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Contents
1. B1 5 170 257 1 REEXAMINATION CERTIFICATE ISSUED UNDER 35 U S C 307 THE PATENT IS HEREBY AMENDED AS INDICATED BELOW Matter enclosed in heavy brackets appeared in the patent but has been deleted and is no longer a part of the patent matter printed in italics indicates additions made to the patent AS A RESULT OF REEXAMINATION IT HAS BEEN DETERMINED THAT The patentability of claims 11 14 is confirmed Claim 6 is cancelled Claims 1 3 7 8 and 9 are determined to be patent able as amended Claims 2 4 5 and 10 dependent on an amended claim are determined to be patentable 1 A method of calibrating halftone output images form an imagesetting device comprising providing a halftone input image each said input image including a plurality of requested gray value densities each said input image being a function of image resolution exposure intensity and screen frequency reproducing said halftone images onto a photo graphic media chemically processing said media to manifest the exposure thereon measuring the density of each said requested gray value of each said halftone input image by a densi tometer generating a plurality of calibration sets in accor dance with said measuring step each said calibra tion set corresponding to any variation between said requested gray value density and said respec tive measured density reading for each said half tone input image at various said2. 54 IMETHOD AND APPARATUS FOR CALIBRATING HALFTONE OUTPUT FROM AN IMAGE SETTING DEVICE 75 Inventor James M Burns Austin Tex Southwest Software Inc Austin Tex 73 Assignee Reexamination Request No 90 003 294 Dec 22 1993 Reexamination Certificate for Patent No 5 170 257 Issued Dec 8 1992 Appl No 592 022 Filed Oct 2 1990 Certificate of Correction issued Oct 19 1993 511 Int OL Lillian HO4N 1 23 592 UC ciano 358 298 358 455 358 456 358 458 358 461 58 Field of Search 358 298 455 456 458 358 534 536 406 504 56 References Cited U S PATENT DOCUMENTS 4 075 662 2 1978 Gall 358 280 OTHER PUBLICATIONS Series 2000 Graphics 2000 Linotype sales brochure printed Feb 1988 Copyright 1988 Series 2000 Graphics 2000 User Manual issued Mar 1989 Copyright 1989 Linotronic 300 500 R Imagesetter Interface Manua l issued Feb 1988 Copyright 1988 Linotronic 300 500 CORA Operation Manual issued Apr 1987 Copyright 1987 PAGE DESCRIPTION LANGUAGE Aldus Printer Description File reference AF filed by Burns with Information Disclosure Statement LinoCal 1 0 a k a Calibration Utility of Linotype Hell corresponding to the calibration presently employed in later versions of the Linotype Hell utility program such as utility 6 0 and 7 0 Color Calibration Software by Technical Publishing Servic
3. resultant gray values of each halftone image is a func tion of image resolution exposure intensity and screen frequency A densitometer is used to measure the den sity of each halftone image and a plurality of calibra tion sets are then generated corresponding to the varia tion between the requested gray value density and the respective measured density reading from the densitom eter The calibration sets are then used to convert subse quent plurality of halftone input images to respective plurality of calibrated halftone output images The method of calibrating halftone images further includes storing the calibrations sets in a local memory medium and programmably retrieving a selected calibration set so as to convert the subsequent halftone input image to a calibrated halftone output image Conversion or cali bration can compensate for variation in the imageset ting device s current settings of image resolution expo sure intensity and screen frequency The calibration method can also calibrate depending upon whether or not the calibrated halftone output image is to be positive or negative or is received on either film or paper photo graphic medium As a further embodiment the present invention also contemplates a method of calibrating halftone output images from an imagesetting device comprising provid ing a halftone input image with requested gray value densities wherein each gray value density is a function of image reso
4. 8 1992 U S Patent vse yi EL ol LI LL z xmanysibdou wphliubjopoge ZAXMANLSHOdON Varis ogy ziz ZAXMAN SIDGOU upiftu y p qe JI e ZAXMANLSHDAJON LAS ve L Old 91 vo S 91 O Sb O Idi 12961 uses Bulag insodxz 00 z opSZ j o oz 6l gl z a 5 170 257 Sheet 2 of 3 Dec 8 1992 U S Patent LNASLIO Q34 2 977 2 LZ SLNIWIYNSKIW WILY LOD FIVINTIVIS OP BE YOLYYINID ae T VO 2 TOSTA ALISNILNT ES TENSO XT PE EE YILSYY Bi 0 12 ET o 1 FIVIONY 7 NOI LA HISIT 55 Iva 5 170 257 Sheet 3 of 3 Dec 8 1992 U S Patent c Q 7G V V2Y02 WILY LOT SONIWN 2 06 08 U 001 0E 00 SNNIN 2 7372221 1 METHOD AND APPARATUS FOR CALIBRATING HALFTONE OUTPUT FROM AN IMAGESETTING DEVICE BACKGROUND OF THE INVENTION 5 170 257 Incorporated herein is a computer program listing printout appendix of source code used to generate cali bration sets and calibration transfer functions of a test pattern to enable calibration of halftone output images according to the present invention Copyright 1990 Softwest Software Inc A portion of the disclosure of this patent document contains mat
5. Not only are conventional imagesetters and transfer functions therein lacking in necessary complexity so as to compensate for numerous variations in e g the reso lution of the image to be calibrated the exposure inten sity of the to be developed image the screen frequency etc but conventional devices also cannot produce pre cision transfer functions applicable to color image pro cessing Current imagesetters are black and white de vices and process color reproductions are produced by overprinting the colored inks yellow cyan megenta and black The colors of an image are separated into yellow cyan megenta and black components and are then outputted on black and white film in precise per centages The films called color separations represent the amount of process color needed in each area of the page and are used to make the printing plates The imagesetter produces black and white and the ink used with the final printing plate produces color The RIP and imagesetter mechanism that produces tints and halftones also produces color separations The key dif ference is that each color separation is rotated with respect to the others and its respective line per inch specification is slightly different The rotation and change in lines per inch specification avoids the dots lining up in a moire pattern While gray scale calibration is mostly insensitive to rotation it is a function of the halftone lines per inch Gra
6. said exposure inten sity and said screen frequency generating a plurality of calibration sets correspond ing to the variation between said requested gray value density and the density measured by said densitometer for each said halftone input image at varying values of said image resolution said expo sure intensity said screen frequency and whether the resultant said halftone output images are placed on either paper or film mapping subsequent halftone input images through a transfer function process to produce positive or negative sense gray values for any subsequent half tone input image storing said calibration sets and said transfer function having in a local memory medium inputting said subsequent halftone input images each said subsequent halftone input image having a plu rality of gray value densities and converting said gray value densities of each said sub sequent halftone input image to a respective gray value densities of said calibrated halftone output images according to programmed retrieval of se lected said stored calibration sets and said transfer function
7. 20 25 30 35 40 45 50 55 60 65 4 fer curves required conventional methods require limit ing the number of output conditions which can be speci fied For instance if film negatives at high resolutions of 150 lines per inch was the only output condition then only one transfer curve per application per imagesetter would be required This is typically the case for closed reproduction systems such as that mentioned in U S Pat Nos 4 075 662 and 4 500 919 The limitation on output conditions however is unrealistic and impracti cal with multi function RIPs and imagesetters such those used in Model Nos RIP 30 and Linotronic 330 A typical production environment using this equipment may require 24 transfer curves for 5 applications for 2 imagesetters This would yield 240 separate transfer curves 24x 5x 2 240 If these curves were distrib uted among four front ends then the operator would have to manage 960 separate files In practice the num ber of transfer curves used is a balance of output flexibil ity versus the number of curves to manage The second method by which conventional calibra tion adjustments are made or RIP based calibration was formed as an attempt to reduce the number of curves and files required to support a minimum of out put flexibility Thus RIP based calibration is an ap proach used to calibrate the RIP and the imagesetter as a unit so that all front ends and all application
8. between said requested gray value density and said respec tive measured density reading for each said half tone input image at various said image resolutions said exposur intensity and said screen frequency and converting subsequent plurality of halftone input images to a respective plurality of calibrated half tone output images according to changes made to said subseguent halftone input images by said cali bration sets 2 The method as recited in claim 1 wherein said plurality of calibrated halftone output images each being a function of image resolution exposure intensity and screen frequency and each said plurality of cali brated halftone output images being programmable by at least one said calibration sets 3 The method as recited in claim 1 further comprises storing said calibration sets in a local memory medium 4 The method as recited in claim 1 wherein said converting step further comprises converting either positive or negative sense uncalibrated halftone input images to positive sense calibrated halftone output im ages 5 The method as recited in claim 1 wherein said converting step comprises converting either positive or negative sense said uncalibrated halftone input images to negative sense calibrated halftone output images 6 The method as recited in claim 1 wherein said step of converting further comprises programmably select ing a particular calibration set of said plurality of cali bration sets
9. film and mapping said halftone input images through a transfer function process capable of transferring either posi tive or negative gray value densities of said half tone input images to either positive or negative gray value densities of said halftone output images 9 A method of producing calibrated halftone output images from an imagesetting device comprising providing a halftone input image each said halftone image having a set of requested gray values corre sponding to varying density readings wherein each gray value is dependent upon image resolution exposure intensity and screen frequency selected reproducing said halftone images onto a photo graphic media chemically processing said media to manifest the exposure thereon measuring the density of each gray value of each halftone input image by a densitometer for chosen values of said image resolution said exposure inten sity and said screen frequency generating a plurality of calibration sets correspond ing to the variation between said requested gray value density and the density measured by said densitometer for each said halftone input image at varying values of said image resolution said expo sure intensity said screen frequency and whether the resultant said halftone output images are placed on either paper or film mapping subsequent halftone input images through a transfer function process to produce positive or negative sense gray values for any subsequent ha
10. image resolutions said exposure intensity and said screen frequency and converting a subsequent plurality of halftone input images to a respective plurality of calibrated half tone output images according to changes made to said subsequent halftone input images by said cali bration sets by programmably selecting a particular calibration set of said plurality of calibration sets to be used to convert one of said subsequent plurality of halftone input images depending upon said imageset ting device current settings of said image resolution said exposure intensity and said screen frequency 3 The method as recited in claim 1 further com prises comprising storing said calibration sets in a local memory medium 7 The method as recited in claim 1 wherein said step of generating said calibration sets further comprises selecting photographic the type of photographic media according to said exposure intensity 5 10 15 20 25 30 35 40 45 55 60 65 2 8 A method of calibrating halftone output images from an imagesetting device comprising providing a halftone input image each said halftone image having a set of requested gray value densi ties wherein each gray value density is dependent upon image resolution exposure intensity and screen frequency of said input image reproducing said halftone images into a photographic media chemically processing said media to manifest the exposure ther
11. match This method has the significant advantage of not allowing the operator to vary the number of calibration sets for various fre quencies depending upon the operator s accuracy re quirements As gray values 32 are specified in page description language 25 where halftone input images are to be cali brated gray values are mapped through transfer func tion 33 Transfer function 33 is a convenient way to map user gray values to values that are presented to dot generator 35 of RIP 26 so that the end result is a cali brated halftone output image Transfer function 33 however has multiple uses Page description language 25 may specify an inverted transfer function 33 in order to produce a negative image In this case the invention concatenates calibration function 52 with the one speci fied through page description language 25 Care is taken to ensure that calibration function 52 is not concate nated more than once The output sense can be provided by either the de fault transfer function 33 or of one specified via page description 56 provided to calibration function 52 along with the sense of conditional RIP invertor 57 and the sense of conditional recorder invertor 58 These various data inputs provide the present invention with bases for calibrating or mapping within calibration function 52 Mapping or calibration according to sense of conditional RIP invertor 39 and conditional recorder invertor 41 is described with the aid of F
12. recog nize The page description language is what is then sent to the imagesetter Many conventional imagesetter systems consist of raster image processors RIP and recorders An exem plary RIP is one sold under the Model No RIP 30 by Linotype Company of Hauppauge N Y The RIP per forms the third step in the process by converting the page description language sent to it from the front end into a raster or a grid of lines Each line contains a series of pixels which can be considered as a list of dots which turn on or off to form the image In the fourth step the RIP sends this raster to the recorder which uses a mod ulated light source to expose the image onto either photographic film or paper An exemplary recorder is one sold as Model No Linotype 330 also by Linotype Company The photographic medium in a light tight takeup cassette is then removed from the recorder and photo developed by a conventional chemical process All imagesetters have a light source intensity adjust ment to provide proper exposure of the film Exposed areas turn black on both photographic film medium or paper medium after development The amount or de gree of blackness called density is a function of how much the film is exposed Strictly speaking density is a measure of how much light is blocked by a piece of film Conversely if photographic paper is used density is the 20 25 45 60 65 2 measure of how much light is not refle
13. tive labor intensities tedious and therefore error prone Current methods require an inordinate amount of atten tion from the operator for success SUMMARY OF THE INVENTION The problems outlines above are in large part solved by the method and apparatus of the present invention That is the halftone output image calibration method and apparatus hereof provides programmable conve nience capability in programming imagesetters with variable output conditions More specifically the pres ent invention is capable of calibrating halftone output images at various image resolutions exposure intensities and screen frequencies Thus by simply changing in software input variables such as resolution intensity frequency sense polarity and photographic medium film or paper a more precisely calibrated halftone output is achieved from standard imagesetting devices The present invention takes into account many vari ables in its calibration method and apparatus so that a more precisely calibrated output is achieved Broadly speaking the present invention contemplates a method of calibrating halftone output images obtained from a standard imagesetting device In this method of calibrating halftone output images a halftone input image is provided with targeted or requested gray value density The halftone images are then chemicaliy pro cessed and reproduced on photographic media The 10 15 20 35 40 45 55 60 65 6
14. to be used to convert one of said subsequent plurality of halftone input images depending upon said imagesetting device current settings of said image reso lution said exposure intensity and said screen fre quency 7 The method as recited in claim 1 wherein said step of generating said calibration sets further comprises selecting photographic the type of photographic media according to said exposure intensity 8 A method of calibrating halftone output images from an imagesetting device comprising providing a halftone input image each said halftone image having a set of requested gray value densi ties wherein each gray value density is dependent upon image resolution exposure intensity and screen frequency of said input image reproducing said halftone images onto a photo graphic media chemically processing said media to manifest the exposure thereon measuring the density of each gray value of each halftone input image by a densitometer for chosen values of said image resolution said exposure inten sity and said screen frequency 5 170 257 15 generating a plurality of calibration sets correspond ing to the variation between said requested gray value density and the density measured by said densitometer for each said halftone input image at varying values of said image resolution said expo sure intensity said screen frequency and whether the resultant said halftone output images are placed on either paper or
15. 0 dot value area all of the pixels are exposed Both 0 and 100 are always obtainable therefore a non linear relation ship exists between the commanded dot area or gray value and the dot area actually achieved A series of tint boxes in 5 intervals between 0 and 100 is commonly called a gray scale The non linear response of the gray scale is a function of the resolution of the recorder pixel grid the light source intensity the photographic media film or paper and the screen grid These however are not the sole contributors to gray scale non linearity Contributing factors include the recorder light source different film batches and photoprocessor chemicals 5 170 257 3 Page description languages referred to hereinabove usually contain a mechanism to correct for non lineari ties in gray scale and halftone output An exemplary page description language is one sold under the trade name PostScript Q by Adobe Systems Inc of Moun tain View Calif The mechanism Adobe included in PostScript is called the transfer function and it provides the lowest level means to calibrate the image setter to correct for non linear gray scale response Postscript Q transfer functions suffers an important disadvantage in not having the ability to compensate for all the numerous variables which typically cause the non linear response e g resolution variation in media resolution screen frequency exposure intensity etc
16. Attorney Agent or Firm Arnold White amp Durkee 57 ABSTRACT An improved method and apparatus for calibrating halftone output images is provided which calibrates subsequent halftone input images with varying image setter outputs such as image resolutions exposure inten sities and screen frequencies Calibration is programma bly achieved by programmably selecting a specific cali bration set for each corresponding uncalibrated half tone input image The selected calibration set and map ping transfer function automatically compensates or adjusts the uncalibrated input image for any value of resolution intensity and screen frequency to produce a calibrated halftone output image in either positive or negative sense output and usable on either paper or film photographic medium Once the calibrated halftone output is programmably produced it can then be photo processed according to conventional chemical process ing means The present invention represents a substan tial improvement in precision and speed by which half tone output images are calibrated in the typeset indus try 14 Claims 3 Drawing Sheets 22 PERCENTAGE DOT AREA MEASUREMENTS 4 ai 4 MULTIPLE LPL mi 7 Sy N MNN OSN RSS Sx 207 ES UNCTION GENERATOR Fara 3 RR 1E PHOT 25 37 ASTER aZ rear Azzo saz 28 38 eol RECORDER RASTER IMAGE PROCESSOR v 27 CALIBRATED OUTPUT 5 170 257 Sheet 1 of 3 Dec
17. IG 3 Shown in FIG 3 is a graphical depiction of the half tone response contained in a typical calibration set The ordinate and abscissa are in minus dot area a typical and widely used halftone measurement for negatives Dot area measurements are usually spoken of in terms of printed dots on the page That is a 10 dot area means that 10 of a given area is covered by dots or is mostly white But for a negative that would be used to produce a printing plate that would make a 10 dot area the area is mostly black or 90 of the area on the negative is covered by dots For convenience this area is called a 10 minus dot area since the negative pro duces a 10 dot Negative curve 59 of FIG 3 results if the halftone measurement taken on gray scales 15 and 16 of FIG 1 were taken from a negative rather they were minus dot measurements If so positive curve 60 can be con structed from curve 59 In the alternative if halftone measurements taken on gray scales 15 and 16 were taken from a positive then they are plotted on curve 60 20 25 30 35 45 50 55 65 12 and negative curve 59 is constructed from positive curve 60 Curve 61 is the unity function and is shown for reference Curve 60 is a complementary curve constructed from curve 59 in the following manner for each minus dot area commanded such as point 62 there is a corre sponding point 63 for positive dots which is simply expressed as 100 less
18. United States Patent 115 Burns 54 METHOD AND APPARATUS FOR CALIBRATING HALFTONE OUTPUT FROM AN IMAGESETTING DEVICE 75 Inventor James M Burns Austin Tex 73 Assignee Southwest Software Inc 21 Appl No 592 022 22 Filed Oct 2 1990 51 Mei ricada HO4N 1 23 52 U S Cl 358 298 358 455 358 456 358 458 358 461 58 Field of Search 358 298 455 461 358 406 80 214 346 108 160 395 107 109 56 References Cited U S PATENT DOCUMENTS 4 004 079 1 1977 Boston 358 302 4 075 662 2 1978 Gall 4 314 281 2 1982 VViggins et al 4 500 919 2 1985 Sehreiber 4 757 334 7 1988 Volent 4 833 627 5 1989 Leszezynski 395 145 4 903 141 2 1990 Morton et al OTHER PUBLICATIONS Listing of an Aldus Printer Description File Jun 9 1988 Aldus Preprint Program User s Guide Mar 1990 Adobe Photoshop Program User s Guide Feb 6 1990 EXPOSURE 54 INTENSITY 55 RESOLUTION ACCOR AU US005170257A 11 Patent Number 45 Date of Patent 5 170 257 Dec 8 1992 Color Studio User s Guide and Retouching Separation Package Mar 6 1990 Color Calibration Software for PostScript Image Set ters Feb 22 1990 Desktop to Press vol 1 No 5 Mar 1990 Primary Examiner George H Miller Jr Assistant Examiner Scott A Rogers
19. ame absolute gray value output Again the use of two curves is not mandatory but is helpful for explanation purposes The invention herein described assumes short term stability in the photoprocessor 28 photographic media 43 and photo exposure mechanism 42 Significant effort within the current art is devoted to assuring the stability of these three items In practice the calibration sets 50 which are a function of these three items have been shown to be accurate for periods exceeding four weeks provided there are good controls on the photographic processes In the event of major change such as a change in the type of film chemicals or processor the existing calibration sets 50 may need to be discarded and new ones created The present invention provides for easy verification of the calibration set accuracy and can be easily changed The current embodiment of the present invention also assumes that dot generator 35 on FIG 2 produces a fixed dot shape that varies only with gray value re quested In another embodiment of the invention the dot shape is allowed to change This replicates the groups 45 46 and 47 for each different dot shape Dot shape is also included in the calibration sets 50 and the dot shape is taken into account by selector 51 with input from dot generator 35 The foregoing description of the present invention has been directed to particular preferred embodiments It will be apparent however to those skilled in t
20. bstantially similar to the second method above and uses the same transfer as the second method in order to calibrate In practice these methods are multiplicate in nature That is each of the three output sense settings shown above are set either positive or negative and combine to produce an image The negative of a negative is a posi tive For example the first method shown above places the output sense setting as positive and the second and third method each set output sense setting as positive and the resultant image is positive However if method one places sense setting as negative but method two places sense as negative and method three as positive the resultant image is also positive The transfer curve for these two cases however are different and this must be recognized by the operator for successful calibration Thus regardless of which method is used or whether or not the methods are multiplicated such that more than one method is used an operator must set the methodol ogy each time positive or negative output sense condi tions are specified Accordingly conventional methods shown above which set sense output are often prone to operator error The net result of conventional art and all the com plexities described hereinabove is that halftone calibra tion for imagesetters with multiple output conditions such as e g resolution media halftone frequency screen intensity and sense polarity output is non intui
21. claim 11 wherein said photographic media is paper 14 The apparatus as recited in claim 11 wherein said photographic media is film g g x UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO 5 170 257 DATED December 8 1992 INVENTOR S James M Burns It is certified that error appears in the above indentified patent and that said Letters Patent is hereby corrected as shown below Column 16 claim 12 line 48 after resolution insert Signed and Sealed this Nineteenth Day of October 1993 Attest VS ence Antics BRUCE LEHMAN Attesting Officer Commissioner of Patents and Trademarks UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO 5 170 257 DATED December 8 1992 INVENTOR S James M Bums t is certified that error appears in the above identified patent and that said Letters Patent is hereby corrected as shown below Column 1 line 19 after whatsoever insert A complete copy of the Program Printout Appendix is included Column 13 after line 63 and before What is claimed is insert the Appendix as shown on the attached pages Signed and Sealed this First Day of April 1997 BRUCE LEHMAN Attesting Officer Commissioner of Patents and Trademarks US005170257A REEXAMINATION CERTIFICATE 2471st United States Patent m Burns un B1 5 170 257 45 Certificate Issued Feb 7 1995
22. cted Higher density values correspond to a blacker piece of film These films are typically used in the publishing indus try to make contact printing plates For reasons of qual ity and ease of production in printing plate manufacture it is desirable that the maximum density measurement on a film be in excess of 4 00 The maximum density available is a function of the light source intensity the photographic medium and the process and chemicals used for development Since conventional printing processes cannot repro duce continuous tone tints or images gray scales tints and halftones are approximated in page description languages by overlying a halftone grid on top of the recorder s pixel grid The halftone grid often called a screen is specified by the number of halftone cells per inch or lines per inch and the angle from the vertical by which the halftone grid is rotated Each cell of the half tone grid encompasses a specific number of pixels of the recorder s pixel grid A continuous tone image is simulated by assigning a gray value to each halftone cell contained within the image Thus imagesetters which use PostScript are unable to produce shades of gray directly generally approximate shades of gray by utiliz ing the aforementioned halftone technique Halftone grids overlying the top of the pixel grids allows for a shade of gray to be approximated by turning a certain percentage of pixels on The gray value of the ha
23. ed value for 25 positive and 75 negative are identical It is worth mentioning at this point that the use of two curves is not mandatory The mathematical relationship between positive and negative dot area measurements is straight forward but the use of two curves here is advantageous for recording the halftone 5 170 257 13 response measurement and to help understand the oper ation of transfer function 33 and calibration of function 52 If Ntf is positive and invertors 39 and 41 are nega tive the dot area value from transfer function 33 is used directly For this case however calibration function 52 intersects the positive curve 60 even though a negative image will ultimately be formed From an input value of 90 the curve is intersected at point 71 for a com manded value at point 72 The command value at point 72 is used directly and is sent along path 34 to the dot generator 35 Still further if Ntf is negative and inver tors 39 and 41 are negative the dot area value from transfer function 33 is inverted by subtracting from 100 For an input value of 90 this converts to 10 The calibration function intersects the negative curve 59 even though a positive image will ultimately be formed From an input value of 10 the curve is inter sected at point 73 The value to command from point 73 is then subtracted from 100 and is sent along path 34 to dot generator 35 Note that the last two examples shown above pro duce the s
24. eon measuring the density of each gray value of each halftone input image by a densitometer for chosen values of said image resolution said exposure inten sity and said screen frequency generating a plurality of calibration sets correspond ing to the variation between said requested gray value density and the density measured by said densitometer for each said halftone input image at varying values of said image resolution said expo sure intensity said screen frequency and whether the resultant said halftone output images are placed on either paper or film and programmably mapping said halftone input images through a transfer function process capable of transferring either positive or negative gray value densities of said halftone input images to either positive or negative gray value densities of said halftone output images 9 A method of producing calibrated halftone output images from an imagesetting device comprising providing a halftone input image each said halftone image having a set of requested gray values corre sponding to varying density readings wherein each gray value is dependent upon image resolution exposure intensity and screen frequency selected reproducing said halftone images onto a photo graphic media chemically processing said media to manifest the exposure thereon measuring the density of each gray value of each halftone input image by a densitometer for chosen values of said image resolution
25. erial which is subject to copyright protection The copyright owner has no objection to the facsimile reproduction by anyone of the Program Printout Appendix as it appears in the Pa tent and Trademark Office Patent file or records but otherwise reserves all copyright rights whatsoever This invention relates to a method and apparatus for calibrating halftone output images from an imagesetting device In particular the present method and apparatus comprises steps and devices involved in producing a plurality of calibration sets and in using those calibra tion sets to adjust the uncalibrated halftone input images to produce calibrated output images as a function of image resolution exposure intensity and screen fre quency Further the calibration sets can be generated to produce halftone output images on either film or paper and for either positive or negative representation of the halftone input image Typically there are four fundamental steps involved in producing typeset quality output from a computer front end publishing system such as e g a PostScript imagesetter or typesetter First an application pro gramed a drawing or page layout package is used to find what is wanted or actually printed on the paper or film Second the print option from the program is used to start the printing process Typically the print option translates what is seen on the screen into a page description language that the imagesetter can
26. es Inc Kita Calibration System for Linotronic May 1989 Goertzel G and Thompson G Digital halftoning on the IBM 4250 Printer IBM J Res Develop vol 31 No 1 1987 Webster J Radius opens color calibration system MacWeek 1990 Postscript Language Reference Manual Aug 1986 Primary Examiner Scott A Rogers 57 ABSTRACT An improved method and apparatus for calibrating halftone output images is provided which calibrates subsequent halftone input images with varying image setter outputs such as image resolutions exposure inten sities and screen frequencies Calibration is programma bly achieved by programmably selecting a specific cali bration set for each corresponding uncalibrated half tone input image The selected calibration set and map ping transfer function automatically compensates or adjusts the uncalibrated input image for any value of resolution intensity and screen frequency to produce a calibrated halftone output image in either positive or negative sense output and usable on either paper or film photographic medium Once the calibrated halftone output is programmably produced it can then be photo processed according to conventional chemical process ing means The present invention represents a substan tial improvement in precision and speed by which half tone output images are calibrated in the typeset indus PERCENTAGE DOT AREA MEASUREMENTS CALIBRATED OUTPUT
27. esentation onto a photo graphic media a photoprocessor configured to receive said photo graphic media for chemically developing said me dia a densitometer for measuring amount of density of each said gray value of said developed photo graphic medium a computer for receiving a plurality of programmed calibration sets said sets include variations be tween measured density of said densitometer and corresponding said requested density a subsequent uncalibrated halftone input image said subsequent halftone input image converted into said page description language and inputted to said computer a selector accessed by said computer for receiving said subsequent halftone input images and selecting a corresponding said calibration set stored in said computer to programmably adjust said uncali brated halftone input image to said calibrated out put halftone image and mapping means for mapping either positive or nega tive sense representations of said subsequent uncali brated halftone input images through said raster image processor said recorder said photoproces sor and outputted as calibrated said halftone output images on said photographic media 12 The apparatus as recited in claim 11 wherein each said calibration set comprises means for calibrating gray values of said subsequent uncalibrated halftone input images as a function of image resolution exposure inten sity and screen frequency 13 The apparatus as recited in
28. h has the net effect of providing unadjusted half tones This method of operation has the advantage of not disabling output in the event that halftone input images could not be calibrated a desirable outcome to many operators In another embodiment of the present invention selector 51 terminates current page descrip tion 25 if a calibration set could not be found 5 170 257 11 Note that in the current art page description 25 may specify any halftone screen frequency 47 from the prac tical limits of 40 lines per inch to 300 lines per inch It would be unreasonable and unworkable to construct a calibration set 50 for every halftone frequency that could be specified Evidence from testing has shown that a given calibration set at a given halftone screen frequency is accurate at a frequency significantly differ ent from that at which the set was measured For in stance a calibration set measured at 150 lines per inch may be accurate within a tolerance of 1 for 15 lines per inch It is a feature of the present invention that the operator may specify a halftone frequency range for which a calibration set may apply Selector 51 matches the current halftone frequency to a halftone range con sisting of a starting frequency spanning to an ending frequency inclusive The halftone ranges for calibra tion set at a given exposure intensity 21 and a given resolution 46 are not allowed to overlap thereby yield ing the possibility of only one
29. halftone output from image setting devices In one aspect of the invention a special test strip pattern is constructed from an uncalibrated halftone input image into page description language of the imagesetting device The test strip pattern contains gray scale requests This pattern is outputted in an un calibrated or unadjusted fashion from the imagesetter developed with a photoprocessor and then measured with a densitometer for its actual density or percent dot area These measurements called the halftone response function combine with the output sense resolution exposure intensity and screen frequency with which the test strip was created in order to ultimately produce a polarity of calibration sets The calibration set construction process is repeated for each combination of resolution exposure intensity and screen frequency of which the operator wishes to calibrate The calibration set is used in a transfer func tion process in which all requested gray values are mapped to values so chosen to provide calibrated out put that is gray value output measured with a densi tometer as being equal to that gray value requested Thus the present invention achieves gray value calibra tion by programmably compensating or adjusting via calibration sets the uncalibrated halftone input image The transfer function used for calibration is the inverse of the halftone response function the halftone response function is inverted through a l
30. he art that modifications and changes in both apparatus and method may be made without departing from the scope and spirit of the invention For example the present invention contemplates functioning with any standard raster image processor for any form of output media Furthermore the present invention also contemplates use in any standard personal computer which can pro grammably receive calibration sets and apply those calibrations sets to any form of input page description language Therefore it is the applicant s intention in the following claims to cover all such equivalent modifica tions and variations which fall within the true spirit and scope of this invention What is claimed is 1 A method of calibrating halftone output images from an imagesetting device comprising providing a halftone input image each said input image including a plurality of requested gray value tea 5 20 25 35 40 45 50 65 14 densities each said input image being a function of image resolution exposure intensity and screen frequency reproducing said halftone images onto a photo graphic media chemically processing said media to manifest the exposure thereon measuring the density of each said requested gray value of each said halftone input image by a densi tometer generating a plurality of calibration sets in accor dance with said measuring step each said calibra tion set corresponding to any variation
31. ion 56 Ntf is positive and invertors 39 and 41 produce a positive for a positive image 2 Ntf is negative and the invertors 39 and 41 produce a posi tive for a negative image 3 Ntf is positive and the invertors 39 and 41 produce a negative for a negative image and 4 Ntf is negative and the invertors 39 and 41 produce a negative for a positive image The above four principal sense conditions are de scribed hereinbelow In the case where Ntf is positive and invertors are also positive the dot area value from transfer function 33 is in positive dot so it is first con verted to minus by subtracting from 100 For an input value of 50 calibration function follows line 66 until in intersects curve 60 at point 67 This occurs at a given percent dot area to command point 68 The value to command from point 68 is then subtracted from 100 and output along path 34 to dot generator 35 of FIG 2 Other transfer values are computed in like manner by intersecting curve 60 In the case where Ntf is negative and invertors 39 and 41 are positive the dot area is used directly For an input value of 50 calibration function 52 follows line 66 until in intersects curve 59 at point 69 This occurs at a given percent dot area to command point 70 The value to command from point 70 is then used directly and output along path 34 to dot generator 35 of FIG 2 Note that the commanded values for 50 positive and 50 negative are identical and the com mand
32. l ues and three halftone screen frequencies Upon obtaining media 45 resolution 46 and screen frequencies 47 of uncalibrated output 29 the operator can then measure the test pattern gray scales 15 and 16 shown in FIG 1 with a standard dot area densitome ter 48 The record of these measurements along with output sense either positive or negative media expo sure intensity resolution and halftone frequency consti tute a calibration set 49 A plurality of calibration sets 50 may be created stored in RIP 26 and used with transfer function 33 to linearize the imagesetter s halftone re sponse One or more calibration sets 49 are transferred to RIP 26 in a manner well known in the art RIP 26 is constructed with a selector 51 and calibration function 52 Thus selector functions to select a given calibration set 50 within RIP 26 to provide linearized calibrated halftone response output Once calibration sets 50 are obtained and transfer function 33 is in place subsequent halftone input images can then be calibrated in accordance with calibration sets 50 and transfer function 33 Accordingly at the start of processing any subsequent halftone input image in the form of subsequent page description language 32 selector 51 is called upon to select a corresponding calibration set 50 appropriate to the current system conditions such as e g exposure intensity 21 media 45 resolution 46 screen frequency 47 etc Current media 45 is deter
33. lf tone input image storing said calibration sets and said transfer function having local memory medium inputting said subsequent halftone input images each said subsequent halftone input image having a plu rality of gray value densities and converting said gray value densities of each said sub sequent halftone input image to a respective gray value densities of said calibrated halftone output images according to programmed retrieval of se lected said stored calibration sets and said transfer function 10 The method as recited in claim 9 wherein said converting step further comprises applying said transfer function to at least one gray value density of each said 10 15 20 25 30 35 45 55 65 16 subsequent halftone input image to generate either posi tive or negative sense of said calibrated halftone output image 11 An apparatus for generating calibrated halftone output images from an imagesetting device comprising a halftone input image including a plurality of gray values each said gray value having a requested density value means for converting said input image into a page description language a raster image processor having a channel for receiv ing said page description language and converting said language into a raster representation of said halftone input image a recorder connected to said raster image processor said recorder includes a modulated light source to expose said raster repr
34. lftone is obtained by averaging the gray continuous tone val ues contained within the dimensional boundaries of the halftone cell Each halftone cell approximates a shade of gray by turning on in dot shape a certain percentage of the pixels contained within the cell As the cell needs to be darker more of the pixels are turned on Theoreti cally a 50 gray tint is made by turning on 50 of the pixels in a half tone cell These conventional techniques are well known in the art Associated with halftones is the problem commonly discovered by densitometers A densitometer is a scien tific instrument that measures density or the ability of a piece of film to block light The blacker the film the more light it absorbs and the higher the density reading on the densitometer When measured with a densitome ter a halftone area that was supposed to be 50 gray will typically for paper positive be darker because the higher exposure intensity needed for a higher maximum density Dmax tends to expand the pixel size causing the overall halftone dot to be larger than it actually is Thus halftone images when uncalibrated produce a gray value which is typically darker than that which the gray value should be That is when 50 of the pixels within a halftone cell are exposed the resulting dark area covers more than 50 of the area contained by the halftone cell At a dot area value of 0 none of the halftone cell s pixels are exposed at 10
35. lution exposure intensity and screen fre quency of the input image The uncalibrated halftone input images are chemically processed and thereby exposed on photographic medium so that they can be measured by a densitometer A plurality of calibration sets are then generated corresponding to the variation between the requested gray value density and the den sity measured by the densitometer for each halftone input image and at varying values of image resolution exposure intensity screen frequency and photographic medium which is to receive the resultant halftone out put image Subsequent halftone input images can be mapped through a transfer function curve or process capable of transferring either positive or negative gray value densities of the subsequent halftone input image to either positive or negative gray value densities of cali brated halftone output image The present invention also contemplates an apparatus for generating calibrated halftone output images from an imagesetting device comprising a halftone input image having a plurality of gray values representing various shades of the halftone input image Means is provided for converting the input image into a page description languages recognizable by a computer A raster image processor within the computer is config ured for receiving the page description language and converting that language into a raster representation of the halftone input image A recorder is also connected
36. mined by a one to one mapping process with the exposure intensity 54 which is determined from recorder 27 by query For example all film test strips 10 are exposed at the same exposure intensity setting 21 and all paper test strips are exposed at the same expo sure intensity setting 21 A requirement of the present invention is that exposure intensity settings 21 be differ ent for each different media 45 paper or film In prac tice this causes no difficulty In the event that two different media are properly exposed at the same expo sure intensity setting 21 the present invention requires that one media be shifted in exposure intensity This shift is at most 0 4 a negligible amount In this man ner selector 51 can determine which group of calibra tion set is appropriate to the current media 45 by exam ining the exposure intensity setting only Next selector 51 narrows the list of possible sets by examining the current resolution 53 from recorder 27 The halftone parameters 55 notably halftone screen frequency are determined either from default within RIP 26 or may be overridden by page description lan guage 25 Once media 45 resolution 46 and screen fre quency 47 are determined selector 51 examines the group of calibration sets 50 for a match and if a match is found it presents the found calibration set to the calibration function 52 In the event that a calibration set match is not found selector 51 presents a no set whic
37. nd halftone frequency 47 First the gray values in test pattern 10 are mapped through transfer function 33 Note that line art and text through black in a positive page image is still considered a gray value of 100 and is passed through transfer function 33 Not shown and not germane to this discussion are the mechanisms that RIP 26 uses to generate the actual text in line art images included in a complete page description For purpose of forming calibration sets and charac terizing the halftone response transfer function 33 is set to the unitary function that is the output gray value equals the input value Also possible and equally us able is an inverse linear transfer function in which the percent gray value is subtracted from 100 to invert the image The output value equals 100 minus the input value Unity or inverse transfer function 33 is set at user direction through page description language 25 After transfer function 33 which may have caused a negative image gray values 34 pass to halftone dot generation section 35 RIP 26 then creates a separate halftone dot proportional in size usually linear to each gray value 34 Halftone dots representing each gray value 36 are placed in raster memory 37 Raster mem ory 37 contains a pixel by pixel translation of page de scription 25 When page description 25 has been com pletely processed page image 38 is passed through a conditional invertor 39 Invertor 39 is enabled to
38. ook up table combined with an interpolation process In another aspect of the present invention the plural ity of calibration sets are deposited in a raster image processor in such a fashion that once deposited they no longer require any operator attention for successful use other than periodic checks to verify their continued accuracy Further since the calibration sets are depos ited in the raster image processor each is available for use independent of the application program or the page description A calibration set and its resulting transfer curve is automatically selected for use depending upon the current imagesetter setting or resolution exposure intensity and halftone screen frequency In still a further aspect of the present invention the present method and device provides means by which a transfer curve is programmably provided so that half tone output image sense is mathematically derived from the halftone input image and all the output sense condi tions along with resolution intensity and screen fre quency Namely a single calibration set at a resolution exposure intensity and halftone screen frequency is used to calibrate both positive and negative images whether or not the images are produced with mathematical or electronic inversion included 20 25 30 35 40 45 50 55 60 65 8 BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 is a plan view of a sample test strip pattern used by the presen
39. pro duce a negative at user direction through page descrip tion language 25 After the pixel stream 40 leaves RIP 26 it enters recorder 27 and is passed through a similar conditional invertor 41 This invertor 41 is also enabled under user direction through page description language 25 From invertor 41 the pixel stream is used to modulate a light source scanning in raster fashion 42 which then pro duces a photographic media of the latent image corre sponding to test pattern 10 representation of the uncali brated halftone input image The film with the exposed test pattern 43 correspond ing to halftone input image test pattern 10 is then devel oped by photoprocessor 28 Photoprocessor 28 has been used extensively in the art and therefore it is not dis cussed herein The processor 28 which produces uncalibrated test pattern images 44 is activated for each combination of media resolution and halftone frequency desired 45 46 and 47 respectively Test pattern films 43 are divided 15 20 25 35 40 45 50 55 60 65 10 into a specific media category 45 film or paper resolu tion 46 high or low and halftone screen frequencies 47 multiple lines per inch Note that while FIG 2 shows two types of media 45 two resolution values 46 and three halftone screen frequencies 47 it is clearly within the scope and spirit of this invention to include multiple values beyond two types of media two resolution va
40. programs assume the device produces a linear gray scale This process requires an operator to construct a transfer curve under identical output conditions to the present and to properly identify and select that curve for use in the RIP and imagesetter In the existing art the opera tor loads the single curve into violative memory within the RIP This is not repeated for each application pro grammable front end In our example this has the net effect of reducing a number of transfer curves needed from 240 to 48 and the number of files from 960 to 48 Although RIP based calibration is effective in reducing the number of transfer curves current RIP base calibra tion techniques do not utilize many of the variable in puts which can effect the transfer curve Furthermore conventional RIP based calibration requires manual selection and loading of the transfer curve appropriate to the current output conditions Manual selection and loading for each transfer curve is not only inefficient but difficult to incorporate and thus it is error prone Accordingly current calibration methodology re quires that separate transfer curves be constructed to account for the plurality of variable inputs Included with those inputs is either positive or negative output sense conditions Images can be outputted in either a positive or negative format one format or sense being the inversion of the other Images lines and type in a page description language are
41. ray values ranging between 2 and 100 0 is capable of being read from any clear space 17 on pattern 10 It is important to note that pattern 10 is merely a sample and does not indicate all the various forms of test patterns which can be obtained by the present invention Fur thermore pattern 10 is placed upon a photographic media such as a film so that density readings can be obtained therefrom by a standard densitometer It is envisioned however that other forms of photographic media may be used provided densities can be read there from by a standard densitometer Thus photographic media such as paper or photographic plates are also possible Included for identification of an operator viewing test pattern 10 is the imagesetter name 18 and various pa rameters which effect the calibrated halftone output image such as e g screen frequency 19 angle of rota tion 20 of the halftone image with respect to the type set page the imagesetter light source exposure intensity 21 and the imagesetter pixel grid resolution 22 Also shown is indicator 23 useable for determining if the gray scale shown on pattern 10 is uncalibrated output as shown in FIG 1 or an attempt at calibrated output not shown Thus FIG 1 illustrates uncalibrated halftone test pat tern 10 usable for establishing calibration sets disclosed hereinbelow Included under each gray scale tint box 2 100 is an area 24 usable for recording gray scales with positive ou
42. t invention in setting the imagesetter exposure intensity setting and generating halftone re sponse function for calibration set construction FIG 2 is a diagrammatic block representation of the present invention showing various components along with the flow of data and steps used in the calibration process and FIG 3 is a graph depicting the method by which given a calibration set selected for resolution exposure intensity and halftone screen frequency the transfer function is derived for the proper output sense condi tion DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings FIG 1 illustrates a sample test strip pattern used by the present invention In par ticular FIG 1 illustrates an exemplary uncalibrated halftone input image processed in the form of a test pattern 10 representing numerous indicia usable for subsequent calibration of halftone input images In cluded within pattern 10 is a solid area 11 along with targets 12 13 and 14 Targets 12 13 and 14 are useful in setting the exposure intensity of the halftone image to as a high a value as possible without overexposing the image placed on a media such as film Solid area 11 and targets 12 13 and 14 are examined on pattern 10 for evidence of film overexposure so that corrections can be made prior to the imagesetting process Also in cluded in test pattern 10 are two gray scales 15 and 16 usable in measuring various degrees of densities or g
43. the minus dot point value At point 62 the difference of the commanded value to the actual value recorded the distance A is transferred to the corresponding point 63 in opposite sense as the distance A In like manner point 63 s distance to the negative curve B is transferred to point 62 in opposite fashion as distance B the magnitude of A equals A and the magnitude B equals B for each corresponding point along curve 61 At points 64 and 65 A equals A equals B equals B It is a feature of the present invention that the mea surement points are symmetric about 50 That is a measurement of 2 corresponds to a measurement of 98 a 5 to 95 a 7 to 93 etc While not manda tory for construction of the complementary curve this arrangement merely simplifies the algorithms and as sures accuracy The purpose of the calibration function 52 is to invert the halftone response function from the selected calibra tion set 50 depending upon the output sense conditions 56 57 and 58 The present invention relies on the fact that the measured halftone response is usable regardless of the state of the three output sense conditions 56 57 and 58 Calibration functions 52 is dependent on the individual output sense conditions Because the effect of the conditional pixel invertors 39 and 41 can be com bined through the use of the exclusive or function there are four principal sense conditions to deal with 1 the transfer funct
44. to the raster image processor to expose the raster repre sentation onto a photographic media such as paper film etc A photoprocessor is then configured to re ceive the media for chemically developing the photo graphic media A densitometer is used to measure the density of each gray value developed photographic media The variations between measured density of the densitometer and corresponding actual density in en tered into a computer in the form of programmed cali bration sets Thus subsequent uncalibrated halftone input images which are converted into page description language and inputted into the computer are applied to a corresponding selected calibration set stored in the 5 170 257 7 computer The selected calibration set programmably adjusts the uncalibrated halftone input image to pro duce a calibrated halftone output image Mapping means is provided for mapping either positive or nega tive sense representations of the subsequent uncali brated halftone input images through the raster image processor recorder photoprocessor and then outputted on the photographic medium as calibrated positive or negative sense output halftone images The raster image processor in conjunction with programmably selected calibration sets and transfer function provides for rapid and precise calibration of halftone output images before they are photoprocessed Therefore the present application contemplates a new system for calibrating
45. tput sense Area 25A is used for recording gray scale areas with negative output sense Thus the sample test pattern 10 shown in FIG 1 is produced with a positive output since area 25A contains positive output sense recorded test patterns Turning now to FIG 2 calibrated halftone output images are created by computer front ends in a page 5 170 257 9 description language 25 as shown Language 25 is then sent to an imagesetter comprised of a raster image pro cessor or a RIP 26 RIP 26 then converts language 25 into a raster format which is then sent to recorder 27 Recorder 27 records the halftone input image on photo graphic media and delivers that media to photoproces sor 28 for chemical processing and development Out put from photoprocessor 28 is then characterized in one of two categories namely uncalibrated halftone output images 29 or calibrated halftone output images 30 Un calibrated output 29 is then used to characterize the halftone response of the imagesetter film and photo processor system for different combinations of media resolution and halftone frequency 45 46 and 47 respec tively Test pattern 10 shown in FIG 1 can thereby be en coded from page description language 25 to begin the calibration process Namely description language of a halftone image corresponding to test pattern 10 is sent via channel 31 to RIP 26 Test pattern 10 is then ulti mately imaged at a predetermined media 45 resolution 46 a
46. usually specified from the front end in a positive sense The RIP and imagesetter can output any positive sense or can switch to a nega tive independent of the page description The negative sense is useful in many printing processes Conventional methods for specifying a negative output sense are as follows 1 Mathematically through the transfer function and its interaction with the page description language This method converts a gray value to negative by subtract ing the gray value from 100 prior to halftone dot gener ation Hence a 0 gray value becomes 100 25 becomes 75 etc 2 Electronically by inverting the pixel stream in the RIP prior to it leaving the RIP A gray valve is con verted to a raster based halftone dot the halftone dot is 5 170 257 5 placed in the RIP s raster memory and after the com plete page description is processed the raster memory is transferred from the RIP in serial fashion to the re corder Each bit in the pixel stream is inverted that is if the bit were on it is turned off if the bit were off it is turned on This produces a negative image but be cause it has the effect of turning the halftone dot inside out the transfer curve required to produce the negative calibrated output is substantially different from that used with the mathematical negative 3 Electronically by inverting the pixel stream in the recorder immediately after it enters the recorder This method is su
47. y scale calibration is well known in the industry A series of pre determined gray scale tint boxes at pre determined dot areas are requested of the RIP and imagesetter through the page description language and each tint box on the resulting film is measured with a dot area densitometer described hereinabove The re sulting measurement called a transfer curve is then used to adjust the requested dot area by an amount necessary to achieve calibrated output However as mentioned above conventional transfer curves suffer many inherent disadvantages Existing art divides the method in which the gray scale calibration adjustments are effected into two categories applica tion program based and RIP based In application pro gram based gray scale calibration an attempt is made to compensate for the non linearities by sending an ad justed transfer curve immediately before the print job This process is tedious and error prone because it re quires an operator to construct a transfer curve under identical output conditions to the present and to prop erly identify and select a specific curve to be sent with the print job This process is duplicated for each appli cation program and for each imagesetter If there are multiple front ends sending the imagesetter print jobs then the files containing the curves must be distributed and managed The number of transfer curves can be come unruly In an effort to reduce the number of trans
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