JP2001309175A - Image processing unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processing unit that can provide more detailed gradation reproducibility and attain recording with high image quality. SOLUTION: The image processing unit 11 consists of an image input section 13, an image processing section 14, and an image output section 15 or the like. The image processing section 14 consists of an analog/digital conversion section 21, a shading correction section 22, an input gradation correction section 23, a color correction section 24, an image area separation processing section 25, a black generation under color reduction section 26, a spatial filter processing section 27, and a medium tone output gradation processing section 28 or the like that conducts gradation correction processing and error spread processing. The gradation correction processing corrects input 8-bit image data into output 12-bit image data, and the multi-value error spread processing applies 4-bit 16-value error spread processing to the 12-bit image data by each color component.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus for performing a gradation correction process and an error diffusion process.

[0002]

2. Description of the Related Art For example, if 8-bit multi-valued image data is
An error diffusion method is known as a binarization technique when a binary image is recorded by converting it into a bit representation. The error diffusion method is based on R.
Proposed by Floyd et al. (1975 SID International Sy
mposium Digest of Technicalpapers 4.3, p36-p37 (197
5)), the binarization error generated when the multi-valued image is binarized with the threshold is distributed and added to the peripheral pixels, and the area gradation density is preserved.

[0003] A multi-valued error diffusion method with further improved gradation and resolution has been known. A multi-valued image is multi-valued, and the quantization error generated at this time is distributed and added to peripheral pixels to obtain an average. Save the concentration.

On the other hand, in an image output device, a tone correction process is performed before the above-described error diffusion process so as to obtain a desired output characteristic. Generally, the gradation correction process performs gradation conversion on multi-valued image data having the number of gradations N, and outputs multi-valued image data having the same number of gradations N.

[0005]

In the above-described multi-valued error diffusion method, the average density of a multi-valued image can be stored before and after the process. The same correction value may be output for different input values, and as a result, the gradation expression is reduced.

SUMMARY OF THE INVENTION An object of the present invention is to provide an image processing apparatus which shows finer gradation reproducibility and is capable of high-quality recording.

[0007]

SUMMARY OF THE INVENTION The present invention provides image input means for inputting an image, halftone output tone processing means for performing tone correction processing and error diffusion processing on the input image, and halftone output tone processing means. Image output means for outputting an image subjected to output gradation processing, wherein the number of gradations N of the image before the gradation correction processing is provided.
1. The gradation number N2 of the image after the gradation correction processing and before the error diffusion processing and the gradation number N3 of the image after the error diffusion processing are N
An image processing apparatus characterized by satisfying a relationship of 3 <N1 <N2.

According to the present invention, finer gradation reproducibility can be maintained because the number of gradations N2 after processing in the gradation correction processing is larger than the number of gradations N1 before processing. In addition, fine gradation reproducibility can be maintained as a whole.

Further, the present invention is characterized in that the number of gradations N2 is a power of two. According to the present invention, when image data after gradation correction processing is handled by a digital circuit, the maximum value of the number of circuit bits and the number of gradations can be matched, so that the processing capability of the circuit can be effectively utilized. .

Further, the present invention is characterized in that the number of gradations N3 is a power of two. According to the present invention, when image data after error diffusion processing is handled by a digital circuit, the maximum value of the number of circuit bits and the number of gradations can be matched, so that the processing capability of the circuit can be effectively utilized.

Further, according to the present invention, the number of gradations N2 = (N3-1)
× 2 ^m +1 (m is a natural number), and the quantized value before processing in the error diffusion processing is a value obtained by multiplying each output value after processing by 2 ^m .

According to the present invention, if the number of bits required to handle image data before error diffusion processing is Ma and the number of bits required to handle image data after error diffusion processing is Mb, Ma bits The remaining bits obtained by removing Mb bits from the most significant bit (MSB) correspond to the error, so that error calculation can be simplified and speeded up.

Further, the present invention is characterized in that the threshold value used for quantization before the processing in the error diffusion processing is a median of adjacent quantized values.

According to the present invention, if the number of bits required to handle image data before error diffusion processing is Ma and the number of bits required to handle image data after error diffusion processing is Mb, Ma bits By adding the bit value of the (Mb + 1) th bit from the most significant bit to the value indicated by the Mb bit from the most significant bit (MSB) of the above, the quantization in the error diffusion process can be executed. Simplification,
Higher speed can be achieved.

Further, the present invention is characterized in that the image is a color image composed of a plurality of color components, and a gradation correction curve in the gradation correction processing is individually set for each color component.

According to the present invention, a fine gradation expression corresponding to the characteristics of the color components can be realized by individually setting the gradation correction curve for each color component.

Further, the present invention is characterized in that the image is a color image composed of a plurality of color components, and the number of gradations N2 is set individually for each color component.

According to the present invention, by setting the number of gradations N2 after gradation correction processing individually for each color component, it is possible to realize fine gradation expression according to the characteristics of the color components.

Further, according to the present invention, the image is a color image composed of a plurality of color components, and the number of bits of a circuit for processing image data after gradation correction processing and before error diffusion processing is set individually for each color component. It is characterized by that.

According to the present invention, the number of bits of a circuit that handles image data after gradation correction processing and before error diffusion processing is individually set for each color component, thereby effectively utilizing the processing capacity of the circuit. And a fine gradation expression according to the characteristics of the color components.

Further, the present invention is characterized in that the image is a color image composed of a plurality of color components, and the number of gradations N3 is set individually for each color component.

According to the present invention, an appropriate error diffusion process can be realized according to the characteristics of the color components by individually setting the number of gradations N3 after the error diffusion process for each color component.

Further, the present invention is characterized in that the image is a color image composed of a plurality of color components, and the number of bits of a circuit for processing the image data after error diffusion processing is individually set for each color component. .

According to the present invention, the number of bits of a circuit that handles image data after error diffusion processing is individually set for each color component, so that the processing capability of the circuit is effectively utilized and the characteristics of the color components are improved. It is possible to realize a fine gradation expression corresponding to it.

[0025]

FIG. 1 is a block diagram showing an embodiment of the present invention. The image processing device 11 is configured as, for example, a digital color copying machine, and includes an image input unit 13,
It comprises an image processing unit 14, an image output unit 15, and the like.

On the upper surface of the image input unit 13, a transparent document table 1 is provided.
11 and an operation panel 16 are provided. On the document table 111, a double-sided automatic document feeder 112, which can also be used as an openable document holding cover, is mounted.

The image input unit 13 includes a scanning unit 113,
114, an optical lens 115, and a photoelectric conversion element C
It is configured by a CD (Charge Coupled Device) line sensor 116 and the like, reads an image of a document placed on the document table 111, and outputs an image signal.

The scanning unit 113 includes an exposure lamp for exposing the original and a mirror for deflecting the reflected light from the original to the scanning unit 114. The scanning unit 113 extends along the original table 111 while keeping a certain distance from the lower surface of the original table 111. Reciprocate in parallel. The scanning unit 114 includes two mirrors for deflecting the document reflected light from the scanning unit 113 to the optical lens 115, and reciprocates at half the moving speed of the scanning unit 113.

The optical lens 115 includes a scanning unit 114
Of the original document from the CCD line sensor 11
No. 6 forms a reduced image on the light receiving surface. CCD line sensor 116
Is composed of, for example, a three-line color CCD, and separates an original image into red (R), green (G), and blue (B), and outputs an analog image signal for each color component.

The image processing section 14 performs predetermined processing on the RGB image signals supplied from the image input section 13,
It outputs digital image data composed of cyan (C), magenta (M), yellow (Y), and black (K) color components to the image output unit 15.

The image output section 15 includes a paper feed mechanism 211, a transfer / conveyance belt mechanism 213, image forming sections Pa to Pd,
It is composed of a fixing device 217 and the like. The paper feed mechanism 211
The paper P stored in the tray is taken out,
1. The sheet is fed to the image forming unit Pa via the registration roller 212. In the case of duplex printing, the fixing device 217 is used.
Is turned over, and then the image forming unit Pa is again turned over.
Feed to.

The transfer / transport belt mechanism 213 includes a transfer / transport belt 216 stretched between a driving roller 214 and a driven roller 215, and the sheet passing through the registration roller 212 is electrostatically attracted by a charger 228 to form an image. Forming part P
The sheet is conveyed in the order of a to Pd, and is carried out to the fixing device 217.

The image forming units Pa to Pd have substantially the same configuration except for the color of the recording toner to be used, and include photosensitive drums 222a to 222d, chargers 223a to 223d,
Developing devices 224a to 224d and transfer members 225a to 225d
25d, cleaning devices 226a to 226d, and scanner units 227a to 227d. The scanner units 227a to 227d include a semiconductor laser element (not shown) and polygon mirrors 240a to 240d.
40d, fθ lenses 241a to 241d, and mirror 2
42a to 242d and 243a to 243d.

The image forming unit Pa is in charge of black (K) image recording, the image forming unit Pb is in charge of cyan (C) image recording, and the image forming unit Pc is in charge of magenta (M) image recording. The image forming unit Pd is in charge of recording the yellow (Y) image, and the recording timing is adjusted so that there is no color shift on the sheet.

For example, in the image forming section Pa, the charger 223a charges the photosensitive drum 222a, and modulates the laser beam from the semiconductor laser element based on the black (K) digital image data supplied from the image processing section 14. The rotation of the polygon mirror 240a causes the photosensitive drum 222 to rotate.
When a is exposed, an electrostatic latent image corresponding to the black image is formed, and the developing device 224a containing the black toner develops the electrostatic latent image, and the transfer member 225a transfers the black toner image onto the transfer conveyance belt 216. Transfer to paper. Image forming unit Pb
To Pd transfer the cyan toner image, the magenta toner image and the yellow toner image by the same process.
6, the paper is peeled off the belt by the neutralizer 229, and finally the fixing device 217
, Each color toner is fixed on the paper.

For the sheet after fixing, a switching path 218 selects a discharge path to the discharge tray 220 or a transport path 221 for switchback.

In the above configuration, an electrophotographic system using a laser scanner has been described. However, the present invention can be applied to an electrophotographic system using an LED head including a light emitting diode array and an imaging lens array, an ink jet system, and the like. .

FIG. 2 is a block diagram showing an electrical configuration of the image processing apparatus 11 of FIG. The image processing device 11 includes an image input unit 13, an image processing unit 14, an image output unit 15, and the like. The instruction signal from the operation panel 16 is supplied to the image input unit 13, the image processing unit 14, the image output unit 15, and the like.

The image processing section 14 includes an A / D (analog / digital) conversion section 21, a shading correction section 22, an input gradation correction section 23, a color correction section 24, an image area separation processing section 25, It is composed of a generation under color removal unit 26, a spatial filter processing unit 27, a halftone output gradation processing unit 28, and the like.

The A / D converter 21 converts an analog signal input from the image input unit 13 into an RGB digital signal. The shading correction unit 22 removes various types of distortion generated in the illumination system, the imaging system, the imaging system, and the like of the image input unit 13.

The input tone correction section 23 performs color balance processing on the RGB signals subjected to shading correction, converts them into density signals suitable for image processing, and further performs density correction processing by tone conversion for each color.

The color correction unit 24 converts the RGB density signals from the input tone correction unit 23 into CMY density signals, and when the spectral characteristics of the CMY color materials used in the image output unit 15 have unnecessary absorption components, Remove CMY
Performs processing to enhance the color reproducibility of the signal.

The image area separation processing section 25 performs image area separation processing for separating a document image into a character area, a photograph area, and the like based on the CMY density signal from the color correction section 24. Black generation and under color removal section 26, spatial filter processing section 27, halftone output gradation processing section 28, and image output section 15
Supplied to

The black generation and under color removal unit 26 performs the color-corrected CM
A K (black) signal is generated from the Y signal and the original CM
A new CMY signal is generated by subtracting the K signal from the Y signal.

The spatial filter processing unit 27 performs a spatial filter process using a digital filter on the CMYK signal from the black generation and under color removal unit 26. For example, in a case where characters and photographs are mixed in a document, in order to improve the reproducibility of characters, edge enhancement processing is performed on a character area, and smoothing processing is performed on a photographic area. Since the spatial frequency characteristics of the image can be corrected by such processing, it is possible to prevent the image output by the image output unit 15 from being deteriorated in graininess or blurred.

The halftone output tone processing section 28 performs tone correction processing and halftone generation processing such as multi-level error diffusion processing on the CMYK signal from the spatial filter processing section 27.

As described above, the image output unit 15 is constituted by a color printer such as an electrophotographic process or an ink jet, and performs color recording on a sheet based on the CMYK signal from the halftone output gradation processing unit 28.

FIG. 3 is a flowchart showing the operation of the halftone output gradation processing section 28. Halftone output gradation processing unit 2
The tone correction processing at 8 is for correcting, for example, input 8-bit image data (0 to 255) to output 12-bit image data (0 to 4095). , The image gradation number N1 = 256, and the image gradation number N2 = 4096 after the gradation correction processing.

Further, a reference correction curve 41 as shown in FIG.
Are stored in advance in the curve storage unit 31 of FIG. 2, and a plurality of correction amounts for the reference correction curve 41 are stored in the correction amount storage unit 3 of FIG.
2 is stored in advance.

First, in step s 1, when the CMYK image data is supplied from the spatial filter processing unit 27, the halftone output gradation processing unit 28 performs the plurality of corrections stored in the reference correction curve 41 and the correction amount storage unit 32. Using the quantity
As shown in FIG. 4, a tone correction curve 42 to be actually used is created, and this curve 42 is stored as a tone conversion table in the tone conversion table storage unit 33 of FIG. It is preferable that such a tone correction curve be set individually for each color component.

Next, in step s2, the processing unit 28
Using the gradation conversion table stored in the gradation conversion table storage unit 33, an input image having 256 gradations is corrected to an output image having 4096 gradations.

Next, in step s3, the processing unit 28
A multi-level error diffusion process is performed, for example, a 4-bit 16-level error diffusion process is performed on each 12-bit image data for each color component. At this time, the image gradation number N2 before the error diffusion processing
= 4096, and the number of image gradations N3 = 16 after the error diffusion processing.

More specifically, the gradation values of 0 to 4095 are equally divided, and the 16 quantization values are set to 0, 273, 546, 81
9, 1092, 1365, 1638, 1911, 218
4, 2457, 2730, 3003, 3276, 354
The processing unit 28 outputs to the image output unit 15 16 values from 0 to 15 corresponding to the respective quantized values. Note that the threshold value used for quantization is the median of adjacent quantization values, and is 137, 410, 683, 956, 122.
9, 1502, 1775, 2048, 2321, 259
4, 2867, 3140, 3413, 3686, 395
9 respectively.

As shown in FIG. 5, the error caused by the quantization of the processing pixel is diffused to the next pixel A to be processed and the adjacent pixels B, C, and D on the next line. Pixel A can be set to 7/16, pixel B to 3/16, pixel C to 5/16, and pixel D to 1/16 as coefficients.

For example, if the processing pixel data before the error diffusion processing is 1400 and the sum of the errors diffused by the diffusion processing before the processing pixel is 150, the total 1550 (= 1400 + 150) is obtained. Be subject to quantization processing. This 1550 is equal to or greater than the threshold 1502 and the threshold 177
Since it is less than 5, the quantization value becomes 1638, and the 4-bit output value 6 is output to the image output unit 15. At this time, the error of the processing pixel is calculated as 1550-1638 = -88, and this error is diffused to the pixels A to D.

Next, in step s4, it is determined whether or not the processing has been completed for all the pixels of the original image, and steps s2 and s3 are repeated until all the processing is completed.

The case where the image gradation number N2 before the error diffusion processing is a power of 2 has been described (2 ¹² = 4096). Next, the gradation number N2 = (N3-1) × 2 ^m +1 (M is a natural number) will be described.

For example, when N3 = 16 and m = 8, the number of gradations N2 = 3841. Therefore, after performing the gradation correction processing of 3841 values, the gradation values of 0 to 3840 are equally divided, and the 16 quantization values are multiplied by 256, that is, 0, 2
56, 512, 768, 1024, 1280, 153
6,1792,2048,2304,2560,281
6, 3072, 3328, 3584, and 3840, and the halftone output gradation processing unit 28 outputs 16 hexadecimal values of 0 to 15 extracted from the upper 4 bits of each value in correspondence with each quantized value. Output to

The threshold value used for quantization is the median value of the adjacent quantized values, and the fifth bit from the most significant bit of each value is 1 and the bit values after the 6th bit are all 0. , 128, 384, 640, 896, 1152, 14
08, 1664, 1920, 2176, 2432, 26
88, 2944, 3200, 3456, and 3712, respectively.

In this case, when the fifth highest bit of the value obtained by adding the sum of the diffusion errors to the 12-bit gradation correction value is 1, the value obtained by adding 1 to the upper 4 bits is the 4-bit output value. To the image output unit 15, while the fifth bit is 0
In this case, the upper 4 bits are output to the image output unit 15 as a 4-bit output value as it is. Therefore, the output value to the image output unit 15 can be obtained only by adding the value of the fifth most significant bit to the value of the upper four bits, and the error calculation can be simplified and speeded up.

Further, the lower 8 bits of the 12-bit gradation correction value are diffused as errors.

The gradation number N1 of the image before the gradation correction processing in the halftone output gradation processing section 28, the gradation number N2 of the image after the gradation correction processing (same as before the error diffusion processing), the error diffusion processing The subsequent image gradation number N3 can be set individually for each color component of the CMYK signal, and fine gradation expression according to the characteristics of the color components can be realized.

[0063]

As described above in detail, according to the present invention, in the gradation correction processing, the gradation number N2 after processing is larger than the gradation number N1 before processing, so that finer gradation reproducibility is maintained. it can. In addition, fine gradation reproducibility can be maintained as a whole.

Since the number of gradations N2 is a power of 2, when the image data after the gradation correction processing is handled by a digital circuit, the maximum value of the number of circuit bits and the number of gradations are matched. Therefore, the processing capacity of the circuit can be effectively used.

Further, since the number of gradations N3 is a power of 2, when the image data after the error diffusion processing is handled by the digital circuit, the maximum value of the number of circuit bits and the number of gradations can be matched. Therefore, the processing capability of the circuit can be effectively used.

Also, the number of gradations N2 = (N3-1) × 2 ^m +
Since 1 (m is a natural number) and the quantized value before processing in the error diffusion processing is a value obtained by multiplying each output value after processing by 2 ^m , the error calculation can be performed only by the bit calculation. Can be simplified and speeded up.

Further, since the threshold value used for quantization before processing in the error diffusion processing is the median value between adjacent quantized values, error calculation can be performed only by bit calculation, thereby simplifying and speeding up the calculation. Can be achieved.

Further, by setting the processing contents in the gradation correction processing and the error diffusion processing individually for each color component,
Fine gradation expression according to the characteristics of the color components can be realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of the present invention.

FIG. 2 is a block diagram illustrating an electrical configuration of the image processing apparatus 11;

FIG. 3 is a flowchart showing an operation of a halftone output gradation processing unit 28;

FIG. 4 is a graph showing a reference correction curve 41 and a gradation correction curve 42;

FIG. 5 is an explanatory diagram showing a diffusion method in an error diffusion process.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Image processing apparatus 13 Image input part 14 Image processing part 15 Image output part 21 A / D (analog-digital) conversion part 22 Shading correction part 23 Input gradation correction part 24 Color correction part 25 Image area separation processing part 26 Black generation Color remover 27 Spatial filter processor 28 Halftone output tone processor

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) H04N 1/52 H04N 1/46 BF term (Reference) 5B057 AA11 BA25 CA01 CA07 CA08 CA16 CB01 CB16 CE13 CE17 CH11 DB02 DB06 5C077 LL19 MP08 NN11 PP15 PP33 RR06 RR08 RR14 5C079 HB03 LA02 LA12 LC09 MA19 NA05

Claims (10)

[Claims] 1. An image input means for inputting an image, a halftone output tone processing means for performing a tone correction process and an error diffusion process on the input image, and a halftone output tone process. Image output means for outputting an image, wherein the number of gradations N1 of the image before the gradation correction processing, the number of gradations N2 of the image after the gradation correction processing and before the error diffusion processing, and the number of gradations N2 of the image after the error diffusion processing An image processing apparatus, wherein the number of gradations N3 satisfies a relationship of N3 <N1 <N2. 2. The image processing apparatus according to claim 1, wherein the number of gradations N2 is a power of two. 3. The image processing apparatus according to claim 1, wherein the number of gradations N3 is a power of two. 4. The number of gradations N2 = (N3-1) × 2 ^m +1 (m is a natural number), and in the error diffusion processing, the quantized value before processing is obtained by multiplying each output value after processing by 2 ^m . 4. The image processing apparatus according to claim 1, wherein the value is a value. 5. The image processing apparatus according to claim 4, wherein the threshold value used for quantization before processing in the error diffusion processing is a median value between adjacent quantization values. 6. The image according to claim 1, wherein the image is a color image composed of a plurality of color components, and a gradation correction curve in the gradation correction processing is individually set for each color component. An image processing device according to any one of the above. 7. The image according to claim 1, wherein the image is a color image composed of a plurality of color components, and the number of gradations N2 is individually set for each color component. Processing equipment. 8. An image is a color image composed of a plurality of color components, and the number of bits of a circuit for processing image data after gradation correction processing and before error diffusion processing is set individually for each color component. The image processing apparatus according to claim 1, wherein: 9. The image according to claim 1, wherein the image is a color image composed of a plurality of color components, and the number of gradations N3 is set individually for each color component. Processing equipment. 10. The image according to claim 1, wherein the image is a color image composed of a plurality of color components, and the number of bits of a circuit for processing the image data after error diffusion processing is individually set for each color component. The image processing apparatus according to any one of claims 9 to 9.