JP3937645B2 - Image processing apparatus and image processing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to an image processing apparatus and an image processing method for forming an image by converting gradation of multi-value input image data.
[0002]
[Prior art]
An error diffusion method (hereinafter referred to as ED), an average density preservation method (hereinafter referred to as MD), and the like are generally known as image forming methods for performing halftone expression. These are intended to represent halftones in a macro manner by expressing area gradations using a small number of gradations. That is, it is a pseudo halftone expression method. Since gradation can be expressed with a small number of bits, this has the effect of reducing the load on hardware that handles image data.
[0003]
In addition, depending on the characteristics of the printer, there are cases where isolated dots cannot be accurately output in the n-value gradation conversion process. Stabilization of image quality has been achieved through the process of concentration.
[0004]
[Problems to be solved by the invention]
However, when the dots are concentrated, there is a problem that the resolution is approximated to a reduced state. In other words, the effect of concentrating dots is great in places where importance is placed on gradation such as photographs, but jaggies become conspicuous if the dots are concentrated in places where importance is placed on the resolution of characters and the like. There is a problem, and the required processing differs between the photo area and the character area.
[0005]
[Means for Solving the Problems]
Therefore, in order to solve the above problems, the image processing apparatus according to the present invention controls the threshold value of gradation conversion according to the result of gradation conversion performed in the past, and the photographic area and the character area. The threshold generation parameter is changed.
[0006]
  That is, an image processing apparatus having means for inputting multi-value data having shading information for each pixel, and means for gradation-converting the input multi-value data,
  Quantization error generated when the multi-value data is output after gradation conversionFirst calculationMeans,
  Based on the quantization error, a quantization error near the target pixel is calculated.Second calculationMeans,
  The quantization error value in the vicinity of the target pixel is added to the target pixel to correct the error.correctionMeans,
  in frontFloorStorage means for storing the result of the key conversion;
  Stored in the storage meansAverage value of gradation conversion results for multiple pixelsAccording to the generation means for generating a threshold value of the gradation conversion,
  Control means for controlling a threshold value generated by the generating means in response to a signal indicating a character area and a photo area;
  The control means controls the threshold value to disperse dots for character areas, and controls the threshold value to concentrate dots for photo areas.And
  The first calculation means performs gradation conversion on the multi-value data by subtracting the multi-value data and an average value of the results of gradation conversion for a plurality of pixels stored in the storage means. Calculate the quantization error that occurs during outputIt is characterized by that.
[0008]
  Thereby, texture control of gradation conversion processing can be performed in an arbitrary area, a photograph area, and a character area.
  Further, an image processing method executed in an image processing apparatus having means for inputting multivalued data having grayscale information for each pixel and means for converting the gradation of the inputted multivalued data,
  Quantization error generated when the multi-value data is output after gradation conversionFirst calculationProcess,
  Based on the quantization error, a quantization error near the target pixel is calculated.Second calculationProcess,
  The quantization error value in the vicinity of the target pixel is added to the target pixel to correct the error.correctionProcess,
  in frontFloorA storage step of storing the result of the key conversion in the storage means;
  Store in the storage meansAverage value of gradation conversion results for multiple pixelsAccording to the generation step of generating a threshold value of the gradation conversion,
  A control step of controlling a threshold value generated by the generation step in response to a signal indicating a character region and a photo region;
  The control step controls the threshold value to disperse dots for character areas and controls the threshold value to concentrate dots for photo areas.And
  The first calculation step performs gradation conversion on the multi-value data by subtracting the multi-value data and an average value of the results of gradation conversion for a plurality of pixels stored in the storage unit. Calculate the quantization error that occurs during outputIt is characterized by that.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
[First embodiment]
Embodiments of an image processing apparatus according to the present invention will be described below with reference to the drawings.
[0010]
<Process outline>
FIG. 1 is a block diagram showing an outline of the overall configuration of an image processing apparatus according to the present invention.
[0011]
The image reading unit 109 includes a CCD sensor 102, an analog signal processing unit 103, and the like. A document image formed on the CCD sensor 102 via the lens 101 is converted into R (Red), G (Green), It is converted into an analog electric signal of B (Blue). The converted image information is input to the analog signal processing unit 103, and is subjected to analog-digital conversion (A / D conversion) after sample & hold, dark level correction, etc. for each of R, G, B colors. Is done. The digitized full color signal is input to the image processing unit 104.
[0012]
The image processing unit 104 performs correction processing necessary for the reading system such as shading correction, color correction, and γ correction, smoothing processing, edge enhancement, other processing, processing, and the like, and outputs the result to the printer unit 105.
[0013]
The printer unit 105 includes an exposure control unit (not shown) including a laser, an image forming unit (not shown), a transfer paper conveyance control unit (not shown), and the like. Record an image on paper.
[0014]
The CPU circuit unit 110 includes a CPU 106, a ROM 107, a RAM 108, and the like, and controls the image reading unit 109, the image processing unit 104, the printer unit 105, and the like, and comprehensively controls the sequence of the apparatus.
[0015]
<Image processing unit>
Next, the image processing unit 104 will be described. FIG. 2 is a configuration block diagram of the image processing unit 104.
[0016]
The digital image signal output from the analog signal processing unit 103 in FIG. 1 is input to the shading correction unit 201. The shading correction unit 201 corrects variations in sensors for reading a document and light distribution characteristics of a document illumination lamp. The corrected image signal is input to the gradation correction unit 202 to generate density image data in order to convert the brightness signal into density data. The image signal converted into density data is input to the color / monochrome conversion unit 203 and output as monochrome data. The data output from the color / monochrome conversion unit 203 is input to the gradation conversion processing unit 204, and error diffusion processing (ED processing) or average density storage processing (MD processing) is performed as a pseudo halftone expression.
[0017]
Next, the overall configuration of the gradation conversion processing unit 204, which is the point of the present invention, will be described with reference to FIG.
[0018]
<Tone conversion processing unit>
FIG. 3 is a detailed block diagram of the gradation conversion processing unit as a point of the present invention. In the first embodiment, the 1-bit MD method will be described as an example.
[0019]
The error correction unit 302 receives a signal DR obtained by adding a random number (to be described later) to the input multivalued image signal D of the color / monochrome conversion unit 203 and error data E generated by the binarization processing unit 301 (to be described later). The image signal DE is calculated by performing correction, and output to the binarization means 301.
[0020]
The binarization unit 301 inputs an image signal DE, a binarized slice value S described later, and an average density calculated value m described later, and compares the image signal DE with the binarized slice value S. After obtaining the binary output N, the binarization error data E is calculated by subtracting the image signal DE and the average density calculation value m.
[0021]
The binarization result delay unit 303 receives the binary output N, delays by a predetermined number of lines, and calculates the average density calculation unit 304 and the threshold value calculation unit 309 as a plurality of line binarization results Nmn and B * ij. Send data to.
[0022]
The average density calculation unit 304 receives a multi-line binarization result Nmn, performs a product-sum operation on a preset coefficient and the delayed binary result, and performs binarization unit 301 and addition unit 310. Then, the average density calculation value m is output to the selector 311.
[0023]
The threshold value calculation means 309 receives the output B * ij of the binarization result delay means 303 excluding the result immediately before the target pixel, the output T of the hysteresis control amount calculation means 308, and the multilevel signal D. The binarized slice value S ′ is calculated without referring to the (binarization) result immediately before the target pixel, and is output to the adding means 310.
[0024]
The signal value S ′ obtained by the threshold value calculating means 309 is input to the adding means 310 together with the output m of the average density calculating means 304 to be added.
[0025]
At this time, when the signal of S ′ is 255, the binarized slice value S is output as 255, and otherwise, the calculation of S = S ′ + m is performed and output. If the process is coded as a program, it is as shown in FIG.
[0026]
The selector 311 is a point of the present invention, and a process of switching between the output value of the average density calculating unit 304 and the output value of the adding unit 310 is performed in accordance with a character / photo switching signal input from the outside.
[0027]
This processing is performed by switching the dot concentration control signal of the adding unit 310 and the signal of the average density calculating unit 304 to change the dot concentration amount between the character region and the photo region.
[0028]
As a result, it is possible to form an image with emphasis on resolution without performing dot concentration control in the character region, and it is possible to perform gradation-oriented processing with dot concentration control in the photo section.
[0029]
Next, the random number generation unit 306 generates an m-sequence random number R between −17 and +17 by a method to be described later, and the addition unit 307 performs addition processing with the image signal D input to the gradation processing unit 204. Done. At this time, although not shown, limiter processing is performed within a range indicated by “<0” and “> 255” so that the addition result DR falls within the range of 0 to 255.
[0030]
The hysteresis control amount calculating unit 308 is configured to perform threshold control according to the output signal DR of the adding unit 307 by a method described later. Details of each processing means will be described below.
[0031]
<Binarization>
The binarization means 301 inputs an image signal DE, a binarized slice value S, which will be described later, and an average density calculation value m, which will be described later, and compares them to obtain a binary output N and a binarization error. Data E is output and has a configuration as shown in FIG.
[0032]
The input image signal DE is divided into two systems, one being input to the comparison circuit 401 and the other being input to the subtraction circuit 402.
[0033]
The comparison circuit 401 compares the value of the image signal DE and the binarized slice value S,
When DE> S, N = 1 (1)
When DE ≦ S, N = 0 (2)
As a result, a binary output N is output.
[0034]
Further, the subtraction circuit 402 subtracts the value of the image signal DE from the average density calculation value m and outputs the result as binarization error data E. Here, the binarization error data E can be obtained by equation (3).
[0035]
E = m-DE (3)
<Error correction>
The error correction unit 302 receives the image signal DR and the binarized error data E, calculates an image signal DE obtained by performing error correction on the image signal DR, and outputs the image signal DE to the binarization unit 301. This is performed and has a configuration as shown in FIG.
[0036]
The input binarization error data E is halved by the division circuit 5-1. The result is branched into two systems, one input to the subtraction circuit 5-2 and the other input to the line buffer 5-3.
[0037]
The subtraction circuit 5-2 calculates a difference EB (= EE−E / 2) between the binarization error data E and E / 2, and inputs the result to the addition circuit 5-4.
[0038]
The adder circuit 5-4 calculates the sum of EA and EB delayed by one line by the line buffer 5-3 for one line of plural bits (8 bits in this embodiment). 5 is output.
[0039]
The adder circuit 5-5 calculates the sum of the EA and EB delayed by one line and the image signal DR and outputs it as the image signal DE.
[0040]
That is, in the error correction unit 302, as shown in FIG. 5B, the binarization error EA when binarizing A on one line with respect to the pixel of interest and B before one pixel are binarized. At this time, a process of adding the value of the binarization error EB to the data of the target pixel is performed.
[0041]
<Binary result delay>
The binarization result delay unit 303 receives the binary output N, performs a delay of a predetermined number of lines, and outputs to the average density calculation unit 304 and the threshold calculation unit 309 as a plurality of line binarization results Nmn and B * ij. Data is sent and has a configuration as shown in FIG.
[0042]
The input binary output N is sent from the line buffer 6-1 for one bit to one line to the line buffer 6-2, and data is delayed for each line.
[0043]
In addition, the data that is not delayed is delayed one pixel at a time by delay 6-3 to delay 6-8 including a delay circuit for one pixel. The outputs of delay 6-6 and delay 6-7 are output as N14 and N15, respectively.
[0044]
The binarized data delayed by one line by the line buffer 6-1 is delayed by delays 6-9 to 6-14, the outputs of delays 6-9 to 6-13 are changed from N21 to N25, and The binarized data delayed by another line by the line buffer 6-2 is delayed by delay 6-15 to delay 6-20, and the outputs of delays 6-15 to 6-19 are changed from N31 to N35. Is output.
[0045]
At the same time, outputs of delays 6-7 to 6-8 are output as B20 and B30, respectively. The binarized data delayed by one line by the line buffer 6-1 is delayed by delays 6-9 to 6-14 and output as B32 to B02 and Bi12 to Bi32, respectively. Further, the binarized data delayed by another line by the line buffer 6-2 is delayed by delays 6-15 to 6-20 and output as B31 to B01 and Bi1 to Bi31, respectively.
[0046]
<Average concentration calculation>
That is, the average density calculation means 304 is subjected to delay processing of a plurality of lines and a plurality of pixels from data obtained by binarizing a two-dimensional image, and a plurality of lines binarization result Nmn is as shown in FIG. This is input to the average density calculation means 304.
[0047]
The average density calculation unit 304 receives a multi-line binarization result Nmn, performs a product-sum operation on a preset coefficient and the delayed binary result, and performs binarization unit 301 and threshold calculation unit 309. The data m used in the above is output, and has a configuration as shown in FIG.
[0048]
That is, the multiplication circuit 8-1 receives the binarized data N15 and the coefficient M15, and outputs the multiplication result of both. In the multiplication circuit 8-2, the binarized data N14 and the coefficient M14 are input, and the multiplication result of both is output. Similar operations are performed by all the circuits from the multiplier circuit 8-3 to the multiplier circuit 8-12, and all the multiplication results are added by the adder circuit 8-13. The result is output as an average density calculation value m. An example of the coefficient when this processing is performed is as shown in FIG.
[0049]
<Random number generation>
As the random number generation means 306, an m-sequence shift register code sequence generator as shown in FIG. 10 is used. In this case, if the number of stages of the shift register is N, a pseudo random number having a cycle of 2N-1 can be easily generated with simple hardware. In this configuration, a 25-bit 1-bit shift register is used so that periodicity does not appear even when an A3 document is processed at 400 dpi.
[0050]
FIG. 10A shows the configuration as a basic block conceptual diagram, and FIG. 10B shows a processing program. Hereinafter, a description will be given with reference to FIG.
[0051]
Here, the random number generator writes “0” to the register of p [ii]: (0 ≦ ii ≦ 25) at initialization, and sets “1” only to the register of p [12]. And every time before outputting random value,
p [0] = (p [25] ^ p [24] ^ p [23] ^ p [22] & 1) (4)
And the random number calculation of equation (5) is performed.
[0052]
Here, in the processing of equation (4), “p [i] ^ p [j]” indicates an OR logic operation of bits i and j, and “&” indicates an AND logic operation.
[0053]
Similarly, the bits are shifted one bit at a time, and the same processing is executed up to the 24th bit.
[0054]
Random number = (1-2 * p [22]) * (((p [15] * 64 + p [16]
* 32 + p [17] * 16 + p [18] * 8 + p [19] * 4
+ P [20] * 2 + p [21] * 17) / 128) (5)
Therefore, a random number value of −17 to 17 is output.
[0055]
In the present embodiment, random numbers of -17 to 17 are used.
Random number = (1-2 * p [2]) * (p [18] * 8 + p [19] * 4
+ P [20] * 2 + p [21]) (6)
The same effect can be obtained even if it is changed to output random values from -15 to 15. Of course, the value of the random number can be set to 0, but in order to change the binarized slice value and perform texture control naturally, it is preferable to use data with a certain amount of random number added. Here, “*” used in equations (5) and (6) indicates multiplication. The same applies to equations (8) and (9) described below.
[0056]
<Addition>
As described above, the adding unit 307 performs addition processing of the image signal D input to the gradation processing unit 204 and the random number generated by the random number generating unit 306.
[0057]
DR = D + R
if (DR> 255) {DR = 255;} (7)
if (DR <) {DR = 0;}
At this time, the limiter process is performed so that the addition result DR falls within the range of 0 to 255.
[0058]
<Hysteresis control amount calculation>
FIG. 11 is a diagram showing the processing contents of the hysteresis control amount calculation means 308. The hysteresis control amount calculation means 308 changes the value of the constant ALF (= 32) in accordance with the input signal DR and outputs it. This is for adjusting the hysteresis amount in an arbitrary density region. That is, this enables texture control in an arbitrary density region.
[0059]
For example, when the input signal DR is less than or equal to the constant LR1 (= 16), processing is performed to set ll to 0, and when the input signal DR is greater than the constant LR1 and less than or equal to the constant LR2 (= 48) Is obtained from the following equation.
[0060]
ll = ((DR-LR1) * (ALF * 256 / (LR2-LR1))) / 256; (8)
By this calculation, the value of ll gradually approaches 0 from the constant LR1 to the constant ALF as the value of the input signal DR increases from the constant LR1 to the constant LR2.
[0061]
Similarly, when the input signal DR is larger than the constant LR2 and equal to or smaller than the constant LR3 (= 223), 11 is output as a constant constant ALF.
[0062]
Further, when the input signal DR is larger than the constant LR3 and equal to or smaller than the constant LR4 (= 255), 11 is obtained by the following equation.
[0063]
ll = ALF- (DR-LR3) * (ALF * 256 / (LR4-LR3))) / 256; (9)
This indicates that the output 11 gradually approaches 0 from the constant ALF as the value of the input signal DR increases from LR3 to the constant LR4.
[0064]
On the other hand, when the input signal DR is larger than LR4, a process for setting 11 to 0 is performed.
[0065]
A value obtained by subtracting the constant ALFm (= 16) from the above processing result ll is output as the output signal T. The purpose of this subtraction is to change the signal T of the hysteresis control amount calculation means 308 from a negative value to a positive value. This makes it possible to control the texture in an arbitrary density region within a wide range of latitude.
[0066]
<Threshold calculation>
Next, the processing of the threshold value calculation unit 309 will be described with reference to FIGS. Here, the description will be given in the program language C for the sake of explanation.
[0067]
The value of the signal T of the hysteresis control amount calculation means 308 input to the threshold value calculation means 309 is divided by constants LT1 (= 2), LT2 (= 4), LT3 (= 8), and LT4 (= 16), respectively. Variables A (= T / LT1), B (= T / LT2), C (= T / LT3), and D (= T / LT4) are obtained by internal calculation.
[0068]
Next, the threshold value calculation unit 309 performs binarization slice values according to the binarization result arrangement state (pattern) of the output B * ij from the binarization result delay unit 303 in FIG. The value of S ′ is controlled only by variables A, B, C, D and constants. Since the output m of the average density calculation unit 304 is not used here, the calculation of the threshold value calculation unit 309 can be started before the calculation result of the average density calculation unit 304 is obtained.
[0069]
This binarization result arrangement state (pattern) is as shown in FIG. All of these represent past binarization results and do not refer to the pixel immediately before the target pixel. A configuration in which the binarized slice value S is actually controlled according to the binarization result arrangement (pattern) will be described with reference to FIG. FIG. 12A shows the details of processing by a program in C language. Here, “&&” corresponds to “AND” of a logical operation, and “==” indicates processing corresponding to setting of a variable (hereinafter, the same notation is used).
[0070]
The binarization situation around the pixel of interest
B32 == 0 && B22 == 1 && B12 == 0 && B21 == 0 (10)
&& B11 == 1 && B01 = 0
Or
Bi12 == 0 && Bi22 == 1 && Bi32 == 0 (11)
&& B01 == 0 && Bi11 == 1 && Bi21 = 0
In this case, the binarized slice value S is forcibly set to a constant 255 of max and output.
[0071]
This is to make it difficult to forcibly hit dots under the above conditions.
[0072]
Also, the binarization situation around the pixel of interest is
B12 == 0 && B02 == 0 && Bi12 == 0 && Bi22
== 0 && Bi32 == 0 && B11 == 0 && B01 == 0 (12)
&& Bi11 == 1 && Bi21 == 0 && Bi31 == 0
&& B20 == 0 && hi == 1
In addition, even when the input multi-value data D is less than 31 (31 out of 0 to 255), the binarized slice value S is forcibly set to a constant constant 255 of max.
[0073]
This is also for making it difficult to forcibly hit dots under the above conditions. At this time, the result one pixel before the target pixel is not referred to.
[0074]
On the other hand, if the input multi-value data D is 31 (31 out of 0 to 255) or more under the above conditions, the 22-valued slice value S is set to the average density calculated value m and output is performed.
[0075]
This is to prevent the texture control from being performed when the past binarization result has a specific arrangement (pattern). Of course, the constant 31 here is not a fixed value but a parameter and can be set to another value such as 48 or 64. At this time, it goes without saying that if the value of 31 is increased, texture control is more likely to be actively applied. Conversely, if the value is decreased, texture control is less likely to be applied.
[0076]
On the other hand, the binarization situation around the pixel of interest is
B02 == 0 && Bi12 == 0 && B11 == 0 && B01 == 1 (13)
&& Bi11 == 1 && Bi21 = 0 && B20 = 0
In this case, the binarized slice value S is set to a value obtained by subtracting the variable A from the average density calculated value m (S = m−A) and output.
[0077]
This is to make it easier to forcibly hit the dots under the above conditions. Also at this time, the processing is performed without referring to the binarization result immediately before the target pixel.
[0078]
Similarly, the binarized slice value S is binarized using the internal variables A, B, C, D and constants without referring to the result immediately before the target pixel, according to each binarization result pattern. The slice value S ′ is controlled.
[0079]
As a result, when the hysteresis control amount calculation value T is positive, control is performed so that the dot is easily hit, and when the hysteresis control amount calculation value T is negative, control is performed so that the dot is not easily hit. In this embodiment, the control value is controlled so that the dot is easily shot by making the hysteresis control amount calculated value T positive. In other words, control is performed so that dots are concentrated.
[0080]
When the above-described processing is sequentially performed on each pixel, in any density region according to the value of the hysteresis control amount calculation value T and according to the output value B * ij of the binarization result delay means. Control to any shape of texture.
[0081]
The binarized slice value S ′ obtained in this way is input to the adding means 310 together with the output m of the average density calculating means 304 to be added.
[0082]
At this time, when the signal of S ′ is 255, the binarized slice value S is output as 255, and otherwise, the calculation of S = S ′ + m is performed and output. When the processing contents are described by a program, it is as shown in FIG.
[0083]
When the above-described processing is sequentially performed on each pixel, it is arbitrary in accordance with the value of the hysteresis control amount calculation value T and arbitrary in accordance with the value of the binarization result delay unit output value B * ij. It becomes possible to control the texture of the shape.
[0084]
In the present embodiment, control is performed so that the texture becomes a 2 × 2 pixel unit. This makes it possible to form an image in which an arbitrary number of dots are collected and stabilized in an area where one pixel is not stable due to the characteristics of the printer.
[0085]
After such processing is performed by the threshold value calculation unit 309, binarization processing is performed by the binarization unit 301 described above, and the binary signal is output from the gradation conversion processing unit 204, and the printer unit 105 Is printed out.
[0086]
By performing tone conversion that controls the amount of dot concentration based on the character / photo area signal, it is possible to form an image with emphasis on gradation in the photographic part, and image formation with emphasis on resolution in the character part. . As a result, even in a printer in which dots are not stable due to the characteristics of the printer, it is possible to perform processing that does not lower the resolution of the character portion while performing texture control.
[0087]
[Second Embodiment]
The second embodiment will be described in detail with reference to the drawings. Parts similar to those in the first embodiment described above in the basic apparatus configuration in the second embodiment are denoted by the same reference numerals, and description thereof is omitted.
[0088]
In the configuration of the first embodiment shown in FIG. 3, the selector 311 switches and outputs the signal of the adding means 310 and the signal of the average density calculating means 304 by a character / photo switching signal. As a result, image formation is performed with emphasis on resolution in the character portion, and image formation is performed with emphasis on gradation in the photo portion.
[0089]
On the other hand, the configuration of the second embodiment shown in FIG. 14 is a configuration in which a character / photo switching signal is input to the threshold value calculation unit 312 and the generation threshold value between the character part and the photo unit is changed in the threshold value calculation unit 312. It is a thing.
[0090]
Detailed processing of the threshold calculation means 312 is shown as a program in FIG.
[0091]
In the case of FIG. 12A shown in the first embodiment, constants A, B, C, and D are uniformly set.
A = T / LT1; (14)
B = T / LT2; (15)
C = T / LT3; (16)
D = T / LT4; (17)
(T: input signal, LT1, LT2, LT3, LT4: constant)
In this embodiment,
Character area A = T / LT1M; (18)
B = T / LT2M; (19)
C = T / LT3M; (20)
D = T / LT4M; (21)
Photo area A = T / LT1S; (22)
B = T / LT2S; (23)
C = T / LT3S; (24)
D = T / LT4S; (25)
(T: input signal, LT1M, LT2M, LT3M, LT4M, LT1S, LT2S, LT3S, LT4S: constant)
The A, B, C, and D signals are generated by switching by a character / photo switching signal.
[0092]
At this time, positive values are set for the constants LT1S, LT2S, LT3S, and LT4S, and negative values are set for the constants LT1M, LT2M, LT3M, and LT4M. Similar to the first embodiment, density density is controlled by positive and negative.
[0093]
Specific values set in the second embodiment are specifically shown in FIG. In the photo area, the constants LT1S, LT2S, LT3S, and LT4S are set to positive values, as described in the first embodiment, the calculation results A, B, C, D ( (22) to (25)) are set to positive values, thereby controlling the dot concentration amount and enabling output with an emphasis on gradation stability.
[0094]
Similarly, in the character area, the constants LT1M, LT2M, LT3M, and LT4M are set to negative values, as described in the first embodiment, the calculation results A, B, and C of the character area. , D (formula (18) to formula (21)) is set to a negative value, thereby controlling dot dispersion and enabling output with emphasis on resolution.
[0095]
In the present embodiment, the ratio between the gradation-oriented image formation in which dots are concentrated in the photographic area and the resolution-oriented image formation in which dots are dispersed in the character area (dot concentration / dispersion). It is characterized by being able to respond arbitrarily to various printer characteristics.
[0096]
After the processing described above is performed by the threshold value calculation unit 312, processing similar to that of the first embodiment is performed, and a binary signal is output from the gradation conversion processing unit 204 and printed out by the printer unit 105. It has become.
[0097]
In the first and second embodiments, binarization has been described, but the present invention is not limited to this. That is, the present invention can be applied to general multi-value conversion such as quaternarization, 8-level conversion, and 16-level conversion. Further, it is needless to say that the present invention is not limited to the average concentration storage method (MD method) as in the first and second embodiments, but can be applied to a general error diffusion method (ED method).
[0098]
Needless to say, this is applicable to color images.
[0099]
[Other Embodiments]
Note that the present invention can be applied to a system including a plurality of devices (for example, a host computer, an interface device, a reader, a printer, etc.), or a device (for example, a copier, a facsimile device, etc.) including a single device. You may apply to.
[0100]
Another object of the present invention is to supply a storage medium storing software program codes for implementing the functions of the above-described embodiments to a system or apparatus, and the computer (or CPU or MPU) of the system or apparatus stores the storage medium. Needless to say, this can also be achieved by reading and executing the program code stored in the.
[0101]
In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the storage medium storing the program code constitutes the present invention.
[0102]
As a storage medium for supplying the program code, for example, a floppy disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a CD-R, a magnetic tape, a nonvolatile memory card, a ROM, or the like can be used.
[0103]
Further, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also an OS (operating system) operating on the computer based on the instruction of the program code. It goes without saying that a case where the function of the above-described embodiment is realized by performing part or all of the actual processing and the processing is included.
[0104]
Further, after the program code read from the storage medium is written into a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion is performed based on the instruction of the program code. It goes without saying that the CPU or the like provided in the board or the function expansion unit performs part or all of the actual processing, and the functions of the above-described embodiments are realized by the processing.
[0105]
【The invention's effect】
By performing tone conversion that controls the amount of dot concentration based on the character / photo area signal, it is possible to form an image with emphasis on gradation in the photographic part, and image formation with emphasis on resolution in the character part. . As a result, even in a printer in which dots are not stable due to the characteristics of the printer, it is possible to perform processing that does not lower the resolution of the character portion while performing texture control.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating a configuration of an image processing apparatus according to the present invention.
FIG. 2 is a block diagram illustrating an image processing unit.
FIG. 3 is a block diagram illustrating details of a gradation conversion processing unit 204 according to the first embodiment of the present invention.
FIG. 4 is a diagram illustrating the configuration of binarization means 301 in the first embodiment.
FIG. 5 is a diagram for explaining the configuration and processing of error correction means 302 in the first embodiment.
FIG. 6 is a diagram illustrating a configuration of binarization result delay means 303 in the first embodiment.
FIG. 7 is a diagram showing a data configuration used in the binarization result delay unit 303 in the first embodiment.
FIG. 8 is a diagram illustrating the configuration of average density calculation means 304 in the first embodiment.
FIG. 9 is a diagram showing a configuration of coefficients used in average density calculation means 304 in the first embodiment.
FIG. 10 is a diagram for explaining the configuration and processing of random number generation means 306 in the first embodiment.
FIG. 11 is a diagram for explaining processing of means hysteresis control amount calculation means 308 in the first embodiment.
FIG. 12 is a diagram for explaining processing of a threshold value calculation unit 309 in the first embodiment.
FIG. 13 is a diagram for explaining processing of adding means 310 in the first embodiment.
FIG. 14 is a block diagram showing details of an image processing unit 204 in the second embodiment.
FIG. 15 is a diagram for explaining processing of a threshold value calculation unit 312 in the second embodiment.
[Explanation of symbols]
101 lens
102 CCD sensor
103 Analog signal processor
104 Image processing unit
105 Printer
106 CPU
107 ROM
108 RAM
109 Image reader
110 CPU circuit section

Claims (8)

In an image processing apparatus having means for inputting multi-value data having shading information for each pixel, and means for gradation-converting the inputted multi-value data,
First calculation means for calculating a quantization error that occurs when the multi-value data is gradation-converted and output;
Second calculating means for calculating a quantization error in the vicinity of the target pixel based on the quantization error;
Correction means for correcting an error by adding a quantization error value in the vicinity of the target pixel to the target pixel;
Storage means for storing the results of the previous Machinery scale conversion,
Generating means for generating a threshold value for gradation conversion according to an average value of gradation conversion results for a plurality of pixels stored in the storage means;
Control means for controlling a threshold value generated by the generating means in response to a signal indicating a character area and a photo area;
Wherein, for the character region controls the threshold in order to disperse the dots, for the photograph region controls the threshold in order to centralize the dots,
The first calculation means performs gradation conversion on the multi-value data by subtracting the multi-value data and an average value of the results of gradation conversion for a plurality of pixels stored in the storage means. An image processing apparatus that calculates a quantization error that occurs during output .   The image processing apparatus according to claim 1, wherein the control unit is capable of controlling the threshold value in an arbitrary density region.   The image processing apparatus according to claim 1, wherein the storage unit includes a line buffer for delaying data in units of lines and a delay circuit for delaying data in units of pixels.   The image processing apparatus according to claim 3, wherein outputs of the line buffer and the delay circuit correspond to a pixel arrangement pattern in the vicinity of the target pixel. An image processing method executed in an image processing apparatus having means for inputting multi-value data having grayscale information for each pixel, and means for gradation-converting the input multi-value data,
A first calculation step of calculating a quantization error generated at the time of outputting the multi-value data by tone conversion,
A second calculation step of calculating a quantization error in the vicinity of the target pixel based on the quantization error;
A correction step of correcting an error by adding a quantization error value in the vicinity of the target pixel to the target pixel;
A storage step of storing the results of the previous Machinery scale conversion in the storage means,
A generation step of generating a threshold value of the gradation conversion according to an average value of gradation conversion results for a plurality of pixels stored in the storage unit;
A control step of controlling a threshold value generated by the generation step in response to a signal indicating a character region and a photo region;
The control step controls the threshold value to disperse dots for character areas, and controls the threshold value to concentrate dots for photo areas ,
The first calculation step performs gradation conversion on the multi-value data by subtracting the multi-value data and an average value of the results of gradation conversion for a plurality of pixels stored in the storage unit. An image processing method characterized by calculating a quantization error that occurs during output .   The image processing method according to claim 5, wherein the control step is capable of controlling the threshold value in an arbitrary density region.   6. The image processing method according to claim 5, wherein the storing step controls a line buffer for delaying data in line units and a delay circuit for delaying data in pixel units.   The image processing method according to claim 7, wherein outputs of the line buffer and the delay circuit correspond to a pixel arrangement pattern in the vicinity of the target pixel.

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