GB2196508A

GB2196508A - Method of forming halftone dots

Info

Publication number: GB2196508A
Application number: GB08723606A
Authority: GB
Inventors: Tadashi Miyakawa
Original assignee: Fuji Photo Film Co Ltd
Current assignee: Fujifilm Holdings Corp
Priority date: 1986-10-13
Filing date: 1987-10-08
Publication date: 1988-04-27
Also published as: GB8723606D0; DE3734686A1; JPS6397067A

Description

GB2196508A 1 SPECIFICATION to form a desired image. The formed image has

gradations of tone which do not exceed in Method of forming halftone dots number the gradations of each halftone dot of the reproduced image, Le, 64 gradations of

BACKGROUND OF THE INVENTION 70 tone.

The present invention relates to a method of forminWhalftone dots in a platemaking scan- SUMMARY OF THE INVENTION ner, and more particularly to a halftone-dot The present invention results from studies forming method utilizing a template memory to provide a method of forming halftone dots having a threshold pattern with a plurality of 75 by employing a template memory capable of predetermined minute points (referred to halfreproducing detailed and sufficient gradations tone-dot elements) for forming halftone dots. of tone even if the screen ruling is large.

Reproduced halftone-dot images are One aspect of the present invention pro- smoother in outlines and more detailed as the vides a method of forming halftone dots by number of halftone dots per unit length, i.e., 80 employing a halftone- dot forming template the screen ruling, is greater. As disclosed in having a plurality of divided minute grid ele Japanese Laid-Open Patent Publication No. 60 ments with respective predetermined threshold (1985)-132465, for example, each halftone levels assigned thereto, and comparing the dot comprises a plurality of divided minute signal level of an image signal with the thresh grid elements. It is however difficult to reduce 85 old level of each of the grid elements, the the area of each of the grid elements below a method comprising the steps of provding a certain size due to optical or mechanical limita- composite halftone-dot forming template com tions. Even if the area of each grid element posed of two or more (n) of the halftone-dot were made smaller than such a certain size, it forming template arranged side by side, and would take a considerable time to convert the 90 varying one of the n threshold pattern tem halftone dots to black dots. Since it would be plates of the composite halftone-dot forming quite time-consuming to obtain a desired im- template according to a change in the level of age, the area of each grid element is limited the image signal commensurate with one of necessity. Where it is desired to reproduce threshold gradation of the composite halftone an image with a wide range of gradations of 95 dot forming template.

tone, a halftone-dot forming template may be Another aspect of the present invention pro- employed which has many grid elements per vides a method of forming halftone dots, halftone dot. The greater the number of grid wherein the threshold levels of the respective elements in one halftone dot, however, the grid elements of the composite halftone-dot smaller the screen ruling becomes, making it 100 forming template are of different values from more difficult to reproduce an image with a each other, those grid elements the difference smoother outline and greater image detail. between which corresponds to one threshold Conversely, the greater the screen ruling, the gradation belonging to different ones, respec smaller the number of minute grid elements tively, of the the halftone- dot forming tem making up a halftone dot, resulting in an insu- 105 plates.

fficient number of gradations of tone. The above and other features and advan- FIG. 1 of the accompanying drawings illus- tages of the present invention will become trates a conventional threshold pattern tem- more apparent from the following description plate for forming a halftone dot. The threshold when taken in conjunction with the accom pattern template can produce 64 gradations 110 panying drawings in which a preferred em (1, 2,., 64) of tone. The threshold pattern bodiment of the present invention is shown template has a plurality of minute grid ele- by way of illustrative example.

ments with respective threshold levels being expressed by respective numerals. When an BRIEF DESCRIPTION OF THE DRAWINGS image signal having a level higher than the 115 FIG. 1 is a diagram showing a halftone-dot indicated threshold level of a grid element is forming template having 64 threshold levels; applied to a halftone dot generator, the gener- FIG. 2 is a block diagram of an arrangement ator produces a halftone-dot blackening signal for carrying out a method according to the to blacken the grid element. When an image present invention; signal having a level lower than the indicated 120 FIG. 3 is a diagram of a halftone-dot formthreshold level of a grid element is applied to ing template having 128 threshold levels, the halftone dot generator, the grid element is which can be employed in the method of the not blackened. The threshold levels of the re- present invention; spective grid elements become lower toward FIG. 4 is a diagram of a halftone-dot form- the center of the dot region and higher toward 125 ing template for an image signal having 8 bits the outer edge of the dot region. = 256 gradations of tone, which can be emIt has been conventional practice to employ ployed in the method of the present invention; as many such templates combined together as and desired to meet the size of an original image, FIG. 5 is a diagram showing a halftone-dot and to blacken selected minute grid elements 130 template with selected minute grid elements 2 GB2196508A 2 blacked by the method of the present inven- such that the threshold levels of the respec tion. tive grid elements become lower toward the center of the dot region and higher toward the DETAILED DESCRIPTION OF THE PREFERRED outer edge of the dot region, as with the con

EMBODIMENT 70 ventional pattern shown in FIG. 1. However, FIG. 2 shows in block form an arrangement the successive threshold levels are allotted alfor carrying out a halftone-dot forming method ternately to the righthand and lefthaq.0 halves of the present invention. In FIG. 2, the arrows of the template, as can easily be understood directed to and away from various compo- by confirming the numerals in the grid ele nents or blocks of the arrangement indicate 75 ments of FIG. 3. Halftone dots formed by us the directions in which signals flow, and the ing the halftone-dot forming threshold pattern lines with respective single short slanting lines template shown in FIG. 3 can have 128 gra accompanied by nearby numerals Z (= 3, 6, dations of tone, for example, which are twice 8) represent Z-bit signal lines. An X counter the 64 gradations of tone produced even if 20 for counting pixels in a main scanning di- 80 two of the conventional halftone-dot forming rection X of a halftone-dot forming template threshold pattern template shown in FIG. 1 comprises a programmable modulo-N counter. are arranged side by side.

A Y counter 24 for counting pixels in a sub- The line synchronizing signal LSYNC shown scanning direction Y of a halftone-dot forming in FIG. 2 is a pulse indicating the starting of template comprises a programmable modulo-M 85 one scanning line in the main scanning line X.

counter. The X counter 20 has a clock input The pixel clock pulse signal PCLK is a clock terminal CK supplied with pixel clock pulses pulse corresponding to one pixel. The tem PCLK each generated per pixel, and an initializ- plate data signal TDATA is a threshold signal ing terminal CLR to which a line synchronizing which is produced in synchronism with the signal LSYNC is applied via an inverter 22. 90 pixel clock signal PCLK. As described above, The line synchronizing signal LSYNC is also the X counter 20 and the Y counter 24 are applied to a clock input terminal CK of the Y programmable modulo-N and modulo-M coun counter 24. ters, respectively, and are set to the sizes (the The X counter 20 produces 6-bit address number of pixels) of the threshold pattern data XA as output data which is applied to an 95 template in the main scanning direction X and address input terminal A0, of a template the subscanning direction Y. For example, memory 26. The Y counter 24 produces 6-bit where the threshold pattern template contains address data YA as output data which is ap- 16 pixels in the main scanning direction and 8 plied to an address input terminal A,.,, of the pixels in the subscanning direction, the X template memory 26. The template memory 100 counter 20 is set as a modulo16 counter, 26 has another address input terminal A12-14 and the Y counter 24 as a modulo-8 counter.

which is supplied with a page selection signal The page selection signal PAGE is a signal PAGE. representative of which type of template is to The template memory 26 produces as an be used among a plurality of templates stored output signal 8-bit template data TDATA that 105 in the template memory 26. A suitable tem is applied to an input terminal B of a comparaplate type is selected by the page selection tor 28, with its other input terminal A being signal PAGE according to the characteristics of supplied with an image data signal PDATA a photoelectric transducer for scanning an from an 8-bit A/D converter (not shown). The original image to read the same, the screen comparator 28 produces a halftone-dot black- 110 ruling, the screen angle, and other factors. As ening signal which is sent to a halftone-dot an example, FIG. 4 shows a threshold pattern generator (not shown). template for forming a halftone-dot template Operation and advantages of the arrange- to be blackened by the 8-bit image data ment, thus constructed, for carrying out the PDATA shown in FIG. 2. The threshold grada halftone-dot forming method will be described 115 tion between one threshold level and a next with reference to FIGS. 3 through 5. threshold level is predetermined as 2/256.

FIG. 3 shows, by way of example, a half- The threshold pattern template is prepared in tone-dot forming template for forming halftone the template memory 26 so as to be output dots before they are blackened which are therefrom, by being addressed by the page formed by the halftone-dot forming method. 120 selection signal PAGE. The X counter 20 is The template has a size equal to the com- initialized by the line synchronizing signal bined size of two templates, arranged horizon- LSYNC (address data XA 0), and applies tally side by side, each of which is equivalent address data XA in the X direction of the to the conventional template illustrated in FIG. template to the address input terminal A0, of 1. The halftone-dot forming template is char- 125 the template memory 26 in each cycle of 16 acterized in the order or sequence of thresh- pixels. Address data in the Y direction of the old levels assigned to the respective minute template is now updated during one scanning grid elements of the template. The pattern of period. With the page selection signal PAGE threshold levels in each of the righthand and and the address data XA, YA being used as lefthand halves of the template is arranged 130 an address, one item of template data TDATA 3 GB2196508A 3 relative to the threshold level of each grid ele- provding a composite halftone-dot forming ment of hte halftone-dot forming threshold template composed of two or more (n) of said pattern template is issued from the template halftone-dot forming template arranged side by memory 26, and then compared with an im- side; and age signal PDATA by the comparator 28, 70 varying one of the n threshold pattern tem- which then sends a halftone-dot blackening plates of said composite halftone-dot forming signal DOT to the non-illustrated halftone dot template according to a change in the level of generator. said image signal commensurate with one The threshold levels of the respective grid threshold gradation of said composite half- elements of the threshold pattern template 75 tone-dot forming template.

memory 26 correspond to 8 bits = 256 (the 2. A method according to claim 1, wherein threshold levels are incremented by a 2/256 the threshold levels of the respective grid ele threshold gradation). Therefore, if 35 (obtained ments of said composite halftone-dot forming as a result of conversion from 8-bit data to a template are of different values from each decimal number) is applied as the image signal 80 other, those grid elements the difference be- PDATA to the comparator 28, the comparator tween which corresponds to one threshold 28 compares the numeral 35 successively gradation belonging to different ones, respec with the threshold levels of the grid elements tively, of the said halftone-dot forming tem according to the sequence of the address plates.

data. 85 3. A method according to claim 1 substan- Now, A = 35 and B = a threshold level at tially as described herein with reference to the input terminals of the comparator 28. Figures 2, 3 or 4 of the accompanying draw When A 2-- B, i.e., 35 - threshold level, a ings, halftone-dot blackening signal DOT is gener- 4. A halftone-dot image obtained by em- ated by the comparator 28 for producing a 90 ploying a method as claimed in any one of blackened halftone-dot template as shown in claims 1 to 4.

FIG. 5.

While the number of minute grid elements Published 1988 at The Patent Office, State House, 66/71 High Holborn, London WC 1 R 4TP. Further copies may be obtained from of the halftone-dot forming template is 128 in The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD.

Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.

the illustrated embodiment, a halftone-dot forming template with a smaller number of minute grid elements than 128 may be em ployed, and in such a case, two or more grid elements of the halftone-dot forming template may have an identical threshold level.

With the method of the present invention, as described above, there are provided two or more n threshold pattern templates arranged side by side for forming halftone dots, and one of the n combined threshold pattern tem plates is varied according to one threshold gradation change in an image signal. There fore, a composite halftone-dot forming tem plate is constructed which has gradations of tone that are n times the number of grid ele ments of each of the n halftone-dot forming templates.

A composite halftone-dot forming template may comprise a vertical array of halftone-dot forming templates.

Although a certain preferred embodiment has been shown and described, it should be understood that many changes and modifica tions may be made therein without departing from the scope of the appended claims.

Claims

1. A method of forming halftone dots by employing a halftone-dot forming template having a plurality of divided minute grid ele ments with respective predetermined threshold levels assigned thereto, and comparing the signal level of an image signal with the thresh old level of each of the grid elements, said method comprising the steps of: