JPH0383177A - Dither image enlargement system - Google Patents

Abstract

PURPOSE:To realize a dither image of good quality wherein a border generated owing to density variation is inconspicuous by finding the density of an image to be converted from a dither image while the number of gradations and finding final converted image density from the area rate of converted picture elements to be projected to picture elements to be converted. CONSTITUTION:This system is equipped with an input means 1 for the picture elements to be converted, a density calculating means 2 for the picture elements to be converted, a converted picture element density calculating means 3, and a dither binary-coding means 4. The converted picture elements corresponding to conversion magnification are projected on an original image consisting of a 1st dither matrix and the density of the picture elements to be converted is increased to a desired number of gradations and calculated from data on a group of picture elements to be converted which are positioned within the same size; and the converted picture element density is calculated from the density of the picture elements to be converted by weighting at the partial area ratio of the projection of the converted picture elements having the same area with the picture elements to be converted to an image plane to be converted and binary coding is carried out finally with a 2nd dither matrix corresponding to the increased number of gradations. Consequently, the border of a part which varies slightly in density while the gradations are increased when the dither image is enlarged.

Description

[Detailed Description of the Invention] [Summary] Regarding an enlargement method for a dithered image composed of systematic dithering, an object of the present invention is to make the boundaries of parts where the density changes slightly at the same time as the gradation increases when enlarging the dithered image less noticeable. Then, a converted pixel according to the conversion magnification is projected onto the original image composed of the first dither matrix, and from the data of a group of converted pixels located within the same size as the first dither matrix of this converted pixel, The density of the converted pixel is increased to the desired number of gradations, and the converted pixel density is calculated from the converted pixel density by weighting based on the partial area ratio of the converted pixel having the same area as the converted pixel projected onto the converted image surface. is calculated and finally binarized using a second dither matrix corresponding to the increased number of gradations.

[Industrial Field of Application] The present invention relates to an enlargement method for systematic dithered images, and in particular, the density of the converted pixel is first determined from a group of converted pixels near the converted pixel, and the converted pixel is expanded based on the converted pixel density. The present invention relates to a method for enlarging a dithered image that determines the density of a dithered image and performs second dithering.

Image input/output equipment that currently performs black and white binary representation,
For example, facsimiles and scanners are increasingly used not only for characters and line drawings, but also for dithered and other pseudo-halftone images obtained by controlling the black point density distribution. Density conversion processing between different devices and scaling processing in image editing play an important role.

[Prior Art] Conventionally, known scaling methods used for characters, line drawings, etc. include the SPC method, the logical sum method, the nine-part method, the high-speed projection method, and the distance inverse proportional method. However, when these methods are applied to a dithered image, moiré occurs and the image quality deteriorates.

Furthermore, as a conventional enlargement method for a dithered image, there is a method of determining the density of a converted pixel from data of a group of converted pixels located near the converted pixel, and re-dithering it (Japanese Patent Laid-Open No. 62-216476). .

FIG. 8 shows a principle block diagram of a conventional dither image enlargement processing method, which is composed of a non-converted pixel input means 1011, a converted pixel density determination means 102, and a dither binarization means 1[13]. FIG. 9 shows the processing flow of the conventional method. In Sl, the density data of the non-converted pixel group is input, in S2, the density of the converted pixel is determined, and in S3, the dithering threshold is used to re-dither. become

To explain in more detail, as shown in FIG.
In the case of ×4 dither matrix, 166 pixels around the converted pixel R
The density of the converted pixel is determined by counting the number of black pixels. In this case, since 6 pixels out of 16 pixels surrounding the converted pixel R are black pixels, the density of the converted pixel R is determined to be 6.

[Problems to be Solved by the Invention] However, when enlarging a dithered image using such a conventional method, it is difficult to connect four pixels to be converted, as shown in the 4 times enlarged example in FIG. The 12 converted pixels indicated by the x marks that are newly created within the frame according to the enlargement magnification have the same density because the density is determined from the same reference pixel, and the edge parts (gradation levels) of hair etc. that appear in the dithered image are (portions that are not represented by patterns) are also represented by average density, resulting in blurring when enlarged. Also, the first
As shown in FIG. 1(b), there was a problem in that the boundaries were noticeable in areas where the uniform density of the converted pixel group changed.

The present invention has been made in view of these conventional problems, and provides a method for enlarging a dithered image that allows boundaries of areas where the density changes slightly at the same time as the gradation increases to become independent when enlarging the dithered image. The purpose is to

[Means for Solving the Problems] FIG. 1 is a principle explanatory diagram showing the configuration and processing flow of the present invention.

First, the present invention is directed to an image enlarging method composed of systematic dithering (NX-N dither matrix).

Regarding such a dithered image enlargement method, in the present invention, converted pixels according to the conversion magnification are projected onto the original image composed of the systematic dithering, for example, a 4×4 dither matrix, and the converted pixels a converted pixel input means 1 (processing step Sl) for inputting data of a converted pixel group located within the same size as the first systematic dither in the vicinity of the first systematic dither; a converted pixel density calculation means 2 (processing step S2) that calculates the density of the converted pixel in a scaled form; a portion where the converted pixel having the same area as the converted pixel is projected onto the converted image surface; a converted pixel density calculation means 3 (processing step S3) that calculates a converted pixel density from the converted pixel density by weighting according to the layer stack ratio; A dither binarization means 4 (processing step S4) for binarizing with a second systematic dither corresponding to the key, for example, an 8×8 dither matrix, is provided.

[Effect] Next, the present invention will be explained in detail.

When enlarging a dithered image according to the present invention,
As shown in Figure 1, first, in step S1, each data of the converted pixel located near the converted pixel is calculated based on the position where the converted pixel to be converted to a predetermined magnification is projected onto a plane composed of the converted pixels. will be placed manually one by one. For example, as shown in FIG. 4, A, B, and C surrounding the converted pixel R are pixels located near the converted pixel R at a predetermined magnification.
.

The four pixels to be converted of D and the 12 pixels to be converted around them are input.

Subsequently, in step S2, the input pixel group A to be converted is
- from P to the nearest converted pixel around the converted pixel R, for example, the density (multi-value) of the converted pixel C, to a desired number of gradations, for example 1
It is calculated by increasing the number of gradations from 6 to 64, which is four times the number. Third
The figure shows the reference converted pixel group when calculating the density of the converted pixel C, but each of the remaining pixels A, B, and D is also determined by the pixel C from the relative positional relationship with the converted pixel R. Next, in step S3, the density of the converted pixel A, B, and D for the converted pixel R is determined as the density when the converted pixel R is at the position of the converted pixel A.

Conversion pixel R based on the density determined for B, C, and D
Find the concentration of

For example, as shown in FIG. 4, the density of a converted pixel R indicated by an x mark is calculated from the product-sum calculation of the partial ratio of the area projected onto the converted image plane and the density of the four surrounding converted pixels. demand. Unlike the normal projection method, this method is characterized in that the area of the converted pixel R to be projected is the same as each of the converted pixels A, B, C, and D, and the converted image is
Weighting Wa according to the difference in the overlapping area when projected onto the three converted pixels A, B, C, and D,
Wb, We.

The density of the converted pixel R is determined by multiplying Wd by the already determined density of the converted pixels A-D and taking the sum. This has the effect of reducing unevenness on the stairs at a multilevel level when enlarged.

Furthermore, in the final step S4, step S2
A gradation that can be expressed by binarizing using a second systematic dither of a size corresponding to the number of gradations increased by calculating the density of pixels A-D to be converted, for example, a threshold value of an 8 × 8 dither matrix. Increase key.

[Embodiment] FIG. 2 is a block diagram showing an embodiment of the present invention.

In FIG. 2, 11 is a converted pixel input circuit, 12 is a converted pixel density determination circuit, 13 is a converted pixel density calculation circuit,
14 is a dither binarization circuit.

For example, taking an original image composed of a 4×4 dither matrix as the first systematic dither, the converted pixel input circuit 11 inputs dithered image data from a scanner, computer, etc. that is directly read from the original image. For example, four converted pixels A and B located near the converted pixel R shown in FIG.
, C, D and the surrounding 12 pixels to be converted E-P are shifted to shift registers 111, 112, 113, 1 for each line.
14.

The converted pixel density determination circuit 12 includes a converted pixel density calculation circuit 121 using a ROM that calculates the converted pixel density from a dither pattern, and a converted pixel density calculation circuit 121 that uses a ROM to calculate the converted pixel density from a dither pattern. A multiplication circuit 122 that increases by a magnification corresponding to the gradation;
A line memory 123 that stores density data obtained from
will be provided.

That is, if the conversion pixel R based on the predetermined conversion magnification shown in FIG. 3, the 16 reference pixels A to P shown in FIG. 3 are input to the conversion pixel determining circuit 12. In the conversion pixel determination circuit 12, each time the 16 pixels from the first line to the fourth line are processed, the pixel to be processed is selected as the pixel of interest and black among the 16 reference pixels surrounding the target pixel is processed. The density of the pixel to be converted at that time is determined from the number of pixels, and the multiplier 122 multiplies the density by an increasing rate for increasing the gradation, for example, a magnification of 4 to increase the gradation from 16 to 64, and performs the next stage of conversion. Register 1 of the pixel density calculation circuit 13.
31-3 and simultaneously stored in the line memory 123.

In the case of FIG. 3, since the pixel of interest for the converted pixel R is the converted pixel C, when the density of the converted pixel C is determined by the converted pixel density determination circuit 12, the next converted pixel Register 1 provided in the concentration calculation circuit 13
The respective densities of the determined pixels A to D to be converted are stored in 31-1 to 131-4.

Specifically, the density of the pixel C to be converted, which is the pixel of interest, is determined by the pixel density determining circuit 12, and the density is stored in the register 131-3.
At the time when the density of the previous converted pixel B has already been stored in the register 131-4, the density of the converted pixel B that has already been stored in the line memory 123 from the previous line processing The density of pixel A is stored in the register 131-2, and the density of the pixel to be converted is stored in the register 131-1.

Converted pixel A- for registers 131-1 to 131-4
The converted pixel density calculation circuit 13 that has received the density calculated for D converts the converted pixel R into a pixel to be converted as shown in FIG. 4 at the same time as the converted pixel R shown in FIG. The partial area ratios Wa, Wb, Wc, and Wd of the converted pixel R occupying each of the four converted pixels A to D when projected as the same area as each of A and -D are calculated.

Therefore, the four multiplication circuits 1 of the converted pixel density calculation circuit 13
32-1 to 132-4 are registers 131-1 to 131
The respective densities of the converted pixels A to D increased to the predetermined gradation stored in each of -4 and the partial area ratios Wa to Wd are multiplied together.

The multiplication results by these multiplier circuits 132-1 to 132-4 are added through three adder circuits 133-1 to 133-3 to obtain a total sum.
The converted pixel density of the converted pixel R to be processed is calculated as the output of step 3.

The converted pixel density of the converted pixel R calculated by the converted pixel density calculation circuit 13 is given to the dither binarization circuit 14, and is converted into, for example, an 8x8 Bayer type dither matrix (shown in FIG. 6).
the comparator circuit 14 by a threshold based on the second systematic dither)
1 and taken out as the final converted pixel output.

Next, the operation of the embodiment shown in FIG. 2 will be specifically explained using FIG.

Process■ First, assuming that the first systematic dither is a 4×4 dither matrix, the density of the pixel of interest C closest to the converted pixel R is calculated from the density of the surrounding 16 converted pixels as shown in the figure. It becomes 9.

Similarly, the density of the pixel A to be converted is determined to be 9, and the density of the pixel to be converted B is determined as 8 for every 101 pixels to be converted.

Process ■ Here, if you want to increase the number of gradations from 16 gradations using the 4x4 dither matrix to 64 gradations, which is 4 times as much, you can The density of pixel C is converted to density 36 at 64 gradation levels. In addition, in this conversion calculation, the decimal point is rounded off to 64
All densities at gradation levels are integers.

Steps (2) and (2) of this process are performed by the converted pixel density determining circuit 12 shown in FIG.

Process ■ Converted pixels A, B, C, D calculated in process ■
Product-sum calculation of the partial area ratios Wa, Wb, Wc, and Wd for each density and the converted pixel R having the same area as the converted pixel for the four converted pixels A to D shown in FIG. 5 is determined by projection. The density of the converted pixel R is determined by . In this case, as shown in FIG. 5, the partial area ratio is, for example, Wa=15/64
, Wb = 9/64, Wc = 15/64, Wd = 25
/64 (however, Wa+Wb+Wc+Wd=1), the density 8.4 weighted by the partial area ratio Wa-Wd is multiplied by each density of pixels A-D.
375.5.625.8.4375.12.5 are calculated, and the density 3 of the converted pixel density R is calculated as the sum of these.
5 is required.

Such process (2) is performed by the converted pixel density calculation circuit 13 shown in FIG.

Process ■ By comparing the density 35 of the converted pixel R obtained in process ■ with the threshold value 44 obtained from, for example, an 8×8 Bayer type dither matrix as shown in FIG. Since it is small, the value 0 of the binarized dithered image is finally found.

The above processing can be carried out, for example, as shown in FIG. 11 (a
) is repeated for all converted pixels indicated by X marks.

FIG. 7 shows a comparison between determining the density of a converted pixel according to the conventional method and determining the density of a converted pixel according to the present invention.

That is, in the conventional conversion pixel density determination shown in FIG. 7(a), since the conversion pixels between the pixels to be converted have a constant value, the gradation changes stepwise. On the other hand, in the pixel conversion density determination of the present invention shown in FIG. It is possible to determine a converted pixel density whose gradation increases substantially linearly with respect to a change in position by weighting by position (weighting by area in two dimensions) corresponding to the average density.

As a result, according to the present invention, the number of gradations to be expressed increases, and at the same time, even in areas where the density changes slightly, the densities of adjacent converted pixels are all different, so even if re-dithering is performed after enlargement, the dither pattern remains the same. As a result, it is possible to obtain a high-quality dithered image in which boundaries that occur in areas where the density changes are less noticeable.

[Effects of the Invention] As described above, according to the present invention, the density of the converted pixel is determined from the dithered image by increasing the number of gradations, and the density of the converted pixel is calculated based on the area ratio of the converted pixel projected to the converted pixel. to find the final converted pixel density and calculate the second value corresponding to the number of increased gradations.
The quality of the dithered image is improved by increasing the number of gradations expressed, and at the same time, even in areas where the density changes slightly, the density of adjacent converted pixels is always different. The same dither pattern does not appear even if the dither pattern is changed, and it is possible to obtain a high-quality dither image in which boundaries caused by density changes are not noticeable.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the principle of the present invention; FIG. 2 is a configuration diagram of an embodiment of the present invention; FIG. 3 is an explanatory diagram of a pixel of interest and surrounding reference pixels that determine the density of the pixel of interest in the present invention; FIG. 4 is a diagram showing the positional relationship between the converted pixel and the converted pixel and the weighting of the projected area; FIG. 5 is a diagram showing the process of the present invention for determining the converted pixel density; FIG. 6 is an explanatory diagram of the Sx g Bayer type dither matrix ; Figures 7A and 7B are explanatory diagrams comparing the conversion pixel density settings of the conventional method and the present invention; Figure 8 is a principle block diagram of the conventional method; Figure 9 is a principle flow diagram of the conventional method; Figure 10 is a diagram showing the principle of the conventional method; A diagram showing determination of reference pixel and converted pixel densities in the conventional method; FIG. 11 is an explanatory diagram of an example of 4 times enlargement in the conventional method. In the figure, 1: Converted pixel input means 2: Converted pixel concentration calculation means 3: Converted pixel concentration calculation means 4; Dither binarization means 11: Converted pixel input circuit 12: Converted pixel concentration determination circuit 13: Conversion Pixel density calculation circuit 14: Teaser binarization circuits 11 to 114 Shift register 21: Converted pixel density calculation circuits 22, 132-1 to 132-4: Multiplication circuit 23, Line memories 31-1 to 13+-i: Register 33 -1 to 133-3: Addition circuit 41/comparison circuit (ROM)

Claims (1)

[Claims] (1) In a method of enlarging a dithered image made up of systematic dithering, a converted pixel according to the conversion magnification is projected onto the original image made up of the first systematic dithering, and the area near the converted pixel is Said first
a converted pixel input means (1) for inputting data of a group of converted pixels located within the same size as the systematic dither of; a converted pixel density calculating means (2) for calculating the density of the converted pixel; a converted pixel density calculating means (3) for calculating a converted pixel density from the density; converting the converted pixel density calculated by the converted pixel density calculating means (3) by a second systematic dither corresponding to the number of increased gradations; A method for enlarging a dithered image, comprising: a dither binarization means (4) for binarizing;