JPH0620239B2 - Image processing device for electrophotographic printer - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to image processing for processing image information read by a scanner so as to be optimal for image reproduction, and particularly to image processing read by a scanner by an electronic device such as a laser printer. The present invention relates to an image processing method for processing according to the characteristics of a photographic printer.

2. Description of the Related Art Recently, a digital copying machine has been developed in which a laser printer records and reproduces an image based on image information of an original read by a scanner in units of pixels.

FIG. 7 shows the configuration of the laser printer. In the figure, 1 is a laser oscillator, 2 is a polygon mirror, 3 is an fθ lens, 4 is a reflection mirror, 5 is a photosensitive drum, 6 is a charger, 7 is a developing device, 8 is a transfer / separation charger, 9
Is a cleaner.

In the laser printer configured as described above, the photoconductor surface is exposed by the laser output light modulated according to the binary image information BS sent from the scanner side, and the latent image formed by the exposure is formed. An image is formed by a so-called electrophotographic process in which the image is visualized by toner development and transferred onto the recording paper 10.

However, in such an electrophotographic printer, the eighth
As shown in the figure, the linearity between the amount of light incident on the photoconductor surface and the recorded image density is poor, so that the recorded image is crushed or dots are skipped, and irregularities in gradation are particularly generated. The reproducibility of halftones has become poor. In particular, the image is crushed in the high density portion and the dots are skipped in the low density portion, so that the component of high image frequency is lost.

Conventionally, when recording / reproducing is performed by a printer based on image information read by a scanner, various image processes such as shading correction and MTF correction are performed in order to improve image quality. All of these are only for correcting the image deterioration that occurs when the image is read by the scanner, and even if such image processing is performed, the effect due to the non-linearity of the light quantity-image density transfer characteristic in the printer is not corrected, the quality is improved. A good recorded image cannot be obtained.

An object of the present invention is to correct deterioration of frequency characteristics due to non-linearity of light quantity-image density transfer characteristics in an electrophotographic printer to obtain a recorded image of good quality according to density changes. .

Structure In order to achieve the object of the present invention, by multiplying the values of the weights of the corresponding matrix elements of the Laplacian filter and the digital image information read by the scanner in pixel units and quantized multi-valued. In the image processing apparatus of the electrophotographic printer that performs the spatial filter processing on the pixel of interest at the center of the Laplacian filter to record and reproduce by summing, when the density of the digital image information of the pixel of interest is low, The value of the center weight of the Laplacian filter is increased according to the increasing function of the density, and when the density of the digital image information is medium density, the value of the center weight is increased and the density of the digital image information is increased. At the time of density, a control means is provided for decreasing the value of the center weight according to the density decreasing function.

An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

In the light quantity-image density transfer characteristics on the printer side shown in FIG. 8, when the light quantity shown by A in the drawing is small, that is, the characteristics in the high density area have a small fluctuation range of the image density with respect to the fluctuation of the light quantity. Has become. Similarly, the characteristics in the portion C where the light amount is large and the density is low are the same. Therefore, in such areas of the A and C portions, the component having a high image frequency is compressed, and the transfer characteristic is offset in the low frequency direction. This means that the recorded image is crushed or skipped, resulting in poor resolution of details and deterioration of gradation. Further, the portion having substantially linearity shown by B in the drawing is a region in which the image frequency is transmitted relatively well. The transfer characteristic of such an image frequency is shown in FIG. In the figure, A, B and C are characteristics corresponding to the respective areas A, B and C in FIG.

As described above, it is necessary to perform a process of improving the frequency characteristic according to the value of the digital image information regarding the density of the original image which is read in pixel units by the scanner and is multi-value quantized.

Therefore, in the present invention, the weight of the center value in the Laplacian filter is changed according to the digital image information of the pixel to be processed, and the spatial filter processing having different frequency characteristics is performed.

FIG. 1 (a) shows an example of a Laplacian filter having a 3 × 3 configuration, and its center value Z is a positive value, which is extracted according to the Laplacian filter configuration as shown in FIG. 1 (b). It is a function of the digital image information in the target pixel D (2,2) to be processed in the fixed pixel area. The numerical array around the center value Z is constant without change. At that time, the surrounding values are also the target pixel D (2,
Although it may be handled as a function of the digital image information in 2), changing the center value of the Laplacian filter and changing the values in the vicinity of the Laplacian filter give relatively same results, Here, only the central value is changed. In FIG. 1 (b), X indicates the main scanning direction of the scanner.

The spatial filter processing performed here is a digital calculation by so-called convolution integration, and the calculation formula is as follows.

That is, new digital image information at the spatially filtered target pixel is obtained by multiplying the corresponding matrix elements of the Laplacian filter and the input pixel, summing them, and normalizing by (Z-12). D
_N (2,2) will be obtained.

At that time, in the present invention, the value of the weight Z of the center value in the Laplacian filter is averaged with the value D (2,2) of the pixel of interest or the values D (1,1) to D (3,3) of the surrounding pixels. The spatial filtering process is sequentially performed by appropriately changing the value according to the value. FIG. 2 shows an example of a change state of the weight Z of the center value in the Laplacian filter with respect to the value of the pixel of interest D (2,2). The relationship is the light amount-image density peculiar to the electrophotographic printer to be used. It is optimally set according to the transfer characteristic (γ characteristic) so as to eliminate the influence. Note that here, a case is shown in which the digital image information is quantized into 256 values (8 bits) of 0 to 255. The distribution of the dynamic characteristics shown in FIG. 2 is similar to the differential waveform of the light quantity-image density transfer characteristics on the printer side shown in FIG.

It should be noted that the above-described spatial filter processing is performed while sequentially extracting each pixel data in the 3 × 3 pixel area over the entire image surface read by the scanner.

FIG. 3 shows the spatial frequency characteristic of the Laplacian filter shown in FIG. 1 (a). Here, the horizontal axis represents the image frequency (spatial frequency) and the vertical axis represents the MTF value. As illustrated, the spatial frequency characteristic changes according to the value of the center value Z. The value of the image frequency N / 2 changes according to the sampling pitch of the corresponding pixel. For example, if the sampling pitch is 125 μm, N / 2 = 1000/125
The frequency is × 2, and in this case, the scanning density by the scanner is 4 pixels / mm.

Specifically, the calculation for such spatial filter processing is performed in the arithmetic processing unit by the CPU of the system. FIG. 4 shows a flow of processing in the arithmetic processing unit at that time.

As described above, in the image processing method according to the present invention, by changing the value of the center value weight Z in the Laplacian filter in accordance with the digital image information which is read pixel by pixel by the scanner and is multi-value quantized, the low frequency component of the low frequency component is changed. The transfer characteristic is changed to relatively change the transfer characteristic of the high frequency component. Therefore, the high-pass filter processing is performed dynamically in the low-density portion or the high-density portion according to the density of the image read by the scanner, and the weak high-pass filter processing (having a high DC component transmissibility) is performed in the intermediate density portion. The loss of image information read by the scanner is reduced, and the optimum preprocessing is performed according to the electrophotographic printer to obtain a recorded image with good resolution and gradation reproducibility in the electrophotographic printer. Will be able to.

Further, FIG. 5 shows another example of the Laplacian filter. Here, it is composed of the central value Z of the 3 × 3 configuration and four filter elements above and below, and to the left and right,
The spatial filter processing result in this case is as follows.

Therefore, as compared with the case where the Laplacian filter of FIG. 1 (a) is used, the calculation for the spatial filter processing becomes simpler. The value of the surrounding filter element is not limited to -1, but this is also determined by the relative value with respect to the central value Z, and either the central value Z or the peripheral value may be fixed.
FIG. 6 shows the spatial frequency characteristic by the Laplacian filter in this case.

In this case also, the value of the center value Z of the Laplacian filter serves as a high-pass filter, and the transmissibility of the low frequency component can be changed.

As described above, according to the image processing apparatus of the electrophotographic printer according to the present invention, the deterioration of the frequency characteristic due to the non-linearity of the transfer characteristic of the light amount-image density in the electrophotographic printer is corrected, and it is possible to respond to the density change. It is possible to obtain a high quality recorded image.

[Brief description of drawings]

FIG. 1 (a) is a diagram showing an example of a Laplacian filter, FIG. 1 (b) is a diagram showing digital image information in a fixed pixel region extracted corresponding to the Laplacian filter, and FIG.
FIG. 4 is a characteristic diagram showing an example of the variation characteristic of the weight of the center value of the Laplacian filter according to digital image information, FIG. 3 is a diagram showing the spatial frequency characteristic of the Laplacian filter, and FIG. 4 is a diagram showing the image processing according to the present invention. A flow chart, FIG. 5 is a diagram showing another configuration example of the Laplacian filter, FIG. 6 is a diagram showing the spatial frequency characteristics of the Laplacian filter, FIG. 7 is a simplified configuration diagram of a general laser printer, and FIG. FIG. 9 is a diagram showing a light quantity-image density transfer characteristic in an electrophotographic printer, and FIG. 9 is a view showing an image frequency transfer characteristic.

Claims (1)

[Claims] 1. A scanner is used to read each pixel,
-Multi-quantized digital image information and Laplacian fi
The weight values of the corresponding matrix elements
Laplacian filter by crossing and summing
Spatial filtering is applied to the pixel of interest at the center of
Image processing device of electrophotographic printer for playback
If the density of the digital image information of the pixel of interest is low,
The weight of the center of the Laplacian filter
The digital image is increased according to the increasing function of the density.
When the information density is medium, the weight value of the center is increased.
If the density of the digital image information is high,
Decrease the value of the center weight according to the decreasing function of the concentration
An electrophotographic printer, which is equipped with a control means for
Image processing device.