Pre-press process and system for accurately reproducing color images

A pre-press process employs color accurate instant photography so that the customer and/or his photographer adjust lighting and exposure at the photography session to achieve approved reflection originals. The photographs are judged under controlled lighting against a standard background in device such as a portable illuminator. Since any deficiencies are corrected by recreating an improved original, the session produces a job output which is 100% suitable for color accurate scanning and separation without the intervention of the pre-press operator. Originals are suitable as proofs for the printer to match. Novel means are employed to achieve rapid, colorimetrically calibrated scanning with standard equipment, and the colorimetry of the prior art is improved to deal effectively with whitepoint and blackpoint misalignment.

FIELD OF THE INVENTION 
The present invention relates generally to apparatus for and methods of 
automated pre-press for color printing, and more particularly to apparatus 
for and methods of automatically generating a color accurate printed 
facsimile of a reflection input color image. 
BACKGROUND OF THE INVENTION 
Color printing typically involves interaction between a customer, a 
photographer, a pre-press house, and a printer. The process generally 
begins with a photography session or "photo-shoot", during which 
photographs of the desired scene are taken. The photographic film 
typically used is of the type for producing color transparencies, or 
slides, because using negatives with color prints would add an extra level 
of complexity, loss of sharpness and color distortion to the process. Once 
approved, the developed transparencies are then sent to the pre-press 
house where the required color separation process is performed and a set 
of film, typically one for each subtractive color (cyan, magenta, yellow 
and black), known as half-tones are produced. The acceptability of the 
color separation can be checked by producing a positive proof from the 
half-tone films. The positive proof is indicative of how the final printed 
product will look. Typically, the positive proof is not produced by an 
offset press because of the substantial cost of configuring the offset 
press, which is usually only justified if it is determined in advance that 
the customer will be satisfied with the printed product. Instead the 
proofs are often produced by a manual process requiring semi-skilled labor 
and costs of materials. If the customer approves the proof, then the 
half-tones are sent to the printer. If the proof is not acceptable, then 
the color separation must be redone with new parameters, or alternatively, 
a new photo-shoot is required. The proof becomes a contract specification 
for the printer, i.e. the printer must match the final print to the proof. 
One problem with this process is that it requires skilled craftsmen at the 
pre-press house to make a subjective judgment, typically through an 
iterative process, to achieve what they consider to be a good color 
reproduction of the photographic original (the transparency) taking into 
account corrections requested, or presumed to be desired, by the customer. 
Another problem is that the color separation process is expensive and 
requires expensive equipment, e.g., expensive scanners for scanning the 
slides. Yet another problem with this process is that the proof of half 
tones is not available for review and approval by the customer until well 
after the photo-shoot photography session. Since slight errors of lighting 
or exposure can produce small but unacceptable color cast or tonal errors, 
even the best of several shots of the same subject often requires 
correction in the scanning and separation process. Such editorial 
improvements upon the original can only be accomplished with the human 
judgment of a trained operator, and may be unacceptable to the customer. 
In order to confirm and insure that the customer is satisfied with the 
composition and lighting of the photographs taken during the photo-shoot, 
often, during the photo-shoot the photographer takes an instant photograph 
of the scene, typically using self-developing film of the type 
manufactured and sold under the trademark Polaroid by the Polaroid 
Corporation of Cambridge, Mass., prior to exposing the slide film. 
However, since both the customer and the photographer know that several 
corrections and human judgments will be made during the separation 
process, there has been no motivation to make the Polaroid instant 
photograph strongly resemble the desired printed product. 
Although the theoretical possibility exists for converting reflection 
originals for accurate reproduction by mechanical processing means, 
without the trained human who traditionally creates the match, this 
possibility has not been pursued because: 
1. The heavily predominant practice is to use transparency materials for 
originals. When reflection materials are involved, they are usually pan of 
a mixed job that also involves transparencies. 
2. When reflection materials are intended for color reproduction, they 
frequently require correction as stated above, and so exact reproduction 
of the color original is not desirable. 
3. The range of colors that can be produced in a particular printing 
process are the "in gamut" colors, while those outside the process are 
"out of gamut" colors. Gamut mis-match between original and reproduction, 
particularly with respect to the extremes of the neutral scale, requires 
that for good results, even the in-gamut colors must be reproduced without 
exactly matching conventional colorimetry. Conventional color science does 
not teach how to deal with this problem effectively. 
4. Unless all, or very nearly all, of the images in a job can be processed 
mechanically, much of the attraction of a mechanized procedure is lost. 
5. Since the reflection original cannot be trusted to represent the 
customer's desires, the advantage of being able to substitute it for a 
separation proof is lost. 
OBJECTS OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide an 
apparatus for and method of automatically generating a color separation 
from a reflection original, such as a Polaroid print. 
Another object of the invention is to provide an automated system for 
creating the appearance of a color printed image that is a guaranteed good 
color match to the appearance of a reflection original image. 
And another object of the present invention is to provide an apparatus for 
and method of automatically generating a color separation from a Polaroid 
instant photograph. 
Still another object of the present invention is to reduce the degree of 
human judgment required to perform a color separation from a print 
provided from a photo-shoot. 
Yet another object is to provide a pre-press method and apparatus which 
allows a reflection color original produced by instant photography to 
function as the prior art proof. 
And still another object is to provide a method of and apparatus for 
generating color separations at reduced cost. 
Other objects and advantages of the present invention will become readily 
apparent to those skilled in this art from the following detailed 
description wherein a preferred embodiment is shown and described, simply 
by way of illustration of the best mode of the invention. As will be 
realized, the invention is capable of other and different embodiments, and 
its several details are capable of modifications in various respects, all 
without departing from the invention. Accordingly, the drawings and 
description are to be regarded as illustrative in nature, and not 
restrictive. 
SUMMARY OF THE INVENTION 
These and other objects are achieved by an improved pre-press process and 
system for reproducing color images from reflective images. In accordance 
with the present invention, employing color accurate instant photography, 
the customer and/or his photographer adjust lighting and exposure at the 
photography session to achieve approved reflection originals. The 
photographs are judged under controlled lighting against a standard 
background in device such as a portable illuminator. Since any 
deficiencies are corrected by recreating an improved original, the session 
produces a job output which is 100% suitable for color accurate scanning 
and separation without the intervention of the pre-press operator. 
Originals are suitable as proofs for the printer to match. Novel means are 
employed to achieve rapid, colorimetrically calibrated scanning with 
standard equipment, and the colorimetry of the prior art is improved to 
deal effectively with whitepoint and blackpoint misalignment.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a block diagram a system 10 used for implementing the present 
invention for producing an ink-printed color image, indicated as an 
ink-printed output 28, which has an appearance which is an accurate 
reproduction of the appearance an original color image of a reflection 
print 12. The color image of a reflection original color print 12 is the 
input to system 10. Print 12 may be a photographic print, and is 
preferably of the instant photographic variety, such as a Polaroid print. 
In the preferred embodiment, print 12 is taken using Polaroid's color 
accurate PRO-100 film. Alternatively, print 12 can be an ink-printed 
image. 
The color image provided on print 12 is scanned by an image scanner 14 
which generates an electronic representation of the color image on print 
12. Scanner 14 generates a digital output representative of a plurality of 
pixel values of the color image of print 12. One preferred scanner 14 is 
the Polaroid CS-500 scanner, also sold by the Polaroid Corporation of 
Cambridge, Mass. 
The output of scanner 14 is then fed to a first transform device 16 which 
applies a predefined transform to the scanner output. The transform, which 
will be discussed further below, is such that device 16 calculates a set 
of intermediate values for every pixel in the image. The transform applied 
by transform device 16 is dependent on the type of reflection original 12 
fed into the system and in particular the dyes from the film processes 
from which the original image is provided. The user input 18 informs the 
transform device 16 which type of original 12 is being used, and therefore 
defines which transform device 16 applies. 
Preferably, and in accordance with the present invention scanner 14 is 
calibrated so that the output data of scanner 14 is expressed in terms of 
the intermediate set of standardized primary colors approximating red, 
green, blue in a color space as further discussed below. When the scanner 
is capable of such calibration, and is so calibrated as discussed below, 
the function of transform device 16 becomes incorporated into the scanner, 
and is available in the scanner output. 
The output of the first transform device 16 is then sent to a second 
transform device 20, which converts the intermediate values of each pixel 
provided by the transform device 16 to an output type dependent 
representation. Transform device 20 also has user input 22, which allows 
the user to specify which type of printing process will be used to print 
the final product. As with transform device 16, transform device 20 
implements a different transform depending on the particular 
characteristics of the output printing process to be used. Transform 
device 20 converts the output of transform device 16 to digital files 
which can be used by a pre-press house to generate a set of half tone 
transparencies 24. The half-tone transparencies 24 can then be sent to a 
printer 26 for generating the final ink-printed product 28. 
The system 10 guarantees that the color reproduction provided by the final 
ink-printed output 28 will substantially resemble the original image of 
reflection print 12, thereby eliminating the need for traditional proofs. 
Reflection print 12 functions as the proof, and since print 12 can be a 
Polaroid instant photograph, the invention eliminates the delay between 
the photo-session and the production of a proof. If the customer is 
unsatisfied with a particular Polaroid print, the customer and 
photographer can adjust the scene and re-shoot until a satisfactory 
Polaroid instant photograph is produced. Thus at the end of the 
photo-shoot, the customer can know how the final printed product will 
look. 
Once the print 12 has been produced, the process of generating the color 
separation is fully automatic, thus system 10 eliminates the need for 
operator judgmentin producing the color separation. System 10 also 
eliminates the expensive scanners traditionally used by pre-press houses 
when performing color separations. The only scanner used by system 10 is 
scanner 14 which can be a relatively inexpensive color image scanner. 
Thus it is feasible for the customer to own scanner 14, and transform 
devices 16, and 20. Transform devices 16 and 20 can be implemented on an 
appropriately programmed digital computer, processor or the like, such as 
desk top computer. In this case, a data storage device 30a (such as a disk 
drive) can be used to store the output of the transform device 16 on a 
storage medium such as a disk. The data can then be easily transferred to 
the pre-press house where it enters the data to a tramform device 20 
through a comparable storage and retrieval device so as to generate the 
half-tone films 24. Alternatively, the pre-press house may own transform 
devices 16, and 20. In this case the system can include a data storage 
device 30b for storing the data, for example on a disk provided from the 
scanner 14. The customer can then send the pre-press house the output of 
scanner 14. The pre-press house would set the transform devices 16 and 20 
through inputs 18 and 22 and automatically generate half-tone 
transparencies 24. It can also store the data in a storage device 32, 
similar to devices 30a and 30b. In any case the traditional role of the 
pre-press house is greatly reduced since operator judgment is eliminated 
from the process of generating the half-tone transparencies 24. 
Typically, the customer will own scanner 14 and the transform device 16 
will be integrated as a part of the scanner. The customer may wish to edit 
the image using a graphics editor 34 of a prior art type comprising 
graphic editing software packages which typically run on digital 
computers, shown in FIG. 1, prior to storing the data on a storage device 
30a. This would eliminate the need for storage device 30b. 
The calculations performed by scanner 14, and transform device 16, will now 
be discussed in detail. System 10 provides a guaranteed good match between 
the appearance of original image 12, and the appearance of reproductions 
28 of that image. The desired good match is one that a human observer 
finds satisfying. That is, a human observer comparing the original image 
12, and the printed product 28, should feel that the two images are 
substantially similar. This definition of a good match is different from 
the concept of "colorimetric match" which is typically discussed in 
reference works on color science. While a colorimetric match is an 
objective criterion based on the physical properties of light reflected 
from the images, the idea of a good match is a subjective criterion that 
takes into account how human perception of color and tone is affected by a 
surrounding image context. In particular, it is necessary to consider how 
the image context of an area affects the perception of error in its tone 
or color. As there may be colors and/or aspects of tonal range in an 
original image, which it is either impossible or undesirable to replicate, 
it is also necessary to consider which of the infinite variety of possible 
accon unodations is least likely to yield perceived error in what the user 
considers to be a "good match". 
Both in principle and in practice, objective, mechanizable means of color 
measurement can guarantee the subjectively identical appearance of two 
images only under stringently matched viewing conditions. In particular, 
the brightness, color and perceived source of illumination must be the 
same for both, and the image surround must be the same, out to the 
peripheral limits of the observer's field of view. 
Since transparency film is designed and optimized for radically different 
viewing conditions from printed reflection reproductions, different 
observers can naturally disagree as to what reflection reproduction best 
matches the transparency. An objective mechanized method of matching 
transparency originals therefore cannot be defined even in principle. 
However, this process is easy with reflection originals. Since 
photographic images and ink-printed images have different properties, it 
is virtually impossible to produce an ink-printed product that will look 
exactly like a reflection original under all lighting conditions. 
Therefore, the concept of a good match implies a standard lighting 
condition. The preferred choice of standard lighting condition is provided 
by the well known 5000.degree. K. viewing apparatus. Various types of such 
viewing apparatus are available from Graphic Technologies, Inc. of 
Newburgh, N.Y. Therefore, when the user selects a print to function as the 
proof, the user should view the prints with the viewing apparatus. 
Typically, the whitest white and the blackest black achievable with a 
photographic reflection original 12 will not exactly match those which are 
achievable by the printing process with the inks, paper and press 
conditions typically used for reproduction. In addition, some of the most 
intense colors in the original may fall outside the range, or "gamut", of 
what can be produced by any available combination of printing inks. 
Although the whitepoint/blackpoint alignment problem and the intense color 
problem both deal with misalignment of the original and reproduction 
gamuts, the perceptual consequences of these two problems are sufficiently 
different to require two very different solutions. 
Broadly, these problems can be addressed either by: 1) clipping, that is, 
by replacing colors in the original which are outside the reproduction 
gamut with the "nearest" reproducible one, while leaving in-gamut colors 
untouched, or by 2) smoothly distorting all the colors in the original, so 
that a smooth progression of color and tone from one edge of the original 
gamut through to the opposite edge is matched by a similar transition in 
the reproduction. 
Clipping has the disadvantage of potentially replacing many different 
colors in the original by the same color in the reproduction, leading to 
loss of detail and texture in the out-of-gamut material. Also, where the 
gamut of the reproduction process is larger than that available to the 
original, a clipping transformation will never put any of that extra range 
to use. 
A smoothly distorting transformation, on the other hand, has the 
disadvantage of introducing some error into every color in the image, even 
when the particular, original image happens to contain no non-reproducible 
colors. 
It has been determined in accordance with the present invention that 
preferably a smoothly defined alteration of all tones in the original is 
necessary to deal with the whitepoint/blackpoint alignment problem, while 
a clipping approach, using a sufficiently sophisticated definition of the 
"nearest in-gamut color ", works best for handling overly intense colors 
in the original. 
An attempt to create an "objectively correct" ink-printed product 28 might 
lead one to try to duplicate in a reproduction exactly those luminanee 
weighted reflectance values found at corresponding points of the original. 
However, for a photographic original with a 1% reflectance in deepest 
shadow, and a printing process which gives a minimal reflectance of 3%, 
for example, this approach would render all shadow detail in the 1% to 3% 
range of the original as the same featureless black. This "plugging up in 
the shadow" is both readily visible and unacceptable. Similarly, if the 
paper is darker than the brightest white of the original, reflectance 
matching will blow out the highlights. 
Alternatively, if the whitest area representing a pure white object in the 
original is present at a reflectance of 70%, and the paper on which the 
reproduction is printed has a reflectance of 90%, an exact reproduction of 
reflectances will be perceived as too dull. If the printing process is 
capable of a deeper black than can be made within the medium of the 
original, the failure of objective reflectance matching to utilize this 
deeper black will not produce the same sense of a defective image, and of 
a failure to match the original, as with blocking up of shadow or of 
blowing out the highlight. Still, a reproduction which stretches the tonal 
range of the reflection original to use the entire range of the 
reproduction (assuming the image contains at least some deep shadow or 
other psychological reference "black") is subjectively judged by the user 
to be superior, even when his announced goal is exact matching. 
If the hues of the two blackpoints are not the same, the attempt to 
duplicate reflectances can lead to a visible color shift going into deep 
shadow. If the color of the paper being printed is different from "white" 
as present in the original, an attempt to print a colorimetrically 
matching white may fail to produce matching appearance due to the 
visibility of any surrounding unprinted paper. An attempt to shift the 
white definition provided by the paper also runs into color problems where 
a bright white pushes toward 0% printing dot, as the screens will not drop 
out together. 
To handle these problems, whitepoint and blackpoint determinations (along 
with color measurements to be described later) must be made for each type 
of film used for originals, and for each significantly different category 
of press reproduction. By coupling input to output through a standardized 
electronic image representation, characterization of the two sides can be 
treated separately through transform devices 16 and 20. When scanning an 
original, then, the only user input through input 18 is a single discrete 
choice, indicating the film type to transt brm device 16. After the user 
combines this image with text, graphics, and other images to form a 
complete "electronic document", he or she (or a properly equipped printer 
or service bureau) "separates" the document for printing, using software 
which generates the necessary printing screens of half tones 24 in a 
manner responsive to the requirements of the printing process. Again, 
during this output process, the only operator provided information 
provided through input 22 to transform device 20 is a selection of the 
press type, ink, and paper being targeted. 
Preferably, whitepoint, blackpoint, and all other color measurements 
referred to herein are to be made with standard colorimetry apparatus, and 
are referred to here (unless otherwise stated) in terms of energy 
proportional tristimulus values "X", "Y", and "Z", or in terms of the 
conunoniy employed chromaticity ratios x=X/(X+Y+Z) and y=Y/(X+Y+Z) . The 
illuminant employed must be of constant intensity and color. 
The best possible subjective match between original and reproduction has 
been found to occur when the following relations are true for every pixel 
of the original image and the corresponding pixel of ink-printed output 
image 28. 
##EQU1## 
wherein X.sub.O, Y.sub.O, and Z.sub.O represent the tristimulus values of 
any pixel of an original image; 
X.sub.R, Y.sub.R, and Z.sub.R represent the tristimulus values of the 
corresponding pixel of the image on its reproduction; 
X.sub.OK, Y.sub.OK, Z.sub.OK and X.sub.RK, Y.sub.RK Z.sub.RK represent the 
tristimulus values of the blackpoints of original and reproduction images, 
respectively; and 
X.sub.OW, Y.sub.OW, Z.sub.OW and X.sub.RW, Y.sub.RW, Z.sub.RW are the 
tristimulus values of the whitepoints of original and reproduction images, 
respectively. 
All measurements are taken under the same intensity of the same reference 
illuminant. 
In equations (1a-c), the X, Y, and Z values are preferably expressed in 
accordance with the well known Commitee International D'Eclariarge (CIE) 
standard tristimulus curves. As those skilled in the art will appreciate, 
these X, Y, and Z values are related to the visual sensing elements of the 
human retina. Preferably, the illuminant used should match the 
5000.degree. K. standard widely used in the viewing apparatus mentioned 
above. Preferably, tristimulus values are integrated from the tristimulus 
curves of the CIE 1964 10.degree. observer, due to the greater sensitivity 
of the eye to image color errors at this larger subtense. The basic 
characteristics of different subtractive ink and dye sets are sufficiently 
similar, however, that equipment based upon the more widely used 1931 
2.degree. observer is also satisfactory. Thus, by insuring that Equations 
(1a-c) are true for every pixel, system 10 insures that the appearance of 
the image ink-printed output 28 will be a good match to the appearance of 
the image of the input print 12. 
Images satisfying these conditions may be termed "matched by endpoint 
aligned colorimetry". This form of colorimetry provides a novel objective, 
mechanizable, and effective specification for image matching. In 
particular, colorimetric, and other objective, mechanizable forms of image 
matching in the prior art do not deal with blackpoint alignment, although 
some form of it has long been known to be necessary in practical image 
reproduction. Colors for which these conditions cannot be satisfied are 
"out of gamut" with respect to the matching definition of Equations 
(1a-c), in conjunction with the film type and printing process in 
question. The phrase "out of gamut", as further used here, is used in this 
sense. 
While the relationships of Equations (1a-c) have been established 
empirically, it is helpful to understand, as best as possible, why they 
should take this particular form. Illumination intensity varies greatly 
from one part of a typical scene to another. Color varies as well; even 
when there is only one light source. Indirect reflections tend to have 
different color from the directly incident light, and the mix varies from 
place to place. Since the eye and brain are excellent at ignoring this 
effect when perceiving object color, it can be understood that if an 
object in different representations of the same image is to be perceived 
the same way, the whitepoint of each representation must be scaled to be 
consistent with the immediately surrounding context. That the best 
correction for differing blackpoints is a simple subtraction of apparent 
blackpoint energy may seem more surprising, especially in view of the many 
highly non-linear relationships in the psychophysics of vision. There is a 
diffuse scattering of light within the human eye, however, which adds 
about 2% of the average scene intensity to all points of the retinal 
image. For the live scenes with which we mostly contend, however, 2% of 
"average scene intensity" can vary from much less than 2% of object white, 
for a mostly dark field of view, to much more than 2% of object white, for 
a field of view which includes the glare of one or more bright 
illumination sources. Furthermore, since the exposure of different parts 
of the retina to other, differently illuminated parts of the eye's 
interior must vary with retinal location and the distribution of scene 
brightness, the diffuse fill must vary some from one area to another. Thus 
the brain's ability to perceive a constant object black must require the 
equivalent of a scene and context dependent offset subtraction. This 
explains how a printed "black" can have several percent reflectance and 
still be a subjectively. satisfying black, so long as nothing blacker is 
nearby (in particular, nothing blacker is perceived to be on the satne 
surface). It may also explain why, for matching purposes, the residual 
black reflectance is best treated as though it were a diffuse fill 
throughout the image. 
Blackpoints are defined by the colorimetry of the darkest area a process 
can render when attempting to produce a high quality black. For a 
photograph, this normally corresponds to a completely unexposed region of 
the original image, while for four color printing, it corresponds to a 
neutral tone at the Under Color Removal (UCR) limit. The whitepoint of the 
printed reproduction is taken as a defined fraction, such as for example, 
90%, of the tristimulus values of the unprinted medium upon which the 
image is printed. The fractional reduction allows detail to be held in 
highlight areas without screen dropout, and preserves the character of 
specular highlights on light toned surfaces. It may be adjusted in 
accordance with the capabilities of the specific printing process being 
characterized. For the original, the whitepoint is taken to have the 
chromaticity of the background against which the originals are viewed, 
selected, and with the preferred use of instant photography, the originals 
are also preferably adjusted by the customer. This viewing background is 
preferably a piece of standard com nercial grade proofing stock, against 
which the images are placed in a portable 5000.degree. K. viewing booth. 
The luminance tristimulus value Y.sub.RW is taken from the brightest white 
at which the film type of the original can hold acceptable highlight 
detail, then the other two tristimulus values are calculated by combining 
this datum with the previously measured chromaticities of the background 
used. 
As described above, the digital electronic signals, as provided by the 
output of the scanner 14, are preferably standardized so as to facilitate 
the solving of equations (1a-c). The standardized representation should 
facilitate storage of images over time, and communication of images from a 
user who scans them and forwards them to a service bureau that prints 
them. It should also allow the use of popular software tools for image 
editing, compression, etc. without extra file translations. In order to 
provide the standardized values for the pixels of the image, it is 
necessary to perform at least one, and preferably two transformations, as 
generally described above. 
As generally described above and more specifically described below, the 
digital data representing the image is preferably defined in terms of a 
linear additive red, green and blue color space. Accordingly, the X, Y and 
Z values must be converted to the primary color component values. More 
particularly, the representation of the color of a pixel is therefore 
defined by specifying the normalized value of the three additive primary 
colors, hereinafter referred to as R', G', and B' (where the value R' 
represents the normalized value of the primary red component, the value G' 
represents the normalized primary green component, and the value B' 
represents the normalized primary blue component) which will add together 
to provide the designated color. 
As will become more evident hereinafter, it is preferable for the standard 
image space to be an R', G', B' space rather than normalized values of an 
X', Y', Z' space, because images represented in the R', G', B' space can 
be edited with popular prior art software tools for image editing, 
compression, etc. without extra file transformations. Further, the R', G', 
B' space provides a standard representation that facilitates the storage 
of images over time, and the communication of images from a customer who 
scans them, and forwards the data to a service bureau that prints the 
images from the data. 
Two primary considerations influence the choice of using primary color 
component values: 
1. The desire to represent all reproducible colors. This may require the 
use of primary color values somewhat more saturated than can exactly match 
CRT phosphors. 
2. The desire to match the chromaticities of the CRT phosphors widely in 
use, at least approximately, so that popular image editing software may be 
used on stored images without further file conversions. 
Within any latitude that remains, these secondary considerations apply: 
1. The desire to minimize the computational burden in capturing image data 
from some predominant image source, such as a heavily used scanner type. 
2. The desire to maintain archival compatibility with a previous standard. 
Once the primary component values have been selected, a tramformation 
between X, Y, Z space and R', G', B' space can be developed. In developing 
this transformation it is useful to first define a normalized intermediate 
X', Y', Z' space with respect to each pixel as defined by the following 
equations (2a-c) wherein: 
##EQU2## 
wherein X, Y and Z are the measured tristimulis values of the original 
image, 
X.sub.K, Y.sub.K and Z.sub.K represent the values of the blackpoint of the 
original image, and 
X.sub.W, Y.sub.W and Z.sub.W, represent the values of the whitepoint of the 
original image, all of the values preferably being measured using standard 
colorimetry apparatus. 
A transformation from X', Y', Z' space to R', G', B' space preferably is of 
the form of a three by three matrix M1, where M1 satisfies the following 
equation (3). 
##EQU3## 
The elements of matrix M1 will be specified further below. 
Prior to storage of an image as a digital computer file, the R', G', B' 
values are preferably normalized and gamma encoded. In one embodiment, the 
file storage parameters R.sub.file, G.sub.file and B.sub.file, are 
represented as unsigned 8-bit bytes. The parameters R.sub.file, G.sub.file 
and B.sub.file are then given by the following equation (4): 
EQU R.sub.file =255(fR').sup..gamma.f, G.sub.file =255(fG').sup..gamma.f, 
B.sub.file =255(fB').sup..gamma.f (4) 
wherein 
the whitepoint is f times the energy of the maximum code, 
.gamma..sub.f represents the gamma encoding, and 
final values for R.sub.file, G.sub.file and B.sub.file are rounded to the 
nearest integer. 
Further, values larger than 255 are clipped at 255, and values less than 
zero are clipped to zero. A preferred choice for .gamma..sub.f is 0.4545 
which provides compensation for displays with a gamma of 2.2, and 
therefore closely approximates common digital image standards, although 
other values can be used. Preferably, f is 0.900, providing an 11% 
overrange capability in any highlights. A range of values for f may be 
successfully used so long as a single value is consistently observed. 
The scanner 14 must be calibrated via transform device 16 so that scanner 
14 and transform device 16 provide output data in the form of the standard 
color space. Unfortunately, the great majority of image input scanners, 
and of other sources of digital images, are not colorimetrically 
calibrated. Typically, a set of square cut separation filters is employed 
to provide an approximate RGB representation. Since the capture process 
thereby employs a set of taking sensitivities which are not linear 
combinations of the CIE tristimulus curves, a colorimetric calibration is 
not possible for a broad class of source materials. For a single image 
medium, however, there is a specific set of three image forming dyes, and 
thus a one-to-one relation between the scanner readings, the dye 
densities, and the colorimetry of each point in the image. A separate 
colorhnetric calibration is thus possible (within the limits of equipment 
stability) for each type of image medium to be used. 
In the preferred embodiment, a colorimetric calibration is preformed for 
each and every type of original reflection print 12 that will be provided 
at the input to system 10. Such a calibration can be approached in a 
number of ways. Each starts with a calibration image (or images) in the 
medium in question, such image consisting of many different uniform color 
swatches, evenly sparming the gamut of the medium. One preferred 
calibration image for use in calibrating Polaroid PRO100 film is provided 
in the Kodak Q60 Scanner Alignment Kit. 
These swatches are first measured to determine their tristimulus X, Y, Z 
values using standard colorimetry apparatus. As will be described further 
below, the X, Y, Z values can be used to compute the corresponding 
standardized image space values, R , G', and B'. 
Once the R', G', and B' values have been computed for each swatch on the 
calibration image, the calibration image is scanned to determine the r, g 
and b values returned by the scanner. Next an optimization is performed to 
find an optimum transform between the r, g, b, space output by the 
scanner, and the standard image space, R.sup., G', and B'. 
In one method of determining the optimum transform, a very large number of 
swatches are prepared and measured, and the results are used to create a 
look-up table which relates r, g and b values as input to the desired R', 
G' and B' values as an output. High order input bits may be used to build 
the table address, while low-order bits are used for interpolation. Known 
methods, extended by the novel use of endpoint aligned colorimetry, may be 
used to accomplish this. 
In the preferred method, blackpoint values R.sub.K, G.sub.K, B.sub.K, 
whitepoint values R.sub.W, G.sub.W, B.sub.W, neutral correction exponent 
values .gamma..sub.R, .gamma..sub.G, .gamma..sub.B, and matrix M2, are 
determined in a best fit manner, such that the following simplified 
transformations of equations (5) and (6) may be applied. 
##EQU4## 
In a preferred refmement of this method, to avoid redundant setting of the 
whitepoint, and more parameters than necessary in the fit, the matrix M2 
is required to have row sums of unity. This reduces it to six degrees of 
freedom, and forces it to preserve the brightness and color (or rather, 
lack of it) of the neutral scale. In this method, the matrix M2 is 
preferably given by the following equation (7): 
##EQU5## 
The best fit parameters are given by the following procedure: 1. Given the 
current best estimate of M2, (which may initially be the identity matrix) 
calculate the values of R.sub.W, G.sub.W and B.sub.W, and R.sub.K, G.sub.K 
and B.sub.K ; 
2. Using measurements from a full gray scale of swatches, preferably 
equally spaced for CIELAB "L", find the values of .gamma..sub.R, 
.gamma..sub.G, .gamma..sub.B, yielding the least sum of squares of errors 
in .DELTA.E (where .DELTA.E is defined in terms of the CIELAB standard); 
3. Using the full set of measurements, find that M2 yielding the least sum 
of squares of errors in .DELTA.E; and 
4. Repeat the above three steps until there is no further significant 
change in the calculated parameters. 
The multi-dimensional optirnizations required in steps 2 and 3 may be 
performed by any of a variety of techniques. The "downhill simplex" 
method, as described by Press, et al. in Numerical Recipes in C (Cambridge 
University Press, 1988), is preferred for its simplicity and robustness, 
and has been successfully used to achieve rapid results on an inexpensive 
personal computer. It is necessary to perform the optimizations of steps 2 
and 3 separately, as the standard CIELAB .DELTA.E calculation which is to 
be employed does a rather poor job of capturing the objectionability of 
color error in an image context (it is primarily intended to deal with the 
perceptibility of color difference in a swatch comparison context). In 
particular, .DELTA.E alone understates the both relative visibility and 
objectionability of color errors near neutral. By separating step 2, which 
corrects the gray balance, from step 3, which corrects more saturated 
colors, this problem is effectively minimized. 
One advantage of this method is that the calculations required by equations 
(5) and (6) can be merged into single uninterpolated lookup tables for 
each color component. Further, this method can be implemented on existing 
scanners, such as the preferred Polaroid CS-500, such that processing the 
image into standard form does not require any separate software 
procedures. This facilitates the use of popular pre-existing image capture 
environments, such as Adobe's PhotoShop. Also, even with the computations 
required by equation (5), this method is less computationally intensive 
than other methods. 
In yet another method of performing the optimization, the first two methods 
are combined in reverse order. An intermediate estimate of R, G, B is 
further refined by interpolated table look-up. Due to the relatively high 
accuracy of the intermediate estimate, this table may have many fewer 
entries (and be derived from many fewer swatch measurements) than called 
for by the first method. 
One preferred choice for M2 and .gamma..sub.R, .gamma..sub.G, 
.gamma..sub.B, for use with the Polaroid CS-500 scanner and Polaroid 
PRO100 film are as follows. 
##EQU6## 
Once scanner 14 has been properly calibrated as described above, scanner 14 
effectively transforms input image 12 into a representation within the 
standard R', G', B' space. The additional calculations of equation (4) can 
also be programmed into scanner 14 so that the output of the scanner 14 
includes digital file representations of the input image 12 in the form of 
R.sub.file, G.sub.file, B.sub.file. This facilitates the use of popular 
image editing software, thus allowing customers in the preferred 
embodiment to edit the output of scanner 14 with editor 34 prior to 
feeding the data to transform device 18. 
The transformation between R', G', B' space and X', Y', Z' space will now 
be discussed. The transformation from unnormalized X, Y, Z space to R', 
G', B' space is in the form of a three by three matrix M3 such that M3 
satisfies the following equation (10): 
##EQU7## 
where M3 is of the form given by the following equation (11). 
##EQU8## 
As can be seen from equation (11), the columns of M3 define the tristimulus 
values of the set of three additive primaries with respect to which image 
color is encoded. Therefore, once the primary colors R', G', B' have been 
selected, the matrix M3 is completely constrained. 
Matrix M3 must be normalized to provide a transformation between R', G', B' 
space and X', Y', Z' space. This normalization is accomplished by use of a 
diagonal three by three matrix M4, which represents the tristimulus values 
of the standard 5000.degree. K. illuminant, where M4 is given by the 
following equation (12). 
##EQU9## 
The matrix M5 which is equal to the inverse of (M4)(M3) provides the 
desired transformation from R', G', B' space to X', Y', Z' space. Further, 
the reverse transformation from X', Y', Z' space to R', G', B' space 
referred to as M1 in equation (3) is given by the inverse of M5. 
The values calculated by device 16 are then fed to transform device 20 
which generates the color separation. In general, a transformation between 
a defined additive color space and a color separation for a known printing 
process can be developed by known techniques given the following 
additional considerations: end point aligned colorimetry in accordance 
with equations (2a-c) must be applied to printed calibration swatches, and 
the colorimetry for the standardized image input files to which such 
swatches are to correspond must be interpreted using matrix Ms. Since most 
offset printers use the cyan, yellow, magenta, black (CYMK) printing 
process, the output of device 20 is generally in the form of four digital 
files such that each file corresponds to the four printing inks. Other 
printing processes may be used, as long as the type of process is well 
characterized. The type of printing process used is specified by input 22. 
Since all the matrices and variables required for the calculations 
performed by scanner 14, transform device 16, and transform device 20, can 
be precalculated as described above, the process of transforming an input 
image to a color separation is entirely automatic, i.e. , no human 
judgment is required. The system 10 further guarantees that the final 
ink-printed product 28 generated from the color separation produced by 
device 20, will be a good match to the original input image 12. 
Since certain changes may be made in the above apparatus without departing 
from the scope of the invention herein involved, it is intended that all 
matter contained in the above-description or shown in the accompanying 
drawing shall be interpreted in an illustrative and not in a limiting 
sense.