots or a size modulated halftone pattern
• CPC Class Codes: H04N 1/4057 the pattern being a mixture of differently sized sub-patterns, e.g. spots having only a few different diameters multi-toning H04N1/40087

Abstract

Halftone images are formed from dots selected from a dot character font memory storing information representing halftone dots of different sizes and shapes corresponding to different gray scale tone values or image densities. An original image is first scanned to sample its tone values at minute intervals exceeding the desired dot resolution of the reproduction. These sample values are digitized by an A/D converter which converts them to multibit binary numbers. Adjacent samples are compared to determine the average image density in a particular area, as well as the rapidity of rate of change of image density within that area. If the rate of change of image density in a given area is below a selected threshold value, the area is designated as low detail and the multibit binary number representing the result of the average density calculation over the area is used to select a dot character from the font memory for reproducing that area of the image. If, however, the rate of change of image density exceeds the selected threshold value, indicating an area of relatively high detail, the area samples are used to select special dot shapes and positions for use in reproducing that sampled area. This technique achieves a higher resolution halftone image for a given amount of stored data than prior techniques, as well as achieving a higher "apparent" resolution to the observer.

Claims

1. A photocomposing system for producing a half-tone replica of an original image comprising in combination:

means for scanning said original image to produce a digital representation of the tonal density of each of a plurality of points of said image;

comparison means for comparing digital representations of adjacent points within an area forming a cell for generating a "flag" signal when the differences in tonal density between said points exceeds a certain threshold;

arithmetic means responsive to the digital representations of points within a cell for producing a single digital weighted value for that cell;

dot character storage means storing a plurality of different halftone dot characters;

means responsive to each digital weighted value for selecting one of said halftone dot characters from said dot character storage means;

output means responsive to each halftone dot character selected for producing a visible representation of that dot character positioned at substantially the center of a cell area; and

means responsive to the generation of a "flag" signal associated with a cell for altering the horizontal and vertical positions of the visible representation produced in the area of that cell.

2. The photocomposing system of claim 1 wherein said arithmetic means includes means for arithmetically averaging the digital representation of points within a cell to produce said single digital weighted value.

3. The photocomposing system of claim 1 wherein said dot character storage means stores dot characters of different size and shape.

4. The photocomposing system of claim 1 wherein each of said cells contains four points successively displaced by 90°

from one another and wherein the tonal densities of said four points are respectively represented by D_N, D_E, D_S, D_W and wherein said comparison means generates said "flag" signal if the tonal density of any one of said four point differs by more than a threshold value from the tonal density of any other one of said points.

5. The photocomposing system of claim 1 wherein each of said cells contains four points successively displaced by 90°

from one another and wherein the tonal densities of said four points are respectively represented by D_N, D_E, D_S, D_W and wherein said arithmetic means for producing said weighted value includes means responsive to ##EQU4## where Σ

=K (D_N +D_E +D_S +D_W) where K represents a constant.

6. The photocomposing system of claim 1 wherein each of said cells contains four points successively displaced by 90°

from one another and wherein the tonal densities of said four points are respectively represented by D_N, D_E, D_S, D_W and wherein said means for altering the horizontal and vertical positions include means for altering the horizontal position Δ

X proportional to L (D_N -D_S) and the vertical position Δ

Y porportional to L(D_E -D_N) where L represents the distance from the center of a cell to the center of one of said points.

7. A photocomposing system for producing a half-tone replica of an original image comprising in combination:

means for scanning said original image to produce a digital representation of the tonal density of each of a plurality of points of said image;

comparison means for comparing digital representations of adjacent points within an area forming a cell for generating a "flag" signal when the differences in tonal density between said points exceeds a certain threshold, each of said cells containing four points successively displaced by 90°

from one another, the tonal densities of said four points beings respectively represented by D_N, D_E, D_S, D_W ;

arithmetic means responsive to the digital representations of points within a cell for producing a single digital weighted value for that cell, said digital value being dependent upon Σ and

Σ

is proportional to the sum of D_N +D_E +D_S +D_W ;

dot character storage means storing a plurality of different halftone dot characters of different size and shape;

means responsive to each digital weighted value for selecting one of said halftone dot characters from said dot character storage means;

output means responsive to each halftone dot character selected for producing a visible representation of that dot character positioned at substantially the center of a cell area.

8. The photocomposing system of claim 7 wherein said digital value produced by said arithmetic means is further responsive to the relationship of D_N +D_S compared to D_E +D_W.

9. A method for producing a halftone replica of an original image comprising the steps of:

scanning said original image to derive a digital representation of the tonal density at each of a plurality of points;

comparing digital representations of tonal density of adjacent points defining a cell area to ascertain whether the difference therebetween exceeds threshold so as to identify a high detail cell or is less than said threshold so as to identify a low detail cell;

selecting for each cell area a particular one of a plurality of stored dot characters in accordance with the values of the digital representations of points within that cell area; and

displaying each of said selected dot characters on an output medium at a certain position within the cell area of low detail cells and at a variable position within the cell area of high detail cells which position is dependent upon the digital representations of points within that cell area.

10. A photocomposing system for producing a halftone replica of an original image comprising in combination:

means for scanning said original image to produce a digital representation of the tonal density of each of a plurality of points of said image;

comparison means for comparing digital representations of adjacent points within an area forming a cell to identify a cell as being of either high or low detail;

arithmetic means responsive to the digital representations of points within a cell for producing a single digital weighted value for that cell;

dot character storage means storing a plurality of different halftone dot characters;

means responsive to each digital weighted value for selecting one of said halftone dot characters from said dot character storage means;

output means responsive to each halftone dot character selected for producing a visible representation of that dot character positioned at substantially the center of a cell area; and

means responsive to said comparison means determining a cell is of high detail for altering the horizontal and vertical positions of the visible representation produced in the area of that cell.

11. A photocomposing system for producing a halftone replica of an original image comprising in combination:

means for scanning said original image to produce a digital representation of the tonal density of each of a plurality of points of said image;

comparison means for comparing digital representations of adjacent points within an area forming a cell to identify a cell as being of either high or low detail;

a digital memory;

arithmetic means responsive to the identification of a low detail cell for processing the digital representations of the points therein to produce a single digital weighted value for that cell;

means for storing said single digital weighted value in said digital memory;

means responsive to the identification of a high detail cell for storing the digital representations of each of the points therein in said digital memory;

dot character storage means storing a plurality of different halftone dot characters;

means for accessing said digital memory;

means responsive to the accessing of a digital weighted value associated with a low detail cell for selecting one of said halftone dot characters from said dot character storage means;

means responsive to the accessing of digital representation of points within a high detail cell for selecting one of said halftone dot characters from said dot character storage means;

output means responsive to each halftone dot character selected for producing a visible representation of that dot character positioned at substantially the center of a cell area; and

means responsive to the accessed digital reoresentations associated with a high detail cell for altering the horizontal and/or vertical position of the visible representation produced in the area of that cell.

12. The photocomposing system of claim 11 wherein each of said cells contains four points successively displaced by 90°

from one another and wherein the tonal densities of said four points are respectively represented by D_N, D_E, D_S, D_W and wherein said comparison means identifies a cell as being of high detail if the tonal density of any one of said four points differs by more than a threshold value from the tonal density of any other one of said points.

13. The photocomposing system of claim 11 wherein each of said cells contains four points successively displaced by 90°

from one another and wherein the tonal densities of said four points are respectively represented by D_N, D_E, D_S, D_W, and wherein said means responsive to the accessing of digital representations of points within a high detail cell includes means responsive to ##EQU5## where Σ

=K (D_N +D_E +D_S +D_W) where K represents a constant.

14. The photocomoosing system of claim 11 wherein each of said cells contains four points successively displaced by 90°

from one another and wherein the tonal densities of said four points are respectively represented by D_N, D_E, D_S, D_W and wherein said means for altering the horizontal positio Δ

X proportional to L (D_N -D_S) and the vertical position Δ

Y proportional to L (D_E -D_N) where L represents the distance from the center of a cell to the center of one of said points.

15. In a photocomposing system for producing a halftone replica of an original image, an image read subassembly comprising:

means for scanning an original image to produce a digital representation of the tonal density of each of a plurality of points of said image;

comparison means for comparing digital representations adjacent points within a cell area of said image to identify a cell as being of either high or low detail;

a digital memory;

means responsive to the indentification of a low detail cell for processing the digital representations of the points therein to produce a single digital weighted value for that cell;

means for storing said single digital weighted value in said digital memory;

means responsive to the identification of a high detail cell for storing the digital representations of each of the points therein in said digital memory.

16. In a photocomposing system for producing a halftone replica of an original image in response to data stored in a digital memory representing a low detail cell in the form of a single digital weighted value and a high detail cell in the form of multiple digital representations of adjacent points within a cell area, an image record subassembly comprising:

dot character storage means storing a plurality of different halftone dot characters;

means for accessing said digital memory;

means responsive to the accessing of a digital weighted value representative of a low detail cell for selecting one of said halftone dot characters from said dot character storage means;

means responsive to the accessing of multiple digital representations representative of a high detail cell for selecting one of said halftone dot characters from said dot character storage means;

output means responsive to each halftone dot character selected for producing a visible representation of that dot character positioned at substantially the center of a cell area; and

means responsive to the accessing of digital representations associated with a high detail cell for altering the horizontal and/or vertical position of the visible representation produced in the area of that cell.

17. A photocomposing system for producing a halftone replica of an original image comprising in combination:

means for scanning said original image to produce a digital representation of the tonal density of each of a plurality of points of said image;

comparison means for comparing digital representations of adjacent points within an area of said original image forming a cell to identify each cell as being of either high or low detail;

a digital memory;

arithmetic means responsive to the identification of a low detail cell for processing the digital representations of the points therein to produce a single digital weighted value for that cell;

means for storing said single digital weighted value in said digital memory;

means responsive to the identification of a high detail cell for storing the digital representations of each of the points therein in said digital memory;

dot character storage means storing a plurality of different halftone dot characters;

means for accessing said digital memory;

means responsive to the accessing of a digital weighted value associated with a low detail cell for selecting one of said halftone dot characters from said dot character storage means;

means responsive to the accessing of digital representations of points within a high detail cell for selecting one or more of said halftone dot characters from said dot character storage means; and

output means responsive to each halftone dot character selected for producing a visible representation of that dot character within a cell area of said replica.

18. The system of claim 17 wherein said means responsive to the accessing of digital representations of points within a high detail cell includes means for selecting one of said halftone dot characters in response to the digital representation of each such point.

19. The system of claim 18 wherein said output means includes means for positioning each visible representation of a point within a high detail cell within said replica cell area at a location corresponding to the location of the point within the high detail cell of said original image.

20. In a photocomposing system for producing a halftone replica of an original image in response to data stored in a digital memory representing a low detail cell in the form of a single digital weighted value and a high detail cell in the form of multiple digital representations of adjacent points within a cell area of said original image, an image record subassembly comprising:

dot character storage means storing a plurality of different halftone dot characters;

means for accessing said digital memory;

means responsive to the accessing of a digital weighted value representative of a low detail cell for selecting one of said halftone dot characters from said dot character storage means;

means responsive to the accessing of multiple digital representations representative of a high detail cell for selecting one or more of said halftone dot characters from said dot character storage means; and

output means responsive to each halftone dot character selected for producing a visible representation of that dot character positioned within a cell area of said replica.

21. The system of claim 20 wherein said means responsive to the accessing of multiple digital representations representative of a high detail cell includes means for selecting one of said halftone dot characters in response to each such accessed digital representation.

22. The system of claim 21 wherein said output means includes means for positioning each visible representation of a point within a high detail cell within said replica cell area at a location related to the location of the point within the high detail cell of said original image.