JP7430554B2 - Optical information reading device, optical information reading method, optical information reading program, computer readable recording medium, and recording device - Google Patents

Description

The present invention relates to an optical information reading device, an optical information reading method, an optical information reading program, a computer readable recording medium, and a recording device.

It receives light from a mark called a code or symbol (hereinafter referred to as a "code") such as a barcode or QR code to be read, acquires image data of the code using an imaging module, and converts the image data into 2. Description of the Related Art Optical information reading devices called code readers are known, which read information according to a code by analyzing it. For example, in the logistics industry, stationary code readers are used to read barcodes attached to various items to be delivered while the items are being conveyed on a conveyor. This kind of stationary code reader illuminates the object being transported on a conveyor with illumination light from a remote location, captures the image with a camera, searches for the barcode in the obtained image, and decodes it. There is.

When the imaging unit captures an image of a conveyed object including a code with an imaging device to generate image data, the imaging device converts an input value with a high number of bits into an output value with a lower number of bits. A code reader is known that can switch conversion characteristics when converting the number of bits (Patent Document 1). By changing the standard conversion characteristic to a conversion characteristic aimed at expanding the dynamic range, it is possible to effectively expand the dynamic range of the imaging unit and obtain a code image having the contrast necessary for decoding.

However, if the brightness of the image data fluctuates greatly due to differences in height of the baggage to which the code is attached, or if the code itself has low contrast, the above method does not necessarily yield a code image with sufficient contrast. There were cases where it was not possible.

Japanese Patent Application Publication No. 2018-136853

One object of the present invention is to provide an optical information reading device, an optical information reading method, an optical information reading program, a computer readable recording medium, and a recording device that can stably read codes. It's about doing.

Means for solving the problem and effects of the invention

According to a first aspect of the present invention, the optical information reading device reads a code attached to a conveyance object moving on a conveyance line, and the illumination unit irradiates the code with light. an imaging unit that receives light emitted from the illumination unit and reflected by the code to obtain an input image; and a larger input value for each pixel forming the input image obtained by the imaging unit; a conversion unit that calculates an output value by reducing the number of gradations from the input value based on a characteristic conversion formula in which the width of change in the output value becomes smaller with respect to a change in the value, and generates a code image in which each pixel has the output value; and a decoding unit that decodes information encoded in the code based on the code image generated by the conversion unit, and the characteristic conversion formula is such that at a position where the input value is 0, the output value is in a negative direction. The input value is offset on a graph showing the relationship between the input value and the output value so that For pixels larger than , the output value changes based on the magnitude of the input value based on the characteristic conversion formula , and the larger the input value becomes from the threshold value, the more the output value changes with respect to the change in the input value. For a pixel that outputs an output value whose width is smaller and the input value is smaller than the threshold, the input value is relatively smaller than the pixel whose input value is larger than the threshold, regardless of the characteristic conversion formula. It can be configured to output a fixed value as an output value. With the above configuration, stable reading can be performed even if the contrast of the code changes.

According to the optical information reading device according to the second aspect, in addition to the above configuration, the characteristic conversion formula is set to the output value when the luminance value serving as the input value is in a range from 0 to a predetermined value . The brightness value can be set to be 1/5 or less of the number of gradations . With the above configuration, the output value is limited for dark pixels with a low input value of the imaging section, and although the expression of the output value is restricted in areas where the input value is small, the difference between dark and bright areas, which is important in code reading, is restricted. This provides the advantage of ensuring contrast.

Furthermore, according to the optical information reading device according to the third aspect, in addition to any of the above configurations, the characteristic conversion formula can have an upwardly convex curved shape.

Furthermore, according to the optical information reading device according to the fourth aspect, in addition to any of the above configurations , a characteristic conversion formula to be used during operation is further determined from the plurality of characteristic conversion formulas in which the threshold values are different from each other. The tuning section includes a tuning section for performing the operation by referring to each reading result by the decoding section of the code image generated by the converting section based on each of the plurality of characteristic conversion formulas. It can be configured to determine the characteristic conversion formula that is sometimes used.

Furthermore, according to the optical information reading device according to the fifth aspect, in addition to any of the above configurations, the brightness and darkness of the code image is determined from a plurality of conversion characteristic curves having different offset amounts created in advance. It is possible to include a tuning unit for selecting one of them based on the variation in the code image and its ability to respond to a code image with a low contrast difference.

Furthermore, according to the optical information reading device according to the sixth aspect, in addition to any one of the above configurations, the tuning section is configured to adjust the conversion characteristic curve in the conversion section based on a plurality of conversion characteristic curves having different offset amounts. The code images obtained by performing gradation conversion may be decoded by the decoding section, and the parameters of the plurality of conversion characteristic curves may be determined based on the reading results.

Furthermore, according to the optical information reading device according to the seventh aspect, in addition to any of the above configurations, the tuning section selects conversion characteristic curves whose parameters are close to each other among the plurality of conversion characteristic curves. The configuration can be configured to select one of the conversion characteristic curves by referring to the reading results. With the above configuration, it is possible to increase the robustness to variations in brightness of the code image.
Furthermore, according to the optical information reading device according to the eighth aspect, the optical information reading device reads a code attached to a conveyed object moving on a conveyance line, and the illumination unit irradiates the code with light. an imaging unit that receives light emitted from the illumination unit and reflected by the code to obtain an input image; and a larger input value for each pixel forming the input image obtained by the imaging unit; a conversion unit that calculates an output value by reducing the number of gradations from the input value based on a characteristic conversion formula in which the width of change in the output value becomes smaller with respect to a change in the value, and generates a code image in which each pixel has the output value; a decoding unit that decodes information encoded in the code based on the code image generated by the conversion unit; and a tuning unit that determines a characteristic conversion formula to be used during operation from among the plurality of different characteristic conversion formulas. and, for a pixel in which the input value is larger than a different threshold value for each characteristic conversion formula, the conversion unit converts an output value that changes based on the magnitude of the input value based on the characteristic conversion formula. , and for a pixel for which the input value is smaller than the threshold value, outputs a constant value that is relatively smaller than a pixel for which the input value is larger than the threshold value, and the tuning unit The characteristic conversion formula used during the operation can be determined by referring to each reading result by the decoding unit of the code image generated by the conversion unit based on each of the plurality of characteristic conversion formulas.

Furthermore, according to the optical information reading method according to the ninth aspect, the optical information reading method uses an optical information reading device that reads a code attached to a conveyed object moving on a conveyance line, A step of irradiating the code with illumination light from the illumination unit and receiving the reflected light reflected by the code at the imaging unit to obtain an input image, and a step of obtaining a large input value for each pixel constituting the obtained input image. On the graph showing the relationship between the input value and the output value, the width of change in the output value becomes smaller with respect to the change in the input value, and the output value is in the negative direction at the position where the input value is 0. When generating a code image in which the number of gradations is reduced from the input value and an output value in which each pixel has the output value is calculated based on the characteristic conversion formula to be offset , the input value is A pixel larger than a threshold value determined based on the offset amount changes based on the magnitude of the input value based on the characteristic conversion formula , and the larger the input value is from the threshold value, the more the input value changes. A pixel that outputs an output value such that the width of change in the output value becomes smaller with respect to a change in value , and the input value is smaller than the threshold value is a pixel whose input value is smaller than the threshold value , regardless of the characteristic conversion formula. A step of generating a code image having the output value for each pixel by outputting a constant value relatively smaller than that of a large pixel as an output value, and a step of generating a code image having the output value for each pixel, and The method may include a step of decoding information encoded into a code in a decoding unit based on the information encoded in the code. This allows stable reading even if the contrast of the code changes.

Furthermore, according to the optical information reading program according to the tenth aspect, the optical information reading program reads a code using an optical information reading device that reads a code attached to a conveyed object moving on a conveyance line. The function includes a function of emitting illumination light from an illumination unit to a code, receiving reflected light reflected by the code at an imaging unit to obtain an input image, and for each pixel constituting the obtained input image, The relationship between the input value and the output value is set so that the larger the input value is, the smaller the change width of the output value is with respect to the change in the input value, and the output value is in the negative direction at the position where the input value is 0. An output value obtained by reducing the number of gradations from an input value is calculated based on a characteristic conversion formula that is offset on the graph shown in FIG. A pixel whose characteristic conversion formula is larger than a threshold determined based on the offset amount is changed based on the magnitude of the input value based on the characteristic conversion formula , and the input value is changed from the threshold. An output value is output such that the larger the input value is, the smaller the change width of the output value is with respect to a change in the input value, and a pixel whose input value is smaller than the threshold value is determined by the input value regardless of the characteristic conversion formula. A function of generating a code image having the output value for each pixel in a conversion unit by outputting a constant value that is relatively smaller than a pixel whose output value is larger than the threshold value, and a function of generating a code image in the imaging unit. Based on the encoded code image, the computer can realize a function of decoding information encoded into a code in a decoding section. With the above configuration, stable reading can be performed even if the contrast of the code changes.

Furthermore, the computer-readable recording medium or device storing the computer-readable recording medium according to the eleventh aspect stores the above-mentioned program. Recording media include CD-ROM, CD-R, CD-RW, flexible disk, magnetic tape, MO, DVD-ROM, DVD-RAM, DVD-R, DVD+R, DVD-RW, DVD+RW, Blu-ray, HD. Included are magnetic disks such as DVD (AOD) and UHD (all product names), optical disks, magneto-optical disks, semiconductor memories, and other media capable of storing programs. In addition to programs that are stored and distributed in the recording medium described above, programs that are distributed by downloading through a network such as the Internet are also included. Further, the stored device includes a general-purpose or dedicated device in which the program is implemented in an executable state in the form of software, firmware, or the like. Furthermore, each process and function included in the program may be executed by computer-executable program software, or the processing of each part may be executed by hardware such as a predetermined gate array (FPGA, ASIC) or program software. It may also be realized in a mixed format with a partial hardware module that realizes some of the hardware elements.

1 is a schematic diagram showing the configuration of an optical information reading system according to Embodiment 1 of the present invention. FIG. 2 is a perspective view of an optical information reading device. 3 is a front view of the optical information reading device of FIG. 2. FIG. 3 is a side view of the optical information reading device of FIG. 2. FIG. 3 is a top view of the optical information reading device of FIG. 2. FIG. 3 is a bottom view of the optical information reading device of FIG. 2. FIG. FIG. 2 is a block diagram of the optical information reading device of FIG. 1. FIG. FIG. 3 is a schematic diagram showing how the reading area and the amount of light change depending on the distance between the optical information reading device and the object to be read. It is an image diagram showing how the brightness and size change between a cord placed nearby and a cord placed far away. It is a graph showing an HDR conversion formula. FIG. 3 is an image diagram of a code image generated using standard conversion characteristics. FIG. 3 is an image diagram of a code image generated using HDR conversion characteristics. It is a graph showing a conversion formula for converting a 12-bit input value into 8-bit. 14 is a semi-logarithmic graph showing the contrast with respect to the brightness value of the white part in FIG. 13. FIG. 15 is a semi-log graph showing a dynamic range when a standard contrast code is read using the standard conversion characteristics shown in FIG. 14. FIG. 15 is a semi-log graph showing a dynamic range when a standard contrast code is read using the HDR conversion characteristics shown in FIG. 14. FIG. 15 is a semi-log graph showing a dynamic range when a low contrast code is read using the standard conversion characteristics shown in FIG. 14. FIG. 15 is a semi-log graph showing a dynamic range when a low contrast code is read using the HDR conversion characteristics shown in FIG. 14. FIG. It is a graph which shows the conversion characteristic curve of HDR1, 2, and 3. 20 is a graph showing a hypothetical region in which the output value is negative in FIG. 19. 20 is a semi-logarithmic graph showing the relationship between the brightness value of the white portion of the code and the contrast regarding the conversion characteristic curve of FIG. 19. 22 is a semi-log graph showing the dynamic range when reading a standard contrast code with the standard conversion characteristics shown in FIG. 21. FIG. 22 is a semi-log graph showing the dynamic range when reading a standard contrast code with HDR1 in FIG. 21. FIG. 20 is a semi-logarithmic graph showing the dynamic range when reading a low contrast code using the standard conversion characteristics shown in FIG. 19. 20 is a semi-logarithmic graph showing the dynamic range when reading a low contrast code with HDR1 in FIG. 19. It is a graph showing a conversion formula for gamma correction. It is a graph showing curves of HDR conversion formulas with γ=4, 8, 16, 32, and 64. 28 is a semi-logarithmic graph showing the relationship between the brightness value of the white portion of the code and the contrast in the HDR conversion formula of FIG. 27. It is a graph showing an HDR conversion formula in which parameter value β is changed. 7 is a flowchart showing a method for optimizing HDR during tuning. FIGS. 31A to 31E are distribution charts showing how the distribution of brightness values changes when the brightness of the code image is changed. It is an image diagram showing an example of a tuning screen of an optical information reading program. FIG. 6 is an image diagram showing image data obtained when the distance to the conveyed object changes with the optical information reading device according to the first embodiment and the optical information reading device according to the comparative example.

Embodiments of the present invention will be described below based on the drawings. However, the embodiments shown below illustrate an optical information reading device and an optical information reading method for embodying the technical idea of the present invention, and the present invention is not limited to the optical information reading device and the optical information reading method. The formula information reading method is not specified as follows. Moreover, this specification does not in any way specify the members shown in the claims to the members of the embodiments. In particular, the dimensions, materials, shapes, relative positions, etc. of the components described in the embodiments are not intended to limit the scope of the present invention, unless otherwise specified, and are merely illustrative. Just an example. Note that the sizes, positional relationships, etc. of members shown in each drawing may be exaggerated for clarity of explanation. Furthermore, in the following description, the same names and symbols indicate the same or homogeneous members, and detailed descriptions will be omitted as appropriate. Furthermore, each element constituting the present invention may be configured so that a plurality of elements are made of the same member so that one member serves as a plurality of elements, or conversely, the function of one member may be performed by a plurality of members. It can also be accomplished by sharing.
[Embodiment 1]

An optical information reading system 1000 according to Embodiment 1 of the present invention is shown in the schematic diagram of FIG. The optical information reading system 1000 registers and verifies data by reading marks called codes and symbols (hereinafter collectively referred to as "codes") such as barcodes and two-dimensional codes. The optical information reading system 1000 shown in FIG. 1 includes an optical information reading device 1, a computer 100, a programmable logic controller (PLC) 101, and a conveyor belt BC.

In this example, during operation, a plurality of workpieces WK are placed on the upper surface of the conveyor belt BC and are transported in the direction of arrow Y in FIG. An optical information reading device 1 according to Embodiment 1 is installed. The workpiece WK is, for example, a conveyed object such as a baggage, and is attached with a code such as a bar code or a two-dimensional code. Information such as the name of the package and the destination is encoded and recorded in the code. Furthermore, if the work WK is a product, the product name, serial number, price, etc. are recorded in a code to ensure traceability.

The optical information reading device 1 is a code reader configured to image a code attached to a workpiece WK, decode the code included in the captured image (decode processing), and read the information. It is. The optical information reading device 1 may be used by being fixed to a bracket or the like so that it does not move during operation, or may be held and operated by a robot, a user, or the like. Further, the code of the workpiece WK in a stationary state may be read by the optical information reading device 1. Note that during operation, the operation is performed to sequentially read the codes of the workpieces WK conveyed by the conveyor belt BC.

A code is attached to each workpiece WK. A one-dimensional code such as a barcode or a two-dimensional code can be used as the code. Examples of two-dimensional codes include QR code, micro QR code, Data matrix (Data code), Veri code, Aztec code, PDF417, Maxi code, etc. (all product names). Two-dimensional codes include stack type and matrix type, and the present invention can be applied to either type of two-dimensional code. The code may be attached by directly printing or stamping on the workpiece WK, or may be attached by printing on a label and then pasting it on the workpiece WK, and the means and method thereof are not limited.

The optical information reading device 1 is wire-connected to the computer 100 and the programmable logic controller (PLC) 101 through signal lines 100a and 101a, respectively, but the invention is not limited to this. A communication module may be built into the PLC 101, and the optical information reading device 1, the computer 100, and the PLC 101 may be wirelessly connected. The PLC 101 is a control device for sequentially controlling the transport belt conveyor BC and the optical information reading device 1, and a general-purpose PLC can be used. The computer 100 can be a general-purpose or dedicated computer, a portable terminal, or the like.

Further, during operation, the optical information reading device 1 receives a reading start trigger signal that defines the start timing of code reading from the PLC 101 via the signal line 101a. The optical information reading device 1 then images and decodes the code based on this reading start trigger signal. Thereafter, the decoded result is transmitted to the PLC 101 via the signal line 101a. In this way, when the optical information reading device 1 is in operation, the reading start trigger signal is input and the decoding result is output between the optical information reading device 1 and an external control device such as the PLC 101 via the signal line 101a. It is done repeatedly. Note that the input of the reading start trigger signal and the output of the decoding result may be performed via the signal line 101a between the optical information reading device 1 and the PLC 101, as described above, or via other signal lines. It may also be done through For example, a sensor for detecting the arrival of the workpiece WK and the optical information reading device 1 may be directly connected, and a reading start trigger signal may be input from the sensor to the optical information reading device 1.

The appearance of the optical information reading device 1 is shown in FIGS. 2 to 6. In these figures, FIG. 2 is a perspective view of the optical information reading device 1, FIG. 3 is a front view of the optical information reading device 1 of FIG. 2, and FIG. 4 is a side view of the optical information reading device 1 of FIG. 5 shows a top view of the optical information reading device 1 of FIG. 2, and FIG. 6 shows a bottom view of the optical information reading device 1 of FIG. 2.

The optical information reading device 1 includes an illumination section 4, an imaging section 5, a display section 6, a power supply connector 7, and a signal line connector 8. Furthermore, the device body 2 is provided with an indicator 9 shown in FIG. 5, an aimer 10 shown in FIG. 3, a select button 11 shown in FIG. 5, and an enter button 12.

The device main body 2 includes a casing 2a having a substantially rectangular box shape that is elongated in the vertical direction. A control unit 29, a decoding section 31, etc. shown in FIG. 7 are provided inside the casing 2a. On the front side of the casing 2a, there is an illumination section 4 that illuminates at least the code of the workpiece WK by irradiating light toward the front of the optical information reading device 1, and a lighting section 4 that illuminates at least the code of the workpiece WK in front of the optical information reading device 1. An imaging unit 5 for imaging at least the code is provided. Further, an aimer 10 made of a light emitting body such as a light emitting diode (LED) is provided on the front surface of the casing 2a. The aimer 10 is used to indicate the imaging range of the imaging section 5 and the optical axis of the illumination section 4 by emitting light toward the front of the optical information reading device 1 . The user can also install the optical information reading device 1 by referring to the light emitted from the aimer 10.

A display section 6 is provided on the upper surface of the casing 2a, as shown in FIG. Further, on the upper surface of the casing 2a, a select button 11 and an enter button 12, which are used when setting the optical information reading device 1, etc., are provided. The select button 11 and the enter button 12 are connected to a control unit 29, and the control unit 29 can detect the operation states of the select button 11 and the enter button 12. The select button 11 is a button that is operated when selecting one from a plurality of options displayed on the display section 6. The enter button 12 is a button operated when confirming the result selected with the select button 11.

Further, as shown in FIG. 4, indicators 9 are provided on both left and right sides of the upper surface of the casing 2a. The indicator 9 is connected to the control unit 29 and can be composed of a light emitting body such as a light emitting diode. The operating state of the optical information reading device 1 can be notified to the outside by the lighting state of the indicator 9.

On the lower surface of the casing 2a, as shown in FIG. 6, there are a power connector 7 to which power wiring for supplying power to the optical information reading device 1 is connected, and signal lines 100a and 101a connected to the computer 100 and PLC 101. An Ethernet (trade name) connector 8 is provided. Note that the Ethernet standard is just one example, and signal lines of standards other than the Ethernet standard can also be used.
(Lighting section 4)

The illumination unit 4 includes a plurality of first light emitting diodes 16 and a plurality of second light emitting diodes 17, as shown in FIG. The first light emitting diode 16 and the second light emitting diode 17 are electrically connected to a control unit 29 and individually controlled by the control unit 29, so that they can be turned on and off separately.

A block diagram showing the configuration of the optical information reading device 1 is shown in FIG. As shown in this figure, the optical information reading device 1 includes an illumination section 4, an imaging section 5, a conversion section 30, and a decoding section 31. The illumination unit 4 irradiates the cord with light. The imaging unit 5 receives the light emitted from the illumination unit 4 and reflected by the code, and acquires an input image. The converting unit 30 generates a code image from the input image obtained by the imaging unit 5. The code image is an output that reduces the number of gradations from the input value by using a characteristic conversion formula for each pixel that makes up the input image, in which the larger the input value, the smaller the change width of the output value in response to a change in the input value. This is the calculated value. The HDR conversion formula described later can be used as the characteristic conversion formula. For pixels whose input value is larger than a predetermined threshold, the conversion unit 30 outputs an output value that changes based on the magnitude of the input value based on the characteristic conversion formula. Here, the input value refers to the pixel value of each pixel forming the input image. An example of the input value is a brightness value. On the other hand, a pixel whose input value is smaller than the threshold value is configured to output a relatively smaller constant value as an output value than a pixel whose input value is larger than the threshold value. Here, the relatively small constant value allows for some negligible variation when reading the code.

Finally, the decoding unit 31 decodes the information encoded into the code based on the code image generated by the converting unit 30. By doing so, stable reading can be performed even if the contrast of the code changes. In particular, code images are generated using conversion characteristics that maintain a constant contrast value between white and black areas of the code even when the brightness of the code image fluctuates, improving code readability. do. This makes it possible to improve the ability to respond to cases where the height difference between the workpieces is large.
(Configuration of imaging unit 5)

The imaging unit 5 is attached to the workpiece WK and includes an imaging element 5a that images the code illuminated by the illumination unit 4, an optical system 5b having a lens, etc., and an autofocus mechanism (AF mechanism) 5c. ing. Light reflected from the coded portion of the workpiece WK is incident on the optical system 5b. The image sensor 5a is an image sensor made of a light receiving element such as a CCD (charge-coupled device) or a CMOS (complementary metal oxide semiconductor) that converts an image of the code obtained through the optical system 5b into an electrical signal. The image sensor 5a is connected to a control unit 29, and the electrical signal converted by the image sensor 5a is input to the control unit 29. Further, the AF mechanism 5c is a mechanism that performs focusing by changing the position and refractive index of a focusing lens among the lenses constituting the optical system 5b. The AF mechanism 5c is also connected to the control unit 29 and is controlled by the AF control section 29a of the control unit 29.
(Display section 6)

The display section 6 is composed of, for example, an organic EL display or a liquid crystal display. The display section 6 is connected to the control unit 29, and can display, for example, the code imaged by the imaging section 5, a character string that is a decoding result of the code, a reading success rate, a matching level, and the like. The read success rate is the average read success rate when reading processing is executed multiple times. The matching level is a reading margin indicating the ease of reading a code that has been successfully decoded. This can be determined from the number of error corrections that occur during decoding, and can be expressed, for example, as a numerical value. The fewer error corrections are made, the higher the reading margin is, while the more error corrections are made, the lower the reading margin is.
(Conversion unit 30)

The optical information reading device 1 includes a converting section 30. The conversion section 30 is connected to the control unit 29. The conversion unit 30 is a part that generates a second image by converting pixel values of the first image acquired by the image sensor 5a into pixel values according to predetermined conversion characteristics. The first image can also be called a pre-conversion image. The second image can also be called a converted image. The conversion characteristics used by the conversion section 30 when converting the pixel values of the first image are stored in advance in a conversion characteristic storage section 35d of the storage device 35, which will be described later.
(Decode unit 31)

The optical information reading device 1 includes a decoding section 31 that decodes black and white binary data. For decoding, a table showing the contrast of encoded data can be used. Further, the decoding unit 31 checks whether the decoded result is correct according to a predetermined checking method. If an error is found in the data, the error correction function is used to calculate correct data. The error correction function differs depending on the type of code.

In this embodiment, the decoding unit 31 decodes the code included in the second image converted by the converting unit 30. The decoding unit 31 is configured to decode the code and write the obtained decoding result into the storage device 35. Further, the decoding unit 31 performs image processing such as various image processing filters on the second image before decoding.
(Communication department 32)

The optical information reading device 1 has a communication section 32 . The communication unit 32 is a part that communicates with the computer 100 and PLC 101. The communication unit 32 may include an I/O unit connected to the computer 100 and the PLC 101, a serial communication unit such as RS232C, and a network communication unit such as a wireless LAN or wired LAN.
(control unit 29)

The control unit 29 shown in FIG. 7 is a unit for controlling each part of the optical information reading device 1, and can be configured with a CPU, MPU, system LSI, DSP, dedicated hardware, etc. The control unit 29 is equipped with various functions as described below, but these may be realized by a logic circuit or by executing software.

The control unit 29 includes an AF control section 29a, an imaging control section 29b, a tuning section 29c, and a UI management section 29e. The AF control section 29a is a unit that controls the AF mechanism 5c, and is configured to be able to focus the optical system 5b using conventionally well-known contrast AF or phase difference AF.

The imaging control section 29b is a unit that adjusts the gain, controls the light amount of the illumination section 4, and controls the exposure time (shutter speed) of the imaging device 5a. Here, the gain is an amplification factor (also called magnification) when the brightness of the image output from the image sensor 5a is amplified by digital image processing. The amount of light from the illumination unit 4 can be changed by controlling the first light emitting diode 16 and the second light emitting diode 17 separately. The gain, the amount of light of the illumination section 4, and the exposure time are the imaging conditions of the imaging section 5.

The tuning unit 29c is a unit that changes imaging conditions such as gain, light amount and exposure time of the illumination unit 4, and image processing conditions in the decoding unit 31. The image processing conditions in the decoding unit 31 include the coefficients of the image processing filter (strength and weakness of the filter), switching of the image processing filter when there is a plurality of image processing filters, a combination of different types of image processing filters, and the like. Appropriate imaging conditions and image processing conditions vary depending on the influence of external light on the workpiece WK during transportation, the color and material of the surface to which the code is attached, etc. Therefore, the tuning section 29c searches for more appropriate imaging conditions and image processing conditions, and sets the processing by the AF control section 29a, the imaging control section 29b, and the decoding section 31. As the image processing filter, various conventionally known filters can be used. In addition, during tuning by the tuning unit 29c, the code images obtained by performing gradation conversion in the conversion unit 30 are decoded by the decoding unit 31, respectively, based on a plurality of conversion characteristic curves having different offset amounts. Based on the reading results, parameters of a plurality of conversion characteristic curves can be determined.

The tuning unit 29c uses the first image acquired by the imaging unit 5 at the time of setting the optical information reading device 1 before operating the optical information reading device 1, and instructs the conversion unit 30 to perform conversion characteristic storage. By generating a second image according to each of a plurality of different conversion characteristics stored in the 35d, and analyzing the results of decoding codes included in the generated second image by the decoding section 31, a plurality of different conversion characteristics can be generated. It is configured to select one of the conversion characteristics. The processing procedure by the tuning unit 29c will be explained in detail based on the flowchart described later, but the outline is as follows.

That is, before operating the optical information reading device 1, settings of the optical information reading device 1 are performed as a preparation stage for operation. When setting the optical information reading device 1, various settings are performed by transmitting various setting commands from the computer 100 connected to the optical information reading device 1 via the signal line 101a. At the time of this setting, tuning is performed by the tuning section 29c. When tuning, the code attached to the workpiece WK is imaged by the imaging unit 5 to obtain a first image. A second image is generated by converting the pixel values of the obtained first image into pixel values according to one of the conversion characteristics stored in the conversion characteristic storage section 35d. Further, a second image is generated by converting the pixel values of the obtained first image into pixel values corresponding to other conversion characteristics among the conversion characteristics stored in the conversion characteristic storage section 35d. If the number of conversion characteristics stored in the conversion characteristic storage section 35d is three or more, the conversion section 30 can be caused to generate three or more second images.

Then, the codes included in the plurality of generated second images are each decoded by the decoding unit 31, and the tuning unit 29c analyzes the reading margin indicating the ease of reading the codes that have been successfully decoded. The tuning unit 29c selects the conversion characteristic of the second image that has a high reading margin as a result of analyzing the reading margin. After the settings of the optical information reading device 1 are completed, when the optical information reading device 1 is in operation, the conversion unit 30 uses the first image acquired by the imaging unit 5 to perform the conversion selected by the tuning unit 29c. A second image according to the characteristics is generated and decoded by the decoding unit 31.

When setting the optical information reading device 1, the tuning unit 29c causes the imaging unit 5 to acquire one first image, and causes the conversion unit 30 to use the one first image and store it in the conversion characteristic storage unit 35d. It may be configured to generate a plurality of second images according to each of a plurality of different conversion characteristics. In this case, when setting the optical information reading device 1, as many second images as the number of conversion characteristics stored in the conversion characteristic storage section 35d are generated.

Further, when setting the optical information reading device 1, the tuning unit 29c causes the imaging unit 5 to acquire a plurality of first images, and causes the conversion unit 30 to store each of the plurality of first images in the conversion characteristic storage unit 35d. The plurality of second images may be generated in accordance with each of the plurality of different conversion characteristics. In this case, since a plurality of first images are obtained when setting the optical information reading device 1, the number of first images is multiplied by the number of conversion characteristics stored in the conversion characteristic storage section 35d. A number of second images will be generated.

Further, the tuning unit 29c may be configured to cause the imaging unit 5 to capture a plurality of images under different imaging conditions to obtain a plurality of first images when setting the optical information reading device 1. The imaging conditions include the gain, the light amount of the illumination unit 4, the exposure time, and the like.

The UI management unit 29e shown in FIG. 7 causes the display unit 6 to display various interfaces, codes imaged by the imaging unit 5, character strings that are decoding results of the code, reading success rate, matching level, etc., and selects a select button. 11 and the enter button 12, and controls lighting of the aimer 10.
(Storage device 35)

The storage device 35 is composed of a memory, a hard disk, and the like. The storage device 35 is provided with a decoding result storage section 35a, an image data storage section 35b, a parameter set storage section 35c, and a conversion characteristic storage section 35d. The decoding result storage section 35a is a section that stores the decoding result that is the result of decoding by the decoding section 31. The image data storage section 35b is a section that stores images captured by the image sensor 5a.

The conversion characteristic storage section 35d is a section that stores a plurality of different conversion characteristics used by the conversion section 30 when converting pixel values of the first image. The conversion characteristics are gradation conversion characteristics, and in this embodiment, three conversion characteristics are stored in advance in the conversion characteristic storage section 35d, as shown in FIG. 19, which will be described later.

The parameter set storage unit 35c shown in FIG. 7 is a part that stores setting information set by a setting device such as the computer 100, setting information set by the select button 11 and the enter button 12, and the like. The parameter set storage unit 35c stores at least one of the imaging conditions of the imaging unit 5 (gain, light amount and exposure time of the illumination unit 4, etc.) and the image processing conditions of the decoding unit 31 (type of image processing filter, etc.). A parameter set including a plurality of constituent parameters can be stored.

In recent years, one-dimensional codes such as barcodes and two-dimensional codes such as QR codes, which record information such as product name, serial number, and price, have been widely used to quickly recognize product management information. There is. An optical information reading device is used as a member that reads and decodes such codes and performs processing such as registration and verification of data recorded in the codes. Optical information reading devices are broadly classified into handheld scanners, handy terminals, business-use PDAs, and stationary code readers.

Stationary optical information reading devices are widely used in distribution centers and the like. At logistics distribution centers, codes that include destination information attached to each item are read and used for automatic sorting of items transported on conveyors or the like. Therefore, it is required to read the code reliably, and it is also necessary to appropriately arrange a plurality of optical information reading devices. Additionally, because the amount of deliveries is increasing year by year, it is necessary to read conveyed items at high speed.

However, the size of the objects to be transported at the distribution center is not fixed, and varies from large and tall objects to small and short objects. When using an optical information reading device for sorting packages by reading codes on irregularly shaped packages at high speed, the following two issues exist in order to perform stable reading.
(Issue 1: Suppressing brightness fluctuations due to height differences in luggage)

In order to read the code reliably, it is necessary to clearly photograph the moving baggage. However, the photographing distance varies greatly due to differences in the height of the baggage, the distance between the optical information reader and the conveyor, and other factors. Generally, it is known that brightness decreases in proportion to the square of the distance from the optical information reading device 1, as shown in FIG. Therefore, as shown in the lower right corner of Figure 9, when the code is close, the image tends to be too bright; conversely, when the cord is far away, as shown in the upper left corner of Figure 9, the image tends to be too dark. There is a tendency to

Furthermore, when a linear image is acquired, as the object to be imaged becomes brighter, the contrast between the code and other parts also increases. Therefore, if the image is dark, the contrast will be low and it will not be possible to decode it. On the other hand, when the image is bright, the contrast of the code and the contrast of other parts increases, causing a problem in that processing time is increased.
(Issue 2: Dealing with low contrast codes)

Another problem is when the code itself has low contrast. For example, the narrow bar on the black background may become thinner if the amount of ink remaining is low when printing the code, or if sufficient ink cannot be applied due to high-speed printing. Alternatively, the printed surface of the code attached to the conveyed object may be rubbed off by various objects during the conveyance of the conveyed object, and the surface may be peeled off, resulting in a subsequent reduction in contrast.

Regarding problem 1, even if an automatic focusing mechanism or the like is used to improve out-of-focus, this problem still remains, and it is desirable to solve it.

In addition, issue 2 is difficult to improve on the code side, so it is desirable to deal with it by improving the code reading performance.

In order to solve this problem, conventional techniques have been used to devise conversion characteristics when converting image data with a high number of bits read by the imaging unit 5 into image data with a low number of bits that can be easily handled by the converting unit 30. to avoid loss of contrast. Specifically, a conversion characteristic (also called an HDR curve, a LOG curve, etc.) as shown in FIG. 10 was prepared to ensure a contrast difference. Here, a code image generated using standard conversion characteristics is shown in FIG. 11, and a code image generated using HDR conversion characteristics is shown in FIG. 12, respectively. In this way, by expressing the image using an HDR curve, it is possible to significantly improve white collapse and black collapse, ensure a large dynamic range, and obtain a code image with clear contrast.

Here, the conversion characteristics will be explained based on FIGS. 10 to 13. FIG. 10 shows four conversion characteristics in which the gamma value of gamma conversion is changed. The conversion characteristics shown in this figure are HDR characteristics (first conversion characteristics) with a gamma value larger than 1.0 (for example, 2.0), and super HDR characteristics with a gamma value larger than the gamma value of the HDR characteristics. A characteristic (first conversion characteristic), a standard characteristic with a gamma value of 1.0, and a contrast enhancement characteristic (second conversion characteristic) with a gamma value of less than 1.0 (for example, 0.5). The gamma value of each conversion characteristic is an example, and is not limited to the above values. Since the HDR characteristic and the super HDR characteristic have a gamma value larger than 1.0, they are so-called log conversion, and the brightness of the first image acquired by the imaging unit 5 during operation of the optical information reading device 1 This is a characteristic consisting of a substantially logarithmic curve for suppressing fluctuations. Even if the brightness of the code of the workpiece WK changes, the brightness fluctuation of the second image for decoding is suppressed. Furthermore, by using HDR characteristics and super HDR characteristics, halation is less likely to occur, so information is less likely to be lost. For example, when the effective field of view of the imaging section 5 is wide or when the depth becomes long, the distance between the workpiece WK and the imaging section 5 will vary widely. It is conceivable that the brightness of the first image obtained by increases.

Since the standard characteristic is that the gamma value is 1.0, the pixel values of the first image acquired by the imaging unit 5 during operation of the optical information reading device 1 are output as they are. The standard characteristics have characteristics between HDR characteristics and contrast enhancement characteristics. The greater the number of bits in the first image, the more information can be compressed and acquired, so 12-bit input from the image sensor 5a is richer in expression than 10-bit input. This is preferable because brightness fluctuations are suppressed and reading accuracy is increased. Note that the signal input from the image sensor 5a in 12 bits or 10 bits is converted to 8 bits.

Since the gamma value is less than 1.0, the contrast enhancement characteristic is a characteristic consisting of a substantially exponential curve for enhancing the contrast of the first image acquired by the imaging unit 5 during operation of the optical information reading device 1. . As a certain input value, the contrast between the cell part and the space part is small, such as when the pixel value of the cell part (black part) of a two-dimensional code is 120 and the pixel value of the space part (white part) is 130. In this case, by converting using the contrast enhancement characteristic, the pixel value of the cell portion (black portion) becomes 23 and the pixel value of the space portion (white portion) becomes 164 as output values, so that the contrast can be emphasized.

However, there is a limit to the range that can be supported even with such general HDR conversion, especially when conveyed objects with large height differences such as those handled in the logistics industry, when the reading distance is long, or when there are large variations in brightness. was still insufficient for stable code reading.

For example, for an image taken with an image sensor, a 12-bit (4096 tones) input value for each pixel that makes up the image data is converted to 8 bits (256 tones) using a certain conversion characteristic and output. think. The conversion formula in this case becomes a graph as shown in FIG. In addition, a graph in which the horizontal axis is the brightness value of the white part of the code at this time and the vertical axis shows the difference in brightness value from the white part to the black part, that is, the contrast can be obtained, is as shown in Figure 14. Become.

In order to read a code stably, a certain degree of contrast (brightness difference between the white and black parts of the code) is required. Here, the contrast value threshold required for reading the code is called edge strength. When this edge strength is high, the code is easy to recognize and can be read at high speed, but when the brightness value changes, it quickly becomes impossible to read. That is, the ability to respond to changes in the brightness value of the object to be read is weak. On the other hand, when the edge strength is low, the contrast value of the code is buried in noise, making it difficult to read. Generally, a certain amount of noise components included in an image sensor such as a CMOS sensor used in the image capturing section 5 are superimposed on the captured image, so if the edge strength is made too low, reading becomes difficult. In other words, it is not suitable for high-speed reading. On the other hand, the ability to respond to changes in brightness values is improved. In this way, there is a trade-off relationship between high-speed reading and the ability to respond to changes in luminance value, so it is important to set an appropriate threshold value.

Here, FIGS. 15 and 16 show graphs in which noise components of the image sensor are overlapped with diagonal lines on the graph of FIG. 14. Here, a case is shown in which a general code (for example, PCS=0.85 and MTF=0.15 is referred to as a "standard contrast code") is read at standard brightness. Here, PCS (Print Contrast Signal) is an index representing the quality of a barcode, generally the contrast. It is calculated as PCS={(white reflectance)-(black reflectance)}/(white reflectance), and the ideal code is PCS=1.0. Further, MTF (Modulation Transfer Function) is an index indicating the degree of blur when capturing an image. The MTF is not a code, but a value determined by the lens and focus position. MTF=0.15 indicates that 100% contrast would be obtained without blur, but only 15% contrast would be obtained due to blur. Furthermore, the edge strength is set to 6 (indicated by a broken line on the vertical axis). FIG. 15 is a semi-log graph showing the dynamic range when a standard contrast code is read using the standard conversion characteristics shown in FIG. 14, and FIG. 16 is a semi-log graph showing the dynamic range when reading a standard contrast code using the HDR conversion characteristics shown in FIG. It is a logarithmic graph. As shown in these figures, although it is difficult to obtain a large contrast with the HDR curve than with the standard curve, it is possible to secure a wide dynamic range. For example, in the case of the standard curve shown by the solid line in FIG. 15, reading is possible in a range of brightness values from 512 to 4096, and the dynamic range is eight times as large. Furthermore, in the case of the HDR curve shown by the broken line in FIG. 16, it is possible to read the luminance value in a range of 256 to 4096, and the dynamic range is 16 times larger.

However, if the object to be read is a low contrast code (for example, PCS=0.70 and MTF=0.12 is called a "low contrast code"), the results will be as shown in FIGS. 17 and 18. Compared to the standard contrast code, the low contrast code has a lower PCS from 0.85 to 0.70, making it a code with lower contrast. Furthermore, the MTF has decreased from 0.15 to 0.12, assuming that the image was captured under conditions where the out-of-focus condition becomes even greater. For example, in the case of the standard curve shown by the solid line in Fig. 17, it is possible to read within a luminance value range of 700 to 4096, which is narrower than the dynamic range of the standard contrast code in Fig. 15 (indicated by the broken line arrow in Fig. 17). I understand that. Furthermore, in the case of the HDR curve shown by the broken line in Fig. 18, it is possible to read the brightness value in the range of 512 to 4096, which is narrower than the dynamic range of the standard contrast code in Fig. 16 (indicated by the arrow shown by the broken line in Fig. 18). I know that there is.

Thus, low-contrast codes have a reduced dynamic range compared to standard codes, resulting in less stable reading. Therefore, there is a need for an optical information reading device that can ensure a wide dynamic range so as to be able to handle such low contrast codes.

Therefore, in the optical information reading device according to the present embodiment, a conversion characteristic is provided in which the slope doubles when the brightness is halved. This makes it possible to maintain contrast even if brightness changes. Specifically, the conversion characteristic equation is set so that the output value is zero when the input value of the imaging unit 5 is in the range from 0 to a predetermined value. The input value range from 0 to a predetermined value is a level of darkness that makes it impossible to read the code, and it is set depending on the environment in which the optical information reader is installed, such as the surrounding brightness and the required accuracy. be done. Originally, for brightness that makes it impossible to read codes, even if there is an output value, it is virtually unusable, so there is no practical problem even if it is set to zero. Further, in the present invention, the output value is not necessarily limited to zero, but may be a predetermined value. This predetermined value is also preferably a fixed value set in a low luminance range that is substantially difficult to read. For example, in the case of 8 bits and 256 gradations, the range is 0 to 50, which is 1/5 or less of the number of gradations. Further, the value is not limited to a fixed value, but may be a variable value. However, the upper limit of the variable value is set to a luminance value such as the above-mentioned 0 to 50, which makes it virtually difficult to read the code.

Here, as shown in Figure 19, conversion characteristic curves for HDR1, 2, and 3 are prepared as conversion characteristics for converting an input value with a high number of bits (for example, 12 bits) to an output value with a low number of bits (for example, 8 bits). are doing. Note that in FIG. 19, for comparison, a standard conversion characteristic for linearly converting an input value into an output value is also shown by a solid line. This standard conversion characteristic passes through the origin of input value 0 and output value 0, whereas HDR1, 2, and 3 do not pass through the origin. This is because the conversion characteristic curves of HDR1, 2, and 3 are offset in the negative direction on the vertical axis, as shown in FIG. 20, which shows a hypothetical region in which the output value is negative in FIG. be. In other words, HDR1, 2, and 3 are offset so that when the input value is 0, the output value is in the negative direction.

By offsetting the conversion characteristics in this way, it is possible to expand the range of brightness and darkness that can be handled, although some of the performance of the image sensor is sacrificed. Such a concept is not normally seen in the technical field, which attempts to take full advantage of the capabilities of an image sensor and reproduce optical images captured with high resolution by cameras and the like. In other words, unlike optical images, this is due to circumstances unique to stationary optical information reading devices. When observing an optical image, output values can be determined even in areas where input values are small. On the other hand, according to the configuration according to the present embodiment, in an area where the input value is small, the brightness value becomes 0 and cannot be expressed at all. In other words, some of the capabilities of the image sensor are discarded. However, in the field of optical information reading, reading the code is important, and for this purpose, the contrast between dark and bright areas (generally white and black areas) is important; dark areas cannot be seen. However, there is no problem in practical use. The present invention adopts a unique configuration that discards brightness values in dark areas, which are not used in optical image observation, by providing conversion characteristics specialized for reading such codes.As a result, it can be used even under various environments. This ensures stable code reading by ensuring contrast.

FIG. 21 is a graph showing the relationship between the brightness value of the white part of the code and the contrast, that is, the difference in brightness between the white part and the black part of the code, for these conversion characteristic curves. In this figure as well, the conversion characteristics are shown by solid lines for comparison.

Furthermore, the dynamic range when reading a general code (PCS=0.85, MTF=0.15) at the standard position using the standard conversion characteristic shown by the solid line in the conversion characteristic curve in FIG. is shown in FIG. In this figure, when the edge strength, which is the reading threshold, is set to 5, the point where the broken line indicating the vertical axis 5 and the standard conversion characteristic curve intersect becomes the lower limit of reading. Therefore, it is possible to read in a range of brightness values from 512 to 4096, and the dynamic range is eight times as large.

On the other hand, FIG. 23 shows the dynamic range when a general code is read at the standard position using the HDR1 conversion characteristic indicated by the broken line in the conversion characteristic curve of FIG. 21. In this figure, the point where the horizontal broken line indicating edge strength 5 intersects with the vertical broken line indicating HDR1 conversion characteristics is the lower limit of reading, so reading is possible within a brightness value range of 64 to 4096. The dynamic range is 64 times greater.

Next, the case where a low contrast code (PCS=0.70, MTF=0.12) is read using the conversion characteristic curve of FIG. 19 is shown in FIGS. 24 and 25. First, among the conversion characteristic curves in FIG. 19, the dynamic range when reading a low contrast code with the standard conversion characteristic is shown in the semi-logarithmic graph of FIG. As shown in this figure, the lower limit of reading is the point where the dashed line indicating edge strength 5 intersects with the standard conversion characteristic, so reading is possible in the range of brightness values from 700 to 4096, and the dynamic range is six times larger. .

On the other hand, among the conversion characteristic curves in FIG. 19, the dynamic range when reading a low contrast code with HDR1 is shown in the semi-logarithmic graph in FIG. As shown in this figure, the lower limit of reading is the point where the broken line indicating edge strength 5 intersects with the solid curve indicating HDR1 conversion characteristics, so reading is possible in the range of brightness values 64 to 4096, and dynamic The range will be 64 times. That is, the same dynamic range as the standard contrast code shown in FIG. 23 can be secured even when reading a low contrast code.

In this way, by selecting HDR1 as the conversion characteristic curve, a wide dynamic range can be ensured. On the other hand, since there is no margin for threshold values and noise, it is difficult to read low-contrast input images with high stability. In order to read a low contrast input image with high stability, HDR2 is selected as the conversion characteristic curve. In this case, the dynamic range is narrower than in HDR1. Because of this trade-off, in order to read with high stability, it is necessary to select appropriate conversion characteristics according to the code to be read and the usage environment.

Therefore, the tuning unit 29c optimizes the trade-off between the available brightness and darkness range, that is, the dynamic range, and the ability to handle low contrast codes, thereby realizing highly stable reading. For example, the tuning unit 29c performs tuning by trying out a plurality of conversion characteristic curves so that the optimum value is determined by ensuring a mutual margin. In other words, by tuning the system to select the optimal conversion formula according to conditions such as the actual conveyed object and surrounding brightness, it is possible to perform appropriate readings under various different environments. become.
(gamma correction)

Here, gamma correction will be explained as an example of a characteristic conversion formula. Gamma correction is a nonlinear correction used for contrast adjustment. When the pixel value of the input image is I _in , the pixel value of the output image is I _out , and the maximum brightness is I _max , the conversion formula for gamma correction is defined by Equation 1.

The gamma correction conversion curve defined by Equation 1 is as shown in FIG. As shown in this figure, when γ>1, the conversion curve becomes an upwardly convex curve. Furthermore, in the image after conversion, the difference in bright areas is small, the difference in dark areas is large, and the contrast becomes brighter overall.

When γ=1, the conversion curve becomes linear, and the contrast of the image after conversion is unchanged.

When γ<1, the conversion curve becomes a downwardly convex curve. Furthermore, in the image after conversion, the difference in bright areas is large, the difference in dark areas is small, and the contrast becomes darker overall.
(HDR conversion formula)

Next, as an example of a characteristic conversion formula using gamma correction, an HDR (gamma correction) conversion formula for converting a 12-bit input value into an 8-bit output value is shown in Equation 2.

In number 2,
Y: Output value X: Input value 1/γ: Gamma coefficient 255: 8 bits 4095: 12 bits.
(Formula for contrast and white brightness value)

The HDR conversion characteristic used in the optical information reading device according to the present embodiment using such gamma correction is expressed by Equation 3.

In number 3,
X: Input value (gradation from 0 to 4095)
Y: Output value (gradation from 0 to 255)
α, β: Parameter values for adjusting the HDR curve are respectively shown.
Here, too, a conversion formula for converting a 12-bit image into an 8-bit image is shown. Note that in Equation 3, when Y<0, Y=0.
(Contrast calculation method)

Also, a method of calculating contrast will be explained. Here, if we define the white luminance value of the 12-bit image as W_12, the black luminance value of the 12-bit image as B_12, the white luminance value of the 8-bit image as W_8, and the black luminance value of the 8-bit image as B_W, then the contrast of the 12-bit image is defined as Contrast=W_12-B_12, and the contrast of an 8-bit image is defined as Contrast=W_8-B_8, respectively.

The white luminance and black luminance after converting a 12-bit image into an 8-bit image are expressed by Equation 4 and Equation 5, respectively.

From the above, the contrast is expressed by equation 6.

However, W_8<0 and B_8<0 are excluded.

By using such an HDR conversion formula, when the brightness of an input image is halved, the brightness is doubled using the conversion characteristics. This allows the slope to be maintained constant. The details will be explained below. First, if the parameter value α in Equation 3 above is replaced by γ, it can be expressed by the following equation.

By differentiating both sides with respect to x, the following equation is obtained.

Here, in order to express the state where the input value x is multiplied by a, if x is replaced by ax, the following formula is obtained.

Transforming this, we get the following formula

Further transformation yields the following equation.

Here, if γ is set to ∞, the following equation is obtained.

In this way, it can be seen that in an ideal state where γ is infinite, when the input value is multiplied by a, the slope (which can be approximated to contrast here) becomes 1/a. Therefore, by using this conversion formula, the contrast can be kept constant regardless of the input value. Furthermore, by adjusting the α and β values so that Y'(x)<1 when Y=0, a conversion formula that can use all 8-bit (256 steps) gradation values is realized.

Furthermore, when the parameter value α in Equation 3 is increased, the magnitude of the output value when the input value is small increases significantly, and the expressive power of the luminance value decreases. In other words, when the parameter value α is increased, the image becomes more horizontal when converted into a contrast value. Therefore, by adjusting the parameter value β, the zero point of the curve is offset and adjusted.
(parameter value α)

In Equation 3, the parameter value α corresponds to the parameter value γ in general gamma correction (Equation 1 and Equation 2).

Here, in Equation 3, the curve of the HDR conversion formula with parameter values α of 4, 8, 16, 32, and 64 is shown in the graph of FIG. Further, FIG. 28 shows a semi-logarithmic graph showing the relationship between the brightness value of the white part of the code and the brightness difference between the white part and the black part, that is, the contrast, using this HDR conversion formula.

In the relationship between the input value and the output value shown in FIG. 27, increasing the parameter value α results in a more upwardly convex HDR curve. In other words, in a section where the input value is small, a small change in the input value results in a large output value. Therefore, the output luminance value is thinned out in that section, and the expressive power of brightness deteriorates, which is not preferable.

Conversely, when the parameter value α is decreased, the HDR curve becomes closer to a straight line. That is, even in the section where the input value is small, gradual input-output conversion is performed, so the expressive power of brightness is high, which is preferable.

On the other hand, regarding the contrast value with respect to the luminance value of the white part of the code shown in FIG. 28, when the parameter value α is increased, the contrast change becomes smaller. Therefore, a certain amount of contrast can be obtained with respect to a wide range of brightness value changes in the white portion of the code, which is preferable.

Conversely, when the parameter value α is decreased, the contrast change increases. That is, the contrast also varies depending on the change in the brightness value of the white part of the code, which is not preferable.

In this way, the parameter value α shows a trade-off relationship in terms of the expressiveness of brightness and the maintenance of contrast against changes in the brightness value of the white portion of the code. Therefore, it is necessary to take these points into account and find the optimum value.

From the viewpoint of contrast uniformity shown in FIG. 28, it is desirable that the contrast change be as small as possible. Therefore, it is desirable that the parameter value α is 8 or more. Further, it is more desirable that the γ value is 32.

On the other hand, from the viewpoint of thinning out the output values shown in FIG. 27, it is desirable that the γ value be as small as possible.

Considering the above viewpoints, in the optical information reading device according to the present embodiment, the parameter value α in Equation 3 is set as a fixed value, and the parameter value β is changed. Here, α=5.

Further, FIG. 29 shows a graph of the HDR conversion formula in which the parameter value β is changed. In this example, HDR when parameter value β is changed in 16 ways: 11, 12, 13, 14, 15, 17, 19, 21, 23, 25, 27, 30, 34, 39, 45, 52. The curves of the conversion equations are shown respectively. The curves of these HDR conversion equations are optimized through tuning. In addition, each graph of FIG. 29 has parameter values β as 11x (-128) = -1408; 12x (-128) = -1536; 13x (-128) = -1664; 14x (-128) = -1792...・This is a change. Furthermore, if the parameter value β is set to a value smaller than 11x (-128) = -1408, the slope of the conversion equation exceeds 1 and cannot be expressed in 8 bits, so such β is omitted from the optimization target. . Further, in FIG. 29, the curve of HDR5_19 shows that α=5 (that is, γ=2 ⁵ =32) and β=19x(−128)=−2432.
(How to optimize HDR during tuning)

Here, a method for optimizing HDR during tuning will be explained based on the flowchart of FIG. 30. First, in step S3001, brightness is tentatively determined.

Next, in step S3002, parameters are initialized. Here, the HDR index value is 0, the parameter value β (index) is 0, and the number of successive successes is 0. An HDR number is assigned to each conversion characteristic curve, and a different value of β is assigned to each conversion characteristic curve. For example, the value of β is assigned so that the smaller the HDR value is, the stronger it is against changes in brightness, but the weaker it is against codes with low contrast, so that the edge strength increases each time the HDR number changes.

Next, in step S3003, the image capturing unit 5 captures an image using the set parameter value β, the converting unit 30 generates a code image, and a reading process is executed in step S3004. Then, in step S3005, it is determined whether reading is possible at the set parameter value β. If reading is not possible, the process advances to step S3006, and the number of successive successes is set to zero. The reason why the number of successive successes is set to 0 in step S3006 is to evaluate that continuous reading is possible. If it cannot be read even once, initialize it to zero to count the number of times it can be read continuously. As mentioned above, if the edge strength is low, there is a trade-off that makes it impossible to read low-contrast images with high stability, but it is resistant to changes in brightness. Therefore, by counting the number of consecutive readings, it is possible to Evaluate whether it is possible to read the code with high stability in response to the contrast of the code. Then, the process advances to step S3007, and the parameter value β is changed. That is, 1 is added to the HDR number. Thereafter, the process returns to step S3003 and the above steps are repeated.

On the other hand, in step S3005, if reading is possible with the set parameter value β, the process advances to step S3008, and the number of consecutive successes is counted. Here, 1 is added to the number of consecutive successes. Then, in step S3009, it is determined whether the number of consecutive successes has reached a predetermined threshold. In this embodiment, the threshold value for the number of consecutive successes is set to 4. If the number of successive successes does not reach the threshold number of successive successes, the parameter value β is increased in step S3007, that is, the HDR number is increased stepwise, and the process returns to step S3003 to repeat the above steps. For example, each time step S3007 is passed, the parameter value β is changed to 11x(-128)=-1408, 12x(-128)=-1536, 13x(-128)=-1664, 14x(-128)=-1792, ...increase in order.

If the number of successive successes exceeds the threshold number of successive successes in step S3009, the process advances to step S3010, and this parameter value β is adopted.
(How to optimize brightness during tuning)

Next, a method of optimizing brightness when performing tuning by the tuning section 29c will be described. As described above, the brightness of the code image is adjusted by adjusting the exposure time, illumination intensity, etc. Here, FIGS. 31A to 31E show how the distribution of brightness values changes when the brightness of the code image is changed. In these figures, the brightness of the code image gradually becomes brighter from FIG. 31A to FIG. 31E. Note that the frequency of the brightness value being less than 0 is added to 0.

As shown in these figures, when the brightness of the captured image is changed to brighter while the HDR curve is fixed, the brightness distribution of the image in the code area (code area) in the image data changes as the brightness value changes. While the spread remains constant, it shifts from dark to bright areas. In view of these characteristics, it is preferable to select brightness such that the distribution of brightness values is centered around the median brightness value of 128, as shown in FIG. 31C.

Based on this tendency, in order to optimally set the brightness during tuning, the brightness values of the white part and the black part of the code area should be distributed in a well-balanced manner. Specifically, after the code image has been successfully read, if the representative value of the bar area (black part) in the code area is set to blackValue, and the representative value of the space area (white part) is set to whiteValue,

intensityBalance=ABS(128-(whiteValue+blackValue/2)
Calculate. It is assumed that the closer the intensityBalance in the above equation is to zero, the better the balance between the brightness values of the white part and the black part is. From this point of view, the tuning unit 29c sets the brightness to be optimal.
(Method of calculating scores used for tuning)

Here, the following equation is used to calculate the score used during actual tuning.

contrast=whiteValue-blackValue;

score=contrast-intensityBalance/2

Here, since the contrast value is a value that does not change with changes in brightness, only the intensityBalance substantially determines the optimal brightness. Note that the reason why the contrast value is included in the score is to express the margin of contrast.

Note that in the above example, robustness is determined based on whether or not reading was successive successively. With this configuration, when successive readings are successful, it is possible to select the parameters up to that point, which has the advantage of reducing the number of parameter changes. However, the present invention is not limited to this method. For example, after changing the parameters once, the parameter that shows the best result among those that were successfully read may be selected. Furthermore, even if reading is not successive successively, robustness can be considered by making a judgment based on the results of conversion characteristic curves of similar parameters.
(Tuning screen)

Tuning for setting optimal parameter values in the tuning unit 29c is performed by, for example, operating an optical information reading program to display a tuning screen on the display unit. An example of such a tuning screen is shown in FIG. 32. In this figure, there are provided an operation area on the left side in which tools such as buttons for various operations are arranged, an image display area on the upper right side for displaying image data, and a graph display area on the lower right side. With the "reading" tab selected, the operation area displays a "start monitoring" button, an "autofocus" button, and a "start tuning" button. In the image display area, image data of the conveyed object including the code, which is actually imaged by the imaging unit 5, is displayed. The graph display area displays a graph showing the relationship between the brightness of the data area obtained when tuning is performed and the score (reading margin), which is the evaluation value obtained when HDR optimization is performed. ing. The higher the reading margin, the more stable the code image can be read. On this screen, when the user presses the "Start Tuning" button, tuning starts for the image data displayed in the image display area, the HDR optimization method shown in FIG. 30 is executed, and the tuning results are are sequentially plotted in the graph display area. Further, a code area recognized as a code from among the image data displayed in the image display area is displayed surrounded by a frame.

Specifically, when the "Start Tuning" button is pressed, black circles are plotted in the graph shown in the graph display area provided at the lower right in FIG. 32 at the locations where the reading has been completed. The horizontal axis of this graph represents the brightness of the image, and the vertical axis represents the reading margin. The brightness of the image is determined by indexing the total brightness (combination of exposure time and gain) preset by the optical information reading device. Furthermore, readability is an index of readability that indicates whether contrast is clearly displayed.

Further, the amount of light is inversely proportional to the square of the distance between the optical information reading device and the conveyed object. Therefore, by determining the brightness at the part where the score peak is the maximum, during operation, the brightness can be set to ensure a well-balanced margin for changes to both the near and far sides. be able to.

Here, FIG. 33 shows image data obtained when the distance to the conveyed object changes using the HDR conversion characteristics according to Example 1 and the HDR conversion of FIG. 10, which is a conventional technique. The left side is a code image generated by HDR conversion in FIG. 10, and the right side is a code image generated by the conversion characteristics optimized by the tuning unit 29c described above. As shown in these figures, it is confirmed that a certain amount of brightness can be maintained even if the distance between the conveyed object and the optical information reading device changes.

The optical information reading device, optical information reading method, optical information reading program, computer readable recording medium, and recorded equipment of the present invention can be used in barcodes, etc. used in warehouses, factories, stores, hospitals, etc. It can be suitably used for stationary code readers, handheld scanners, handheld terminals, business PDAs, etc. that read symbols such as two-dimensional codes to register and verify data.

1...Optical information reading device 2...Device body 2a...Casing 4...Illumination section 5...Imaging section 5a...Imaging element 5b...Optical system 5c...Autofocus mechanism 6...Display section 7...Power connector 8...Signal line connector; 8b ...Ethernet connector; 8c...USB connector 9...Indicator 10...Aimer 11...Select button 12...Enter button 16...First light emitting diode 17...Second light emitting diode 29...Control unit 29a...AF control section 29b...Imaging control section 29c... Tuning section 29e...UI management section 30...Converting section 31...Decoding section 32...Communication section 35...Storage device 35a...Decoding result storage section 35b...Image data storage section 35c...Parameter set storage section 35d...Conversion characteristic storage section 40...Tuning Screen 41...Operation area 42...Image display area 43...Graph display area 44..."Read" tab 45..."Monitor start" button 46..."Autofocus" button 47..."Tuning start" button 100...Computer 100a...Signal line 101 ...PLC
101a...Signal line 1000...Optical information reading system BC...Transportation belt conveyor CD1, CD2...Code CI1, CI2...Code image WK...Work

Claims (11)

An optical information reading device that reads a code attached to a conveyed object moving on a conveyance line,
a lighting section that irradiates the cord with light;
an imaging unit that receives light emitted from the illumination unit and reflected by the code to obtain an input image;
For each pixel constituting the input image obtained by the imaging unit, the number of gradations is calculated from the input value based on a characteristic conversion formula in which the larger the input value, the smaller the change width of the output value with respect to the change in the input value. a conversion unit that calculates the reduced output value and generates a code image in which each pixel has the output value;
a decoding unit that decodes information encoded in the code based on the code image generated by the conversion unit;
Equipped with
The characteristic conversion formula is offset on a graph showing the relationship between the input value and the output value so that the output value is in a negative direction at a position where the input value is 0,
The conversion unit is
For pixels for which the input value is larger than a threshold value determined based on the amount by which the characteristic conversion formula is offset , the characteristic conversion formula is changed based on the magnitude of the input value , and outputting an output value such that the larger the input value becomes from the threshold value, the smaller the change width of the output value with respect to the change in the input value ;
For a pixel whose input value is smaller than the threshold value, the pixel is configured to output a relatively smaller fixed value as an output value than a pixel whose input value is larger than the threshold value, regardless of the characteristic conversion formula. Optical information reading device. The optical information reading device according to claim 1,
Optical information in which the characteristic conversion formula is set such that the luminance value serving as the input value is in the range from 0 to a predetermined value and the luminance value serving as the output value is 1/5 or less of the number of gradations. reading device. The optical information reading device according to claim 1 or 2,
The optical information reading device, wherein the characteristic conversion formula has an upwardly convex curved shape. The optical information reading device according to any one of claims 1 to 3 , further comprising:
comprising a tuning unit for determining a characteristic conversion formula to be used during operation from a plurality of characteristic conversion formulas having different threshold values;
The tuning unit determines the characteristic conversion formula to be used during the operation by referring to each reading result by the decoding unit of the code image generated by the conversion unit based on each of the plurality of characteristic conversion formulas. An optical information reading device configured as follows. The optical information reading device according to claim 1 , further comprising:
A tuning unit is provided for selecting one of a plurality of conversion characteristic curves created in advance with different offset amounts based on the ability to respond to variations in brightness and darkness of the code image and to a code image with a low contrast difference. An optical information reading device. The optical information reading device according to claim 5 ,
The tuning unit decodes each code image obtained by performing gradation conversion in the conversion unit based on the plurality of conversion characteristic curves having different offset amounts in the decoding unit, and reads the reading results. An optical information reading device configured to determine parameters of the plurality of conversion characteristic curves based on. The optical information reading device according to claim 6 ,
The optical information reading device is configured such that the tuning unit selects one of the plurality of conversion characteristic curves by referring to reading results of conversion characteristic curves having similar parameters. . An optical information reading device that reads a code attached to a conveyed object moving on a conveyance line,
a lighting section that irradiates the cord with light;
an imaging unit that receives light emitted from the illumination unit and reflected by the code to obtain an input image;
For each pixel constituting the input image obtained by the imaging unit, the number of gradations is calculated from the input value based on a characteristic conversion formula in which the larger the input value, the smaller the change width of the output value with respect to the change in the input value. a conversion unit that calculates the reduced output value and generates a code image in which each pixel has the output value;
a decoding unit that decodes information encoded in the code based on the code image generated by the conversion unit;
a tuning unit for determining a characteristic conversion formula to be used during operation from the plurality of different characteristic conversion formulas;
Equipped with
The conversion unit is
For pixels for which the input value is larger than a different threshold value for each characteristic conversion formula, outputting an output value that changes based on the magnitude of the input value based on the characteristic conversion formula;
For pixels where the input value is smaller than the threshold, output a relatively smaller fixed value as an output value than pixels where the input value is larger than the threshold;
The tuning unit determines the characteristic conversion formula to be used during the operation by referring to each reading result by the decoding unit of the code image generated by the conversion unit based on each of the plurality of characteristic conversion formulas. An optical information reading device configured as follows. An optical information reading method using an optical information reading device that reads a code attached to a conveyed object moving on a conveyance line,
irradiating the code with illumination light from the illumination unit and receiving the reflected light reflected by the code at the imaging unit to obtain an input image;
For each pixel constituting the obtained input image, the larger the input value, the smaller the change width of the output value with respect to the change in the input value , and at the position where the input value is 0, the output value is in the negative direction. An output value is calculated by reducing the number of gradations from the input value based on a characteristic conversion formula that is offset on a graph showing the relationship between the input value and the output value , so that each pixel has the output value. When generating the code image,
A pixel whose input value is larger than a threshold determined based on the amount by which the characteristic conversion formula is offset is changed based on the magnitude of the input value based on the characteristic conversion formula , and outputting an output value such that the larger the input value is from the threshold, the smaller the range of change in the output value with respect to the change in the input value ;
A pixel whose input value is smaller than the threshold value outputs a relatively smaller constant value as an output value than a pixel whose input value is larger than the threshold value, regardless of the characteristic conversion formula ,
a step of generating a code image having the output value for each pixel in a conversion unit;
a step of decoding information encoded into a code by a decoding unit based on the code image generated by the imaging unit;
Optical information reading method including. An optical information reading program that reads a code using an optical information reading device that reads a code attached to a conveyed object moving on a conveyance line,
A function of emitting illumination light from the illumination unit to the code and receiving the reflected light reflected by the code at the imaging unit to obtain an input image;
For each pixel constituting the obtained input image, the larger the input value, the smaller the change width of the output value with respect to the change in the input value , and at the position where the input value is 0, the output value is in the negative direction. An output value is calculated by reducing the number of gradations from the input value based on a characteristic conversion formula that is offset on a graph showing the relationship between the input value and the output value , so that each pixel has the output value. When generating the code image,
A pixel whose input value is larger than a threshold determined based on the amount by which the characteristic conversion formula is offset is changed based on the magnitude of the input value based on the characteristic conversion formula , and outputting an output value such that the larger the input value is from the threshold, the smaller the range of change in the output value with respect to the change in the input value ;
A pixel whose input value is smaller than the threshold value outputs a relatively smaller constant value as an output value than a pixel whose input value is larger than the threshold value, regardless of the characteristic conversion formula ,
a function of generating a code image having the output value for each pixel in a conversion unit;
a function of decoding information encoded into a code by a decoding unit based on the code image generated by the imaging unit;
An optical information reading program that enables computers to realize A computer-readable recording medium or a device storing the program according to claim 10.

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