Note: Descriptions are shown in the official language in which they were submitted.
WO 96/3~29~ o Is l7
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APPARATUS FOR USE IN COLOR CORRECTION IN AN
ELECTRONIC IMAGING SYSTEM
FIELD OF THE INVENTION
This invention, in general, relates to the field of electronic imaging devices
and, more ~ ul~ly, to apparatus and methods for use in such devices for editing
tone range and correcting for color errors.
BACKGROUND OF THE INVENTION
It is well understood that the perceived colors from objects depend on thei}
spectral reflection 1 ~ t ~ - ` and the spectral content of the sources which
illu~ninate them. Objects that are neutral in color, that is, those which reflect equal
amounts of an adopted set of primary colors, will appear neutral gray when
min~t~d by a source that is spectrally flat. For example, a neutral gray object
min~t.~d by a spectrally flat source such as noontime sunlight, which has a
uniform spectral power ~ tnhlltifm will appear more or less neutral to an observer.
If a light source differs from neutral, it is said to have a "color cast" since its spectral
content is no longer flat with wavelength, but rather, contains more of one "color"
than another. It, therefore, has a dominant "cast" which makes it less 'pure" than a
neutral "white" light. For example, tungsten sources have relatively more red light
than blue so will have a "reddish cast". Fluorescent lights, on the other hand, tend to
the "greenish" part of the spectrum, and hence have "green casts".
Obviously, the color appearance of even neutral objects, whether in
reflectance or ~ , e, is influenced dramatically by the cast of the source
min~tinE them. And, where objects inherently are of one cast themselves, that
cast can be distorted in color appearance to be something other than it would appear
under neutral lighting.
While color cast issues are relatively easy to .~ l_lr to human observers
since they can see what is taking place when differently reflecting objects are
illurninated by sources of different casts, what takes place when a nonhuman
detector does not have the same spectral response ~1, . ,.. ~ . ;~1;. . of the human visual
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ayStem is somewhat more subtle, but in principle similar if one cam mlagine
}eplacing the spectral response of the receiver for that of a human observer. Like the
human visual system other light detectors have preferential spectral a~l~;Livi~
which also vary with W~ L lly,LII. Video cameras and film scanners, for example,differ ai~l;fi~,~Lly from the hu~nan visual system in their response to color, yet all
can "see" color but in different ways. Co~ ..ly, the response of the detector ofcolor can introduce or modify the "color cast" of an image, and that color cast
component will be seen by an observer as am additiorl~l color distortion. Thus, color
cast problems are pervasive in electronic imaging systems" " r ~ themselves
in a variety of ways which make images have an 'bnreal" or 'bnnatural" color
appearance to a human observer.
One example of color cast occurs in the electronic scarming and display of
images. When scanrling color ~ .l, materials (reflection prints or
Llf~ h ~) usmg am electronic scalmer, and ~ f ~ ly, displaying the
scalmed image ~fia a monitor after applying a color transform of some sort, the
displayed image often appears with an ~ 1 lF color cast which is l uvl~,~ullle
because it is unnatural in ~rpf.~r~nrr Such color casts can be attributed to a variety
of sources mcluding, but not limited to, scene illlmninrtinn conditions during
a~ exposure,variationinthel ,~ ofthel.l...lu~ ~ . materials
themselves, ambient ~ conditions, or variations in subsequent chemical
processing.
The color cast problem has been dealt with in a variety of ways depending on
the context in which it appears. In labs for the processing and prmting of
CUIl~,llLiull~ r films, controls for color cast removal are provided to the
operator who assesses the degree of cast and manually adjusts for its removal. In
many such systems today, automatic color cast removal is;, . ,l,l. . ". ~t 1 by way of a
low-resolution video camera which observes the processed negative, performs a
calculation on the resulting data, and provides some form of correction to the
negative-printing system. The rnmrllf~tinnc typically are based on computing the
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average color level m a negative which, on ave}age, will produce a neutral gray in
the print.
Consumer camcorders use a slightly different approach which deals primarily
with variations in the illuminamt color t~ laLul~. Most operate by attempting todetermine a white object in the scerle, and then infer a scene color cast from the
white pomt cast. Capture circuitry is then 1~ ' ' ' "on-the-fly" as recording
continues. These mPthr~ lovj~-~ are sometimes referred to as automatic gain control
(AGC) and automatic white point control for correcting for differences in the
illumin~mt's color ~ .,LaL~i, a measure of the color content of the illuminant.
Color copiers may employ steps similar to those above or various
~mhin~tirmc of them.
Image processmg software often provides for user adjustment of color cast by
way of manual control over color content through the use of color sliders for R, G, B
content. However, in the _ands of am unskilled and,., I~ ~ . cd operator, such
controls can be ~ lr and as l..vl.~ as they are beneficial.
While a number of approaches to the color cast problem have appeared m a
variety of settings, it is a primary object of this invention to provide a systematic,
highly automated m~th~ f logy for the removal of color cast for use in electronic
imaging systems.
It is another object of the present invention to provide for the systematic
l,,."~I'",.,.rl,nn display,andremovalofcolorcastsfromscannedhardcOPYimages
displayed on color monitors.
It is yet another object of the present invention to provide for the automated
editing of the tonal r~mge in a scarlned color hard copy of an image to assure that
neutrals m the image appear visibly gray.
Other objects of the invention will appear obvious and will appear hereinaftff
when reading the following detailed ~ rirtinn
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~, ;.
SUMMARY OF THE INVENTION
The invention is related to electrorlic color processing of images im the form
of electronic signals and, in particular, to systems which are not wholly automatic
but involve some operator sdection. It is applicable to electronic scanners used to
scan ~ or other materials, ~ , electronic still cameras, video
cameras, video or still picture image-editing software, or in the related alts. It may
be used with systems usmg tbree or more color signals (e.g. RGB or CMYK).
Although the preferred r~ o~ 1 involves the use of digital signals, amalog signal
processing to perform tEIese operations could aiso ~e . ~ Il It- 1 Regardless of
the signal source, all aspects of the invention involve the use of a color display
which is viewed by an operator.
The inventive system operates to remove color casts or color balance errors
from images. The system consists of three main parts: a set of cast-prediction
operators which operate on the source image data and coarse device profile data, a
preview mechanism whereby an image can be displayed to the operator on a
calibrated color display, and a mechanism for selecting between candidate casts
computed by the cast-prediction operators - which are applied to the images and
displayed to the user for evaludtion. The use of ~ images for image
preview and cast-removal 1~", 1 ,1" . 1 1 ,"1 1 is 1~ . ~111111. . .~1. .1, but not required.
In one aspect of the invention, the operator obtains a source image and
presents it to the system, along with the coarse device profile for the image
acquisition system and a device profile for the display monitor. Using the frrst two
datd sets, the system computes equivalent log-scene exposures (e.g. Red exposure,
Green icxposure, Blue Exposure) for each pixel in the image. The cast predictionoperators compute a color cast from the log-scene exposure images using knowledge
derived from the image datd and the known exposure balance (color balance or cast)
of a neutral object in the scene -- which was provided as pa~t of the coarse profile
datd dCC~ g the image. The cast predictions from each of the operators, now
represented by a log exposure triplet ( e.g., R, G, B exposure) are stored in a
memory.
wo 96/3529~ 947
2Ig21 04
In another aspect of the invention, the operator selects a cast prediction for
preview.
In yet amother aspect of the invention, the image data is processed through
the color processing system the operator normally uses to transform the input image
data for display on the monitor, with mn-lifi~tinnc such that the selected color cast
correction is applied. For example, if the image source is an input scanner having
the property tbat scan data is ~UIU,UUl ~iUII~II to log-scene exposure for each of tbe color
channels, cast correction may be introduced by adding an offset to each channel
before sending the data to the color processor to compute an image for rendering on
tbe display.
In the usual operating mode, an operator repeats the color tr~msform and
preview operation for each of the cast predictions, amd tben selects the cast
prediction having the preferred appear.mce. Additional cast correction could be
performed in ~ ptinn~l cases using CU,~ tiullal means, begimling with the closest
prediction from the above system.
DESCRIPTION OF THE DR~WINGS
The novel features that are considered .1,~ of the invention are set
forthwithl~Li~uLu;L~/intheappendedclaims. Theinvention,itself,however,both
as to its ~ ~ æ~ ;. ", amd method of operation, together with other objects amd
advantages thereof, will best be umderstood from the following detailed description
of the ~mhntlim~-ntc when read in connection with the a, ~u" ~ .yillg drawings
wherein the same numerals have been used throughout the Yarious figures to denote
the same features and wherem:
Fig. 1 is a .1; .~ ~ ; perspective view of an electronic imaging system in
which the present invention is embodied;
Fig. 2 shows a fiow diagram for an ~ o.~;" . .~ of the invention in the form
of a Ll~ llcy scanner amd associated ~ which implement the cast
removal m~-thn~ ngy of the invention and ~ how preview and cast-
removal modules of the invention interact with other system ~
wo 96/35291 2 1 9 2 1 0 ~ ~ ~.,1 c ,~ ~7
Fig.3isaflowdiagramfo}thè~" aL~ u~l moduleofFig.2,showing~e
interaction ofthe cast c....~ .." preview, and cast selection, .... ~
Fig. 4 is a flow diagram of the color cast prediction module of the mvention;
and
Fig. 5 shows a graphical user mterface by which the image preview and cast
selection mf-fh~ni~m can be operated as part of an image-editing wulkakll;ul..
DETAILED DESCRIPTION
The invention in a preferred e",l-o.l;",. .,~ is described in connection with a
color negative and ll~.,~cl~.,r scarmer As will be seen hereinafter, the scannerconsists of a transport light source, color CCD Imear array detector, a transport
mf-e~ .m a preview memory, and a color tr,msform mrrh~ m Output from the
scanner is digital data intended for display on a color monitor with output
similar to the Apple 13 inch color monitor, IllalllJI`f~ by Apple
Computer of Cupertino, California.
This system is designed to provide high quality digital color images from a
wide variety of l IIuL,-~f~ flc negative and Llflla~f ~ Cll~,,r materials. Further, it is
designed to be used by relatively unskilled operators who may or may not have
significant knowledge of color reproduction systems, and need rapid scanning of
their images. l'he major application for the scanner is in the desktop publishing
application where the scarmer is attached to a personal computer used for image
editing and display. As such, the scanner is controlled by and provides output data
to host application software executing on a PC ~e.g. Adobe Photoshop by Adobe
Systems, T~f"'1""- ' '~)-
Reference is now made to Fig. 1 which shows an image-editing wul~al~Liull
20 embodying a preferred form of the invention. C omponents of the workstation 20
comprise a computer 22 used to perform operations on image data amd present a user
with a graphical interface in the fûrm of dialogs on a co~or monitor 24, which is
preferably calibrated. Color monitor 24 is also used for image preview amd display
operations. A pointing device 28 and keyboard 26 fæilitate operator interætion
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with other UUIIIUUII~,Il.~ of wul~Liiull 20. A disk drive 36 can be used for input or
output of image files to or from a storage disk 34 or the like. Input signals can also
originate from a digital still or motion video camera 30, but preferably originate
from a Il~Aual~ "y scanner 32.
In general, the work station 20 allows an operator to place the scan media in
the scanner, specify the material to be scanrled (e.g., color negative)along with the
scan resolution and output size. A low resolution preview scan is performed and
displayed to the user after havmg been color processed for viewing on the calibrated
monitor. The user can then perform ~ on the scan parameters ( e.g.,
sharpening level, color cast removal, brighhness, cropping,...) while viewing aninteractive image preview showing the results of the ~ , tben initiate a final
scan. A high resolution scan is then performed arld the data from the CCD array is
,ly processed tbrough the color processor and sent to the host ~MIir~hnn
Figure 2 is a block diagram showing major ~ . ." ~ 0f the invention as it
resides in ~ r~ 20 and is generally desigrlated as a system 50. As seen,
system 50 comprises rh~togr~rhi~ negative image sampling module 52. Module 52
includes a light source 54 and a sensor 58 in the form of a CCD array. The makria
being scarmed, m tbis case a color negative, is desigrlakd at 56. Output signals are
generated from CCD array 58 as the negative 56 is scalmed. The output signals
consist of tbree analog signals, one eæh for three primaries: red, green, and blue.
The output sigmals are ~,lu~ill~',y linear with respect to optical ~ ofthe negative being scarmed, in each color channel. The output of the sampling
hardware is taken as mput to the digitization block 60, which consists of an
adjustable analog gain stage 62 amd an analog-to-digital converter 64. The analog
gain stage 62 allows adjustment of the signal input to the analog-to-digital converkr
64, which converts the arlalog signals to quantized digital signals having discrete
levels. The output of the digitization stage can be directed to two modules,
depending on the operational mode of the scanner. During preview mode, the output
is directed to the input of a preview and cast correction module 72. Alternately,
when m final scan mode, the output is direckd to the input of a color processor 74.
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2l92lo4
A control line 90 and mode sv~itch 70 direct the signal from the analog-to-digital
converter 64 toward the cast preview and removal module 72 or color processor 74based on the current selected scam mode. When scanner 32 is irl operation, the
outputs of the cast preview and removal module 84, 86, and 88, Ic~ .ly, are
directed to the light source 54, analog gam stage 62 and color processor 74,
~c*~.~Li~ly. In all three cases, the signals are gain or offset signals appropriate for
adjusting the uullc~,ulldillg subsystems to achieve the desired cast removal. When
in fmal scan mode, data from the analog-to-digital converter 64 are processed
through the color processor 74 using mput from the cast preview and removal
module 72 . The resultant image data can then be sent to a nulnber of outputs 76including host application 78 (e.g., Adobe Photoshop) running on the host computer
22, the color display monitor 80, or a storage file 8æ
Fig. 3 is a block diagram showing in more det il cast preview amd removal
module 72. Input to module 72 is stored in digital memory 74. The contents of
memory 74 can be read as r~put to three modules, depending ûn the mûde of
operation of the cast preview and removal module 72. In the furst mode of operation,
the cast predictions are determined by the cast prediction module 76, operating on
the data in the preview memory buffer 74 and stored in a memory buffer 78 for later
use. In a second amd optional mode, an exposure shift is computed by use of a scene
amalysis module 92 operating on the contents of the preview buffer 74. The
operation consists of creating a luminance signal from a color signal 94 and rulming
the scene analysis processor 96. The output of processor 96 is a signal which isapplied equally to all three color channels, and will not intrûduce or remove a color
cast.
Different cast prediction mPthn~ lo~i~c are carried out in block 76 and stored
,Li~,ly, blocks 80 or 82 or 84 or 86 in memory 78. The results ofthe cast
prediction m~tht fl~ c are selectable by a selection mechanism 90 via operator
intervention and the results are combined with the signal from the scene analysis
module 92 at a junction 100. The combined signal is sent to a color processor 102
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~WO 96/35291 ~, ~, PCINS96/04947
and/or to a gain ,u~uLiLiulfil~g module 106. The gain ~LiLiul~illg module 106, in turn,
sends a signal to the light source 54, gain stage 62 and/or color processor 74.
In the tbird operational mode, the contents of the preview buffer 74 is
processed through the color processor 102 with input from junction 100 and the
resulting image displayed on color monitor 104 for cast judgment by the operator.
Cast prediction module 76 will now be described with reference to Fig. 4.
The contents of preview buffer 74 are p}ocessed through a color processor 200 to
form a .~ ; , of scene exposure, which is stored in a memory buffer 202.
Cast operators 206, 208, 210 and 212 take the contents of this buffer as input, and
each output a cast signal which is stored in a CUll-,~,UUlllliU~ memory buffer location
80,82,84,86, ~ ivcly, in the cast memory 78.
Fig. ~ shows a graphical user interface with which a user interacts in
practicmg the invention. An operator interaction dialog 120 is used in the preferred
~ ., .1 .û.~l ", l It consists of a dialog box presented to the user on the color monitor
24 by computer 22. Region 122 is a color image display mto which is mapped the
contents of the preview buffer 74 after processing by color processor 102 operating
in the display mode of the cast preview and removal module 72. Below the previewis a cast selection mecharism 124 consisting of four buttons, 126,128,130,132, each
~ullc~luulldi~ to a cast memory location (80,82,84,86) in the cast buffer 78. Bdow
the selection mechanism is a button for initiating a final scan 140, which sends imput
to the mode switch 70. Region tool 142 allows the operator to select a region in the
memory buffer 74 on which to perform the cast ~ u~ ull~ "uullliug to the
enclosed region in the preview image 122 using the keyboard 26 or pomting device28.
The various color cast prediction m~thn~ c and their use will now be
described. It will be seen that these comprise no cast prediction, whik point, gray
world, and bright paskls. In general, the sequence of events in carrying out theinvention involve- (I) Color Cast Prediction with a low resolution scan, (2) Cast
Selection, (3) Image Preview, and (4) Final Scan, which will now be described.
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wo 96135291 ~ PCr/USs6/04s
2~2~ ,. t-~
1. Color Cast Prediction
A) Low resolution scan
Referring to Figs. I and 2, scanner 32 is first configured for preview
scanning. The gain of the analog gain stage 62 (a Imear amplifier) is set such that
the rr~inimum density (film base density) for a negative of tbe selected film type
produces an input signal to the amalog to digital converter 64 which is mapped to
digit 1024 (out of 1024). Data for tbis adjustment is contained in a device profile for
the sc~,,~ tive c~mh:-~tif)n selected by the user before initiating the sc~m. AsCCD 58 output voltage is nearly linear with respect to incident light, the digital
counts from A/lD converter 64 will correspond to negative optical ~
The low resolution preview scan is made, and a resulting RGB I 0-bit image stored
in the preview buffer 74 shown m Figure 3.
B) Low resolution image data conversion
Using the ..l,-.,.... ;,, ~ ,.", for the ~ a~iY~ pair stored in the device
profile,inthiscaseasetofonell;,l1 ,~;~",i~lLUTsrelatingscenelogexposureto10-
bit digit for each channel: red, green, blue, the image data are converted from digital
coumt to scene log exposure and stored in a computer memory buffer.
C) Cast prediction operators
The following operations are performed on the log exposure data to predict
candidate color casts.
I) None
The first method is to perform no rnmrllt~ti~n and report a color cast of zero
red, zero green amd ~ero blue exposure.
2) White Point
The white point cast prediction method assumes that the brightest object in
the scene is a specular reflection of the illllmin~fil-n source offa neutral object. The
operation is performed in two parts -- isolation of the white, and .1. :. . ",;, ,~1 ;. ." of
cast.
a) Isolating the white
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~WO 96/35291 21 g 2 1 (~ 4 PCT/US96/04947
For every pixel location in the low-res image, compute the rninimum of the
three exposure values at that location
minRGB = Minimum(red,green,blue)
and compare the value of minRGB to the highest of all previous minRGB.
maxMinRGB holds this value. Update it if necessary.
If(minRGB > maxMinRGB)then maxMinRGB = minRGB
b) Recording the cast
If the brightness at the location is higher than all previous pixel locations
fminRGB > maxMinRGB), save the exposure data for that pixel.
whitePt_red = red;
whitePt ~reen = green;
whitePt_blue = blue;
Repeat for every pixel.
3) Gray World
The gray wo}ld cast prediction method is based on the o~plU~ iiUII that for
a large number of scenes, the scene exposure in the tbree channels red, green, and
blue when summed over the iInage area, will be equal. The operation is to simplydeterrnine the mean of all pixels in each of the three channels. That is:
Mean_Red = (nx * ny) ~ R~
Mean-Green=(nx*ny)~ Gy
Mean_Blue= ( * )~ ~ ~^Y B,~
where Rij, Gij, Bij are the red, green, and blue exposures of the ij'th pixel, nx is the
height of the selected region, and ny is the width, specified in low resolution pixels.
4) Bright Pastels
The bright pastel cast prediction method is based on tne observation that a
white point determmed from a specular highlight (as in the White Point method
above) will be incorrect due to the object reflectance not being neutral. For example,
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WO 96/3S291 PCT/US96/04947
red Christmas-tree ball might be the brightest oint found in the image, but
., ~,., P
certainly is not neutral and will cause a poor cast estimate. The bright pastel method
operates by usimg the white point deter~nined above, and averages together all pixels
v~rith exposure within a constant factor PASTELRANGE (e.g. ~) of the white having
inter-chalmel color difference less th3n another factor (e.g. 0.15 logE)
a) determine the exposure of the brightest pixel using the White Point
method described above, then deter~nine the minimum brightness threshold.
minWhitePt = Minimum(whitePt_red,whitePt~reen whitePt_blue);
minbright = minWhitePt * I/PASTELRANGE;
b) reprocess the preview image, sumnung R, G, and B exposures
separately for those pixels which are have brightness between the brightness
threshold and the brightness of the white reference foumd m the image, and whichhave a pastel factor (Minimum(G-RG-B) ) less than the p~ct~ i7:1tinn threshold, and
counting the number of pixels that meet this criteria After the entire low-resolution
image has been processed, calculate the mean R, G, and B exposures which were
included in the sums.
Once the cast exposures haYe been determined for each of the above
methods, they are converted to balance offsets.
rBalance = ( GreenExp- RedExp)
bBalance = ( GreenExp- BlueExp)
and further modified to preserve luminance exposure level
avg = ( rBalance + bBalance ) / 3,
dExpRed = rBalance - avg;
dExpGreen = -avg;
dExpBlue = bBalance - avg;
The exposure changes to be applied to implement the cast removal are then:
newExpRed = OriginalExpRed + dExpRed
newExpGreen = OriginalExpGreen + dExpGreen
newExpBlue = Ori~in~lFxrFllue + dExpBlue
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II. Cast Selection M- ~'
Once the cast prediction operation has concluded, and a set of candidate casts
has been computed, the operator must select the preferred cast removal from the set
of four ( three complex operations, and a null operation). A preferred method for
fæilitating this selection is to generate an image preview from each of the fourcandidates -- which represents the fmal cast the operator will observe on the color
monitor when the selected methods are applied to the final scan operation, regardless
of how the cast-removal operation is actually i,..l,l. ,.,. ,t ~1 The preferred
~" ,l.o.l ,., .~ is to provide the user a set of "buttons" to select a cast (method) and
update the color monitor rapidly to reflect the change in the image.
III. Ima~ePreview
The color monitor used for the preview step must be adjusted prior to use
such that the color of a neutral object in the scene (no color cast) has the appearance
the user is attempting to achieve by the ~ udu-,Licll. In the preferred ~",1,.~1;".. l,
the operator is assumed to have calibrated the display monitor in such a way that the
Apple 13 inch Color Monitor device profile adequately represents the color monitor.
This calibration can be performed using readily available tools (e.g. Knoll Gamma
tool supplied with Adobe Photoshop).
To preview the image, the contents of the preview buffer memory is
processed tbrough the color processor using the ~ SdLiv~ profile appropriate
for the negative currently being scanned. The profile ætually passed to the color
processor has been modified to incorporate the cast change selected by the operator
in the previous step (2 above). The resulting image is displayed on the color monitor
for the operators judgment. This operation is repeated under the user control for all
selections of cast (removal method).
IV. Final Scan
Having selected the appropriate cast removal, the operator initiates a final
scan. The scanner is reconfigured to pass the output(s) of the A/ D converter(s)directly to the color processor unit which has been configured as above to account
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for the selected cast change. If the cast chamg~ is large, the change may be
partitioned between the modified ~ ;aLivc profile and the analog gain stage
ahead of the A/D converted to reduce data loss. A high-resolution scan is then made,
processed through the color processor, and output to the host software ~rFlif~ti~n
Those skilled in the r21evant aits will recogrlize that the invention may be
T~ 1 in yet other ways according to its teachings. For example, in
cu~ ,~iu~ ulu~a~Ly, lab negative printing equipment provides controls for
color cast to a skilled operator, who manually determines and performs the cast
adjustment. In many systems today, an automatic system has been ;~
consisting typically of a low-resolution electronic camera (stiL or video) whichur,~ ' the negative, performs a calculation on the resulting data, amd provides
the result to the negative-printing system. Here, the present known cast correction
", 1, ~ could be replaced by the mvention.
The mvention may also be ;",1/l~ t ~1 in consumer camcorders or the like.
Consumer camcorders use a slightly different approæh, as the main problem is
changing illuminant color ~ Jlc. Most operate by attempting to determine a
white object in the scene, and then deriving color cast from the cast of the white
point. The capture circuitry is then recalibrated "on-the-fly" as ,I.lllU~U~ ,Ull,y
continues. (AGC-automatic gain control, amd auto white point control (corrects for
difference m lighting color ~ UlC))
Color copiers may employ variations of the techniques described above.
Image-processing software allows user adjustment of color cast -- Adobe
Photoshop "Variations" plug in for example. A user is given controls (R,G,B sliders
for example) and can apply a specific cast change to an image.
In addition to the foregoing, other Gl ~ ~h~ may be ;. "1,~ "t ~1 and it
is mtended that such r ~ ol~ be within the scope of the invention.
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