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Description  |
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FIELD OF THE INVENTION
This invention relates to a tonal conversion method of a picture to be used
upon formation of reproduced pictures such as printed halftone pictures
and other binary pictures (which will hereinafter be called 37 reproduced
pictures" collectively) from an original continuous-tone picture such as
painting, monochrome picture or color picture, in other words, upon
reproduction of halftone pictures. The term "picture" as used herein
should be interpreted in a broad sense so that it embraces not only
pictures itself but also images.
More specifically, the present invention is concerned with a novel tonal
conversion method for a picture, which is applicable to various tone
reproduction methods adopted upon reproduction of a continuous-tone
picture, for example, the variable-area gradation method in which a
gradation or tone is expressed depending on the size of each dot as
observed in printed pictures and the like, the density gradation method in
which a gradation is expressed in accordance with the density of each
minute picture element to be recorded (per dot) as seen in the
heat-sensitive transfer (sublimation transfer) method and gravure
plate-making process, the method in which the reproduction of a gradation
or tone is achieved by adjusting the number of dots or the number and size
of ink droplets to be recorded per picture element (unit area), and hybrid
tone reproduction method making use of two or more of these gradation
methods in combination, so that the continuous tone of an original picture
can be converted rationally and scientifically into the halftone of a
picture to be reproduced.
BACKGROUND OF THE INVENTION
Reflecting the ever-increasing demand for pictures having higher quality,
it is required to reproduce the gradation or tone of an original
continuous-tone picture correctly in a reproduced picture upon making a
print or copy from the original picture or printing out the original
picture by means of a printer. The term "original continuous-tone picture"
as used herein should be interpreted in a broad sense, so that it may
embrace soft originals composed of electronic data from image-related
computers, TV sets (including high-definition TV sets), video recorders,
electronic still cameras (video floppies), etc., to say nothing of hard
originals such as paintings, monochrome pictures, color pictures,
handwritten manuscripts, freehand drawings and the like.
The reproduction of original continuous-tone pictures is dealt with in an
extremely wide range of industry and technical field as already mentioned
above. Many problems however still remain in expressing the tone of an
original picture suitably in the tone of a picture to be reproduced,
namely, in the reproduction of the tone of the original picture. It is
hence the current circumstances that no rational and scientific technique
has been established yet for the conversion of a tone.
Taking, for example, printing which is a representative industry and
technical field, the abovementioned problems will hereinafter be
described.
When a printed picture of a halftone is produced from a photographic
original of a continuous tone by using a photomechanical camera or the
like or a printed picture of a halftone is produced from a photographic
color original by performing color separation of the photographic original
by means of an electronic color separation apparatus (monochrome scanner,
color scanner), it is indispensable to convert the tone of the original
picture from the continuous tone to the halftone upon production of the
printed picture as is known very well.
Non-scientific approaches have however been taken in the tonal conversion
of pictures in the conventional photomechanical technology [including both
photomechanical processes for making original films (e.g., halftone
positive films and halftone negative plate. The same definition will be
applied hereinafter.) by using photographic techniques and
electromechanical processes for making original films electronically].
Namely, no satisfactory scientific analysis and study has heretofore been
conducted with respect to such issues as what gradation characteristics a
halftone picture for a photomechanical process should have upon production
of a printed picture of a halftone from an original continuous-tone
picture, how a gradation characteristic curve should be specified as a
standard work curve for obtaining such a halftone picture useful in the
photomechanical process, and how such a gradation characteristic curve
should be obtained rationally. In fact, these issues have heretofore been
dealt with depending primarily on the experiences and perception of
photomechanical technicians or empirical and personal judgements of design
engineers of electronic color separation apparatus (scanners).
The above-mentioned manner of solution still prevails even the present day
of wide spread adoption of total scanners, one of very expensive and
technically advanced photomechanical equipment, in the present field of
art relating to photomechanical processes and printing. Incidentally, a
standard work characteristic curve is employed as a standard or reference
when a tonal conversion is effected by performing halftone scanning or
both color separation and halftone scanning with advanced photomechanical
equipment such as monochrome scanner or color scanner. This standard work
characteristic curve is a characteristic curve of a halftone picture,
which is to be used as a standard or reference upon performing halftone
scanning or color separation. It is also called "halftone characteristic
curve" in connection with monochrome scanners and "color-separation
characteristic curve" in the case of color scanners. Such a standard work
characteristic curve is determined by the knowledge of engineers of
manufacturers of these equipment. Their knowledge is based on experiences
and perception. Under the circumstances, users rely upon a standard work
characteristic curve stored beforehand in the memory of their equipment or
a standard work characteristic curve determined on the basis of the
experiences and perception of the users.
Accordingly, the tonal conversion of the continuous-tone of an original
picture into the halftone of a printed picture is not performed rationally
and scientifically in the photomechanical technology. This has prevented
the rational systematization of the overall photomechanical technology.
From the realities, photomechanical work is carried out by using a variety
of advanced electronic color separation apparatus (scanners) in order to
produce printed pictures. The photomechanical work is however still
accompanied by problems such as those to be mentioned below.
(i) Problems pertaining to the tonal expression of intermediate tone:
(a) In spite of today's wide-spread adoption of advanced photomechanical
equipment, it is difficult to achieve the gray balance (density balance)
of an intermediate tone. The term "intermediate tone" as used herein means
the tone of a halftone area, whose dot area percentage ranges from about
40% to about 80%, on a print.
(b) The color of the intermediate tone is dull and does not have any vivid
tone.
(c) It is difficult to emphasize or stress the intermediate tone.
(d) The delicate gradation of the intermediate tone (which is of course
related closely to the gradation and tone of the entire pattern) can not
be reproduced well.
Reflecting the above problems, re-scanning is required to the following
extents in actual work.
About 10% in color-separation work where usual quality is required.
About 20%-30% in color-separation work where high quality is required.
It is the poor color balance of the intermediate tone in the printed
picture that accounts for a major part of causes for the re-scanning.
Under the circumstances, the poor color balance amounts to about 30% -40%
of the whole causes.
(ii) Problems relating to the preparation of a standard work characteristic
curve for processing a non-standard color original:
A color separation characteristic curve (i.e., tonal conversion curve),
which is a standard work characteristic curve required for the production
of a printed picture having desired quality, must be determined relying
upon the experiences and perception of workers, for example, when it is
desired to perform color separation on a non-standard color original,
namely, a non-standard color original having quality other than that of
standard color originals taken into consideration upon designing a color
scanner or the like (i.e., an original not subjected to proper exposure
and/or development) or when it is desired to perform color separation
similarly by using a color separation characteristic curve other than that
stored beforehand in a color scanner of the like. Further, tremendous
time, cost, labor and experiences are also required upon inputting basic
data in a color scanner in order to obtain such a characteristic curve.
In the case of a non-standard color original containing a color fog in the
high-light area thereof, its color separation work is confused so that the
quality is not stabilized. Such non-standard originals now account for as
much as 30-40%.
(iii) Operational problems of scanners:
(a) A very long period of time is required until an operator learns the
operation technique of a scanner, receives training on the scanner, and
then masters it well.
(b) The product quality varies when the technician or operator is replaced,
whereby the product quality can hardly be unified and stabilized.
(c) The product quality changes considerably depending on the place of work
or the operator, even when the same scanner is used.
(d) The high performance of a scanner may not be fully utilized in many
instances.
(e) It becomes difficult to make products of the same quality when an
original is replaced by another original having different quality or
contents.
(f) It is very difficult to make a product whose quality meets the
customer's desire.
(g) It is difficult to unify the quality of products when a great deal of
originals are received at once and their processing must be performed at
plural work places.
(h) Mutual understanding is not achieved well between scanner manufacturers
and those dealing with plate-making and printing, regarding the equipment
and the quality of products.
(iv) Lack of scientific approach for the production of halftone pictures
suitable for use in the photomechanical process:
(1) Further, design engineers of electronic color separation apparatus
(scanners) and the like do not have full understanding of plate-making and
printing techniques. This lack of understanding has resulted in the lack
of flexibility in work by the apparatus, thereby failing to solve various
problems such that diversified needs of print orderers can hardly be met,
photomechanical engineers have difficulties in experimenting and
developing new creative color separation methods, and jumping takes place
in the gradation of a halftone picture in spite of the use of the
apparatus.
(2) As a more fundamental problem, it has not been attempted to establish a
universal and rational, tonal conversion method for the tonal conversion
of an original continuous-tone picture into a printed halftone picture,
which tonal conversion is indispensable for the production of the printed
picture. For this reason, various confusions have been induced
unnecessarily regarding tonal conversion not only in the photomechanical
business but also in the printing business and even on the side of
orderers for prints.
The picture forming technique described above placing emphasis on printed
pictures will next be summarized from the viewpoint of picture or image
processing techniques including other techniques for the formation of
pictures.
Picture processing techniques adapted to form reproduced pictures such as
binary pictures and multivalued pictures from continuous-tone pictures
include indirect picture processing methods for obtaining a reproduced
picture from pictorial information of an original via an intermediate soft
picture such as color separation characteristic curve or halftone
characteristic curve as well as direct picture processing methods for
obtaining a reproduced picture directly from pictorial information of an
original. A typical example of the former methods is a scanner-dependent
picture processing method employed upon production of a printed picture,
while representative examples of the latter methods include the picture
processing method for obtaining a CRT picture for a color separation
monitor, a heat-sensitive transfer picture, a reproduced ink-jet picture,
a reproduced toner picture or the like.
It is common to these picture processing methods that electronics,
computer, mechatronics and the like are employed as technical means and an
analog processing method and/or digital processing method is relied upon
as a processing method of pictorial information.
However, none of these picture processing methods and picture processing
techniques includes any scientific and rational technique for the tonal
conversion of pictures. Most of the picture processing techniques
described above have a function to control the tone of a picture and the
luminance of each picture element. However, their controls are, as a
matter of fact, dependent heavily on the experiences and perception of a
man.
The adjustment of the tonal conversion of a picture on the basis of the
experiences and perception of a man is in fact the fundamental cause for
the various problems which arise upon formation of a reproduced picture
such as binary picture or multivalued picture from an original
continuous-tone picture. The above adjustment is a fundamental matter
which should be rationalized as soon as possible in the present picture
processing techniques.
OBJECT AND SUMMARY OF THE INVENTION
As has already been described above, the tonal conversion of a picture is
performed based fully on the experiences and perception of a man although
the tonal conversion takes an extremely important role in the entire
picture processing technology. An object of this invention is therefore to
improve the above tonal conversion to a scientific and rational tonal
conversion technique of a picture by using a tonal conversion formula
developed newly for the picture.
The tonal conversion technique will next be described in detail with
respect to a picture processing technique of a photomechanical process for
printed pictures.
The true cause for the occurrence of the above-described various problems
of the present field of art was found to reside in the fact that the tonal
conversion step of an original picture (continuous tone picture), which is
the first step of work upon production of a printed picture (halftone
picture) from the original picture and plays the most important role in
the work, was regarded as "not involving any technically and practically
rational means for the tonal conversion of the original picture and being
performed satisfactorily depending primarily on the experiences and
perception of a man". Paying attention to the above fact, the present
inventors have carried out an extensive investigation under the basic
belief that the establishment of a scientific and rational tonal
conversion technique of a picture is indispensable in order to achieve
eternal rationalization of the photomechanical process and its practice.
In particular, the present inventors have learnt through many cases that
the quality of tonal conversion of a picture directly affect not only the
quality of the gradation of a printed halftone picture but also the
quality of its tone. From this fact, a fundamental error or defect is
believed to exist in the conventional approach that a photographic color
original is subjected to color separation by a scanner to make an original
film for color printing. In the conventional scanner operation, the
analysis of a color has been considered to be most important (this is
called "color-correction-first-ism"), and its has not been taken into due
consideration how much influence a variation in each dot area, which is an
extremely important finding in connection with plate-making and printing,
would be given to the changes in gradation or tone of a printed picture
(this is called "tone-rendering-first-ism"). This can be understood easily
provided that articles on the color separation technology or scanners are
simply referred to [J. A. C. Yule, "Principles of Color Reproduction", PP
110-111, John Wiley & Sons, Inc. (1967)].
The foregoing is however believed to be extremely strange in view of the
objective facts that fundamental elements visualizing a printed picture
are two elements, i.e., "dot area" and "reflection ink density" which
occurs on each dot and the visual sensation of a man can easily
distinguish as a density difference a difference as small as 1% or even
smaller in size between dots, namely, between dot areas.
In this respect, the present inventors have the following view.
Namely, the present color separation technique (which is also the basic
technique for the designing of scanners and the like) is dependent heavily
on the photographic picture and photographic processing technology in view
of the process of its development. For this reason, the basic concept for
the formation of a picture to be printed has inclined too much toward the
technique for processing photographic pictures, so that the understanding,
study and analysis of the nature of printed pictures and the work steps of
printed pictures have been insufficient.
The characteristic features of the present color separation technique
reside in that (1) in the adjustment of a picture, color adjustment, color
correction and the like are considered to be more important than the
control of the gradation and tone of the picture, especially the control
of the gradation of the picture and (2) it is considered to be sufficient
for the control of the gradation of the picture if a halftone
characteristic curve (also called "gradation curve"; this serves as a
standard or reference for tonal conversion by a scanner) defining the
relationship between a density on the original picture and the size of its
corresponding dot, said curve being a work standard curve for producing a
halftone picture, is controlled at three points on the original picture,
namely, at the smallest dot (the brightest point H) in the high-light area
on the original picture, the largest dot of the shadow area (the darkest
area S) and an intermediate-density point (intermediate density: M.sub.1)
(this will hereinafter be called "3-point control method).
As described above, there is however a substantial limitation to the
formation of a printed picture excellent in the reproduction of gradation
and tone of an original picture from the original picture under the
present color separation technique in view of the facts that (1) the basic
elements for the visualization of the printed picture are both "the area
of each dot" and "the reflection ink density" to be observed on the dot,
in other words, the gradation and tone of the printed picture is related
closely to "the area of each dot" and (2) there is a problem in the
reproduction of an intermediate tone (an area having dot area percentages
of about 40-80%) in the operation of a today's advanced scanner, for
example, a point (M.sub.2) having a dot area percentage of 50% is not
controlled in a halftone characteristic curve (the present inventors have
already learnt the importance of the 4-point control method including the
above M.sub.2 together with the above-described H, M.sub.1 and S from many
cases).
As is understood from the three point control method mentioned above, the
conventional tonal conversion method of an original continuous-tone
picture into a halftone picture to be printed makes use of an advanced
color separation apparatus (scanner) but involves no rational means for
correlating scientifically and eternally the value at a desired control
point on the continuous-tone picture with the value of dot area percentage
of dots on its corresponding half-tone picture although their correlation
is considered to be the crux of tonal conversion.
In addition, the study of tonal conversion techniques of an original
picture for the formation of a reproduced picture from the original
picture has been rendered more difficult due to the fact that the
densities of H and S of the original picture, the density gradation
characteristic curve ranging from H to S, the surface reflectivity of a
base material for visualizing a picture to be printed, such as printing
paper sheet and/or the density of a picture visualizing material such as
printing ink always vary in widely. As a result, its study has not been
conducted fully to date.
The present inventors have carried out an extensive investigation with a
view toward breaking up the above-mentioned limitation of the conventional
conversion techniques. As a result, the present inventors have found a
novel tonal conversion method for an original continuous-tone picture,
which can always provide a reproduced picture having natural gradation and
tone to the visual sensation of a man in each case by correlating the
density of a desired control point on the original continuous-tone picture
with the dot area percentage of its corresponding control point on a
printed picture.
In summary, the present invention relates to a method for the tonal
conversion of an original continuous-tone picture (including any
continuous-tone halftone picture such as painting, monochrome photographic
picture, photographic color picture, negative photographic picture,
positive photographic picture, photographic transparent picture, or
photographic reflection picture) into a halftone picture, which is to be
reproduced, on the basis of pictorial information obtained from the
original continuous-tone picture by photoelectrically scanning the
original continuous-tone picture by way of example, which comprises
processing the pictorial information (for example, as electrical signals
of pictorial information) in such a way that the base density (x) (a value
obtained by subtracting the density at the brightest point H of the
original continuous-tone picture from the density at the desired control
point) and the halftone intensity (y) of a corresponding control point on
the picture to be reproduced are correlated, for example, x and y
processed as defined by the following conversion formula (1) are used.
##EQU1##
where x: the base density of the desired control point X on the original
continuous-tone picture, namely, a value obtained by subtracting the
density at the brightest area H of the original continuous-tone picture
from the density at the desired control point X of the original
continuous-tone picture;
y: the halftone intensity of the corresponding control point Y on the
picture to be reproduced, such as a printed picture, as expressed in terms
of dot area percentage or the like;
y.sub.h : a desired halftone intensity set for the brightest area of the
picture to be reproduced, such as the picture to be printed, as expressed
in terms of dot area percentage or the like;
y.sub.s : a desired halftone intensity set for the darkest area of the
picture to be reproduced, such as the picture to be printed, as expressed
in terms of dot area percentage of the like;
.alpha.: the reflectivity of a base material on which the halftone picture
is to be reproduced;
.beta.: the surface reflectivity of a medium for visualizing the halftone
picture; and
k: the ratio of the density range of the halftone picture to be reproduced
to the density range of the original continuous-tone picture.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(a) is a schematic illustration showing one embodiment of this
invention for converting an original continuous-tone picture into a
halftone picture via an intermediate picture compressed in density; and
FIG. 1(b) is a schematic illustration showing another embodiment of this
invention for converting the original continuous-tone picture into a
halftone picture via an intermediate picture compressed in density range.
DETAILED DESCRIPTION OF THE INVENTION
Features of the present invention will hereinafter be described in detail.
The method of this invention for the tonal conversion of an original
picture (which may be either a hard original or a soft original such as
electronic information) can be used either directly or indirectly when it
is desired to change the tone of the original picture rationally in any
technical field where a reproduced picture, such as printed picture,
copied picture or picture printed by a printer, is formed from the
original picture.
The expression "direct or indirect utilization of the method of this
invention for the tonal conversion" as used herein has the following
meaning.
For example, in an electronic plate-making process making use of a scanner
for the production of a print, the direct utilization means a mode of
utilization in which an original picture is photoelectrically scanned by
means of a photomultiplier or CCD (charged coupled device) to obtain
information on the density of the original picture, the information is
converted into electrical signals as pictorial information, such as
voltages, the electrical signals are processed in accordance with the
conversion formula (1) to obtain electrical signals (voltages)
corresponding to dot area percentages, and the exposure is then controlled
on the basis of the latter electrical signals to directly form a halftone
picture to be printed, while the indirect utilization indicates another
mode of utilization in which the continuous tone of an original picture is
converted into another continuous tone by using the conversion formula (1)
and the conversion of the picture of the thus-converted continuous tone
into a halftone picture is effected in accordance with a conventional
method or a further mode of utilization in which a dot area percentage
corresponding adequately to the density of a desired control point of an
original picture is measured in advance by a transmission densitometer
capable of converting the densities of a continuous tone into their
corresponding halftone area percentages or by a similar device, thereby to
determine whether a plate-making operation by a scanner is performed
correctly or not. In the latter modes of utilization, pictorial
information, for example, electrical signals bearing the pictorial
information is considered to be used indirectly.
Although the method of this invention for the tonal conversion of a picture
can be used in a variety of fields as mentioned above, the features of the
present invention will next be described in detail in connection with a
tonal conversion method of a picture for producing a printed picture for
the sake of convenience.
The following objective facts and empirical rules have been known with
respect to a printed picture, namely, a halftone picture. Namely, there
are two basic elements expressing or visualizing the printed picture,
i.e., "dot areas" and "the reflection density of an ink". Regarding "the
reflection density of the ink", it has been known empirically that in
order to conduct printing while reproducing dots in the high-light area H
and shadow area S on a printing plate correctly on a sheet of printing
paper, namely, in order to perform adequate printing, the amount of the
ink must be controlled within about .+-.10% relative to an appropriate ink
amount on a printing machine (in the case of a black plate, the amount of
an ink may be controlled within a range as wide as about .+-.20% in some
instances so as to improve the quality of the picture or black characters
or letters.). From experiences, it has been known that the visual
sensation of a man can easily distinguish a difference as small as 1%
between "dot area" percentages as a density difference and the accuracy of
the visual sensation is hence superior to densitometers. It has also been
known empirically that the area at the same dot may vary by a degree as
much as several tens percentages in a plate-making or printing operation.
In view of these objective facts and empirical rules, it is readily
understood that the control of the area of each dot is extremely important
in the production of a printed picture as a halftone picture.
Incidentally, it is a well-known fact that the size of each dot (dot area
percentage) varies depending on the state of the grain of the surface of a
plate material at the time of plate-making, properties of a photosensitive
material, the degree of deformation of the corresponding dot on an
original printing film, variations of exposure (causes for such
variations: exposure time, reduction of radiation power of a lamp, etc.),
the temperature, viscosity and degree of kneading of an ink at the time of
printing, the composition, temperature and amount of damping water, the
surface conditions of paper, the printing pressure, etc. and hence gives
considerable influence to the quality of the resultant picture.
Diametrical variations of a dot (circular dot, square dot, or the like)
and the degrees of their deviations from a standard dot area are tabulated
below. The importance of control of the dot size is appreciated in view of
the fact that a slightest change (5-10.mu.m) in dot diameter leads to an
appreciable change to the picture quality, in other words, gives influence
to the visual sensation of a man. It is also understood easily that
variations in dot size are greater than the above-mentioned variations in
"the reflection density of a printing ink" as shown in the following
table.
______________________________________
Diametrical Variations of Dot and
Degrees of Deviations from Standard Dot Area
(Rates of increases and decreases of dot area
as calculated based on the standard dot)
(Unit of dot diameter: 1/100 mm)
-1/100 Dot
mm -0.5/100 mm
diameter +0.5/100 mm
+1/100 mm
______________________________________
55.56% 30.56% 3 36.11% 77.78%
36.00% 19.00% 5 21.00% 44.00%
30.56% 15.97% 6 17.36% 36.11%
26.53% 13.78% 7 14.80% 30.61%
20.99% 10.80% 9 11.42% 23.46%
17.36% 8.88% 11 9.30% 19.01%
15.97% 8.16% 12 8.51% 17.36%
14.79% 7.54% 13 7.84% 15.98%
12.89% 6.56% 15 6.78% 13.78%
10.80% 5.48% 18 5.63% 11.42%
9.30% 4.71% 21 4.82% 9.75%
8.16% 4.12% 24 4.21% 8.51%
7.02% 3.54% 27 3.60% 7.27%
______________________________________
Note:
The above figures are all theoretical values. The following equations wer
employed.
(a) When the dot size became larger:
##STR1##
S: Diameter of the standard dot
S.sub.p : Diameter of the enlarged dot
(b) When the dot became smaller:
##STR2##
S: Diameter of the standard dot
S.sub.m : Diameter of the reduced dot
In relation to the matter mentioned above, photomechanical plate-making
work involves the following background characteristics. Namely, original
pictures have a wide variety of quality and details. The step for the
formation of a printed picture, which follows the photomechanical
plate-making work, is diversified and moreover, each of such diversified
kinds of printed-picture forming steps has its own work characteristics.
Base materials such as printing paper sheets nd color materials such as
printing inks, which are both used to express or visualize pictures to be
printed, have various properties. Furthermore, the quality evaluation
standard for printed pictures varies from one orderer for prints to
another.
In order to absorb and overcome such complex and unstable factors involved
in the photomechanical plate making work and printing work, it is
therefore essential to provide means for allowing to choose the smallest
dot (Y.sub.h) at the brightest area and the largest dot (Y.sub.s) at the
darkest area in a halftone picture to be produced (to be printed) as
desired upon conversion of a continuous-tone picture into a halftone
picture and further for allowing to rationally and easily set and control
the tones of a picture, said tones ranging from the brightest area to the
darkest area, to desired tones.
It is the above-described tonal conversion method of this invention,
specifically, the tonal conversion method defined by the conversion
formula (1) that has been created and devised in light of such an
approach. The tonal conversion method will hereinafter be called "the
present conversion technique".
When "the present conversion technique" is applied to a tonal conversion
method of a picture upon production of a printed picture, the above
conversion formula (1) is used to determine the value (y) of the dot area
percentage of the dot at a control point (Y) on a picture, which is to be
printed, from the base density (x) of a corresponding control point (X) on
an original picture while freely selecting the sizes (y.sub.h,y.sub.s) of
dots desired to be placed in the high-light area H and shadow area S of
the picture, which is to be printed, on the basis of the values of the
reflectivity (.alpha.) of printing paper, the surface reflectivity
(.beta.) of a printing ink, and the ratio (k) of the density range of the
picture to be reproduced to the density range of the original picture.
The measurement of these densities is generally performed by using a color
densitometer (transmission type, reflection type, dedicated type, shared
type, or the like). When a scanner is used, they may be measured by a
density measurement device which the scanner is equipped with.
Upon utilization of "the present conversion technique", it is also feasible
to incorporate necessary soft data or information in the processing unit
for pictorial information and/or electrical signals so that densities thus
measured may be employed as they are instead of the base density (x).
The above-described conversion formula (1), which is useful in determining
dot area percentages (y), was derived from the density formula
(photographic density, optical density) accepted widely, namely,
D=log I.sub.o /I=log 1/T
where
I.sub.o : intensity of incident light,
I: intensity of light reflected or transmitted, and
T=I/I.sub.o =reflectivity or transmittance.
The above general formula for the density D may be applied to plate-making
and printing in the following manner.
##EQU2##
where A: the unit area,
d.sub.n : the area of each dot in the unit area,
.alpha.: the reflectivity of the printing paper, and
.beta.: the surface reflectivity of the printing ink.
In the present invention, the density formula (D') for plate-making and
printing was incorporated with the above-described requirement for the
maintenance of correlation between the base density (x) at a desired
control point on a continuous-tone picture and the dot area percentage (y)
of the dot at the corresponding control point on a resulting halftone
picture, whereby the conversion formula (1) has been derived so as to
bring a theoretical value into substantial conformity with its
corresponding value measured actually.
Therefore, the method of this invention for the tonal conversion of an
original continuous-tone picture into a halftone picture to be printed can
be applied to any tonal conversion method so long as the conversion of the
tone of a picture is effected while the aforementioned base density (x) is
correlated with the above-described halftone intensity (y) such as dot
area percentage. The correlation between x and y in the above conversion
formula (1) should therefore be interpreted as illustrative. The present
invention is not necessarily limited to the tonal conversion of a picture
upon production of a printed picture, which has been described above by
way of example. The tonal conversion method of this invention can also be
applied either as is or with a certain suitable modification to the tonal
conversion of an original continuous-tone picture upon production of a
reproduced picture other than printed picture, such as binary picture or
multivalued picture, from the original continuous-tone picture.
It is one advantage of the tonal conversion method of this invention that
one can know extremely easily--as indicated by the conversion formula (1),
for example, by choosing y.sub.h and y.sub.s or .alpha. and .beta. at
will--with what halftone characteristics, namely, with what halftone
characteristic curve a halftone picture, which is to be produced, would be
expressed or shown, irrespective of the quality and details of an original
continuous-tone picture (photographic picture).
"The present conversion technique" may be used by adjusting or modifying it
suitably as needed. The tonal conversion or tonal modification (or
correction) of a picture may be performed as desired by changing the
values y.sub.h, y.sub.s, .alpha., .beta., k, .alpha./(.alpha.-.beta.)
(=.epsilon.) suitably. "The present conversion technique" defined as a
basic embodiment of this invention by the above conversion formula (1)
therefore has an extremely high degree of flexibility.
This advantage will next be described in detail. Upon application of "the
present conversion technique", a user (worker) should keep in mind that
"the present conversion technique" has the following freedom.
Freedom 1:
The conversion formula (1) can be used to obtain a printed picture
conforming its corresponding original picture very well. Namely, the
conversion formula (1) can be applied while giving the priority to the
provision of a printed picture which looks exactly the same as the
original picture to the eyes of a man. Such a manner of tonal conversion
of a picture is described by the term "tonal conversion (of a picture)" in
this invention.
Freedom 2:
The conversion formula (1) can be used to change or modify an original
picture to obtain printed pictures in light of needs from the viewpoint of
photopictures mechanical plate-making and printing techniques, artistic
demands, orderers' needs, etc., whereby printed pictures can be obtained.
Namely, the conversion formula (1) can be applied while giving the
priority to the provision of a visual picture in a form changed (or
modified) when observed through the eyes of a man. Such a manner of tonal
conversion of a picture is described by terms "tonal change (modification)
(of a picture)" in this invention.
In the conversion work of the tone of an original continuous-tone picture
upon production of a halftone picture, which is to be printed, from the
original continuous-tone picture in accordance with this invention, the
above-mentioned tonal conversion or change (modification) can be achieved
with ease, specifically, by changing the y.sub.h, y.sub.s, k value,
.alpha., .beta. and .epsilon. value suitably in the conversion formula
(1).
In the case of multi-color plate-making (the four plates, namely, cyan
plate (C), magenta plate (M), yellow plate (Y) and black plate (BL) are
generally considered to make up a unit), once the standard work
characteristic curve, namely, the standard halftone characteristic curve
of a base plate [in the case of multi-color plate-making, the cyan plate
(C) serves as a base plate as known very well in the art] is determined,
the standard work characteristic curves of the other color plates can
always be determined rationally by multiplying the value y of the base
plate with appropriate adjustment values which are based respectively on
the gray balance ratios of the corresponding individual printing inks.
Moreover, all of the work characteristic standard curves thus determined
for the individual color plates are of course rational characteristic
curves and moreover, the mutual relationship in gradation and tone among
these characteristic curves is also rational and appropriate. So long as
the tonal conversion of a picture is conducted in accordance with "the
present conversion technique", the adjustment and control of the gradation
and tone of a picture to be printed in accordance with a multi-color
printing technique can be effected rationally.
By using "the present conversion method" as described above, it is possible
to get rid of the conventional tonal conversion method which relies upon
experiences and perception, and hence to perform the tonal conversion of a
picture as desired and in a rational manner. As a consequence, rational
conversion is also feasible with respect to the tone which is related
closely and unseparably to the gradation.
In order to rationalize the adjustment or control of a standard work
characteristic curve (halftone characteristic curve) further in response
to the strong demand therefor, particularly, from the standpoint of
photomechanical techniques, the position and number of control points may
be changed easily, for example, a desired number of control points may be
concentrated at a desired position, for example, around the 50% dot area
percentage. The concentration point (point M.sub.2) on an original
picture, which corresponds to the 50% dot area percentage, has been
excluded from consideration in the conventional 3-point control method (H,
M.sub.1, S). The importance of control of the point M.sub.2 has been
proven in the 4-point control method (H, M.sub.2, M.sub.1, S) proposed by
the present inventors.
When "the present conversion technique" is used for the conversion of the
tone of a picture in this invention, absolutely no limitation is imposed
on the purpose, means and method of its utilization. Needless to say, no
limitation is imposed on the picture processing technique such as the
means and method for the input or output of pictorial information, for
example, electrical signals bearing such pictorial information.
When pictorial information (i.e., electrical signals bearing pictorial
information) obtained from an original picture by a photoelectric scanning
means, a solid-state image sensing device such as CCD or a lens system is
processed by a computing means such as halftone-scanning or
color-separating electronic calculator in accordance with "the present
conversion technique", the conversion formula (1) or its term, values
and/or coefficients may be adjusted, modified, converted, omitted, or
otherwise changed suitably so long as the rationality of the tonal
conversion of a picture is not impaired.
The tonal conversion method of this invention has been described taking the
production of a halftone picture, which is to be printed, from an original
picture of continuous tone by way of example. The application field of the
present invention is however not limited to the printing field.
The tonal conversion method making use of "the conversion technique" of
this invention is effective when one wants to reproduce a picture of
non-continuous tone, such as binary picture or multivalued picture, from
an original picture of continuous tone by relying upon an expression mode
such as changes in halftone, i.e., dots or the concentration or luminance
of each unit picture element. Namely, the tonal conversion method can of
course be applied effectively to the following case.
(i) When it is desired to show a gradation or tone by changing the size of
each dot as seen in printed pictures as described above in detail, such as
letterpress, lithography, halftone gravure and silk screen pictures (this
method is called "variable-area gradation method").
The tonal conversion method can also be applied effectively to the
following cases.
(ii) When it is desired to show a gradation or tone by changing the
intensity of a pigment or dye (coloring material), like a printing ink, to
be caused to adhere per picture element (for example, per dot), which has
a uniform area, as seen in fusion-transfer type thermal transfer pictures,
(silver-salt utilized) thermally-developed transfer pictures and
conventional gravure pictures (this method is called "variable-density
gradation method").
(iii) When it is desired to show a gradation by changing the recording
density per unit area, for example, the number of dots, the number and/or
size of ink droplets, or the like as seen in pictures produced by a
digital copying machine (color copies, etc.), printer (ink-jet type,
bubble-jet type, or the like) or facsimile [this method is similar to the
variable-area gradation method (i)].
(iv) When it is desired to obtain a CRT picture from video signals, TV
signals or high-definition TV signals by adjusting the level of luminance
of each unit picture element, or to obtain a halftone print or hard copy
from the CRT picture.
(As is known very well, not only the abovementioned variable-area gradation
method but also a method for expressing or showing a gradation in a
multi-valued fashion such as the double-tone or complementary color method
are used for the production of printed pictures.)
In order to process pictorial information on the densities of a picture
obtained from an original picture of continuous gradation (including a
hard original or a soft original), for example, electrical signals bearing
such pictorial information (which may be either analog or digital signals)
in accordance with "the present conversion technique", it is only
necessary to process the pictorial information at a picture processing
unit (tonal conversion unit) of the above-mentioned equipment in each of
the various application fields, to control the current, voltage or
impression time thereof at the recording unit (recording head) of the
equipment in accordance with a value y (halftone intensity) obtained as a
result of the processing, and then to change the dot area, the number of
dots per unit area (per picture element), the density per unit area (for
example, per dot) to output a picture of non-continuous gradation such as
a dot-gradation picture or density-grada | | |
