JPS60218180A - Median filter circuit - Google Patents

Abstract

PURPOSE:To execute border processing rapidly with a simple device by selecting the value of one picture element out of sort sequence in accordance with the combination of border flags outputted from a sequential sorting circuit. CONSTITUTION:The sequential sorting circuit 1 sequentially sorts plural picture elements in a window by using respective values as keys, arranging the picture elements in the descending or ascending sort sequence, and when a part of the plural picture elements constituting the scanning window exists in the border of a picture, outputs the picture elements together with border flags assigned to invalid picture elements existing outside the border. A selecting circuit 3 selects one picture element out of the sort sequence in accordance with the border flags outputted from the sequential sorting circuit 1.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical Field of the Invention The present invention relates to a median filter circuit used for processing such as noise removal in an image processing apparatus or the like.

(b) Background of the technology In image processing, an original image is divided into elements, or pixels, arranged in a grid pattern with a pinch of, for example, 0.1 mm.
It deals with digital image data that is provided by expressing image information such as the density of each pixel using digital values.

However, the digital image data may contain random noise mixed into the observation system, etc., and it may be difficult to obtain favorable processing results even if some processing is performed on the data.

For this reason, a process called noise removal is often performed prior to the target process.

For example, when the density value of each pixel arranged one-dimensionally as shown in FIG.
by a scanning window containing , pixels. By rask scanning, the leftmost pixels of the window are sequentially read, 1-i
Each time for three consecutive pixels as shown by i...vii...

The pixel corresponding to both sides in the center of the window is set as the pixel of interest, and the value of the pixel of interest is replaced with the median value, that is, the second largest value, of each of the three pixels, and outputted (bl You will get an array like this.

As a result, in the processing of number iii, an isolated 1+jHi pixel like ■ shown by the ↓ mark is considered as noise and the ■ is converted to ■ and output, while the values of other pixels are output as is. Ru.

The means used for such processing is called a median filter or median filter.
The present invention H and others have made a two-second proposal for a circuit that effectively accomplishes this in Japanese Patent Application No. 57-'104379 (filing date: June 1, 1988, old 1).

(c) Prior art and problems The explanation in Figure 1 is an example of data arranged one-dimensionally, but in the case of one image data, each pixel is arranged in a grid, so the scanning line The above-mentioned processing cannot be performed on some pixels at the beginning and end of each image as they are.

That is, Fig. 2 is an example in which a scanning window consisting of five pixels is used, but when the pixel scanned by the center of the scanning window is the pixel of interest, two pixels each at the right end and left end of the two images are used. If processing is performed for a portion (outside the dotted line), a part of the scanning window will protrude outside the image boundary as in (i) or (ii), resulting in the problem that the processing result will be undefined. be.

To deal with this problem, processing is performed on the assumption that there are pixels with specific pixel values (for example, "0") outside the boundaries of one image, as shown in FIG.

Such processing is called boundary processing, and has conventionally been performed in a quadratic manner.

Figure 4 (al indicates an array of image data with n pixels per scanning line, and when scanning this using a scanning window consisting of 5 windows, there will be data before and after each scanning line) (By inserting two pieces of dummy data as shown by

The data was converted into a data string from which the portion corresponding to the dummy data was removed, as shown in Td), and then written into the image memory as shown in (e).

However, this method requires complicated control procedures;
Therefore, there is also the drawback that the processing speed is slow.

(d) Invention 1 The first aspect of the present invention is to perform boundary processing at high speed using a simple and simple device in median filter processing for data arranged in a grid like two-image data. It is in.

(e) Structure of the invention That is, the median filter times that the present invention becomes]? To.

In a digital image, a median filter is used to scan a window containing a plurality of pixels and output a median value selected from the values of a plurality of pixels within the window. A plurality of pixels in the scan window are sequentially arranged in a sort order of sort 1-L by using each value as a key, and a part of the plurality of pixels constituting the scan window 1-L falls on the boundary of the image. A sequential sorting circuit that outputs a boundary flag along with a boundary flag attached to an invalid pixel outside the boundary when a pixel is selected from the width of the sort order according to the combination of the boundary flag output from the sequential sorting circuit. The invention is characterized by comprising means for selecting a value.

(f) Examples of the invention Next, the gist of the present invention will be specifically explained using examples.

FIG. 5 is a diagram explaining the operating principle of the present invention.

One pixel in the scanning window as shown in ial■
If the image exceeds the boundary of the image, O should originally be input to ■ for boundary processing.

Actually, it contains the value X of the leftmost pixel of the next scanning line.

At this time, if the value of each pixel other than ■ in the scanning window shown in FIG. , if these values are sorted and arranged from the left to large h1i, the sorting order will be as shown in (bl), the median value will be 20, and the pixel of interest at the center of the scan-fit will be replaced with 20. It will be done.

However, as mentioned above, the value that enters ■ is actually the pixel X at the beginning of the next scanning line, and the value of X is equal to the median value 2.
If the value is smaller than 0, for example 10, the sorting order will be as shown in tc+ and the median value will be 20 as in the conventional example, so replace the value 10 of the pixel of interest with the median value 20. It shows what should be output.

Also, whether the value X at the beginning of the scanning line of the method is equal to 20 or 2
If the value is greater than 0, for example 40.

The sort l-rank is (shown in dl), and 1 originally should have entered O, but because 40 was entered, 30 was stored where the median value should have been stored. The median value of 20 has been moved to the 4th place. Therefore, in this case, if the value of the pixel ranked 4th in the sorting order is set as the true median value, and the pixel of interest (111fH1 is replaced with 20 and output) It shows good things.

In other words, as shown in Figure 5 fa), if the first pixel ■ in the scanning window protrudes from the image boundary and becomes the beginning of the next scanning line, the value of the pixel that should be in that window, that is, the next If the value X of the pixel number 11 at the end of the scanning line is in the 4th or 5th place in the sorting order, the value in the 3rd place is still the median value. Therefore, the value in the 3rd place is output. do it.

Similarly, as shown in Figure 5 fa), if the first pixel in the scanning window is out of the image boundary and becomes the beginning of the next scanning line, the value of the pixel that should be in that window, that is, the next When the value X of the first pixel of the scanning line is in the first, second or third place in the sorting order,
It is sufficient to output the value of the digit as the median value.

Sort in the same way if the first two pixels of the scan window are at the beginning of the next scan line, and if one or two pixels from the end of the scan window are at the end of the previous scan line. It is possible to determine the rank among the ranks to be output.

FIG. 6 is a truth table that covers which rank among the sorting ranks should be output to obtain the correct median value, and R
D* is a boundary flag indicating that some pixels in the scanning window straddle the scanning line; ST is a scanning line head flag indicating that the pixel of interest is at the tip of the scanning line; and BDa - BDb - BDc.・++11d and BDe are flags indicating whether or not the pixels in the 1st, 2nd, 3rd, 4th, and 5th sorting ranks straddle other scanning lines, respectively; IJ has a flag.
0" indicates no flag; blank indicates whether the flag is present or absent.

That is, (a) indicates that when BD* is 'OJ, that is, no boundary processing is required, so it is sufficient to output the value of the pixel ranked third in the sorting order as is, and (b)
This applies to the example described by FIG. 5TC), and also applies to the example described by fdl.(el and (fl is FIG. 5Td)).

FIG. 7 is a block diagram showing the configuration of an embodiment of the present invention, where 1 is the latest continuous 5 value obtained by scanning and reading the values of each pixel arranged in a square 1ffN shape one pixel at a time.
The elements are sorted sequentially using each value as a key, and the order of sorting is IN+11・T N f21 from the right side of the diagram.
・IN(3)・IN+'11 and I old 5), and when the scanning window is in the boundary area, the boundary flag Br1a - 11Db which is applied to pixels in other scanning lines (=JU) ・Sequential sorting circuit that outputs together with old) c and old] d and BDe, 2 and 3 are boundary flags 1lrl a that are output from sequential sorting circuit 1.
-nr) b. BDc4111d O, l, and Br1e combination IB 'B11; Configure means for selecting the value of one element from the sorting order described in 1 as the median value, 2.
3 is a selection circuit, and 4 is a delay circuit for two clocks.

FIG. 8 is a diagram explaining the operation of the sequential sorting circuit 1,
As shown in (a), the pixel value IN and the boundary flag l'lD are (i) (ii) (iii) (iv
) and (v), each pixel value IN is inputted from the right side as shown in (bl).
I N +31, IN+41 and T N +51,
1; Sea urchin, boundary flags RIla and BDb -B, respectively
rlc -Brld and Br1 (arranged and output with l attached, and then, as shown in (a)', when the next pixel value IN is input as shown in (vi) together with the boundary flag RD, similarly ( The output is changed to the array shown in bl'.

The logical operation circuit 2 receives a scanning line head flag ST obtained from an image data reading section (not shown), and a boundary flag R obtained from an image data reading section (not shown).
Boundary flag 811) l', which is obtained by delaying 11 by 2 L1 times by delay 1111 path 4, and sequential sorting circuit 1
Boundary flag Br1a (old) b-Rr returned by
lc-1 (takes Dd and l1rle as inputs and outputs the output CM given by the truth flCj table of FIG. 6).

The selection circuit rl'83 selects when the output of the logic operation circuit 2 is "00゛・
According to '01' or '10', respectively by sequential saw 1 circuit 1? 3rd, 4th or 5th pixel value in the sorting order, that is, BDc-B11d
Alternatively, select one of 1lrle and output it.

The logical operation circuit 2 can be constructed in a simple and inexpensive manner by storing the truth table shown in FIG. 6 as a table using, for example, a read-only memory.

(g) Effects of the Invention As explained above, according to the present invention, in median filter processing for data arranged in a grid shape such as image data, boundary processing can be performed at high speed with a simple device. I can do it.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram regarding the median filter, Figures 2 and 3
4 and 4 are explanatory diagrams regarding the problems of the conventional example, FIGS. 5 and 6 are explanatory diagrams regarding the principle of the present invention, FIG. 7 is an explanatory diagram of one embodiment of the present invention, and FIG. It is an explanatory diagram of example operation. In the figure, 1 is a sequential sorting circuit, and 2 is a logic operation circuit. 3 is a selection circuit. 2 Tei B ((1) (V) (IV) C11i) ('lri (i)Cd
)' (Vi) (V) OV) (iil) (iripi)

Claims (1)

[Claims] In a median filter circuit that scans a window including a plurality of pixels in a digital image and outputs a median value selected from the values of the plurality of pixels within the window,
A plurality of pixels in the window are sorted sequentially using each value as a key, and sorting in major or minor order 1! a sequential sorting circuit that is arranged at position a and outputs a boundary flag attached to an invalid pixel outside the boundary when a part of the plurality of pixels constituting the scanning window overlaps the boundary of the image; A median filter circuit comprising: means for selecting a value of one pixel from among the sorting orders according to a combination of boundary flags output from the sequential sorting circuit.