JP2006072624A - Optical reading apparatus and method of compressing image data - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical reading apparatus that can be miniaturized and achieve high speeds of analysis, while reducing the cost of manufacture, without reducing the accuracy of analyzing images taken. <P>SOLUTION: The optical reading apparatus comprises an imaging circuit 11 having a number of light receiving elements arranged therein to amplify and output at each of the light-receiving elements a pixel signal generated according to the amount of light received; a light-receiving element resetting circuit 12 for resetting each row of light-receiving elements and extracting element voltages during the resetting; and an offset compensation circuit 13 that corrects the signal voltages of the pixel signals, according to the element voltages for output as image data. The optical reading apparatus includes an A/D conversion circuit 16 that quantizes and converts the image data into M-bit digital data, and an image data compression circuit 24, that converts the M-bit digital data into N-bit digital data with a smaller number of bits according to a folding line 44 whose inclination is not more than one, with the inclination being smaller on the side where light is received more than on the side where light is received less. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical reading apparatus and an image data compression method, and more specifically, an imaging circuit that amplifies and outputs a pixel signal generated by a plurality of light receiving elements according to the amount of received light for each light receiving element, and each light receiving The present invention relates to an improvement in an optical reading apparatus including a light receiving element reset circuit that resets an element for each row and extracts an element voltage, and an offset compensation circuit that corrects a signal voltage of a pixel signal based on the element voltage.

  In recent years, various two-dimensional codes have been developed that can increase the amount of information compared to conventional barcodes and can reduce the size. Such two-dimensional codes include a stack type symbol in which a plurality of bar codes are stacked, and a matrix type symbol encoded depending on whether the intersection of the matrix is black or white. An image sensing type optical reading device is used as a reading device that images such a two-dimensional code and reads information from the captured image. In this image sensing type optical reading apparatus, image data is analyzed and decoded for each frame. In particular, from the viewpoint of reducing manufacturing cost and power consumption, optical reading devices using CMOS (Complementary Metal Oxide Semiconductor) image sensors tend to be widely used.

  In general, in a CMOS image sensor, a large amount of information is required due to a demand for high resolution in a captured image. For this reason, recently, data output of 10 bits is becoming mainstream. However, as a handy-type two-dimensional code reader, there is a limit to the improvement of the processing capability and the expansion of the memory capacity in the main body of the reading apparatus from the viewpoints of reduction in manufacturing cost, miniaturization, and high-speed image processing. Therefore, even in an optical reading device using a CMOS image sensor with a large amount of information, the output data has a bit number so that downsizing and high-speed image processing can be achieved without increasing the manufacturing cost. An optical reading apparatus that converts image data into smaller digital data and captures image data has been proposed (for example, Patent Document 1).

In the optical reading device disclosed in Patent Document 1, out of M-bit data output by the CMOS image sensor, the remaining N-bit data output is used without using the (MN) bit data output, and the effective image data is output. Is taken in.
Japanese Patent Laid-Open No. 2003-669

  In the conventional optical reading device using the CMOS image sensor as described above, image data is lost for some of the bits, so that there is a problem in that the accuracy of analysis of a captured image is lowered.

  The present invention has been made in view of the above circumstances, and an optical reading apparatus capable of reducing the manufacturing cost, achieving downsizing, and speeding up of the analysis processing without lowering the analysis accuracy of the captured image, and An object of the present invention is to provide an image data compression method.

  An optical reading apparatus according to the present invention includes an image pickup circuit in which a large number of light receiving elements are arranged, a pixel signal generated according to the amount of received light is amplified and output for each light receiving element, and each light receiving element is reset for each row. An optical reading device comprising: a light receiving element reset circuit that extracts an element voltage at the time of resetting; and an offset compensation circuit that corrects the signal voltage of the pixel signal based on the element voltage and outputs it as image data. Based on image data quantization means for quantizing image data and converting it into M-bit digital data, and a conversion characteristic having a slope of 1 or less and a slope on the high light reception side smaller than a slope on the low light reception side, Image data compression means for converting the M-bit digital data into N-bit digital data having a smaller number of bits.

  According to such a configuration, the image data is compressed and converted on the basis of conversion characteristics in which the inclination on the high light reception amount side is smaller than the inclination on the low light reception amount side, so that compression on the low light reception amount side is performed on M-bit image data. The image data can be captured with the rate smaller than the compression rate on the high light receiving amount side. Specifically, the image data compression means is configured to perform digital data conversion processing using two or more broken lines having different inclinations as the conversion characteristics.

  In addition to the above-described configuration, the optical reading device according to the present invention is configured such that the image data compression means performs digital data conversion processing based on a polygonal line whose slope on the low light-receiving amount side is 1. According to such a configuration, since the compression rate on the low light reception amount side is 1 for the M-bit image data, it is possible to effectively suppress a decrease in resolution in the image data on the low light reception amount side.

  The image data compression method according to the present invention includes an imaging circuit that amplifies and outputs a pixel signal generated according to the amount of received light for each light receiving element, resets each light receiving element for each row, and extracts an element voltage at the time of resetting An image data compression method in an optical reading device comprising: a light receiving element reset circuit that performs correction of a signal voltage of the pixel signal based on the element voltage, and outputs the image data as image data. Based on the image data quantization step for quantizing and converting to M-bit digital data, and the conversion characteristics of which the slope is 1 or less and the slope on the high light reception side is smaller than the slope on the low light reception side Digital data is converted into N-bit digital data having a smaller number of bits.

  According to the optical reading device and the image data compression method of the present invention, the image data can be captured with the compression rate on the low light reception amount side smaller than the compression rate on the high light reception amount side for M-bit image data. It is possible to suppress a decrease in resolution in the image data on the low light reception amount side. Therefore, it is possible to reduce the manufacturing cost, to achieve downsizing and speeding up of the analysis process, and to suppress a decrease in the analysis accuracy for the captured image.

Embodiment 1 FIG.
FIG. 1 is an external perspective view showing an example of an optical reading device according to Embodiment 1 of the present invention, in which a handy type two-dimensional code reader 1 equipped with a CMOS image sensor is shown. The two-dimensional code reader 1 according to the present embodiment is a small-sized device that performs analysis processing of a captured image obtained by irradiating light to the reading target A2 printed on the subject A1 and receiving reflected light from the reading target A2. It is an electronic device. As the reading object A2, a one-dimensional barcode is used in addition to a two-dimensional code such as PDF417 or a QR code, and these various codes can be read.

  The two-dimensional code reader 1 includes a head unit 2 on which an image sensor module is arranged, a body unit 3 serving as a reading device main body, and a cable 4 for connecting to an external device. The trunk portion 3 is provided with a two-stage push button type switch 5 on the back side.

  The second-stage pushbutton switch 5 starts power supply to the CMOS image sensor when the first-stage pressing operation is performed, lights up a pointer B for positioning the reading target A2, and reads by the second-stage pressing operation. It is a switch that turns on the main light that illuminates the target A2. That is, the switch 5 is a power switch for turning on / off the power at the first stage, and a release switch for instructing to take in image data.

  Here, it is assumed that the power supply is in an on state during the period in which the first stage is pushed in. When the switch 5 is further pushed in from this state, the second stage input is performed. Further, the switch 5 is not limited to turning on / off the main power supply of the reading device including the CMOS image sensor, but may be one that turns on / off only the power supply for the CMOS image sensor when the main power supply is in an on state. . That is, when the CMOS image sensor is in the sleep state, the sleep state may be canceled by a first-stage pressing operation.

  As an external device connected via the cable 4, a liquid crystal monitor for displaying a captured image and an analysis result on a screen in real time, or a personal computer that performs various controls by taking in image data and an analysis result are used.

  FIG. 2 is a block diagram showing an example of a detailed configuration of the main part in the optical reading apparatus of FIG. 1, and shows an image sensor module including a CMOS image sensor 10, a pointer LD 14, and an illumination LED 15. A pointer LD (Laser Diode) 14 is a light emitting element that generates irradiation light serving as the pointer B, and can display a readable rectangular area and its center position on the subject A1. An illumination LED (Light Emitting Diode) 15 is a light-emitting element serving as a main light that generates irradiation light that illuminates the reading target A2 of the subject A1. These light sources are driven based on a control signal from the reader main body.

  A CMOS (Complementary Metal Oxide Semiconductor) image sensor 10 includes an imaging circuit 11, a light receiving element reset circuit 12, an offset compensation circuit 13, and an A / D conversion circuit 16. The imaging circuit 11 is formed of a rectangular semiconductor chip that outputs a pixel signal corresponding to the amount of received light, and a large number of photodiodes are arranged in a matrix as light receiving elements. Each light receiving element is provided with an amplifying element for amplifying the power of a pixel signal generated according to the amount of received light for each light receiving element, and one pixel is constituted by the light receiving element and the amplifying element.

  The light receiving element reset circuit 12 resets each light receiving element for each row, and extracts an element voltage at the time of resetting. The initialization process for each light receiving element is performed based on a vertical synchronizing signal and a horizontal synchronizing signal from the reading apparatus body, and a reset value for each light receiving element is extracted as an element voltage. That is, the reset of the next row is started by the horizontal synchronization signal, and the next image frame is started by the vertical synchronization signal.

  The offset compensation circuit 13 performs a signal voltage correction process on the pixel signal. This signal voltage correction processing is performed based on the element voltage, and the offset-compensated signal voltage is output as image data. In general, in the CMOS image sensor, since the reset value for each light receiving element varies, offset noise compensation in the signal voltage is performed by comparing the signal voltage of the pixel signal and the reset value. Specifically, a CDS (Correlated Double Sampling) circuit that outputs a difference between signal voltages before and after reset is used.

  The A / D conversion circuit 16 is an image data quantization circuit that converts image data (analog data) from the offset compensation circuit 13 into M-bit digital data. Here, it is assumed that the voltage value is quantized to 10 bits, and the amount of received light is represented by 1024 gradations of light and dark. In addition, these circuits 11 to 13 and 16 are integrally formed on one substrate.

  FIG. 3 is a block diagram showing a configuration example of the details of the main part in the optical reading device of FIG. 1, and starts power supply to the CMOS image sensor 10 by the user's operation input and starts the power supply. 1 shows a main body of a reading apparatus that outputs a horizontal synchronizing signal by shortening a pulse repetition interval until a predetermined period elapses.

  The main body of the reading apparatus is a two-stage push button switch 21, an operation input control unit 22, an I / F control unit 23, an image data compression circuit 24, a main control unit 25, a power supply unit 26, a synchronization signal generation unit 27, a frame. It consists of a number counting unit 28, a gain control unit 29, a captured image decoding unit 30 and a memory 31.

  The operation input control unit 22 performs input control based on the operation input of the two-stage pushbutton switch 21 by the user. That is, power supply to the imaging circuit 11 is started based on the first-stage pressing operation on the switch 21 and a control signal for lighting the pointer LD 14 is output, and illumination is performed based on the second-stage pressing operation. A control signal for turning on the LED 15 is output.

  The I / F control unit 23 performs interface control in input / output of image data, control signals, clock signals, and the like. For example, when a clock signal output from the CMOS image sensor 10 is input to the main control unit 25 or when input / output of image data transmitted between the external device and the main control unit 25 via the cable 4 is performed. Control is performed.

  The image data compression circuit 24 is image data conversion means for converting the quantized image data into image data having a smaller number of bits. That is, an operation of converting M-bit digital data input from the A / D conversion circuit 16 in the CMOS image sensor 10 into N-bit (N <M) digital data and outputting the digital data to the main control unit 25 is performed. Here, it is assumed that 10-bit image data is converted into 8-bit digital data having a smaller number of bits. That is, the 1024 gradation image data is converted into 256 gradation image data whose information amount is 1/4 of the image data before conversion.

  In particular, digital data conversion processing is performed based on conversion characteristics in which the inclination is always 1 or less and the inclination on the high light reception amount side is smaller than the inclination on the low light reception amount side.

The reading target A2 is usually composed of a symbol encoded depending on whether it is black or white and a white area surrounding the symbol A2, and an analysis process based on the binarized light / dark pattern in the photographed image, that is, decoding Processing for conversion is performed. Therefore, since the amount of light received per pixel is lower in the area where black is integrated, from the viewpoint of decoding a code pattern such as a two-dimensional code, an image having a higher light reception amount in a gradation in image data having a lower light reception amount. It becomes more important than the gradation in the data. Therefore, in the present embodiment, an integer from 0 to (2 ^M −1) is changed from 0 to (2 ^N −1) by a curve or a broken line whose slope on the low light reception side is larger than the slope on the high light reception side. Compression conversion of image data corresponding to the integers up to is performed.

Specifically, 1024 gradation image data is converted to 256 gradation image data by two or more broken lines having different inclinations. For example, it is conceivable to convert image data (X) into image data (Y) by the following broken line Y = F (X).
Y = X, (0 ≦ X <128)
Y = 1/2 × (X−128) +128, (128 ≦ X <256)
Y = 1/8 × (X−256) +192, (256 ≦ X <512)
Y = 1/16 × (X−512) +224, (512 ≦ X <1024)

  In the polygonal line Y = F (X), the lowest received light amount side is a straight line with a slope of 1, and the high received light amount side is a straight line with a slope of less than 1. Particularly, 0 to 128 are converted on a one-to-one basis. More specifically, it consists of four straight lines with inclinations of 1, 1/2, 1/8, and 1/16 in order from the side of the lower received light amount.

  With such a polygonal line or curve, it is possible to compress and convert the image data of 10-bit image data by setting the weight on the low light reception amount side higher than the weight on the high light reception amount side. It can be effectively suppressed. That is, by changing the compression rate of the image data in accordance with the amount of received light, the resolution on the high light reception amount side in the photographed image can be appropriately reduced without losing the image data necessary for decoding the two-dimensional code. Therefore, the S / N ratio in the captured image can be improved.

  The power supply unit 26 supplies power to each part of the image sensor module. In particular, the power supply to the amplifying element of the imaging circuit 11 is started by the first-stage pressing operation of the two-stage pushbutton switch 21 by the user. The frame number counting unit 28 is an image frame counting unit that counts the number of output frames of image data based on the vertical synchronization signal.

  The synchronization signal generator 27 generates a horizontal synchronization signal and a vertical synchronization signal. Generation of these synchronization signals is started by the first-stage pressing operation of the two-stage pushbutton switch 21 by the user. In particular, during the period from the start of power supply to the image pickup circuit 11 to the end of the output of image data of a predetermined number of frames, the horizontal synchronization signal is generated by shortening the pulse repetition interval. That is, by shortening the pulse repetition interval of the horizontal synchronization signal during the period, the frame period of the vertical synchronization signal can be shortened.

  Specifically, if the second-stage pressing operation of the switch 21 is not performed within the period from the start of power supply to the imaging circuit 11 until the end of the output of image data of a predetermined number of frames, the second-stage pressing Until the operation is performed, the pulse repetition interval of the horizontal synchronization signal is shortened. That is, when the second-stage pressing operation is performed during the above period, the input by the second-stage pressing operation is invalidated, and the horizontal synchronization signal is shortened until the period elapses. On the other hand, if the second-stage pressing operation is not performed during the above period, the shortening of the horizontal synchronizing signal and the vertical synchronizing signal is canceled by the second-stage pressing operation, and image data capture is started. .

  By assigning an operation input for instructing the main control unit 25 to capture image data to the second-stage pressing operation of the switch 21, image data of a predetermined number of frames from the start of power supply to the imaging circuit 11. When the second-stage pressing operation is not performed within the period until the output ends, the period from the pressing operation to the end of the analysis of the captured image can be shortened. In particular, even if the second-stage pressing operation is performed during the period until the output of the predetermined number of frames of image data, the frame period is shortened until the output of the predetermined number of frames of image data is completed. Therefore, it is possible to effectively shorten the period required for capturing the image data while stabilizing the output of the offset compensation circuit 13.

  The number of output frames for determining the period for shortening the cycle of the synchronization signal is determined from the period required for the output of the CMOS image sensor internal circuit such as the offset compensation circuit 13 to be stabilized. Further, the image data output immediately after the activation of the imaging circuit 11 is not suitable for analysis processing because the light receiving period is not determined. Therefore, the predetermined number of frames is preferably 2 or more. Here, it is assumed that a period of two frames is required for the output of the offset compensation circuit 13 to be stabilized, and the synchronization signal is shortened until the output of three or more image frames is completed.

  The gain control unit 29 adjusts the amplification factor for the pixel signal based on the image data. That is, the gain control unit 29 controls the amplification factor (gain) uniformly for all pixels in order to improve the S / N (signal to noise ratio) of the entire pixel and optimize the dynamic range. Specifically, the gain adjustment is performed so that the brightest pixel to the darkest pixel can be accommodated in 10-bit or 8-bit image data.

  The captured image decoding unit 30 performs a decoding process on the image data in order to decode the encoded reading target A2. This decoding process is performed based on the image data fetched into the memory 31, and in particular, the image data output in the second frame after the period of shortening the pulse repetition interval in the horizontal synchronization signal has been analyzed. .

  Specifically, first, gain adjustment is performed based on the image data in the preparation frame using the image data output in the first frame after the elapse of the shortening period of the horizontal synchronization signal as the preparation frame. Since the preparation frame is an image frame that is output first after returning the period of the horizontal synchronization signal to the original period, the amount of received light differs for each row. Gain adjustment is performed using the image data of such a preparation frame. The received light amount calculation process for gain adjustment is performed for each row.

  Next, analysis processing is performed on the image data in the main imaging frame with the gain-adjusted image data output in the second frame after the elapse of the shortening period of the horizontal synchronization signal as the main imaging frame.

  When reading a two-dimensional code, pattern recognition, positioning, identification of any of various codes and decoding are performed in a captured image. Since the image data output in the second frame after the elapse of the shortening period of the pulse repetition interval is the first image data in which the exposure period of each light receiving element is constant for all rows, the captured image can be analyzed. It is possible to appropriately perform the decoding process while shortening the period required for the completion.

  FIG. 4 is a diagram illustrating an example of a reading operation in the optical reading device of FIG. 1, from when the CMOS image sensor 10 is activated by the first-stage pressing operation of the switch 21 until the decoding of the photographed image is successful. Is shown in time series. First, when the first step of the two-step pushbutton switch 21 is pressed by the user, the CMOS image sensor 10 is activated based on this operation input, and the pointer LD 14 is turned on.

  At this time, a horizontal synchronization signal and a vertical synchronization signal are generated, and vertical scanning from the first row to the m-th row is started in synchronization with the horizontal synchronization signal. In this vertical scanning, the pulse repetition interval of the horizontal synchronization signal is shortened, so that the scanning interval for each row is shortened and the number of output image data is reduced. Here, only the image data in the first column is output during the shortening period of the horizontal synchronizing signal.

  Next, when the second-stage pressing operation of the switch 21 is performed during the period from the start of the sensor to the end of outputting the image data of a predetermined number of frames, the pointer LD 14 is turned off based on this operation input, and the illumination is performed. LED 15 is turned on. At this time, the shortening of the horizontal synchronization signal is canceled based on the completion of the output of the predetermined number of frames of image data, and the output of the image data of the preparation frame is started.

  The image data in the preparation frame is output for all pixels, that is, the image data from the first column to the n-th column is sequentially output, and the received light amount calculation process is performed for each row. Gain adjustment is performed based on the processing result of the received light amount calculation, and image data capture in the next main imaging frame is started.

  When the capture of the image data in the main imaging frame is completed, the illumination LED 15 is turned off and the pointer LD 14 is turned on. At this time, the shortening of the horizontal synchronization signal is started again, and the decoding process for the image data of the main imaging frame is started in the CPU.

  When a reading error occurs in the decoding process, a re-imaging command is output by the CPU, and re-imaging is started based on the re-imaging command. That is, when the illumination LED 15 is turned on by the re-imaging command, the pointer LD 14 is turned off, and when the output of the image data output at the start of lighting of the illumination LED 15 is finished, the shortening of the horizontal synchronization signal is released and preparation is performed. Output of frame image data is started. If the captured image data is successfully decoded and the two-dimensional code is decoded, the reading process ends.

  5 and 6 are timing charts showing the details of the main part in the reading operation of FIG. 4. FIG. 5 shows the state of the horizontal synchronization signal and data output immediately after the start of the sensor, and FIG. The state after the elapse of the shortening period of the horizontal synchronizing signal is shown. Immediately after the activation of the CMOS image sensor 10, since the exposure time is indefinite, the content of the pixel output is also indefinite, but after the elapse of the first predetermined period C2 that becomes the data output preparation period in synchronization with the horizontal synchronization signal, Output of the image data in the first column is started.

  In the pulse repetition interval (C2 + C3) of the horizontal synchronization signal, the first predetermined period C2 is a data output preparation period. During this period, pixel signal sampling, reset of each light receiving element, and reset value sampling are performed in this order. Is performed sequentially. Then, after the sampling of the reset value is completed, the image data of the first column is output in the next output period C3.

  When the output period C3 of the first column of image data ends, the data output preparation period for the next row starts. The image data is output in synchronization with a clock signal (dot clock) generated in the sensor.

  After the elapse of the horizontal synchronization signal shortening period, the image data is output for all the columns in the data output period C4 for each row. Specifically, in the case of a CMOS image sensor composed of pixels of 480 rows × 640 columns, C3 = 1 clock with respect to C2 = 150 clocks and C4 = 640 clocks, and the scanning interval for each row is reduced to about 1/5. It becomes. Here, it is assumed that the blanking period in horizontal scanning and vertical scanning is also shortened, and the frame period immediately after the start of the sensor by shortening the pulse repetition interval and shortening or deleting the blanking period in the horizontal synchronization signal. (6 ms) is shortened to about 1 / 5.5 of the normal frame period (33 ms).

  Steps S101 to S110 in FIG. 7 are flowcharts illustrating an example of a reading operation in the optical reading apparatus in FIG. First, when the first-stage pressing operation of the two-stage pushbutton switch 21 is performed, power supply to each part of the image sensor module is started based on this operation input, and the CMOS image sensor 10 is activated (step S101, S102).

  At this time, the pointer LD 14 is turned on, and a horizontal synchronization signal and a vertical synchronization signal are generated (steps S103 and S104). When the second-stage pressing operation of the switch 21 is performed during the period from the start of the sensor to the end of outputting the image data of a predetermined number of frames, the pointer LD 14 is turned off based on this operation input, and the illumination LED 15 is turned on. Light. In this case, the shortening of the horizontal synchronizing signal is canceled at the end of outputting a predetermined number of frames of image data (steps S105 and S110).

  On the other hand, if the second-stage pressing operation of the switch 21 is not performed during the period from the start of the sensor to the end of outputting the image data of the predetermined number of frames, the operation continues until the second-stage pressing operation is performed. (Step S106). After the shortening of the horizontal synchronization signal is canceled, the image data in the preparation frame is sequentially output, and the received light amount calculation process is performed for each row. Gain adjustment is performed based on the processing result of the received light amount calculation, and image data capture in the next main imaging frame is started (step S107).

  When the capture of the image data in the main imaging frame is completed, the illumination LED 15 is turned off and the pointer LD 14 is turned on. At this time, the shortening of the horizontal synchronization signal is started again, and the decoding process for the image data of the main imaging frame is started in the CPU (step S108).

  If a reading error occurs in the decoding process, a re-imaging command is output by the CPU, and the processes in steps S107 and S108 are repeated (step S109). If the captured image data is successfully decoded and the two-dimensional code is decoded, the reading process ends.

  FIG. 8 is a diagram showing an example of the details of the main part in the optical reading device of FIG. 1. The 10-bit image data input from the CMOS image sensor 41 is compressed and converted to the CPU 43 as 8-bit image data. An FPGA 42 as an output image data compression circuit is shown. The FPGA (Field Programming Gate Array) 42 is a logic circuit whose circuit configuration can be easily changed, and performs an operation of converting 1024 gradation image data into 256 gradation image data in hardware. Yes. As a result, the time required for the compression processing of the image data can be shortened as compared with processing using software using the conversion table.

  FIG. 9 is a diagram showing an example of the image data compression operation in the image data compression circuit of FIG. The 10-bit input data is converted into 8-bit image data by the FPGA 42 based on the above-described broken line Y = F (X). Specifically, (1) each integer value as a received light amount from 0 to 127 is an integer value from 0 to 127, and (2) each integer value from 128 to 255 is an integer value from 128 to 191. Each is associated. Also, (3) each integer value from 256 to 511 is associated with an integer value from 192 to 223, and (4) each integer value from 512 to 1023 is associated with an integer value from 224 to 255, respectively.

  The compression ratios of the image data in the sections (1) to (4) are 1, 1/2, 1/8, and 1/16 in this order. In this way, by making the compression rate on the low light reception amount side lower than the compression rate on the high light reception amount side, it is possible to suppress a decrease in resolution on the low light reception amount side. In particular, the resolution of the lowest received light amount section (1) is 16 times the resolution of the highest received light quantity section (4), and noise generated on the high received light amount side in the image data can be effectively removed. it can.

FIG. 10 is a diagram showing an example of conversion characteristics of image data in the image data compression circuit of FIG. 8, and a broken line Y = F (X) 44 representing output data (Y) with respect to input data (X) is shown. ing. This broken line Y = F (X) 44 approximately matches the curve D shown below.
Y = X, (0 ≦ X <128)
Y = 40.865 × ln (X−87.135) −23.7, (128 ≦ X <1024)

  The curve D described above is a straight line with a slope of 1 from 0 to 128 and a logarithmic curve from 128 to 1024. Pseudo logarithmic conversion (substantially log conversion) is performed on the image data by a broken line Y = F (X) 44 that approximately matches such a curve. Thereby, compression of image data can be performed effectively.

  FIG. 11 is a diagram illustrating an example of a captured image based on image data output from the image data compression circuit of FIG. FIG. 12 is a diagram showing the received light amount distribution on the line EE in the captured image of FIG. 11 before and after compression conversion of image data. The captured image 45 obtained from the reading target A2 includes a code area composed of a streaky dark part 46a and a relatively narrow bright part 46b located between the streaky dark part 46a and both sides of the code area. The white area around the code area is located and includes the bright part 46c which is relatively wider than the bright part 46b.

  In the gain control unit 29, the amplification factor is adjusted so that the image data exists in a region (section (1)) in which the inclination of the polygonal line Y = F (X) 44 is 1. For this reason, the amount of received light (brightness) in the code region is relatively lower than the white background region in the captured image 45. Therefore, in the analysis of the reading target a2, the resolution on the low light reception side is higher than the resolution on the high light reception side. It is effective to perform compression conversion of image data at a higher value.

  In other words, as an example, since the bright portion 46c has a value in the vicinity of the input value data 1023 shown in FIG. 9, a compression ratio of 1/16 is applied, while the bright portion 46b is a bright portion 46c. Therefore, the input value data is 511 or less, and a compression ratio of 1/8 or 1/2 is applied. As for the streaky dark portion 46a, a relatively wide one is near the input value data 0, and a compression ratio (non-compression) of 1/1 is applied. In addition, since the input value data of the other relatively narrow streaky dark portion 46a is located between 255 and 127, it is shown in FIG. It can be understood from the values before and after the compression conversion of the received light amount distribution data shown.

  Thereby, since the noise 47 generated in the bright portion 46c in the image data before conversion is flattened by the compression conversion, the analysis accuracy of the captured image 45 can be improved. That is, in the image processing that analyzes the code pattern by binarizing the luminance in the image data with a predetermined threshold, the high-luminance portion is flattened, so that reading errors can be reduced.

  According to the present embodiment, since the image data can be captured with the compression rate on the low light reception amount side smaller than the compression rate on the high light reception amount side for 10-bit image data, the image data on the low light reception amount side can be captured. It is possible to prevent the resolution from being lowered. Therefore, it is possible to reduce the manufacturing cost, to achieve downsizing and speeding up of the analysis process, and to suppress a decrease in the analysis accuracy for the captured image.

1 is an external perspective view showing an example of an optical reading device according to Embodiment 1 of the present invention. FIG. 2 is a block diagram showing a configuration example of details of main parts in the optical reading device of FIG. 1. FIG. 2 is a block diagram showing a configuration example of details of main parts in the optical reading device of FIG. 1. It is the figure which showed an example of the reading operation | movement in the optical reader of FIG. FIG. 5 is a timing chart showing details of main parts in the reading operation of FIG. 4. FIG. FIG. 5 is a timing chart showing details of main parts in the reading operation of FIG. 4. FIG. 3 is a flowchart illustrating an example of a reading operation in the optical reading device of FIG. 1. It is the figure which showed an example of the principal part detail in the optical reader of FIG. FIG. 9 is a diagram showing an example of an image data compression operation in the image data compression circuit of FIG. 8. FIG. 9 is a diagram illustrating an example of conversion characteristics of image data in the image data compression circuit of FIG. 8. It is the figure which showed an example of the picked-up image by the image data output from the image data compression circuit of FIG. FIG. 12 is a diagram showing a comparison of received light amount distributions on a line segment EE in the captured image of FIG. 11 before and after compression conversion of image data.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Two-dimensional code reader 2 Head part 3 Body part 4 Cables 5, 21 Two-stage push button type switch 10 CMOS image sensor 11 Imaging circuit 12 Light receiving element reset circuit 13 Offset compensation circuit 14 LD for pointer
15 LED for lighting
16 A / D conversion circuit 22 Operation input control unit 23 I / F control unit 24 Image data compression circuit 25 Main control unit 26 Power supply unit 27 Synchronization signal generation unit 28 Frame number counting unit 29 Gain control unit 30 Captured image decoding unit 31 Memory 41 CMOS image sensor 42 FPGA
43 CPU
44 Line 45 Photographed image 46 Light / dark pattern 46a Dark portions 46b, 46c Bright portion 47 Noise A1 Subject A2 Reading object B Pointer

Claims (6)

An imaging circuit in which a large number of light receiving elements are arranged, and a pixel signal generated according to the amount of received light is amplified and output for each light receiving element;
A light receiving element reset circuit that resets each light receiving element for each row and extracts an element voltage at the time of resetting;
In an optical reading device including an offset compensation circuit that corrects the signal voltage of the pixel signal based on the element voltage and outputs the corrected image data as image data.
Image data quantization means for quantizing the image data and converting it into M-bit digital data;
An image in which the M-bit digital data is converted into N-bit digital data having a smaller number of bits based on conversion characteristics having an inclination of 1 or less and an inclination on the high light-receiving amount side being smaller than that on the low light-receiving amount side. An optical reader comprising a data compression unit.   2. The optical reading apparatus according to claim 1, wherein the image data compression means performs digital data conversion processing using two or more broken lines having different inclinations as the conversion characteristics.   3. The optical reading apparatus according to claim 2, wherein the image data compression means performs a digital data conversion process based on a polygonal line with a slope of 1 on the low light reception amount side.   2. The optical apparatus according to claim 1, further comprising image analysis means for performing an analysis process based on the N-bit image data obtained from the reading target encoded by black or white. Reader.   2. The gain control means for adjusting an amplification factor for a pixel signal so that the image data exists in a region having a slope of 1 based on the N-bit image data. The optical reading device described. An imaging circuit that amplifies and outputs a pixel signal generated according to the amount of received light for each light receiving element, a light receiving element reset circuit that resets each light receiving element for each row, and extracts an element voltage at the time of resetting, and the above elements In the image data compression method in the optical reading device comprising the offset compensation circuit that corrects the signal voltage of the pixel signal based on the voltage and outputs the corrected signal as image data.
An image data quantization step for quantizing the image data and converting it into M-bit digital data;
Digital that converts the M-bit digital data into N-bit digital data having a smaller number of bits based on a conversion characteristic in which the slope is 1 or less and the slope on the high light reception side is smaller than the slope on the low light reception side An image data compression method comprising: a data conversion step.

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