referring now to fig1 there is depicted in schematic format a particular image reproduction system or set - up useful in explaining the present invention . such set - up may comprise a scanner / digitizer 10 which is used to read an original input image 12 typically by directing and scanning a light beam 14 onto the image and detecting the reflected ( or transmitted ) beam 16 from the input image 12 . the scanner / digitizer 10 may further be able to digitize the detected beam 16 output . in an 8 bit digital system , the digitized image will have discrete values ranging from 0 through 255 for each of the picture elements into which the image is broken down to , the pixel size being a function of the resolution abilities of the scanner / digitizer 10 . this range of digital values corresponds to the gamut or range of optical densities in the original image . a typical black and white photograph has an optical density gamut or range ( defined as the difference between the maxumum optical density and the minimum optical density ) of about 1 . 8 . the output of the scanner / digitizer 10 can then be directed over a line 18 to a digital signal processor unit 20 , which may include a central processing unit ( cpu ) and appropriate software to process the image data prior to sending it over a line 26 to a recorder 28 or other output device . the cpu may perform a number of functions in addition to processing image data in accordance with the present invention . it may include keyboard means 24 to allow an operator to input instruction or additional information , and may include display means 22 to facilitate the communication process between the operator and the cpu . the display means 22 may be a video display terminal for displaying an output image representative of the original input image . the output of the recorder 28 can be an actual half - tone transparency 30 or an actual print . however , as explained above , for the purposes of the present invention , the transparency need not be produced in a film or on a substrate by a recorder or printer , but could remain as data representing the transparency or output image in the system in which case such data is a virtual transparency or virtual output image . the half - tone transparency 30 is used in an exposure unit 32 to expose a lithographic printing plate 34 . following exposure , the plate 34 is developed in equipment not shown herein , then mounted on a printing apparatus 36 where it is used to print a reproduction or copy of the original image onto a receiving medium or substrate 38 referred to as a print . regardless of the apparatus and method employed , input data repesentative of the optical density d o of each picture element or pixel in an orignal image is obtained or generated . the data can be obtained from a system like the one described in reference to fig1 or any other system or method . the data representative of the input image can be created from one or more electronically stored image ( s ) or entirely computer generated . in developing a user - specific system in which a virtual or actual image having predictable preselected characteristics is generated , we separate or divide the operations performed on the input data representing the input image into two distinct categories or steps . the first step or set of operations on the input data is to establish a basic transfer function unique for a set of image reproduction conditions . in the preferred embodiment , a plurality of the basic transfer functions are established for a plurality of image reproduction conditions . the basic transfer function ( s ) can be optionally stored in , for instance , the processor unit 20 . in the case where the output image is a hard copy print , typical image reproduction conditions include the type of printer , receiving medium and ink to be used . for instance , one set of image reproduction conditions for generating an output image which is a print comprise using coated paper on a particular sheet fed press with non - heat set ( or drying oil ) inks . another set includes using uncoated paper ( such as newsprint ) on a particular web fed press with heat set inks . in the case of an output image displayed on a video display terminal , typical image reproduction conditions comprise brightness range of the cathode ray tube ( crt ), the gamma of the crt and the viewing conditions ( e . g ., brightness of the viewing room ). the basic transfer function is established by correlating measured optical densities of a sample input image having a range of optical densities and corresponding optical densities of a sample output image of the sample input image , such that the sample output image generated under the image reproduction conditions from an output of the basic transfer function is an &# 34 ; exact &# 34 ; representation of the sample input image . when the output density range is less than the input density range , a mathematically exact reproduction can not be achieved . however , for the purposes of this invention , the term &# 34 ; exact &# 34 ; means the closest visual reproduction of the imput image . other terms used in the art for &# 34 ; exact &# 34 ; representation include &# 34 ; match copy &# 34 ;, &# 34 ; optimum visual reproduction &# 34 ; and &# 34 ; optimized representation &# 34 ;. the basic transfer function is , thus defined such that when applied to the input data representing an input image , an output image reproduced on a user specified set - up will be generated with the the optical density range or gamut in the input image the same or substantially the same as in the output image . what is meant by the above can be illustrated by and further understood from the following example . assume that a continuous tone original input image has a maximum optical density of 2 . 0 and a mimimum optical density of 0 . 2 , then the total density range in the original image is 1 . 8 . assume further that the particular printing or image reproduction system to be used to make a half - tone print of the continuous tone original image can only produce a maximum apparent optical density ( i . e ., the density of a solid ink area ) of 1 . 6 and a mimimum apparent optical density ( i . e ., the density of a no ink or unprinted area of a receiving medium ) of 0 . 1 , then the spread or gamut of the apparent optical densities of a half - tone print made on the image reproduction system is 1 . 5 . as a result , any optical density in the original continuous tone image will be reproduced in the half - tone print with a half - tone dot area whose apparent optical density range is different , i . e ., less , than the total optical density range as the total optical density of the original image . the following is an example showing how to establish a basic transfer function for one particular set of image reproduction conditions . using the particular set of image reproduction conditions for which the basic transfer function will be developed , a half - tone transparency comprising a test pattern or target of the type shown in fig2 is used to expose and produce a lithographic printing plate of the type used in the particular printing set - up for which the basic transfer function is being developed . the test pattern has at least 4 and preferably 11 distinct areas which are represented in corresponding percent dot areas (% dt ) in the transparency , ranging from 0 % to 100 % in uniform increments of 10 %. the plate is then mounted on the printing press of the particular printing set - up or system and using the chosen operating conditions ( e . g ., the particular receiving medium and the desired ink ), a print is made . the apparent optical densities ( d r ) for the 11 different dot size areas in the printed image of the test target or pattern are then measured with a diffuse reflection densitometer . fig3 shows in graphical format the results of these measurements . in this example , the top apparent optical density of the print is measured as 1 . 5 , and the background apparent optical density of the print was 0 . 1 , thus the half - tone print has a range of apparent optical densities of 1 . 4 . these measured apparent optical density values ( d r ) from the print and corresponding percent dot areas (% dt ) of the transparency are introduced to the cpu where they are stored and a third order polynomial function is calculated representing a best fit curve to the introduced values such that a corresponding % dt is obtained when the function is applied to a specific d r . in fig3 this function is shown for description purposes as a solid curve 42 connecting the measured d r values versus % dt values . in the computer , however , the function resides as an algorithm correlating reflected print densities ( d r ) to % dot area (% dt ) in the transparency . the problem is to correlate the range of reflected print apparent optical densities ( d r ) to the range of the original image optical densities ( d o ). using a typical photographic print density range , e . g ., 1 . 8 ( a preselected value stored in the computer ), or , preferably , the optical density range of the specific input image , a new relationship represented by the curve 44 in fig4 is derived correlating the 1 . 4 print density range illustrated in fig3 to the 1 . 8 density range of the original input image . the relationship or curve 44 is developed by first converting the original optical density range ( e . g ., 1 . 8 ) and the print apparent optical density range ( e . g ., 1 . 5 ) to darkness units v o and v r , respectively , using , for instance , an equation derived from the bartleson - breneman equation , as follows : second , a plurality of the original optical densities { d o 1 , d o 2 , . . . d o n } are converted to darkness units { v o 1 , v o 2 , . . . v o n } using , for instance , equation ( 1 ). third , the darkness units { v o 1 , v o 2 , . . . v o n } are then multiplied by the darkness ratio v r / v o resulting in values { v r 1 , v r 2 , . . . v r n }. fourth , the values { v r 1 , v r 2 , . . . v r n } are converted back to optical density values ( d r 1 , d r 2 , . . . d r n } using , for instance , equation ( 1 ). a plot of the range of values of printed densities ( d r ) versus the range of original densities ( d o ) and a joining of intersecting points results in the relationship or curve 44 as shown in fig4 . with the above information it is now easy to determine the basic transfer function , i . e ., the relationship between the output image , that is reproduced using the image reproduction conditions , considered an &# 34 ; exact &# 34 ; or substantially &# 34 ; exact &# 34 ; duplicate of the input image as defined above . graphically the process is explained with reference to fig5 in which fig3 and 4 have been combined back to back . as each original pixel value is retrieved to be sent to , for instance , a recorder for generation of the corresponding half - tone transparency , the pixel value undergoes the following modifications . assume one of the original optical densities { d o 1 , d o 2 , . . . d o n } is identified by the letter &# 34 ; a &# 34 ;. referring to fig5 &# 34 ; a &# 34 ; is converted to &# 34 ; b &# 34 ;, a corresponding one of the optical density values { d r 1 , d r 2 , . . . d r n } as described above . this can be graphically illustrated by following the arrow 60 from d o = a in fig5 to where it intercepts curve 44 and then following arrow 62 to obtain the apparent optical density ( d r ) that must be printed to give the desired result . to print this optical density ( d r ), we find the point where the arrow 64 intercepts curve 42 &# 39 ; ( the mirror image of curve 42 ) and then follow arrow 66 to obtain the percent dot area in the transparency (% dt ) or value &# 34 ; c &# 34 ;, corresponding to the half - tone to be generated on the transparency . a 45 degree straight line could be used to transfer this % dt value &# 34 ; c &# 34 ; to the output axis shown . optionally , instead of a straight 45 degree line ( because the recorder output is often used as an intermediate half - tone transparency from which a final half - tone transparency is made for use in plate making ), a further correction curve 46 which compensates for this intermediate exposure step may be used . in either case , a % dt &# 39 ; value &# 34 ; d &# 34 ; is obtained . one uses this process for a multiplicity of original optical density values { d o 1 , d o 2 , . . . d o n } and constructs a basic transfer function or curve 48 which may then be used to determine any percentage dot area (% dt &# 39 ;) for a corresponding original optical density value ( d o ). in a computer environment , the above steps occur in a mathematical transfer algorithm , rather than in the graphical format used above to described the process . in summary , each original pixel optical density value d o is first converted to a darkness value v o using equation ( 1 ). it is then multiplied by the darkness ratio v r / v o which is the darkness value corresponding to the reproduced density range ( e . g ., v r = 0 . 865 ) divided by the darkness value corresponding to the original density range ( e . g ., v o = 0 . 933 ). the result is converted back to a density value &# 34 ; d r &# 34 ;. &# 34 ; d r &# 34 ; can then be used in a 3rd order polynomial relationship % dt = f ( d r ) to represent the curve 42 fitting the test points to give the percent dot % dt required to reproduce this pixel &# 39 ; s original density d o after printing . this value may be further corrected by a small factor if the half - tone transparency is to be used as an intermediate , depending on the intended use , to compensate for the slight dot gain ( or loss ) that may occur driving the copying of the one transparency onto another . the combination of the various mathematical algorithms described above is the basic transfer function . a multiplicity of such basic transfer functions may be developed and stored in an addressable memory bank representing basic transfer functions for various image reproduction conditions , to be used when preparing transparencies or other output images for use with each of the conditions . establishing coefficients to be applied to the output of the basic transfer functions in addition to developing a basic transfer function , there is often need to produce a half - tone print so that it differs from the continuous tone original image in one of a number of preselected desired manners . for example , it may be desirable to provide extra detail over a certain portion of the original densities ( d o ) while compressing the balance or otherwise alter the tone or gradation curve of an original image . in this manner , one may want to emphasize the highlights ( lighter ), midtone or shade ( darker ) portions of an input image , for example , to obtain a different more desirable appearance in the output image or print than would be obtained using the basic transfer function alone . to this purpose , the process of the present invention includes establishing a plurality of preselected sets of coefficients corresponding to each one of the established basic transfer functions . each set of coefficients corresponding to each basic transfer function when applied to the output of the corresponding basic transfer function produces a modified output , such that the output image generated under the image reproduction conditions from the modified output is a non - exact representation of the input image which differs in gradation from the input image by a predetermined visual amount . preferably , the sets of coefficients are stored in a selectable accessible memory . depending on the desired effect , a set of the coefficients may be selected and a corresponding one of the coefficients used to mathematically modify the output of the corresponding basic transfer function ( which is in units of percent dot value (% dt )) for each pixel prior to supplying the driving signal , such as , to the recorder 28 . the modified output values corresponding to each set of the coefficients effectively comprise a modified transfer function relating original input density values do to percent output density values % dt &# 39 ;. fig6 shows how such a set of coefficients may be developed . starting with a basic transfer function established as described above , a user decides how to alter a portion of a reproduced density range . for example , the user may decide to provide greater detail in lighter portions of a printed output image in comparison to an original input image . in this case , the user wants to modify the output of the basic transfer function 48 to appear as a new modified transfer function 49 . to obtain a set of coefficients useful in modifying the basic transfer function 48 a user may proceed as follows . assume for each optical density d o of the original , the curve 48 gives a % dt value to be used for an &# 34 ; exact &# 34 ; half - tone copy representation of an original continuous tone image . thus , if 0 . 8 is the optical density d o of one pixel in the original image , as a result of using curve 48 in fig6 a 55 % dot value will be called for on the corresponding area of the transparency to make an exact half - tone copy representation of the pixel in the original continuous tone image . however , if we now want to use the new curve 49 , the 55 % dot value must be modified from the value provided from the basic transfer function 48 . in this example , the value from the basic transfer function 48 is multiplied by a coefficient from a set of coefficients derived by obtaining the difference between the % dot value (% dt ) corresponding to use of curve 48 and the % dot value (% dt ) corresponding to the use of curve 49 and then calculating the corresponding multiplication factor or coefficient . in this instance , this factor is ( 60 % dt / 55 % dt )= 1 . 1 . the same process may be used for a number of other optical densities , to obtain , for instance , four multiplication factors or coefficients and then extrapolate between those multiplication factors or coefficients to obtain intermediate multiplication factors or coefficients . the coefficients may also be applied to the basic transfer curve through other functions as explained above , rather than multiplication . for instance , the new calculated point may equal the value as calculated from the basic transfer function , plus , minus or divided by a numerical value representing the desired % dt change . alternatively , the coefficients may be determined from equations derived using the logic described above . the coefficient sets ( that correspond to each basic transfer function ) correspond to a number of preselected tonal modifications , which preferably are standardized tonal shifts ( or modifications ), which are described by various curves 49 , 50 , 51 and 52 differing from the basic transfer function in the manner illustrated in fig6 . in the preferred embodiment , a family of standardized curves and corresponding sets of coefficients may be adapted to provide standardized modifications to the original image . alternatively , the coefficients may be established by the user based on the type of effect he or she wants to produce and are used to further modify the basic transfer function ( or the corresponding computer algorithm values applied to the data ) to provide predictable results representing preselected gradation variations as follows . correlating the basic transfer functions and the set of coefficients to sets of visual sample output images the basic transfer functions and the sets of coefficients are correlated to sets of visual sample output images such that a user can select the gradation of the output image to be generated and the corresponding basic transfer function and / or the set of coefficients by determining which sample output image has gradation characteristics closest to desired gradation characteristics for the output image to be generated under the image reproduction operating conditions . the sample output images can be generated under a different set of image reproduction conditions with a different basic transfer function than the one ( s ) established in the first step of the present invention . however , the sample output images are preferably generated using the same sets of coefficients established in the second step of the present invention . selecting what output image to generate based on the set of visual sample output images the customer or operator determines which of the sample output images best represents the type of change to the original input image desired . more specifically , the operator selects whether to generate an &# 34 ; exact &# 34 ; representation of the input image or one of the non - exact representations of the input image based on the set of visual sample output images . this selection can be made through use of the keyboard means 24 or other input means to the processor unit 20 . preferably , the selecting step comprises selecting the image reproduction operating conditions with a first control and selecting the gradation characteristics corresponding to the selected sample image with a second control . then using the basic transfer function and / or the set of coefficients , if any , corresponding to the selected sample output image , an output image is generated under the image reproduction conditions based on the selection . in one case , the input image comprises a continuous tone original image and the sample output images comprise half - tone representations . for instance , the continuous tone original image may comprise a black and white continuous tone image . in this case , the generating step comprises using the basic transfer function and / or the set of coefficients corresponding to the selected half - tone representation to generate a half - tone transparency for the continuous tone original image such that a printed half - tone image from the generated half - tone transparency would be an &# 34 ; exact &# 34 ; copy of or differing from the original image in a manner and degree that the selected half - tone representation represented the continuous tone original sample image . in other words , the result is a half - tone transparency which makes a plate that prints an image having predictable characteristics . the resulting half - tone transparency as noted above can be either a virtual or actual transparency . the present invention is equally applicable to whether the output images are half - tone or continuous tone images . further , the present invention is equally applicable to whether an actual output image , a virtual output image or a video display image is produced . those skilled in the art having the benefit of my invention hereinabove set forth may effect various modifications thereto . the invention has been illustrated using a monochrome example , specifically a black and white continuous tone image . however , the same principles may be applied to images comprising more colors than one . these modifications are to be construed as being encompassed within the scope of the present invention as claimed in the appended claims .