JP2008301280A - Image processor and control method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To extract embedded information with high precision from a document image that uses inter-line watermarkings. <P>SOLUTION: The image processor includes an input means of inputting a document image as image data; an image-reducing means of generating reduced image data which are reduced with respect to a first direction from the input image data; a detecting means of scanning the reduced image data generated by the image reducing means in a second direction, having predetermined relationship with the first direction to detect the length of a blank area in the second direction as inter-line distance information; and an extracting means of extracting watermark information embedded in the document image, based on the inter-line distance information detected by the detecting means. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image processing technique for extracting watermark information embedded in a document image based on line spacing of character strings in the document image.

In order to give information such as copyright and copy control to a document image, a method of embedding information using line spacing (hereinafter referred to as line spacing watermarking) as shown in the description on pages 198 to 199 of Non-Patent Document 1. Is known). FIG. 1 is a diagram showing the concept of a line spacing watermark. When extracting embedded information from a document image using a line spacing watermark, the line spacing between character strings in the document image is first obtained. In order to obtain the line spacing, generally, a document image is scanned all over to obtain a histogram, and the line spacing is derived from the histogram. Then, information is extracted according to the derived line spacing and the rules used at the time of embedding. For example, as shown in FIG. 1, the rule is to set U> D for the line intervals U and D when embedding “0” of binary information. On the other hand, when it is desired to embed “1” of the binary information, U <D is set for the line intervals U and D.
Kokoo Matsui “Basics of Digital Watermarking” Morikita Publishing Co., Ltd. 1998

  However, the above-described method for extracting embedded information from a document image using a line watermark has the following problems. That is, in order to measure the line spacing, it is necessary to perform a full scan on the document image and obtain a histogram, and it takes time to perform highly accurate information extraction processing. In particular, when the information to be embedded is copy control information, the copy control information is extracted in the copying machine, it is determined whether or not copying is possible from the extracted information, and then a copy process is performed. Therefore, a considerable time is required for a series of processes for copying one document. Also, if the character string direction and the scanning direction of the input document image are inclined, the line spacing cannot be derived from the histogram. Therefore, a complicated operation such as re-inputting the document image or performing image processing such as rotation operation on the input document image is required.

  In order to solve at least one of the above-described problems, the image processing apparatus of the present invention has the following configuration. That is, an image processing apparatus that inputs a document image and extracts watermark information embedded in the document image based on a line interval of a character string in the document image. Scanning the reduced image data generated by the reduced image data and the reduced image data generated by the reduced image data in a second direction having a predetermined relationship with the first direction, Detection means for detecting the length of the blank area in the second direction as line distance information; and extraction means for extracting watermark information embedded in the document image based on the line distance information detected by the detection means. Prepare.

  In order to solve at least one of the above-described problems, the control method of the image processing apparatus of the present invention has the following configuration. That is, a method for controlling an image processing apparatus that inputs a document image and extracts watermark information embedded in the document image based on line spacing of character strings in the document image, and inputs the document image as image data An input process, an image reduction process for generating reduced image data reduced in the first direction from the input image data, and the reduced image data generated in the image reduction process in advance with the first direction. Based on the detection step of scanning in the second direction having a predetermined relationship and detecting the length of the blank area in the second direction as the inter-line distance information, and the inter-line distance information detected in the detection step, the document An extraction step of extracting watermark information embedded in the image.

  According to the present invention, it is possible to provide a technique capable of more efficiently extracting watermark information embedded in a document image based on the line spacing of character strings in the document image.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. The following embodiments are merely examples, and are not intended to limit the scope of the present invention.

(First embodiment)
As a first embodiment of an image processing apparatus according to the present invention, an image processing apparatus that reads a paper document original with an image reading apparatus (scanner) and extracts watermark information from the read document image data will be described as an example. Explained.

<Device configuration>
FIG. 1 is a block diagram illustrating a main functional configuration of the image processing apparatus 11 according to the first embodiment. As shown in FIG. 1, the image processing apparatus 11 includes an image input unit 101, an image reduction unit 102, an information extraction unit 103, a control unit 110, and an operation unit 111.

  The image input unit 101 is a functional unit that inputs image data corresponding to a document image in which the above-described watermark information is embedded. Specifically, image data based on a paper document original read and generated by an external scanner is input through an external device connection interface such as a USB. Of course, the image data may be input via a network such as Ethernet (registered trademark).

  The image reduction unit 102 performs a first direction (for example, the horizontal direction of the image) and a second direction (for example, a vertical direction of the image) orthogonal to the first direction with respect to the image data input by the image input unit 101. It is a functional part that reduces each. When the input image data is in the bitmap (raster) format, generally, the first and second directions described above coincide with the arrangement direction of the pixels constituting the image data. In addition, although it demonstrated that it was the structure reduced to two orthogonal directions here, it is good also as a structure reduced only to one direction so that it may mention later.

  The information extraction unit 103 is a functional unit that extracts binary (binary) information corresponding to watermark information from the reduced image data generated by the image reduction unit 102. Specifically, it is executed by detecting the presence / absence of significant pixels in the reduced image data generated by the image reduction unit 102. Then, a portion where a significant pixel is detected is determined as a character string region, and a region where a significant pixel is not detected is determined as an interline portion. Then, a binary value (0 or 1) corresponding to each row is determined based on the length (number of continuous pixels) corresponding to each of the plurality of rows determined as the portion between the rows. It is assumed that the watermark information embedding algorithm such as the correspondence between the length of the line spacing portion and the binary value and the information amount (number of bits) is set in the information extraction unit 103 in advance.

  The control unit 110 is a functional unit for controlling the above-described functional units to operate in cooperation with each other. The operation unit 111 is a functional unit for receiving an instruction from the user. Details of the operation of each functional unit described above will be described later.

<Image data with embedded watermark information>
In the first embodiment, the watermark information embedding algorithm will be described as follows. Of course, other algorithms can be applied as long as the watermark information is embedded based on the line spacing of the character string.

FIG. 2 is a diagram exemplarily showing a document original in which watermark information is embedded. In FIG. 2, black characters are recorded on white paper. Also, binary information is embedded by using two types of U and D for each interval (line interval) of a character string composed of a plurality of lines. Specifically, the line spacing is set as follows with respect to the space between two consecutive rows (U _n and D _n : n is a natural number).

When embedding “0”: U _n > D _n
When embedding “1”: U _n <D _n
Then, the watermarked document original is read by a scanner having an optical sensor such as a charge coupled device (CCD), and image data I is generated. Here, the generated document image data I is described as a binary image in which the text portion of the paper document is black and the background portion is white. Hereinafter, the “black pixel” corresponding to the character portion may be referred to as a “significant pixel”.

  FIG. 3 is a diagram exemplarily showing binary image data in which watermark information is embedded. Here, FIG. 3A shows image data in which a horizontally written character document is recorded in the correct direction. On the other hand, FIG. 3B and FIG. 3C are tilted document images. As described above, when a paper document original is read by the scanner, image data as shown in FIGS. 3B and 3C is input due to a deviation from the reference direction or a misplacement of the paper original by the user. May be.

<Operation of the device>
Hereinafter, an operation in which the image processing apparatus 11 extracts watermark information from the image data I will be described in detail with reference to flowcharts.

  FIG. 10 is an operation flowchart of the image processing apparatus 11 according to the first embodiment. Note that the following flow is executed, for example, with an image reading instruction from the user via the operation unit 111 as a trigger.

  In step S <b> 1001, the image input unit 101 receives image data I corresponding to a document image with watermark information from the scanner, and supplies the image data I to the image reduction unit 102.

  In step S1002, the image reduction unit 102 reduces the image data I supplied from the image input unit 101 in the horizontal direction and the vertical direction, respectively, and generates horizontal reduced image data Ish and vertical reduced image data Isv. . Then, the reduced image data Ish and the reduced image data Isv are supplied to the information extraction unit 103.

  FIG. 4 is a diagram illustrating an example of reduced image data Ish obtained by reducing the image data I in the horizontal direction and reduced image data Isv reduced in the vertical direction. 4A is based on the document image in FIG. 3A, FIG. 4B is based on the document image in FIG. 3B, and FIG. 4C is based on the document image in FIG. 3C. Reduced image data generated in this way.

  Here, the reduced image data Ish is reduced in the horizontal direction and is not enlarged / reduced in the vertical direction, and the reduced image data Isv is reduced in the vertical direction and not enlarged / reduced in the horizontal direction. It is data. However, the reduced image data Ish only needs to have a higher reduction ratio in the horizontal direction than in the vertical direction. In the vertical direction, the reduced image data Ish is reduced or enlarged so that the length between character string areas to be described later can be measured. It does not matter to do. The same applies to the reduced image data Isv.

  Here, the reason why the reduced image is generated in the two directions of horizontal and vertical is to cope with the case where the image data I having a 90 degree inclination is input as shown in FIG. is there. Such a situation often occurs when reading with a scanner. Therefore, if the input direction of the image data I is guaranteed in advance, the image data I may be reduced in only one direction corresponding to the character string direction.

  From FIG. 4, it can be seen that a plurality of character strings are expressed as a group (object) of a plurality of significant pixels for a reduced image reduced in the character string direction by reduction by the image reduction unit 102. Hereinafter, the generation of the reduced image will be described in more detail.

  As a specific calculation method of the reduction process executed by the image reduction unit 102, reduction by simply thinning out pixels at a constant period with respect to input image data can be considered. However, in order to more accurately identify the character string portion and the line spacing portion, the following calculation using the bilinear method may be performed.

  In the bilinear method, the pixel value of the corresponding reduced image data pixel is calculated based on the pixel values of four adjacent pixels in the image data I. In this case, if at least one of the four pixels is black, the calculation result is halftone (gray) (left in FIG. 5). Specifically, the result of calculating white as 0 and black as 1 is a value greater than 0 and less than 1.

  Since there is no black pixel that is a significant pixel in the inter-line portion, if there is even one black pixel, it can be estimated that the portion where the four adjacent pixels are present is a character string region. Therefore, the character string area and the line space area can be divided with higher accuracy by converting the portion calculated as the halftone into black as a significant pixel (right in FIG. 5). In other words, there are cases where there are no significant pixels in the character string area (characters with a small number of strokes), but there is generally no significant pixel in the line spacing area.

  The reduction ratio in the horizontal and vertical directions by the image reduction unit 102 may be any reduction ratio that is large enough to recognize an object. The specific reduction ratio value may be a fixed value designated in advance, or may be determined as needed based on additional data (such as reading resolution information by a scanner) of input image data. Although the bilinear method is applied here as an example of an algorithm for generating a reduced image, the present invention is not limited to this. It can be reduced by using various calculation methods such as a near-less tray bar and bicubic.

  In step S <b> 1003, the information extraction unit 103 measures the length (line interval) between the character string regions using one of the reduced image data supplied from the image reduction unit 102. Here, it is assumed that measurement is performed on the reduced image data Ish reduced in the horizontal direction. The specific measurement method is as follows.

  FIG. 6 is an enlarged view of the reduced image data Ish. First, the information extraction unit 103 detects the presence / absence of a significant pixel by scanning the reduced image data Ish in a direction (here, the vertical direction) orthogonal to the reduction direction (here, the horizontal direction). Then, the position where the presence / absence of significant pixels is reversed is determined using the character string area as a boundary with the line spacing area.

For example, when the inversion position is detected along the arrow in FIG. 6, each pixel position of x _{1 to} x ₁₀ is detected. At this time, the length of each of _{U 1, D 1, U 2} , D 2 is a line _{spacing, U 1 = x 3 -x 2} , D 1 = x 5 -x 4, U 2 = x 7 -x ₆ , D ₂ = x ₉ -x ₈ (unit is pixel (pix)).

  As described above, as a result of the reduction process in step S1002, the character string area is reduced in the character arrangement direction, and at the same time, the halftone portion is converted into a significant pixel. As a result, the density of significant pixels (here, black) in the character string region is increased. As a result, the information extraction unit 103 can detect the boundary between the character string area and the line space area with higher accuracy, and can measure the line spacing with higher accuracy.

  The above-described method is effective even when the reduced image data Ish is slightly inclined as shown in FIG. FIG. 8 is a diagram exemplarily showing scanning for the reduced image data Ish in FIG. In this case, the length of the line interval along the arrow is different from the case of FIG. Specifically, when the inclination of the character string is θ (degrees), the length is 1 / cos θ times. However, the relative length relationship between the line intervals does not change.

  Note that when the reduced image data using the above-described bilinear method is used, one scan is generally sufficient. However, when performing detection with higher accuracy, a plurality of scans may be performed on the reduced image, and an average value at a plurality of locations may be used as the line interval. Even when reduced image data is generated by thinning, multiple-site scanning is effective. FIG. 7 is a diagram exemplarily showing a state in which a plurality of positions are scanned with respect to the reduced image data Ish. In FIG. 7, the character string portion indicated by the black object is shown as being closer to the actual reduced image data. That is, it is shown as including an insignificant pixel (white pixel) instead of the ideal rectangular object as shown in FIG. Therefore, the measured value of the distance between objects (inter-line distance information) differs slightly depending on the scanning position.

For example, as shown in the figure, values obtained by performing three scanning positions (arrows (1), (2), and (3))
Arrow (1): U ₁ = 10 [pix], D ₁ = 4 [pix], U ₂ = 4 [pix], D ₂ = 12 [pix]
Arrow (2): U ₁ = 8 [pix], D ₁ = 5 [pix], U ₂ = 6 [pix], D ₂ = 10 [pix]
Arrow (3): U ₁ = 6 [pix], D ₁ = 3 [pix], U ₂ = 5 [pix], D ₂ = 8 [pix]
Suppose that In that case, for example, the average value of each length,
U ₁ = 8 [pix], D ₁ = 4 [pix], U ₂ = 5 [pix], D ₂ = 10 [pix]
May be determined as the distance between objects (line spacing).

In step S1004, the information extraction unit 103 derives watermark information based on the line spacing (interline distance information) derived in step S1003. Specifically, watermark information is calculated in association with an embedding algorithm preset in the information extraction unit 103. For example, in FIG. 6, since U ₁ <D ₁ , U ₂ > D ₂ , the watermark information is derived as “10 (binary)”.

  However, when measurement is performed on the reduced image data Ish shown in FIG. 4C in step S1003, the distance between objects (interval distance information of blank areas) cannot be measured. Therefore, a watermark image cannot be extracted in step S1004. That is, in step S1001, the image data I is input in a state rotated by 90 degrees, and therefore the reduced image data Ish is generally a black solid image.

In step S1005, the control unit 110 determines whether information has been extracted in step S1004. The determination as to whether or not the information has been extracted is, for example, when U _n and D _n can be measured, it is determined that the information has been extracted, and when U _n and D _n cannot be measured, it is determined that the extraction has failed. To do. Alternatively, it is assumed that information cannot be extracted when a value other than a value defined by a preset embedding algorithm is detected. Alternatively, the reading result may be determined by performing a pre-specified erroneous detection / non-detection determination.

  In step S <b> 1006, the information extraction unit 103 measures the length (interline distance information) between the character string regions using the other reduced image data supplied from the image reduction unit 102. That is, here, the measurement is performed on the reduced image data Isv (second reduced image data) reduced in the vertical direction. A specific measurement method is the same as that in step S1003. However, it should be noted that the information extraction unit 103 detects the presence / absence of a significant pixel by scanning the reduced image data Isv in a direction (here, the horizontal direction) orthogonal to the reduction direction (here, the vertical direction). (FIG. 9).

  In step S1003, the scanning is performed in the vertical direction, and in step S1006, the scanning is performed in the horizontal direction. However, the scanning may be performed in the horizontal direction in step S1003, and the scanning may be performed in the vertical direction in step S1006. In addition, these two directions (the first direction and the second direction) are preferably orthogonal, but may not necessarily be orthogonal and may have a slight inclination. For example, the rotation direction may be 85 degrees or 95 degrees. That is, the first direction and the second direction are preferably orthogonal in order to simplify the configuration of the embodiment, but may be in a predetermined relationship so as to be approximately orthogonal.

  In step S1007, the information extraction unit 103 derives watermark information based on the line interval (second interline distance information) derived in step S1006. A specific measurement method is the same as that in step S1004.

  Through the operation flow as described above, the image processing apparatus 11 extracts watermark information from the image data I.

  In the above description, the document image data having the pattern shown in FIG. 3 is described. That is, it is assumed that the document image data is input in a substantially normal direction or rotated by 90 degrees. In addition to these, when the input is performed upside down or horizontally, it is preferable that the scanning in the reverse direction is executed in addition to the scanning direction shown in FIGS. Furthermore, when such a situation is assumed, it is also preferable to use an algorithm configured so as not to depend on the reading start direction as the watermark information embedding algorithm.

For example, if the information to be embedded is “101 (binary)” or “11010 (binary)”, the start bit is “0”, and the stop bit is “11”, the information is “010111 (binary)”, “01101011 (binary)”. Furthermore, if information such as “010111000101 (binary)” and “0110101100101001 (binary)” is embedded so that reading can be performed from either side, information can be extracted by only one scan from the top to the bottom. is there. The latter part of the information is bit-inverted when the determination condition is embedded with “0”: U _n > D _n
When embedding “1”: U _n <D _n
In this case, reading from the opposite direction reverses the order of U _n and D _n and reverses the bit.

  In addition to the above, if the start bit and the stop bit are set to the same “0”, the information to be embedded is “001010 (binary)” when the information to be embedded is “101 (binary)”. When read from the opposite direction, “101011 (binary)” and the start bit is “1”. Therefore, by reversing the information portion and rearranging “010100 (binary)” in the opposite direction, it becomes “001010 (binary)” and can be extracted.

  In the first embodiment, description has been made using an algorithm for embedding watermark information in correspondence with the relative length of one set (two) of line intervals as shown in FIG. However, as described first, the present invention is not limited to this embedding algorithm. For example, on the basis of the first line interval (between the first and second lines of the character string), the line interval after the second (between the second and third lines of the character string) An algorithm that embeds information corresponding to the relative length to the line spacing may be used. Further, instead of the binary information based on the relative line spacing, multi-value information based on the line spacing may be embedded. In other words, this is an effective technique for any arbitrary embedding algorithm using line spacing.

  As described above, according to the image processing apparatus according to the first embodiment, it is possible to extract watermark information with higher accuracy or higher speed. Further, even when the input image data has a tilt, information can be extracted without performing a complicated operation.

(Second Embodiment)
In the second embodiment, a description will be given of a form in which watermark information is extracted from a document image read by a scanner or the like by a program that operates on a computer (PC).

<Equipment configuration>
FIG. 11 is a diagram showing an internal configuration of the PC.

  In the figure, reference numeral 1101 denotes a CPU, which realizes each part of the functional blocks shown in FIG. 1 based on programs and data stored in a RAM 1102 and a ROM 1103.

  Reference numeral 1102 denotes a RAM which stores programs and data loaded from the external storage device 1108. In addition, a program or data downloaded from another computer system 1114 via an I / F (interface) 1115 is temporarily stored. Furthermore, the CPU 1101 has an area necessary for performing various processes.

  Reference numeral 1103 denotes a ROM which stores computer function programs and setting data. Reference numeral 1104 denotes a display control apparatus, which performs control processing for displaying images, characters, and the like on the display 1105. Reference numeral 1105 denotes a display that displays images, characters, and the like. As a display, a CRT, a liquid crystal screen or the like can be applied.

  Reference numeral 1106 denotes an operation input device, which is a device such as a keyboard or a mouse that can input various instructions to the CPU 1101. Reference numeral 1107 denotes an I / O for notifying the CPU 1101 of various instructions input via the operation input device 1106.

  Reference numeral 1108 denotes an external storage device that functions as a large-capacity information storage device such as a hard disk, and stores an OS (operating system), various application programs, and input / output document images. Writing information to the external storage device 1108 and reading information from the external storage device 1108 are performed via the I / O 1109.

  Reference numeral 1110 denotes a printer for outputting documents and images. Output data is sent from the RAM 1102 or the external storage device 1108 via the I / O 1111. Examples of printers for outputting documents and images include ink jet printers, laser beam printers, thermal transfer printers, and dot impact printers.

  Reference numeral 1112 denotes a scanner for reading a document or an image. Input data is sent to the RAM 1102 or the external storage device 1108 via the I / O 1113.

  A bus 1116 connects the CPU 1101, ROM 1103, RAM 1102, I / O 1111, I / O 1109, display control device 1104, I / F 1115, I / O 1107, and I / O 1113.

<Operation of the device>
The external storage device 1108 stores an image processing program for realizing each functional unit and operation flow described in the first embodiment as an application program. The operation flow described in the first embodiment is started based on an instruction to start the image processing program via the operation input device 1106 by the user. Detailed operations are substantially the same as those in the first embodiment, and will not be described.

(Other embodiments)
Although the embodiments of the present invention have been described in detail above, the present invention may be applied to a system constituted by a plurality of devices or may be applied to an apparatus constituted by one device.

  The present invention can also be achieved by supplying a program that realizes the functions of the above-described embodiments directly or remotely to a system or apparatus, and the system or apparatus reads and executes the supplied program code. The Accordingly, the program code itself installed in the computer in order to realize the functional processing of the present invention by the computer is also included in the technical scope of the present invention.

  In this case, the program may be in any form as long as it has a program function, such as an object code, a program executed by an interpreter, or script data supplied to the OS.

  Examples of the recording medium for supplying the program include a floppy (registered trademark) disk, a hard disk, an optical disk (CD, DVD), a magneto-optical disk, a magnetic tape, a nonvolatile memory card, and a ROM.

  In addition, the program of the present invention is encrypted, stored in a storage medium such as a CD-ROM, distributed to users, and key information for decryption is downloaded from a homepage via the Internet to users who have cleared predetermined conditions. Let By using the key information, an encrypted program can be executed and installed in a computer.

  Further, the functions of the above-described embodiments are realized by the computer executing the read program. In addition, based on the instructions of the program, an OS or the like running on the computer performs part or all of the actual processing, and the functions of the above-described embodiments can also be realized by the processing.

  Further, the program read from the recording medium is written in a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer. Thereafter, the CPU of the function expansion board or function expansion unit performs part or all of the actual processing based on the instructions of the program, and the functions of the above-described embodiments are realized by the processing.

It is a block diagram which shows the main function structures of the image processing apparatus 11 in 1st Embodiment. FIG. 3 is a diagram exemplarily showing a document original in which watermark information is embedded. It is a figure which shows exemplarily the binary image data in which watermark information was embedded. It is a figure which shows the example of the reduced image data Ish which reduced the image data I to the horizontal direction, and the reduced image data Isv reduced to the vertical direction. It is a figure which shows illustratively conversion to the black which is a significant pixel of the part calculated as a halftone. It is an enlarged view of reduced image data Ish. It is a figure which shows an example of a mode that multiple times scanning is performed with respect to the reduction image data Ish. FIG. 5 is a diagram exemplarily showing scanning with respect to reduced image data Ish in FIG. It is a figure which shows a mode that a horizontal direction is scanned with respect to reduction image data Isv. 3 is an operation flowchart of the image processing apparatus 11 according to the first embodiment. It is a figure which shows the internal structure of PC.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Image processing apparatus 101 Image input part 102 Image reduction part 103 Information extraction part 110 Control part 111 Operation part

Claims (10)

An image processing apparatus that inputs a document image and extracts watermark information embedded in the document image based on line spacing of character strings in the document image,
An input means for inputting a document image as image data;
Image reduction means for generating reduced image data reduced in the first direction from the input image data;
The reduced image data generated by the image reduction means is scanned in a second direction having a predetermined relationship with the first direction, and the length of the blank area in the second direction is used as the inter-line distance information. Detecting means for detecting;
Extracting means for extracting watermark information embedded in the document image based on the line distance information detected by the detecting means;
An image processing apparatus comprising: The image reduction means further generates second reduced image data reduced from the image data in a direction orthogonal to the first,
When valid line distance information is not detected by the detection means,
The detection means scans the second reduced image data in the first direction, detects the length of the blank area in the first direction as second interline distance information,
The image processing apparatus according to claim 1, wherein the extraction unit extracts watermark information embedded in the document image based on the second interline distance information. And means for detecting the resolution of the image data input by the input means;
Means for determining a reduction ratio in a direction of reduction by the image reduction means based on the detected resolution;
The image processing apparatus according to claim 1, further comprising:   2. The image reduction unit according to claim 1, wherein the image reduction unit generates the reduced image data without reducing the first direction and reducing the second direction. Image processing device.   The image reduction means generates the second reduced image data by reducing in a direction orthogonal to the first direction and without reducing in the first direction. The image processing apparatus according to claim 2.   2. The inter-line distance information is detected by the detecting unit based on lengths of a plurality of blank areas obtained by scanning the reduced image data at a plurality of locations in the second direction. 6. The image processing apparatus according to any one of items 1 to 5.   The image processing apparatus according to claim 1, wherein the image reduction unit sets a pixel detected as a halftone by reduction to a significant pixel. A method for controlling an image processing apparatus that inputs a document image and extracts watermark information embedded in the document image based on line spacing of character strings in the document image,
An input process for inputting a document image as image data;
An image reduction process for generating reduced image data reduced in the first direction from the input image data;
The reduced image data generated in the image reduction process is scanned in a second direction having a predetermined relationship with the first direction, and the length of the blank area in the second direction is used as the inter-line distance information. A detection process to detect;
An extraction step for extracting watermark information embedded in the document image based on the line spacing information detected in the detection step;
A control method comprising: A step of generating second reduced image data reduced from the image data in a direction orthogonal to the first;
When effective line distance information is not detected in the detection step,
Scanning the second reduced image data in the first direction and detecting the length of the blank area in the first direction as second interline distance information;
Extracting watermark information embedded in the document image based on the second inter-line distance information;
The control method according to claim 8, further comprising:   A program for causing a computer to execute each step of the control method for an image processing apparatus according to claim 8 or 9.

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