JP2705934B2 - Image reduction conversion method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reduction conversion method for obtaining a converted image with little image quality degradation from an image expressed in black and white binary. . (Prior Art) Conventionally, for example, SP
C (Selective Processing Conversion) method (Preprints of the 7th Annual Conference of the Institute of Image Electronics Engineers of Japan, Preliminary No. 10, May 19, 1975)
Japan) and area discrimination method (IEICE Technical Report PRL81-92, February 1, 1982)
9) has been announced. The SPC method is a method of substituting the value of a pixel (original pixel) of the original image at a position closest to a pixel of the converted image (converted pixel) when the converted image is mapped to the original image. In the area discrimination method, the four original pixels closest to the conversion pixel are selected, the average density projected by the four original pixels to the conversion pixel position is calculated, the threshold value processing is performed, and the value of the conversion pixel is calculated. It is a method. However, in these conversion methods, if there is a black or white thin line on the original image, the thin line is not the original pixel closest to the converted pixel position, or the average density projected by the four original pixels is thresholded. As a result, if the value of the converted pixel closest to the position of the thin line does not match the value of the thin line, the thin line may disappear on the conversion screen. Figure 2 shows an example of a correspondence between converted pixels and original pixels in the reduced conversion method, (a) represents the case there are fine lines on the left side of Q _j, (b) is the presence of fine lines on the right side of the Q _j is there. In the present example, an example is shown in which the thin line disappears when the conversion ratio α in the horizontal direction is 1/3> α ≧ 1/4. Here, P _{i−4 to} P _{i + 3} indicate original pixels, and Q _{j−1 to} Q _{j + 1} indicate conversion pixels. White circle ○
Is an original pixel having a white value, and a black circle is an original pixel having a black value. That is, in the SPC method or area discrimination method, in the example of FIG. 2, the converted pixel Q _j-1 original pixel P _i-4 The conversion pixel Q in _j original pixel P _i, converted pixel Q _{j + 1} Is replaced with the value of the original pixel P _{i + 3} , respectively.As a result, all of the converted pixels Q _j−1 , Q _j and Q _{j + 1} have white values, and the converted pixel that stores the value of the black thin line is Because of this, the disappearance of the fine line occurs. in this way,
With these conversion methods, cuts due to the disappearance of black thin lines,
Blurring occurs, and crushing occurs due to disappearance of the white thin line, resulting in poor image quality. In addition, in order to prevent the black thin line from disappearing, a method of calculating a logical sum of a plurality of original pixels and assigning the calculation result to a converted pixel (logical sum method) has been considered (facsimile line density). Examination of conversion, IEEJ Journal, Vo17, 197
8). However, this OR method has a drawback in that when a white thin line is present on the original image, the white thin line may disappear on the converted image, resulting in collapse and image quality deterioration. Also, when there is a black thin line having a line width of 1 pixel on the original image, the values of the two converted pixels positioned so as to sandwich the thin line may both be black, and as a result, the thin line of the thin line may appear on the converted image. There is a drawback that the image becomes thicker though the reduction conversion is performed and image quality is deteriorated. Further, in order to solve the drawbacks of the above-described conversion method, a thin line is discriminated, and if there is a thin line, the TP method (Thin Line Preser
vation Method: Fine line loss prevention reduction conversion method, IEICE Technical Report ED84
-26, June 22, 1984). But,
Since the TP method targets only one pixel as the line width of the thin line, when the conversion ratio decreases, in the case of the example shown in FIG. 2, the thin line having the line width of 2 pixels and 3 pixels disappears, The image quality sometimes deteriorated. Thus, in the conventional reduction conversion method, the conversion ratio α is
In the case of 1 / n> α ≧ 1 / (n + 1), there is a disadvantage that thin lines having a line width of n pixels or less disappear and image quality deteriorates. (Object of the Invention) The present invention solves the above-mentioned drawbacks of the reduction conversion method, in which the line width that can be lost when the conversion ratio α is 1 / n> α ≧ 1 / (n + 1) is n pixels. The reference pixels are selected according to the conversion ratio so that all of the following thin lines can be identified, and thin lines are identified using the reference pixels, and saved on the conversion screen so that these thin lines are not lost to prevent image quality deterioration. It is an object of the present invention to provide a reduced conversion method. (Constitution of the invention) (Difference between features of the invention and conventional technology) In the present invention, the conversion ratio in the horizontal direction and the vertical direction is α _x , α _y 1 / n _x > α _x ≧ 1 in order to achieve the above object. _{/ (n x +1), 1} / n y> α y ≧ 1 / (n y +1) However, n _x, n _y in the case of natural numbers, around the converted pixel, each n _x +1 amino left in the horizontal direction The original pixel, and n _y +1 in the vertical direction
With reference to the original pixels, all fine lines having a line width in the original image of nx pixels or less in the horizontal direction and ny pixels in the vertical direction are detected, and when the thin line is detected, the value of the thin line is converted to the shortest-distance converted pixel. It is characterized by substituting. Therefore, the present invention is different from the conventional reduction conversion method except for the above-described TP method in that the present invention has a means for determining a thin line and storing the determined thin line. In addition, the TP method has means for discriminating a thin line and storing the discriminated thin line, whereas the condition for discriminating the thin line is limited to only a thin line having a line width of 1 pixel. Not only the thin line of one pixel but also the area referred to by the original pixel is changed according to the reduction conversion ratio, and the horizontal line width _nx pixel or less and the vertical line width _ny pixel or less that may be lost The difference is that all thin lines are targeted for discrimination. Therefore, as compared with the conventional conversion method, a difference is that thin lines having a horizontal line width of _nx pixels or less and a vertical line width of _ny pixels or less can be prevented from disappearing. (Embodiment) In the following description, a case where the conversion ratio in the horizontal direction and the vertical direction is the same will be described. Even when the two conversion ratios are different, the same reduction conversion can be performed. FIG. 1 is a block diagram showing an embodiment of the present invention.
Position detecting means for determining the positional relationship between converted pixels and original pixels when mapping the converted pixel to the original image, 2 horizontally to the right and left n _{x + 1} pieces of original pixels, respectively around the positions of the target converted pixel Means for reading the value and the values of the n _{y +1} original pixels in the upper and lower directions in the vertical direction, and the line width on the original image is calculated from the values of the pixels in the horizontal and vertical directions read by the reading means. n _x thin line and the line width of the vertical direction is a pixel or less in the horizontal direction is less than n _y pixels thin line detection means, determination means closest converted pixel to the fine line detected by the detection means of the thin line 4, 5 Means for detecting a thin line by the judgment means and substituting the value of the thin line for the conversion pixel of interest when the converted pixel is closest to the thin line; Conversion that is closest to a thin line even if If the pixel is not the conversion pixel of interest, the means for determining the conversion pixel value of interest by a predetermined logical operation method, and 7 are means for determining all the conversion pixel values. FIG. 3 shows an example of the flow of the reduction conversion means according to the present invention shown in FIG. In the reduction conversion process, a value of a conversion pixel is determined in order according to a fixed rule, and the conversion image is created by repeatedly performing the determination. Hereinafter, the embodiment of FIG. 1 will be described with reference to FIG. First, the position determining means 1 maps the converted pixel to the original image,
The position of the conversion pixel of interest on the original screen is obtained ((1) in FIG. 3). Next, the reading unit 2 reads n + 1 left and right original pixels in the horizontal direction and n + 1 upper and lower pixels in the vertical direction as reference pixels around the converted pixel as a reference pixel (third (2)). FIG. 4 shows an example of this reference original pixel in the case where the conversion ratio α is 1/3> α ≧ 1/4.
N in the up, down, left, and right directions around the conversion pixel (marked by X)
Reference is made to + 1 = 4 original pixels. As a specific example of the method, it can be realized by reading out the contents of the line memory of 2n + 2 lines of the original pixel according to the position of the conversion pixel of interest on the original image. Next, a thin line having a line width of n or less existing in the reference pixel is detected by the detecting unit 3. FIG. 5 shows an example of a specific method for detecting a thin line. Original pixels having the same value are m (m is a natural number less than or equal to n)
When the number of original pixels that are continuous and the m original pixels on both sides are different from the values of the m original pixels, a thin line is determined. By executing this judgment for all 2n + 2 original pixels to be referred to for all m from 1 to n, a thin line around the conversion pixel of interest and having a line width of n pixels or less can be eliminated. Can be detected. Further, as another method of thin line detection, as shown in the example of FIG. 2, thin lines of a line width m that may be missing, instead of in the form including the converted pixel Q _j, Q _j As shown in FIG. 6, a thin line having a line width m is formed for each of n + 1 original pixels on one side and one original pixel on the opposite side in the horizontal and vertical directions around the converted pixel _Qj as shown in FIG. By making the determination, it is possible to detect a thin line having a line width of n pixels or less that can disappear around the conversion pixel of interest ((3) in FIG. 3). When a thin line is detected, the conversion pixel determining unit 4 checks whether the conversion pixel of interest is the shortest distance conversion pixel to the detected thin line ((4) in FIG. 3). The value of the thin line is substituted into the converted pixel by the substitution means 5 ((5) in FIG. 3). The determination that the conversion pixel of interest is closer to the thin line than the other conversion pixels can be made by the following method. The center line of the detected thin line is calculated. Next, the closest converted pixel is detected by comparing the distance from the center line of the thin line to each of the nearby converted pixels, and it is determined whether or not the closest converted pixel is the target converted pixel. If the distance between the original pixels is 1, the distance between the converted pixels is 1 / α, so the center between the converted pixels is located at 1 / 2α from both converted pixels. Distance 1 / 2α left / right or up / down from the conversion pixel of interest
By determining whether or not the center line of the thin line is included in the region, it is also possible to determine whether or not the thin line should be stored in the conversion pixel of interest. On the other hand, when the thin line is not detected, or when the converted pixel of interest is not the shortest distance conversion pixel to the detected thin line even if the thin line is detected, the converted pixel is determined by the logical operation method predetermined by the conversion pixel value determining means 6. Is determined ((6) in FIG. 3). As a specific example of the logical operation method, the above-described SPC method, area determination method, or the like can be used. That is, it is determined based on the value of one of the nearest neighbors or a plurality of nearest neighbor original pixels to the conversion pixel of interest. By the above-described method, the value of the conversion pixel can be determined, and the repetition processing is performed on all the conversion pixels by the all conversion pixel value determining means 7 ((7) in FIG. 3). It is possible to create a converted image in which a thin line having a line width of n pixels or less that has a possibility of disappearance is stored. FIG. 7 is an example (2) of the reduced converted image according to the present invention for the original image (1). For comparison, the conversion images of the TP method (FIG. 3 (3)) and the SPC method (FIG. 4 (4)), which are conventional conversion methods, are also shown together. It can be seen that the best conversion surface is obtained even if the disappearance is prevented. (Effects of the Invention) As described above, according to the present invention, even when the ratio is reduced to less than 1/2, the line width existing in the original image is 2 mm.
Fine lines of pixels or more can be discriminated and these fine lines can be stored on the converted image without disappearing, so that a converted image with little image quality deterioration can be obtained. According to the present invention, it is possible to perform any reduction conversion with a different conversion ratio between the horizontal direction and the vertical direction, and it is also applicable to a case where one direction is not converted or a case where expansion conversion is performed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an embodiment of the present invention, and FIGS. 2 (a) and 2 (b) show original pixels for explaining the principle of the present invention and an example where thin lines disappear. FIG. 3 is a diagram for explaining the flow of the conversion process of the present invention, FIG. 4 is a diagram showing a specific example of an original pixel referred to by the present invention, and FIG. 5 is a diagram showing a method for detecting a fine line. FIG. 6 is a diagram showing another example of a thin line detection method, and FIG. 7 is a diagram showing an example of a conversion image by the present invention and the conventional method. 1 ... Position detecting means, 2 ... Reading means, 3 ... Thin line detecting means, 4 ... Converted pixel determining means, 5 ... Substituting means,
6 ... means for determining converted pixel values, 7 ... means for determining all converted pixel values.

   ────────────────────────────────────────────────── ─── Continuation of front page    (56) References JP-A-62-196976 (JP, A)                 JP-A-61-199376 (JP, A)                 IEICE Technical Report ED84-26 P31-36

Claims (1)

(57) [Claims] 1.2 Conversion ratio α in horizontal direction smaller than 1
_x , the conversion ratio α _y α _x smaller than 1 in the vertical direction is 1 / n _x > α _x ≧ 1 / (n _x +1) α _y is 1 / _ny > α _y ≧ 1 / (n _y +1) _nx and _ny are reduced by natural numbers to obtain a converted image. In an image reduction conversion method, pixels of an original image (original pixels) and pixels of a converted image (converted pixels)
Determining a positional relationship between the steps of reading the position the original pixel plus one of the upper and lower respectively n _y in the left and right respectively n _x +1 original pixels and the vertical direction in the horizontal direction around the interest conversion pixel, read the from the value of the original pixel in the horizontal and vertical directions, and detecting a horizontal fine wire fine line and the line width of vertical line width is equal to or less than n _x pixels is less n _y pixels in the original image Determining the conversion pixel closest to the detected thin line; and detecting the thin line, and if the conversion pixel closest to the thin line is the conversion pixel of interest, assigns the value of the thin line to the conversion pixel of interest. And if the fine line is not detected, and if the converted pixel closest to the fine line is not the converted pixel of interest even if the fine line is detected, one of the nearest original pixels or the nearest original pixels to the target converted pixel Determining the value of the conversion pixel of interest based on the value.