Apparatus for generating a screened reproduction of an image

Abstract: A technique for generating a screened reproduction of an image comprising the steps of providing a representation of an original having input density values representing the grey levels of various locations of the original for a given color separation, defining a desired screen dot arrangement for the image, writing screen dots in a line by line fashion, wherein each screen dot is made up of a plurality of lines whose length and location determines the dot configuration and whose length and location is determined by an analog operation employing the input density values of the original and the desired screen dot arrangement. Apparatus employing the method is also provided.

Description: FIELD OF THE INVENTION The present invention relates to screened image reproduction and more particularly to a method and apparatus for electronically generating a screened reproduction of an image. BACKGROUND OF THE INVENTION Electronic screening for image reproduction is well known in the art. According to a well known technique described in U.S. Patent 4,456,924 of the present assignee, for each screened dot, a multiplicity of coordinates of a laser plotter are translated into screen-cell coordinates. A corresponding cell memory is preloaded by threshold values, defining a cell memory matrix. Input digitized scanned density values of an image, such as a color separation, are compared with the threshold values, cell by cell. The results provide a on/off control input for a laser plotter. U.S. Patent 4,149,183 describes an electronic halftone generator wherein a halftone signal is generated by pulse width modulating or comparing the scanned or video signal with a periodic analog signal having two frequencies and phases to create a dot pattern output which is a function of the frequency and phase of the two combined modulating signals. The halftone reproduction generated has variable dot configurations that are controllable to enable both rotation of the dot pattern and geometric modifications of the dot pattern. US-A-4040094 describes a method and apparatus for the electronic screening of a graphic image to be reproduced by printing. The density of the graphic image is determined repetitively during the digital generation of each dot which will form a part of the half tone image when produced and the dot is modified according to the changes in density. Thus the structure of any half tone dot may be modified during its construction. The invention provides a method as set out in claim 1. Preferably, each of said plurality of lines has infinite resolution along its length. In an embodiment the method includes the following steps: storing the input density values of the original at a first spatial resolution; and storing the threshold values of the desired screen dot arrangement in the form of analog functions. In another embodiment the method includes the following steps: storing the input density values of the original at a first spatial resolution; and storing the threshold values of the desired screen dot arrangement at a second spatial resolution different from the first resolution. In an embodiment the spacing between adjacent sections corresponds to the line to line spatial resolution of a plotter used for writing the screen dots. The step of storing the threshold values of the desired screen dot for a given section may include the step of digitally storing the values at a third sampling resolution, which may be different from said first and second resolutions. Each said section may be associated with a line in an x-y plane and each said line in said x-y plane may lie at an angle corresponding to a screening angle. The invention also provides apparatus as set out in claim 8. Preferably each of said plurality of lines has infinite resolution along its length. In an embodiment the apparatus also includes: means for storing the input density values of the original at a first spatial resolution; and means for storing the threshold values of the desired screen dot arrangement in the form of analog functions. In an embodiment the apparatus also comprises: means for storing the input density values of the original at a first spatial resolution; and means for storing the threshold values of the desired screen dot arrangement at a second spatial resolution different from the first resolution. The spacing between adjacent sections may correspond to the line to line spatial resolution of a plotter used for writing the screen dots. In an embodiment the means for storing the threshold values of the desired screen dot arrangement for a given section includes means for digitally storing the values at a third sampling resolution, which may be different from said first and second resolutions. Each said section may be associated with lines in an x-y plane and each line in said x-y plane may lie at an angle corresponding to a screening angle. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more fully understood and appreciated from the following detailed description, taken in conjunction with the drawings in which: Fig. 1 is a simplified block diagram illustration of a process color printing system constructed and operative in accordance with a preferred embodiment of the present invention; Fig. 2 is an illustration of one color separation of a typical color original; Fig. 3 is an enlarged illustration of a small portion of the halftone color separation of Fig. 2; Fig. 4 is an illustration of pixel-by-pixel input density values for the small portion of the color separation of Fig. 2; Fig. 5 is a more detailed illustration of the configuration of screen dots in three dimensional space superimposed on part of the illustration of Fig. 4; Fig. 6A is an even more detailed illustration of a line by line written screen dot produced in accordance with a preferred embodiment of the invention for a uniform input density; Fig. 6B is an even more detailed illustration of a line by line written screen dot produced in accordance with a preferred embodiment of the invention for a non-uniform input density; Fig. 7 is a simplified block diagram illustration of apparatus for screen dot generation operative in accordance with a preferred embodiment of the invention; Fig. 8 is a more detailed simplified block diagram of the apparatus for screen dot generation of Fig. 7; and Figs. 9A - 9D are simplified timing diagrams illustrating the operation of the circuitry of Fig. 8. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Reference is now made to Fig. 1, which illustrates a system for generating a screened reproduction of an image constructed and operative in accordance with a preferred embodiment of the present invention. The system preferably comprises a color separation scanner 10, such as a Scitex Smart Scanner, manufactured and sold by Scitex Corporation Ltd. of Herzlia, Israel, which is adapted to provide a digital color separation output of a color original. The digital output of scanner 10 is normally stored on a image data disc 12 or any other suitable storage medium, which is accessible by a CPU 14, such as an Intel 80386. Interfacing with the CPU 14 is an interactive workstation 16, such as a Scitex Prisma, manufactured and sold by Scitex Corporation Ltd. of Herzlia, Israel. CPU 14 interfaces with screen dot generation circuitry 18, which in turn provides a control output to laser beam control circuitry 24 in a laser plotter 26, such as a Raystar, manufactured and sold by Scitex Corporation Ltd. of Herzlia, Israel. Laser plotter 26 produces halftone film color separations 28 which are employed in a conventional process color printing press 30, to produce process color prints. Reference is now made to Figs. 2 and 3. Fig. 2 illustrates a typical halftone color separation which is stored on disc 12. Fig. 3 illustrates in enlarged detail, a small area 32 indicated on Fig. 2. It is noted that the gray level over area 32 varies thereacross. The halftone color separation in general and the small area 32 in particular are hereinafter termed the input image and are divided into a first multiplicity of pixels 34 which are arranged along coordinates I x and I y . Pixels 34 typically have a resolution of 100 - 400 pixels per inch along one of the coordinate axes of the input image. Each average gray level for a pixel 34 is represented digitally by an input density level. There are typically provided 256 different input density levels, 0 being the lightest and 255 being the blackest. Fig. 4 illustrates the input density values for the pixels 34 of Fig. 3. Fig. 5 illustrates an output dot density threshold distribution 40 for a conventional dot which is used in process color printing. Alongside distribution 40 there is illustrated part of the input density map of Fig. 4, corresponding to a region there identified by reference numeral 42. It is noted that region 42 includes different density values for different input pixels 34. The output dot density threshold distribution 40 is a three dimensional representation of a screen dot arrangement, including a two dimensional space 44 corresponding to the spatial dimensions of the image and a one dimensional value 46 corresponding to the dot density threshold value at each spatial location thereon. Distribution 40 defines a plurality of sections 48 at a selected angle a in the two dimensional space, corresponding to a screening angle, here 90 degrees, for example. The sections 48 are at a section-to-section spacing 50 corresponding to the line to line writing resolution of a plotter, typically 800 - 5000 lines per inch, each section defining threshold values for locations along a line 52 in the two dimensional space. A collection of points, such as those indicated on distribution 40, illustrate values in the three dimensional space which are stored to define the distribution. Alternatively functions which define, inter alia, the collection of points may be stored to define the distribution. It will thus be appreciated that in accordance with a preferred embodiment of the present invention and in contrast to the prior art, the screen dot thresholds are stored either as analog functions or as discrete values in three dimensional space representing such functions. The stored functions or points are used to calculate suitable sections 48 represented by points 54 thereon, corresponding to a selected resolution, mesh (screen dots per unit length) and screen angle. This calculation may involve extrapolation or interpolation as appropriate and is carried out in CPU 14, typically off line. Referring now additionally to Fig. 7, it will be understood that the points 54 are supplied to a screen memory 60, and the input density values of pixels 34 are supplied to a picture memory 62. Screen memory 60 and picture memory 62 are typically controlled by a read/write and timing control unit 61 described in more detail in Fig. 8. The contents of memories 60 and 62 are supplied via respective digital to analog converters 64 and 66 to respective low pass filters 68 and 70. It is noted that under certain circumstances, low pass filter 70 may be omitted. It will be appreciated that the low pass filter 68 is adapted to extrapolate and interpolate the points 54 stored in memory 60 so as to provide an accurate reproduction of the dot density threshold. The inclusion of low pass filter 70 depends on the quality of the input image. The outputs of low pass filters 68 and 70 are supplied to a comparator 72, which provides an output indication to expose control circuitry 24, indicating when the laser plotter 26 is to write. It is noted that there may also be provided various additional control functions, such as intensity control, to enhance the operation of the laser plotter 26. Reference is now made to Figs. 8 and 9A - 9D which respectively are a more detailed illustration of the circuitry of Fig. 7 and a timing diagram therefor. As can be seen from Fig. 8, read/write and timing control unit 61 comprises data buffers 81 and 90, data registers 83 and 92, write control units 84 and 94, address latches 86 and 96 and read control units 88 and 98 for controlling screen memory 60 and picture memory 62, respectively. A timing control unit 100 controls the timing of the signals sent to the digital to analog converters 64 and 66. The analog output of converter 64 for an example multiplicity of screen dots stored in screen memory 60 is shown in Fig. 9A and the analog output of converter 66 for an example plurality of input density values stored in picture memory 62 is shown in Fig. 9B. Fig. 9C illustrates the operation of comparator 72 in comparing the two signals shown in Figs. 9A and 9B, where the signal of Fig. 9B is shown as a dotted line. Fig. 9D illustrates the resultant signal produced by the comparator 72 and used to drive expose control circuitry 24. Reference is now made to Fig. 6A, which illustrates the construction of an output screen dot for a portion 80 of the image illustrated in Figs. 3 and 4. The input density values for the pixels 34 are indicated within each pixel and it is seen that they are all uniform. The laser plotter 26 defines a plurality of parallel lines 82, having a spacing 50, and whose beginning and end are determined by the output of comparator 72. The screen dot is a composite of such lines. In a case wherein the input density values represented by the dot, i.e. in the two dimensional space 42 underlying the density threshold distribution 40 in Fig. 5, are uniform, the dot will have a generally symmetrical shape, as illustrated. In accordance with a preferred embodiment of the present invention, each written line has infinite resolution along its length, because the on/off control inputs to the laser plotter are arrived at by comparison of two generally continuous analog signals. Reference is now made to Fig. 6B, which illustrates the construction of an output dot for portion 42 of the image illustrated in Figs. 3 and 4. The input density values for the pixels 34 are indicated within each pixel in Fig. 4 and it is seen that they differ from each other. It is appreciated from a consideration of Fig. 6B that the length of the lines 82 along each portion of the dot is a function of the input density value for the pixel 34 over which such lines lie. Thus, where the input density value is relatively low, such as 100, the dot lines 82 correspond to a relatively small dot area and where the input density value is high, such as 150, the dot lines 82 correspond to a relatively large dot area. An output dot which overlies pixels 34 having different input density values is thus asymmetric in a manner generally corresponding to the asymmetry of the input density values of the pixels 34 represented by the output dot. It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims which follow:

Claims: A method of generating a screened reproduction of an image comprising the steps of: providing a representation of an original (42) having input density values (34) representing the grey levels of various locations of the original for a given color separation; providing a desired screen dot arrangement for the image, characterized in that said desired screen dot arrangement is a three dimensional screen dot arrangement (40) having any desired shape; defining, based on said three dimensional screen dot arrangement (40), sections (48) having a spacing (50) therebetween; storing said screen dot arrangement (40) with said sections (48) and spacing (50) therebetween; and comparing the input density values of various locations with corresponding ones of threshold values represented by said sections (48); and writing screen dots in a line by line fashion, wherein each screen dot is made up of a plurality of lines, the length and location of each line is determined by said comparing step and the distance between said lines is determined by said spacing (50). A method according to claim 1 and wherein each of said plurality of lines has infinite resolution along its length. A method according to claim 1 and also including the following steps: storing the input density values of the original (42) at a first spatial resolution; and storing the threshold values of the desired screen dot arrangement in the form of analog functions. A method according to claim 1 and also including the following steps: storing the input density values of the original (42) at a first spatial resolution; and storing the threshold values of the desired screen dot arrangement at a second spatial resolution different from the first resolution. A method according to claim 1 and wherein the spacing (50) between adjacent sections (48) corresponds to the line to line spatial resolution of a plotter (26) used for writing the screen dots. A method according to claim 4 and wherein the step of storing the threshold values of the desired screen dot for a given section (48) includes the step of digitally storing the values at a third sampling resolution, which may be different from said first and second resolutions. A method according to claim 1 and wherein each said section (48) is associated with a line in an x-y plane (44) and each said line in said x-y plane (44) lies at an angle corresponding to a screening angle. Apparatus for generating a screen reproduction of an image comprising: means for providing a representation of an original having input density values representing the grey levels of various locations of the original for a given color separation; means for providing a desired screen dot arrangement for the image, characterized in that said desired screen dot arrangement is a three dimensional screen dot arrangement (40) having any desired shape; means for defining, based on said three dimensional dot arrangement (40), sections (48) having a spacing (50) therebetween; means for storing said screen dot arrangement (40) having said sections (48) and spacing (50); means (72) for comparing the input density values of various locations with corresponding ones of said threshold values represented by said sections (48); and means (26) for writing screen dots in a line by line fashion, wherein each screen dot is made up of a plurality of lines, the length and location of each line is determined by said means for comparing and the distance between said lines is determined by said spacing Apparatus according to claim 8 and wherein each of said plurality of lines has infinite resolution along its length. Apparatus according to claim 8 and also including: means (62) for storing the input density values of the original at a first spatial resolution; and means (60) for storing the threshold values of the desired screen dot arrangement in the form of analog functions. Apparatus according to claim 8 and also comprising: means (62) for storing the input density values of the original at a first spatial resolution; and means (60) for storing the threshold values of the desired screen dot arrangement at a second spatial resolution different from the first resolution. Apparatus according to claim 8 and wherein the spacing (50) between adjacent sections (48) corresponds to the line to line spatial resolution of a plotter used for writing the screen dots. Apparatus according to claim 11 and wherein the means for storing the threshold values of the desired screen dot arrangement for a given section includes means for digitally storing the values at a third sampling resolution, which may be different from said first and second resolutions. Apparatus according to claim 8 and wherein each section (48) is associated with lines in an x-y plane and each line in said x-y plane lies at an angle corresponding to a screening angle.

Inventors: GERSHONY MOSHE A, GERSHONY, MOSHE A.
Applicants: SCITEX CORP LTD, SCITEX CORPORATION LTD.
Application year: 1991
Main classification: H04N1
IPC codes: G03F5, H04N1, G06T11