Method of and apparatus for recording halftone image

Abstract: An apparatus records a halftone area and a solid area in a single image with respective screen angles. Encoders (13, 14) and a coordinate value generator (16) generate coordinate values (U, V) on an image plane on a recording drum. An address generator (17), a pattern data memory (18) and a comparator (19) generate a first halftone dot signal (S da ) for the halftone area. An address generator (22) and a pattern data memory (23) generate a second halftone dot signal (S db ) for the solid area. A threshold data setter (20) and a comparator (21) generate a detection signal (S s ) indicating whether or not an image signal (S i ) corresponds to the solid area. One of the first and second halftone dot signals is selected by a selector (24) according to the detection signal.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an apparatus for recording an image, such as color image scanner, and a method thereof, and more particularly, it relates to a method of and an apparatus for recording halftone image while electronically generating halftone dot patterns. Description of the Prior Art Fig. 1 is a block diagram showing the structure of a halftone image scanner of the prior art as described in EP-A-0 292 732 which is a document effective under Article 54 (3) EPC. The image scanner comprises a reading drum 2 and a recording drum 5. A color original 1 is wound around the reading drum 2, and photosensitive material 4, such as a photosensitive film, is wound around the recording drum 5. The reading drum 2 and the recording drum 5 are synchronously rotated by motors 3 and 6 along directions  and , respectively. While those drums 2 and 5 are rotating, a pick-up head 7 is driven by a motor 9 and a feed screw 11 at a prescribed speed along a subscanning direction X parallel to the axial direction of the reading drum 2. The pick-up head 7 reads the original 1 with respect to each scanning line along a circumferential direction (or main scanning direction Y) of the reading drum 5. The pick-up head 7 includes a photoelectric unit for generating color separation signals R, G and B for red, green and blue, respectively, in response to color tone and density of each pixel on the original 1. The color separation signals R, G and B are supplied to a signal processing unit 100 to be converted therein into a light modulation signal S d , as will be described later in detail. While the pick-up head 7 is reading the original 1, a recording head 8 is driven by a motor 10 and a feed screw 12 at a prescribed speed along a subscanning direction U parallel to the axial direction of the recording drum 5. The recording head 8 exposes the photosensitive material 4 and records color separation images according to the light modulation signal S d . The motor 6 is connected to an encoder 13 which generates a clock pulse K in proportion to the angle of rotation of the recording drum 5. The number of clock pulses K indicates a position of the recording head 8 along a main scanning direction V. The motor 10 is connected to an encoder 14 which generates a clock pulse L in proportion to the length of movement of the recording head 8. The number of clock pulses L indicates a position of the recording head 8 along a subscanning direction U. The color separation signals R, G and B are converted into the light modulation signal S d in the signal processing unit 100 as follows. The color separation signals R, G and B are supplied to a color computation circuit 15. The color computation circuit 15 executes color correction and gradation correction on the color separation signals R, G and B to convert them into image signals S i for yellow, magenta, cyan and black. The clock pulses K and L supplied from the rotary encoders 13 and 14 are supplied to a coordinate value generator 16 to be converted into coordinate values U and V. The coordinate values U and V are defined along the subscanning direction U and the main scanning direction V, respectively, and expressed in an unit of pixel on the recording drum 5. The coordinate values U and V are supplied from the coordinate value generator 16 to an address generator 17 and converted into reading address signals S a1 and S a2 . The reading address signals S a1 and S a2 are supplied from the address generator 17 to a pattern data memory 18. The pattern data memory 18 stores halftone pattern data S h representing threshold data which are compared with the signals S i produce halftone signals S d for yellow, magenta, cyan and black printers. The halftone pattern data S h is read out from the pattern data memory 18 in response to the reading address signal S a1 and S a2 , and supplied to a comparator 19. The comparator compares the halftone pattern data S h and the values of the image signals S i supplied from the color computation circuit 15, and generates the halftone dot signal S d for each pixel in accordance with the result of the comparison. The halftone dot signal S d is finally supplied from the comparator 19 to the recording head 8 as a light modulation signal. The recording head 8 performs on-off control of a light beam in response to the light modulation signal S d to expose the photosensitive material 4, whereby color separation images are recorded on the photosensitive material 4 with respect to each scanning line. When the halftone image scanner described above is utilized to produce color separation images for the halftone gravure process, the following problem occurs. In the process of the inverted halftone gravure, different screen angles are applied to a solid area and a halftone area in a single printing plate. Usually, color process utilizes four printing plates for yellow, magenta, cyan and black printers. Halftone areas in the four printing plates are produced with different screen angles from each other, such as 90°, 15°,75° and 45°. On the other hand, solid areas in the four printing plates are all produced with a single screen angle of 45°. The screen angle of the solid areas is determined to prevent a doctor blade, which removes excessive ink on a printing plate, from becoming parallel to walls separating hollowed cells in the printing plate. If the doctor blade became parallel to the walls, excessive ink would not be removed properly and a printed sheet would be mottled consequently. In producing one of the four printing plates, a first halftone film is produced for the solid area while the screen is manually set at about 45°. A second halftone film is produced for the halftone area with the halftone image scanner while the screen angle is automatically set by the scanner. Finally, the first and second halftone films are manually combined to form an overall halftone film with which the printing plate is produced. Those steps for producing the overall halftone film are laborious and inefficient. SUMMARY OF THE INVENTION The present invention is directed to a method of and an apparatus for recording a halftone image including a halftone area and a solid area. Accordingly, an object of the present invention is to provide a method of and an apparatus for producing a halftone image which has respective screen angles for the solid area and the halftone area. This object is achieved by an apparatus according to claims 1 - 4 and a method according to claims 5 - 8. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing the structure of a halftone image scanner of the prior art; Fig. 2A and Fig. 2B are block diagrams showing the structure of a preferred embodiment of the present invention; Fig. 3A and Figs. 4A through 4D illustrate halftone dot arrays in halftone areas; Fig.3B illustrates halftone pattern data assigned in a halftone dot in a halftone area; Fig. 5A illustrates a halftone dot array in a solid area; Fig. 5B illustrates halftone dot data assigned in an halftone dot in a solid area; and Figs. 6A and 6B are block diagrams showing the structure of another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig.2A is a block diagram showing the structure of a halftone image scanner according to the present invention. The halftone image scanner comprises a reading drum 2, a motor 3 therefor, a recording drum 5, a motor 6 therefor, a pick-up head 7, a set of a feed screw 11 and a motor 9 for feeding the pick-up head 7, a recording head 8, a set of a feed screw 12 and a motor 10 for feeding the recording head 8, an encoder 13 for the motor 6 and an encoder 14 for the motor 10, all of which are the same as those shown in Fig. 1. The halftone image scanner further comprises a signal processing unit 101. Fig. 2B is a block diagram showing the internal structure of the signal processing unit 101. The signal processing unit 101 comprises a color computation circuit 15, a coordinate value generator 16, an address generator 17, a pattern data memory 18 and a comparator 19, which are also the same as those shown in Fig. 1. The two encoders 13 and 14, and the coordinate value generator 16 as a whole function as a means A for generating positional information on the recording drum 5. The address generator 17, the pattern data memory 18 and the comparator 19 as a whole function as a means B for generating a halftone dot signal for a halftone area. Fig. 3A illustrates a halftone dot array on an image plane. The halftone dots A H1 are so arrayed as to form a square grid parallel to a main scanning direction V and a subscanning direction U on the recording drum 5. Fig. 3B illustrates halftone pattern data S h assigned in a halftone dot A H1 . The halftone dot A H1 is a square which includes a certain number of pixels A p1 arrayed along address directions i a1 and j a2 . The halftone pattern data S h is assigned to every pixel A p1 in the halftone dot A H1 , and so arrayed that its value increases from the center to the periphery of the halftone dot A H1 . When the coordinate values U and V are supplied from the coordinate value generator 16 to the address generator 17, the address generator 17 generates reading address signals S a1 and S a2 which indicate a position in the halftone dot A H1 along the address directions i a1 and j a2 . The pattern data memory 18 outputs one of the halftone pattern data S h of a particular pixel designated by the address signals S a1 and S a2 . The halftone pattern data S h is supplied to the comparator 19 and compared therein with image signals S i generated by the color computation circuit 15. The halftone pattern data S h functions as a threshold data in determining whether the particular pixel is to be exposed or not. That is, if the value of the image singal S i is greater than the halftone pattern data S h , the pixel is exposed; a halftone dot signal S da becomes "1" level accordingly. Therefore, the greater the value of the image signal S i becomes, the more the size of exposed area expands. The contours A 1 , A 2 and A 3 shown in Figs. 3A and 3B represent shapes of the exposed area in order of the value of the image signal S i . In this embodiment, the value of image signal S i is less than ten. Therefore, the peripheral region which is at the outside of the contour A 3 in halftone dot A H1 is always left unexposed. The halftone dot array of Fig. 3A corresponds to one of the four color separation images employed for the color process. Figs. 4A through 4D illustrates halftone dot arrays H y , H m , H c and H k for yellow, magenta, cyan and black printers, respectively. Those arrays have respective screen angles  s of 0°, 15°, 45° and 75°. The screen angle  s is defined as the angle between the main scanning direction V and one of the two axis of the square grid of the array, where the square grid is formed of lines drawn parallel to the address directions i a1 and j a2 through the center points O y (or O m , O c and O k ) of the halftone dots A H1 . The pattern data memory 18 stores respective halftone pattern data S h for the four color separation images, which have respective screen angles  s of 0°, 15°, 45° and 75°, for example. The signal processing unit 101 has another set of an address generator 22 and a pattern data memory 23. The pattern data memory 23 stores values of a halftone dot signal S db for a solid area. The address generator 22 and the pattern data memory 23 as a whole function as a means E for generating a halftone dot signal for a solid area. Fig. 5A illustrates a halftone dot array on an image plane. Halftone dots A H2 are so arrayed as to form a square grid which is inclined at about 45 degrees to the main scanning direction V. That is, the screen angle of the halftone dot array is about 45 degrees. Fig. 5B illustrates the halftone dot data S db assigned in a halftone dot A H2 . The halftone dot A H2 is a square which includes a certain number of pixels A p2 arranged along address directions i b1 and j b2 . The halftone dot data S db is assigned to every pixel A p2 in the halftone dot A H2 . The value of the halftone dot data S db is zero at the periphery and one at the inside region. That is, the inside region is always exposed while the peripheral region is always unexposed. In the halftone gravure process, a color separation image of the solid area on a printing plate is produced as a halftone image of a certain halftone-area rate in the range of 55 to 75%. The halftone area rate of the solid area depends on various printing conditions, such as a type of ink, especially its viscosity, printing speed, etc.. The halftone area rate of the halftone dot A H2 shown in Fig. 5B is 60%. The halftone, pattern data S h for the halftone area are prepared for respective four color separation images which have respective screen angles  s as was described before. On the other hand, the halftone dot data S db for the solid area is commom to the four color separation images. When the coordinate values U and V are supplied from the coordinate value generator 16 to the address generator 11, the address generator 17 generates reading address signals S b1 and S b2 which indicates a position in the halftone dot A H2 along the address directions i b1 and j b2 . The pattern data memory 23 outputs one of the halftone dot data S db of a particular pixel designated by the address signals S b1 and S b2 . The halftone dot signals S da and S db outputted from the comparator 19 and the pattern data memory 23 are supplied to selector 24, and one of them is selected therein as described later. The signal processing unit 101 further comprises a threshold data setter 20 and a comparator 21 which as a whole function as a means D for detecting whether the image signal S 1 corresponds to a solid area or a halftone area. The threshold data setter 20 stores threshold data D t for the image signals S i . The threshold data D t is used to detect whether the image signal S i corresponds to a solid area or to a halftone area. As was described before, the halftone area rate of the solid area depends on various printing conditions. The threshold data D t is determined considering the printing conditions and stored in the threshold data setter 20 in advance. For example, the threshold data D t may be set at the value of the image signal S i corresponding to the halftone area rate of 75%. The comparator 21 compares the threshold data D t with the value of the image signal S i . When the value of the image signals S i is no less than the threshold data D t , the comparator 21 raises a solid-area indication signal S s to "1" level. On the other hand, when the value of the image signal S i is less than the threshold data D t , the solid-area indication signal S s becomes "0" level. The selector 24 receives the halftone dot signals S da and S db from the comparator 19 and the pattern data memory 23, respectively, and selects one of the signals S da and S db according to the solid-area indication signals S s supplied from the comparator 21. The selector 24 functions as a means F for selecting one of two halftone dot signals corresponding to a solid area and a halftone area, respectively. When the pick-up head 7 is scanning a halftone area in the original 1, the value of the image signal S i is less than the threshold data D t . Therefore, the solid-area indication signal S s becomes "0" level, and the selector 24 selects the signal S da and outputs it as a light modulation signal S d1 . On the other hand, when the pick-up head 7 is scanning a solid area in the original 1, that is, when the value of the image signal S i is no less than the threshold data D t , the solid-area indication signals S s becomes "1" level. Accordingly, the selector 24 selects the signal S db and outputs the same as the light modulation signal S d1 . The recording head 8 receives the light modulation signal S d1 and performs ON-OFF control of a light beam. In the halftone area of the original 1, the recording head 8 exposes photosensitive material 4 according to the halftone pattern data S h stored in the pattern data memory 18. Respective halftone areas of the four color separation images are recorded with the screen angles of 0°, 15°, 45° and 75°, for example; that prevents the moire from appearing on a printed sheet. On the other hand, in the solid area, the recording head 8 exposes the photosensitive material 4 according to the halftone dot data S db stored in the pattern data memory 23. Therefore, respective solid areas of the four color separation images are recorded with the same screen angles of about 45°; that prevents the printed sheet from being mottled in the solid area. Consequently, the halftone area and the solid area in a single color separation image are both produced with the halftone image scanner shown in Figs. 2A and 2B. Fig. 6A is a block diagram showing the structure of a halftone image scanning system constructed as another embodiment of the present invention. The halftone image scanning system comprises a halftone image scanner 300, an image signal generating apparatus 400 and a graphic data generating apparatus 500. The halftone image scanner 300 does not include the reading drum 2, the motor 3, the pick-up head 7, the feed screw 11 and the motor 9 shown in Fig. 2A. Instead, the halftone image scanner 300 receives image signals Sia and Sic from the image signal generating apparatus 400 and the graphic data generating apparatus 500, respectively. The image signal generating apparatus 400 generates the image signal S ia which represents four color separation images about scenery, a human figure or the like. The image signal generating apparatus 400 may be constructed as a reading scanner having a reading drum and a pick-up head similar to those shown in Fig. 2A, a system for editing given image data, or a color page make-up system. The graphic data generating apparatus 500 generates graphic data signal S ic which represents a drawing including an outline and a tint area inside and/or outside of the outline. The tint area is an area having an uniform halftone area rate. The graphic data signal S ic represents a drawing with respect to a pixel whose size is one-fifth of the pixel A p1 defined for the image signal S ia (or S i in Fig. 2A), to reproduce the outline in the drawing minutely. Therefore, frequency of the graphic data signal S ic is five times that of the image signal S ia . Fig. 6B is a block diagram showing the internal structure of the signal processing unit 102 of the halftone image scanner 300. The unit 102 has the structure which is formed by adding an address generator 17c, a pattern data memory 18c, a comparator 19c, selectors 24c and 25, a threshold data setter 20c and a comparator 21c to the signal processing unit 101 of Fig. 2B, and by omitting the color computation circuit 15 from the unit 101. The components 16, 17, 18, 19, 20, 21, 22, 23 and 24 treat the image signal S ia as those of Fig. 2A treat the image signal S i . The components 17c, 18c, 19c and 24c are connected parallel to the components 17, 18, 19 and 24. Upon receiving the coordinate values U and V from the coordinate value generator 16, the address generator 17c generates reading address signals S c1 and S c2 , which are similar to the reading address signals S a1 and S a2 . The pattern data memory 18c stores halftone pattern data S hc with respect to a pixel whose size is one-fifth of the pixel A p1 . The halftone pattern data S hc are prepared for respective four color separation images which have respective screen angles to prevent the moire. The halftone pattern memory 18c outputs one of the halftone pattern data S hc at a particular pixel designated by the address signals S c1 and S c2 . The halftone pattern data S hc is supplied to the comparator 19c and compared therein with the graphic data signal S ic . If the value of the graphic data signal S ic is greater than the halftone pattern data S hc , the halftone dot signal S dc from the comparator 19c becomes "1" level. On the contrary, if the value of the graphic data signal S ic is no more than the halftone pattern data S hc , the halftone dot signal S dc becomes "0" level. The threshold data setter 20c and the comparator 21c has the same functions as the threshold data setter 20 and the comparator 21. The threshold data setter 20c stores a threshold data D tc which is used to detect whether the graphic data signal S ic corresponds to a solid area or to a halftone area. The threshold data D t and D tc stored in the threshold data setters 20 and 20c may be different. When the value of the image signal S ic is no less than the threshold data D tc , the comparator 21c raises a solid-area indication signal S sc to "1" level. On the other hand, when the value of the image signal S ic is less than the threshold data D tc , the solid-area indication signal S sc becomes "0" level. The selector 24c receives the halftone dot signals S dc and S db from the comparator 19c and the pattern data memory 23, and selects one of the signals S dc and S db according to the solid-area indication signal S sc . The graphic data generating apparatus 500 supplies a flag signal S f as well as the graphic data signal S ic to the signal processing unit 102. The flag signal indicates whether or not the apparatus 500 is supplying the graphic data signal S ic . When the flag signal indicates that the graphic data signal S ic is being supplied, the selector 25 selects the output signal of the selector 24c in response to the flag signal S f , and outputs the same as a light modulation signal S d2 . On the other hand, when the flag signal does not indicate that the graphic data signal S ic is being supplied, the selector 25 selects the output signal of the selector 24, and outputs the same as the light modulation signal S d2 . The halftone image scanning system can produce color separation images on the basis of the image signal S ia or the graphic data signal S ic . In either case, a solid area and a halftone area in a single color separation image are recorded with respective halftone data, more specifically, the halftone pattern data S h or S hc representing a changeable shape of a halftone dot in the halftone area and the halftone dot data S db representing a shape of a halftone dot in the solid area, respectively. Because the halftone dot data S db for the solid area has the screen angle of about 45 degrees, the mottling on a printed paper is prevented. The halftone dot data S db may be of other type, such as data representing ground tint, data representing a solid area with different screen ruling from a halftone area, or the like. Namely, any halftone data can be used on condition that different types of halftone data are applied to the solid area and the halftone area. The present invention may be also applied to an electron beam carving apparatus disclosed in Japanese patent Laying Open Gazette JP-A-58 072 450. According to the present invention, a solid area in an original is automatically detected and recorded with different halftone data from a halftone area. Because the solid area and the halftone area in a single image are sequentially recorded without manual labor, such as pasting films of the solid areas on color separation films, printing process can be efficiently executed. Further, when the halftone data with the screen angle of about 45 degrees are used for the solid area commonly to the color separation images in the inverted gravure process, a printed sheet can be prevented from being mottled in the solid area. The features disclosed in the foregoing description, in the claims and/or in the accompanying drawings may, both, separately and in any combination thereof, be material for realising the invention in diverse forms thereof.

Claims: An apparatus for scanning an image-recording medium (4) to record a colour image on said image-recording medium (4) which is usable to obtain colour plates for colour-printing, said colour image including a halftone area and a solid area which is defined as an area having a halftone dot rate larger than a prescribed threshold rate, comprising: image reader means for scanning an image of a colour original (1) to generate colour image signals (Si) representing said image of said colour original for respective colour components, position detector means (13, 14) for generating positional signals (K, L) representing a position on said an image-recording medium (4) currently scanned for image-recording, first memory means (18) for storing predetermined screen threshold signals (Sh) defining halftone dot screens of different angles for respective colour components on said image-recording medium (4), and outputting said screen threshold signal (Sh) in response to said positional signals (K, L), first comparator means (19) for comparing said colour image signals (Si) with said screen threshold signals (Sh) to generate halftone dot signals (Sda) for respective colour components, second memory means (23) for storing predetermined dot signals (Sdb) for recording screen dots of an angle common to respective colour components on said image-recording medium (4), and outputting said constant dot signals (Sdb) in response to said positional signals (K, L), second comparator means (21, 21c) for comparing said colour image signals (Si) with a threshold level (Dt) corresponding to said predetermined threshold rate to generate a switching signal (Ss), selector means (24) for selecting one of said halftone dot signals (Sda) and said predetermined dot signals (Sdb) in response to said switching signal (Ss) to generate image-recording signals (Sd1) for respective colour components, and recording means for scanning said image-recording medium (4) with image-recording beam in response to said image-recording signals (Sd1) to record said colour image for respective colour components on said image-recording medium (4). The apparatus of claim 1, characterized in that said angle common to respective colour components is about 45 degrees on an image plane defined on said image-recording medium (4). The apparatus of claim 2, characterized in that said colour components are yellow, magenta, cyan and black, and said halftone dot screens of different angles are about 0, 15, 45 and 75 degrees on said image plane, respectively. The apparatus of claim 1, characterized in that said image reader means comprises means for scanning said image of a colour original (1), and means for generating a plural sets of colour image signals (Sia, Sic) representing said image of said colour original for respective colour components at different resolutions, said first memory means comprises means (18, 18c) for storing a plural sets of predetermined screen threshold signals (Sh, Shc), defining halftone dot screens of different angles for respective colour components on said image-recording medium (4) at different resolutions, and outputting respective sets of threshold signals (Sh, Shc) in response to said positional signals (K, L), respectively, said first comparator means (19) comprises means (19, 19c) for comparing said plural sets of colour image signals (Sia, Sic) with said plural sets of screen threshold signals (Sh, Shc) to generate a plural sets of halftone dot signals (Sda, Sdc) for respective colour components, respectively, said second comparator means comprises means (21, 21c) for comparing said plural sets of colour image signals (Sia, Sic) with a plurality of threshold levels (Dt, Dtc) to generate a plurality of switching signals (Ssa, Ssc), and said selector means comprises means (24, 24c, 25) for selecting one of said plural sets of halftone dot signals (Sda, Sdc) and said predetermined dot signals (Sdb) in response to said plurality of switching signals (Ssa, Ssc) and a selection signal (Sf) inputted, to generate said image-recording signals (Sd2) for respective colour components. A method for scanning an image-recording medium (4) to record a colour image on said image-recording medium (4) which is usable to obtain colour plates for colour-printing, said colour image including a halftone area and a solid area which is defined as an area having a halftone dot rate larger then a prescribed threshold rate, comprising the steps of: a) scanning an image of a colour original (1) and generating colour image signals (Si) representing said image of said colour original for respective colour components, b) generating positional signals (K, L) representing a position on said an image-recording medium (5), currently scanned for image-recording, c) storing predetermined screen threshold signals (Sh) defining halftone dot screens of different angles for respective colour components on said image-recording medium (4), and outputting said screen threshold signals (Sh) in response to said positional signals (K, L), d) comparing said colour image signals (Si) with said screen threshold signals (Sh) and generating halftone dot signals (Sda) for respective colour components, e) storing predetermined dot signals (Sdb) and recording screen dots of an angle common to respective colour components on said image-recording medium (4), and outputting said constant dot signals (Sdb) in response to said positional signals (K, L), f) comparing said colour image signals (Si) with a threshold level (Dt) corresponding to said predetermined threshold rate and generating the switching signal (Ss), g) selecting one of said halftone dot signals (Sda) and said predetermined dot signals (Sdb) in response to said switching signal (Ss) and generating image-recording signals (Sd1) for respective colour components, and h) scanning said image-recording medium (4) with image-recording beam in response to said image-recording signals (Sd1) and recording said colour image for respective colour components on said image-recording medium (4). A method in accordance with claim 5, characterized in that for said angle common to respective colour components a size of 45 degrees on an image plane defined on said image-recording medium (4) is used. A method in accordance with claim 6, characterized in that yellow, magenta, cyan and black are used as said colour components for said halftone dot screens of different angles about 0, 15, 45 and 75 degrees on said image plane are used, respectively. A method in accordance with claim 5, characterized in that the step a) for scanning an image of a colour original and generating colour image signals comprises the steps a-1) for scanning said image of a colour original, and a-2) for generating a plural sets of colour image signals (Sia, Sic) representing said image of said colour original for respective colour components at different resolutions, the step c) for storing predetermined screen threshold signals (Sh) and outputting said screen threshold signals (Sh) comprises the step c-1) for storing a plural sets of predetermined screen threshold signals (Sh, Shc) defining halftone dot screens of different angles for respective colour components on said image-recording medium (4) at different resolutions, and for outputting respective sets of threshold signals (Sh, Shc) in response to said positional signals (K, L) respectively, the step d) for comparing said colour image signals (Si) with said screen threshold signals (Sh) and generating halftone dot signals (Sda) comprises the step d-1) for comparing said plural sets of colour image signals (Sia, Sic) with said plural sets of screen threshold signals (Sh, Shc) and for generating a plural sets of halftone dot signals (Sda, Sdc) for respective colour components, respectively, the step f) for comparing said colour image signals (Si) with a threshold level (Dt) comprises the step f-1) for comparing said plural sets of colour image signals (Sia, Sic) with a plurality of threshold levels (Dt, Dtc) and for generating a plurality of switching signals (Ssa, Ssc), and the step g) for selecting one of said halftone dot signals (Sda) and said predetermined dot signals (Sdb) comprises the step g-1) for selecting one of said plural sets of halftone dot signals (Sda, Sdc) and said predetermined dot signals (Sdb) in response to said plurality of switching signals (Ssa, Ssc) and a selection signal (Sf) inputted and the step g-2) for generating said image-recording signals (Sd2) for respective colour components.

Inventors: KASAMATSU TOSHIO DAINIPPON SCR, YAMAGUCHI TAKUYA DAINIPPON SCR, KASAMATSU, TOSHIO DAINIPPON SCREEN MFG. CO., LTD., YAMAGUCHI, TAKUYA DAINIPPON SCREEN MFG. CO., LTD.
Applicants: DAINIPPON SCREEN MFG, DAINIPPON SCREEN MFG. CO., LTD.
Application year: 1989
Main classification: H04N1
IPC codes: G03F5, H04N1, G06T5