JP2620373B2 - Digital signal group compression method and apparatus - Google Patents

Description

The present invention relates to a digital signal group compression for compressing an original digital signal group whose value changes in one measurement direction into a compressed digital signal group having a smaller number of signals. Method and apparatus.

[Conventional technology]

In general, to convert an arbitrary amount of change into an original digital signal group whose value changes in one measurement direction, and to improve the efficiency when this original digital signal group is further used,
The tendency of the value change corresponds to the original digital signal group, but the number of signals is compression-converted to a compressed digital signal group smaller than the number of original digital signal groups.

As an example of compressing such a digital signal group,
For example, as shown in FIG. 9, there is a case where an arbitrary image 1 is printed on a sheet 3 as a reproduced image 1a.

More specifically, FIG. 9 shows a case where the image 1 and the character 2 shown at the leftmost portion are edited and printed on the paper 3 as the reproduced image 1a and the reproduced character 2a as shown at the rightmost portion. I have. One character 2 is converted into a digital signal by a character data input device 4 such as a keyboard, and is input to the host computer 6 as character data. The other image 1
Is read as image data by an image data input device 5 such as an image scanner and input to the host computer 6. The image data read from this image 1 is
The density of the image 1 is defined as an original digital signal composed of a set of digital signals measured for each unit pixel along the scanning direction of the scanning line. If the original digital signal is used as it is for printing the image 1, the amount of information is too large, and a large load is imposed on a control device such as a CPU of the host computer 6 or the printer 7, and these are formed with higher performance than necessary. Have to be expensive.

Therefore, conventionally, a compressed digital signal which has a small number of signal lines while changing according to the change of the original digital signal is created, and the host computer 6 edits the image 1 and the character 2 using the compressed digital signal. It is supposed to do it. The print data created by the host computer 6 is sent to a printer 7, and the printer 7 prints the reproduced image 1a and the reproduced characters 2a on the paper 3 based on the received print data.

[Problems to be solved by the invention]

However, the conventional method of compressing the original digital signal group has the following problems.

That is, reading of the density of the image 1 by the image data input device 5 is performed by repeating the measurement in the y direction while scanning the image 1 shown in FIG. 9 in the x direction.

FIG. 10 shows image data consisting of the density of the image 1 along one scanning line obtained by scanning the image 1 in the x direction by the image data input device 5.
This image data is obtained by displaying the tone values of the density measured for each pixel in the x direction of the image 1 as digital signal values and displaying these digital signal values as a line graph, and the values change in the x direction. It is a kind of original digital signal group. When compressing this image data, as shown in FIG. 10, the entire scanning direction range of the image 1 in the x direction is equally divided in the x direction by a number of compression sections having an equal width, and divided in each compression section. The original digital signal group is compressed so as to be one compressed digital signal.

However, like the image data shown in FIG. 10, the original digital signal group often includes a high-frequency component region whose value changes drastically and a low-frequency component region whose value changes gradually. In order to compress the high-frequency component so as to change with good response in response to the change, the length in the measurement direction, that is, the length in the x direction of the compression section to be equally divided must be shortened. However, if the length of the compression section in the x direction is reduced, more compressed digital signals are generated than necessary in the low-frequency component region, which results in poor data compression efficiency including excess data as a whole. In addition, a large load is imposed on various subsequent control processes in the printer 7. For this reason, in the above-mentioned conventional method, it was difficult to determine the number of equal divisions of the entire scanning range, that is, the length of the compression section in the measurement direction, to an appropriate value.

The present invention has been made in view of these points.
A digital signal group that can efficiently compress an original digital signal group whose value changes in two measurement directions into a compressed digital signal group that changes in response to the change in a small compression section It is an object to provide a compression method and apparatus.

[Means for solving the problem]

To achieve the above object, a digital signal group compression method according to the present invention according to claim 1, wherein an original digital signal group whose value changes in one measurement direction is divided into a plurality in the measurement direction. In the digital signal group compression method of compressing using a compressed section, when determining the start point and the end point of the compression section, a pixel position where the degree of change in value in the data of the original digital signal is sharp is defined as a section. The method is characterized in that the section point candidates are narrowed down to a predetermined number, and the narrowed down section point candidates are defined as section points.

3. The digital signal group compression method according to claim 2, wherein the original digital signal group whose value changes in one measurement direction is compressed using a plurality of compression sections divided in the measurement direction. In the group compression method, when determining a section point that is a start point and an end point of the compression section,
Pixel positions where the degree of change in the value in the data of the original digital signal is sharp are defined as section point candidates, the section point candidates are narrowed down to a predetermined number, and the interval between adjacent section point candidates is further separated by a predetermined minimum number of pixels or more. And then narrow down to
When the number is less than the predetermined number, new section point candidates are supplemented so as to reach the predetermined number, and the obtained section point candidates are set as section points.

4. The digital signal group compression method according to claim 3, wherein the original digital signal group whose value changes in one measurement direction is compressed using a plurality of compression sections divided in the measurement direction. In the group compression method, when determining a section point that is a start point and an end point of the compression section,
A pixel position having a sharp change in value in the data of the original digital signal is defined as a section point candidate, the section point candidates are narrowed down to a predetermined number, and the interval between adjacent section point candidates is separated by a predetermined maximum number of pixels or more. In this case, a section point candidate that equally divides the interval within the predetermined maximum number of pixels is added, and the obtained section point candidate is set as a section point.

A digital signal group compression method according to claim 4, wherein the original digital signal is a function g (x), the data value at the position of _i is g _(i), and D _(i) = g _{(i + 1)} - If g _(i) , then (1) D _(i-1) · D _(i) <0 (2) max (| D _(i-1) |, | D _(i) |)> C C is characterized in that the position of i that satisfies both of the two conditions of a constant (predetermined value) is set as a pixel position with a sharp change degree and is set as a section point candidate.

In the digital signal group compression method according to the fifth aspect, a pixel position having a high degree of change is defined as a section point candidate by max (| D _(i-1) |,
| D _(i) |), a predetermined number is selected from those having a large operation value according to the expression of ₍ D), and the section point candidates are narrowed down to a predetermined number.

Digital signal group compression method according to claim 6, when the interval between adjacent sections point candidate is determined as the neighborhood where less than the predetermined minimum number of strokes, and i and j the position where the vicinity of each other, | D _{( i-1)} · D _(i) |> | D _(j-1) · D _(j) | deletes the section point candidate at position j, and | D _(i-1) · D _(i) | <| D _(j-1) · D _(j) | If the section point candidate at the position i is deleted,
The method is characterized in that the intervals between adjacent section point candidates are separated by a predetermined minimum number of pixels or more.

A digital signal group compression method according to claim 7, wherein a change degree calculating means for calculating a change degree of the value in the measurement direction of the original digital signal group whose value changes in one measurement direction; A compression section length determining means for determining a length of the compression section in the measurement direction according to a degree of change of the original digital signal group obtained by the arithmetic means, wherein a section point serving as a start point and an end point of the compression section is determined. When deciding,
Compression section length determining means for setting a pixel position having a sharp change in value in the data of the original digital signal as a section point candidate, narrowing down the section point candidate to a predetermined number, and setting the narrowed section point candidate as a section point; It is characterized by having.

9. The digital signal group compression method according to claim 8, wherein a change degree calculating means for calculating a change degree of the value in the measurement direction of the original digital signal group whose value changes in one measurement direction; A compression section length determining means for determining a length of the compression section in the measurement direction according to a degree of change of the original digital signal group obtained by the arithmetic means, wherein a section point serving as a start point and an end point of the compression section is determined. When deciding,
Pixel positions where the degree of change in the value in the data of the original digital signal is sharp are defined as section point candidates, the section point candidates are narrowed down to a predetermined number, and the interval between adjacent section point candidates is further separated by a predetermined minimum number of pixels or more. And then narrow down to
When the number is less than the predetermined number, a new section point candidate is supplemented so as to become the predetermined number, and a compression section length determining unit using the obtained section point candidate as a section point is provided.

A digital signal group compression method according to claim 9, wherein a change degree calculating means for calculating a change degree of the value in the measurement direction of the original digital signal group whose value changes in one measurement direction; A compression section length determining means for determining a length of the compression section in the measurement direction according to a degree of change of the original digital signal group obtained by the arithmetic means, wherein a section point serving as a start point and an end point of the compression section is determined. When deciding,
A pixel position having a sharp change in value in the data of the original digital signal is defined as a section point candidate, the section point candidates are narrowed down to a predetermined number, and the interval between adjacent section point candidates is separated by a predetermined minimum number of pixels or more. And a compression section length determining means for adding a section point candidate that equally divides the interval within the predetermined maximum number of pixels, and using the obtained section point candidate as a section point. .

(Operation)

According to the present invention, a compressed digital signal group obtained by efficiently compressing an original digital signal group can be obtained by operating the present device in accordance with the present device.

That is, the degree of change of the value in the measurement direction of the original digital signal group is obtained by the degree of change calculating means, and the length of the compression section corresponding to the degree of change is determined by the compression section length determining means. Thereby, each compressed digital signal is created based on each compression section in which the lengths in the measurement direction are unequal within a predetermined length range, and the compression efficiency is improved with responsiveness to the change in the value of the original digital signal group. A group of compressed digital signals is obtained.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to FIGS.

In this embodiment, as shown in FIG. 1 and FIG. 2, as the original digital signal group, image data composed of the shading degree of the image 1 is used as in the related art.

FIG. 1 schematically shows a digital signal group compression device 11 according to the present invention.

The digital signal group compression device 11 of this embodiment is a digital signal group compression unit 11 which is a digital signal group whose value changes in one measurement direction. A degree-of-change calculating means 12 for calculating the degree of change in the x-direction, and compression for obtaining the length in the measurement direction of the compression section, ie, the length in the x-direction, according to the degree of change of the image data obtained by the degree-of-change calculating means 12. Section length determination means 13
Having. Further, a signal compression means 14 for converting the original digital signal into one compressed digital signal in accordance with a predetermined method using the compression section determined by the compression section length determining means 13 and a CPU 15 for operating these means in relation to each other. have.

 Next, the operation of the present embodiment will be described.

In this embodiment, as shown in FIG. 3, the entire scanning direction range of the image data is divided into unequal compression sections having different section lengths, that is, different lengths in the x direction, and the original digital The signal group is converted into one compressed digital signal.

To explain this operation outline flowchart of FIG. 4, when the compression operation is started, first, as shown in step ST _11, the detection scanning in the x direction of a certain concentration of the image 1 is performed by the image data input unit 5 Then, the image data is read, that is, the original digital signal group is read. Next, as shown in step ST _12, the change degree calculation means 12 and the compression section length determining unit 13 by the compression section is set. That is, the change degree calculating means 12 calculates the change degree of the value of the original digital signal group, and the compression section length determining means 13 determines the length of the compression section according to the change degree. Next, in step ST _13, in accordance with a predetermined method using the compression section by the signal compression unit 14,
Create a compressed digital signal group.

Wherein at the setting of the compression section in the step ST ₁₂ include various methods, such as the image data shown in FIG. 3, for the original digital signal group including a high frequency component region, severe degree of change of the value By shortening the length of the compression section of the part and increasing the length of the compression section of the part with a gradual change in the value, the compressed digital signal group that responds well to the change in the value of the original digital signal group Obtainable.

Further, in order to perform the setting of the compression section in the step ST ₁₂ specifically performs smoothly and properly by determining the interval point as a start point and an end point of each compression section indicated with black circles on the x-axis in FIG. 3 be able to.

The selection of the section point may be performed by the change degree calculating means 12 and the compression section length determining means 13 according to, for example, the flowchart shown in FIG.

That is, a plurality selects the x coordinate of the pixel positions or pixels of intense part of the degree of change than the image data of FIG. 3 as a section point candidates in step ST _21. Next, in step ST _22, a plurality of sections points candidate set was elected as the, narrowed down to a predetermined number of sections point candidates. Next, in step ST _23, a manner narrowed section point candidates as said is narrowed down to those that are separated by a predetermined minimum pixel or number. Next, in step ST _24, to replenish the new segment point candidate number corresponding to the number that is reduced by narrowing the step ST _23, obtains the section point candidate of the predetermined number. Next, in step ST _25, additional sections point by remote determination is made. In other words, when the intervals of the predetermined number of section point candidates determined as described above are separated by a predetermined maximum number of pixels or more, the section point candidates that divide the intervals evenly within the maximum number of pixels are set. And a compression section is determined using all of these section point candidates as section points.

An example in which the flowchart of FIG. 5 is further embodied is the flowchart of FIG.

With further description of this FIG. 6, to select a plurality of sections point candidates in step ST _31. This selection is to select a place where the degree of change of the image data is sharp,
As a selection method, for example, a pixel satisfying the following conditions (1) and (2) may be selected.

Now, let the gradation value of the pixel data shown in FIG. 3 be a function g (x), and let the gradation of the pixel at the i position in the x direction be g _(i) .
When D _(i) = g _{(i + 1)} −g _(i) , (1) D _(i-1) · D _(i) <0 (2) max (| D _(i-1) | , | D _(i) |)> C where C is a location that satisfies both conditions consisting of a constant (default).

The conditions (1) and (2) are such that the digital signal of the pixel portion whose x coordinate is (i) changes positively and negatively between the digital signal of each pixel portion before and after the unit pixel by the unit pixel. In addition, the condition is that the product of the absolute values of the amounts of change exceeds the degree of the intensity of the amount of change that is equal to or more than the predetermined C.

Next, in step ST _32, for each section point candidates selected in the step ST _31, the following equation max order from the larger of the calculated value _{(| D (i-1)} |, | | D (i)) Is given. in this case,
If there are a plurality of section point candidates having the same calculated value and it is necessary to select them, for example, the x-coordinate values of the pixels are selected in ascending order.

Next, in step ST _33, from the section point candidate set, a predetermined number (e.g., 30) selects the section point of the upper.

Next, in step ST _34, the section point candidate 30 that is elected, whether the neighborhood condition mutual spacing is within a predetermined minimum pixel count determination is made. If the interval point candidate is located in the vicinity it is determined that YES proceeds to step ST _35, if not in the vicinity proceeds to step ST ₃₈ to be described later is determined to NO.

Next, in step ST _35, to remove the one from the two sections point candidates and neighboring points to each other, leaving the other as a section point candidate. In this case, assuming that the interval point candidates of the points having the x-coordinates i and j are close to each other, | D _(i-1) · D _(i) | and | D _(j-1) · D _(j) Compare with | and keep the larger value and delete the other.

Next, in step ST _36, it is determined whether the total number of sections point candidates have been elected to date is a predetermined number which defines the step ST ₃₃ (30 in this example), described later in the case of YES proceeds to step ST ₃₈ that, in the case of NO proceeds to step ST _37.

Above In this step ST _37, those not in the predetermined number (30) missing minute already near relationship and the section point candidates have been elected a new segment point candidate number to, as in step ST ₃₂ in addition, it elected to order from, and returns to step ST _36.

Operation of the step ST ₃₆ and ST ₃₇ are repeating example until the determination in step ST ₃₆ is is YES.

Next, in step ST _38, each interval of a manner determined interval point as the whether is less than a predetermined maximum number of pixels each of which is determined. In the present embodiment, it is determined whether or not the interval between the interval points is equal to or less than 20 pixels. If the determination is YES, the interval point selection operation is terminated.
Proceed to ST ₃₉ .

In this step ST _39, the distance of the section point of 20 pixels or more sections, the section length is added evenly decomposing section point such that the maximum pixel 20 below. In this division,
It is preferable to increase the compression efficiency by minimizing the necessary number of section points. The section point selection operation is completed by adding this section point.

By determining the section points in this way, as shown in FIG. 3, a compression section formed between the section points is determined. The compression section is set such that the section length becomes shorter in the high frequency component region and becomes longer in the low frequency component region.

Thereafter, in step ST ₁₃ shown in FIG. 4, the compressed digital signal with each compression interval is determined, the compressed digital signal group of one scanning direction as a whole is determined.

By repeating this compression processing operation for all pixels in the y direction, image data of the entire image 1 is created.

The compressed digital signal group obtained in this way is sent to the host computer 6 and the character data input device 4
2 sent to the host computer 6 through
The print data is subjected to a process of creating a print signal for performing editing or the like with the character data, and is printed as a reproduced image 1a on the paper 3 by the printer 7.

The quality of the printed reproduced image 1a is almost equal to the quality of the original image 1. This is because the non-uniformity having a section length within a predetermined length range (in this embodiment, within 20 pixels) determined by the change degree calculating means 12 and the compression section length determining means 13 of the digital signal group compressing device 11. The compressed digital signal group formed by the signal compression means 14 based on the optimal compression section is connected to the reproduced compressed digital signal in such a manner as to smoothly change in the x direction. This is because Further, the compressed digital signal group is compressed and greatly reduced in information amount, that is, the data amount, as compared with the original digital signal group, so that the burden on the host computer 6 and the printer 7 is reduced. High-speed data processing can be enabled.

Next, each compressed digital signal group and each compressed digital signal obtained by performing a compression process on the same original digital signal group by the method of the present embodiment and the conventional method so as to smoothly change in the x direction. The connected reproduced and compressed digital signal groups are compared with each other with reference to FIGS. FIG. 7 shows the result by the method of the present embodiment, and FIG. 8 shows the result by the conventional method. In both figures, the image data to be compressed indicated by the solid line is composed of 256 pixels (there are two pixels on one scale), and the gray scale of the density of the image 1 is represented by a digital signal value for each pixel in the entire scanning direction range. is there.

Then, in the method of this embodiment, as shown in FIG. 7, a total of 49 compression sections (section points on the x-axis) where the entire scanning direction range is within the predetermined length range and the section lengths are different. (Indicated by black circles or white circles), and a compressed digital signal is created using the compression section. The length of the compression section needs to be short in the high-frequency component region, and when the fluctuation of the gradation value is extremely large, the section length may be “0”. (When the section length becomes "0", it is considered that the section points are duplicated on the same x coordinate (measurement direction).) In this embodiment, as shown in FIG. Are present in an overlapping state (points indicated by white circles), and are therefore divided into 49 compressed sections in total. The broken line in FIG. 7 indicates a reproduced compressed digital signal group obtained by connecting the reproduced compressed digital signals in a state where each of the compressed digital signals is reproduced. In addition,
In FIG. 7, when the image data (solid line) and the reproduced compressed digital signal (dashed line) overlap, the image data which is the original digital signal group is illustrated with priority.

In addition, in the conventional method, as shown in FIG. 8, the entire scanning direction range is equally divided into a total of 81 compression sections (section points are indicated by black circles on the x-axis) in which the section length is uniform. Creating a signal group. The broken line in FIG. 8 indicates a reproduced compressed digital signal group obtained by connecting reproduced compressed digital signals in a state where these compressed digital signal groups are reproduced.

As can be seen by comparing FIGS. 7 and 8,
According to the method of the present embodiment, about 1/2 of the number of compression sections of the conventional method is used.
While the original digital signal is compressed with the number of compression sections, the obtained reconstructed compressed digital signal group is not inferior to that of the conventional example having a large number of compression sections,
It responds to the change in the value of the original digital signal group with good responsiveness. Therefore, according to the present embodiment, the original digital signal group can be compressed very efficiently.

In the above-described embodiment, the compression target is the original digital signal group including the shading degree of each pixel of the image 1.
Any digital signal group whose value changes in one measurement direction can be compressed.

As a method of determining the length of the compression section in the measurement direction, a method other than the above-described embodiments may be adopted.

Furthermore, the method and apparatus of the present invention are not limited to the above embodiments, but can be variously modified as needed.

〔The invention's effect〕

As described above, the digital signal group compression method and apparatus according to the present invention are constructed and operated. Therefore, the original digital signal group whose value changes in one measurement direction can be changed in response to the change with good response. The digital signal can be compressed into a group, the compression can be efficiently performed with a small number of compression sections, and the subsequent signal processing can be performed easily and at high speed.

[Brief description of the drawings]

1 to 7 show an embodiment of a digital signal group compression method and apparatus according to the present invention, FIG. 1 is a block diagram schematically showing the apparatus of the present invention, and FIG. FIG. 3 is a block diagram showing a case in which the present invention is applied to a printer which performs image reproduction. FIG. 4 to 6 are flowcharts showing states of the compression processing according to the method of the present invention. FIG. 7 is a diagram similar to FIG. 3 showing a specific example of application of the method of the present invention. FIG. 9 is a diagram similar to FIG. 7 showing a conventional example for comparison with the present invention, and FIGS. 9 and 10 are diagrams similar to FIGS. 2 and 3 showing a conventional example, respectively. 11 ... Digital signal group compression device, 12 ... Change degree calculation means, 13 ... Compression section length determination means, 14 ... Signal compression means, 15 ... CPU.

Claims (9)

(57) [Claims] 1. A digital signal group compression method for compressing an original digital signal group whose value changes in one measurement direction by using a plurality of compression sections divided in the measurement direction. When determining the section points to be the start point and the end point of the above, a pixel position where the degree of change in the value is sharp in the data of the original digital signal is set as a section point candidate, and the section point candidates are narrowed down to a predetermined number. A digital signal group compression method characterized by using point candidates as section points. 2. A digital signal group compression method for compressing an original digital signal group whose value changes in one measurement direction using a plurality of compression sections divided in the measurement direction. When determining the section points that are the starting point and the ending point of the above, the pixel positions where the degree of change in the value is sharp in the data of the original digital signal are set as section point candidates, the section point candidates are narrowed down to a predetermined number, and The interval point candidates are narrowed down to those that are separated by a predetermined minimum number of pixels or more. If the interval point candidates are less than the predetermined number, new interval point candidates are supplemented so as to have the predetermined number. A digital signal group compression method characterized by using candidates as section points. 3. A digital signal group compression method for compressing an original digital signal group whose value changes in one measurement direction by using a plurality of compression sections divided in the measurement direction. When determining the section points that are the starting point and the ending point of the above, the pixel position where the degree of change in the value is sharp in the data of the original digital signal is set as the section point candidate, the section point candidates are narrowed down to a predetermined number, and each adjacent section is When the interval between the point candidates is separated by a predetermined maximum number of pixels or more, a section point candidate that equally divides the interval within the predetermined maximum number of pixels is added, and the obtained section point candidates are defined as section points. A digital signal group compression method. 4. When the original digital signal is a function g (x), the data value at the position of _i is g _(i), and D _(i) = g _{(i + 1)} −g _(i). , (1) D _(i-1) · D _(i) <0 (2) max (| D _(i-1) |, | D _(i) |)> C where C is a constant (default) The digital signal group compression method according to any one of claims 1 to 3, wherein a position of i satisfying both of the following two conditions is set as a pixel point having a drastic change degree as a section point candidate. 5. A predetermined number of pixel positions having a large degree of change are selected as interval point candidates, and a predetermined number is selected from those having a large value calculated by the expression max (| D _(i-1) |, | D _(i) |). 5. The digital signal group compression method according to claim 4, wherein the section point candidates are narrowed down to a predetermined number. 6. If the interval between adjacent section point candidates is less than a predetermined minimum number of strokes, it is determined that the interval is near, and if positions that are near each other are i and j, | D _(i-1) · D _{(i )} |> | D _(j-1) · D _(j) | deletes the section point candidate at position j, and | D _(i-1) · D _(i) | <| D _{(j-1) 3.} If D _(j) |, the interval point candidate at the position i is deleted to narrow down the interval point candidates adjacent to each other to have a predetermined minimum number of pixels or more. 2. A digital signal group compression method according to claim 1. 7. A degree-of-change calculating means for obtaining a degree of change in a value in the measurement direction of an original digital signal group whose value changes in one measurement direction, and an original digital signal obtained by the degree-of-change calculation means. A compression section length determining means for determining a length of the compression section in the measurement direction in accordance with a degree of change of the signal group, wherein when determining a section point to be a start point and an end point of the compression section, Compression section length determining means for setting a pixel position having a sharp change in value in signal data as a section point candidate, narrowing down the section point candidates to a predetermined number, and using the narrowed section point candidates as section points. Signal group compression device. 8. A degree-of-change calculating means for obtaining a degree of change in a value in the measurement direction of a group of original digital signals whose value changes in one measurement direction, and an original digital signal obtained by the degree-of-change calculation means. A compression section length determining means for determining a length of the compression section in the measurement direction in accordance with a degree of change of the signal group, wherein when determining a section point to be a start point and an end point of the compression section, A pixel position having a sharp change in value in the signal data is defined as a section point candidate, the section point candidates are narrowed down to a predetermined number, and the intervals between adjacent section point candidates are separated by a predetermined minimum number of pixels or more. And then, if the number is less than the predetermined number, a new section point candidate is supplemented so as to reach the predetermined number, and the obtained section point candidate is used as a section point. Compression device. 9. A degree-of-change calculating means for obtaining a degree of change in a value in the measurement direction of a group of original digital signals whose value changes in one measurement direction, and an original digital signal obtained by the degree-of-change calculation means. A compression section length determining means for determining a length of the compression section in the measurement direction in accordance with a degree of change of the signal group, wherein when determining a section point to be a start point and an end point of the compression section, A pixel position where the degree of change in value in the signal data is intense is defined as a section point candidate, the section point candidates are narrowed down to a predetermined number, and the interval between adjacent section point candidates is separated by a predetermined minimum number of pixels or more. A compression section length determining means for adding a section point candidate that equally divides the interval within the predetermined maximum number of pixels, and using the obtained section point candidate as a section point.