JP5960097B2 - Image forming apparatus and image forming method - Google Patents

Description

  The present invention relates to an image forming apparatus and an image forming method, and more particularly to an image forming apparatus and an image forming method for performing gradation correction.

Conventionally, there are image forming apparatuses such as multifunction peripherals (MFPs) capable of printing documents and images and single-function printers.
In the image forming apparatus, it is necessary to calibrate in a predetermined period due to secular change or the like to correct the input / output characteristics of the image forming means (hereinafter referred to as “tone correction”).
For such tone correction, the image forming apparatus can usually print a test chart in which a plurality of color samples (hereinafter referred to as “patches”) having different densities are arranged from the image forming unit.
A user such as a serviceman performs gradation correction by comparing a patch of a printed test chart with a patch of a reference chart that has been printed in advance by visual observation, a colorimeter, a densitometer, or the like.

Further, referring to Patent Document 1, a measurement chart output unit that outputs a measurement chart including a plurality of color patches, a reference chart that includes a plurality of patches with known color densities, and the measurement chart output unit outputs the measurement chart. Captured image data acquisition means for acquiring captured image data including the measurement chart, and patch color density holding means for holding the color density of each patch included in the reference chart in association with patch identification information for identifying each patch And the patch color density held by the patch color density holding unit in association with each patch identification information, and the patch specified by the patch identification information among the captured image data acquired by the captured image data acquisition unit The color density for calculating the color density of each patch of the measurement chart based on the color density of the image data of An image forming apparatus comprising the calculating means is disclosed.
According to the technique of Patent Document 1, color correction can be completed without the need for a colorimeter, a densitometer, or the like by performing tone correction from image data captured by an external imaging device such as a digital camera. Color correction can be performed easily and at low cost.

JP 2006-222552 A

  However, in the technique of Patent Document 1, the accuracy of gradation correction is poor due to image data imaging conditions and image distortion derived from an imaging device such as a digital camera.

  The present invention has been made in view of such a situation, and an object thereof is to provide an image forming apparatus that solves the above-described problems.

ここで、ｆｘ，ｆｙ：焦点距離、ｃｘ，ｃｙ：光軸中心であり、検出された前記格子点の座標（ｘｄ，ｙｄ）は、前記レンズ歪みによって前記レンズ歪みの無い理想状態における前記格子点の座標（ｕ，ｖ）とは異なり、下記の式（２）により示され、

ここで、ｋ１，ｋ２，ｋ３：半径方向の歪み係数、ｐ１，ｐ２：円周方向の歪み係数であり、前記式（１）を前記式（２）に代入した式に、検出された前記格子点の座標（ｘｄ，ｙｄ）の値を代入し、前記半径方向の歪み係数及び前記円周方向の歪み係数を算出し、算出された前記半径方向の歪み係数及び前記円周方向の歪み係数を代入した前記式（２）に、前記画像データの各格子点の座標を代入して算出された座標（ｕ，ｖ）に、検出された前記格子点の座標（ｘｄ，ｙｄ）を移動させることを特徴とする。
本発明の画像形成装置は、前記テストチャート出力手段は、テストチャートの位置を示すマーカーを出力させ、前記射影変換手段は、前記画像データから前記マーカーを検出し、前記マーカーの位置により前記画像データを射影変換することを特徴とする。
本発明の画像形成装置は、前記テストチャート出力手段は、テストチャート内の各パッチの大きさをリファレンスチャート内の各パッチと同じ大きさに出力し、リファレンスチャートをテストチャートに並べて配置させるための位置指定画像を出力することを特徴とする。
本発明の画像形成装置は、前記テストチャート出力手段は、ＣＭＹＫのパッチ列毎に、リファレンスチャートと同一行の色値に対応する長方形形状のパッチを出力し、前記色値特定手段は、ＣＭＹＫのパッチ列毎に、テストチャート内のパッチの色値と対応する濃度のリファレンスチャートのパッチの色値とを掛け合わせ、前記画像形成手段からの出力をリニアにするような値のテーブルを、前記階調補正データとして算出することを特徴とする。
本発明の画像形成方法は、画像形成手段の階調特性に対する階調補正を行う画像形成装置による画像形成方法において、前記画像形成装置は、複数の色のパッチを含むテストチャートを前記画像形成手段に出力させ、テストチャートの前記画像形成手段への出力は、テストチャートに、各パッチを囲う格子状の罫線を出力させ、出力させたテストチャートと、色濃度が既知である複数の色のパッチを含むリファレンスチャートとが同時に撮像された画像データを取得し、取得された前記画像データに含まれるレンズ歪みを補正し、前記レンズ歪みの補正において、前記画像データに含まれる格子状の罫線の交点である格子点の座標を検出し、前記レンズ歪みの無い理想状態における前記格子点の座標を（ｕ，ｖ）とすると、前記（ｕ，ｖ）は、下記の式（１）で示され、

ここで、ｆｘ，ｆｙ：焦点距離、ｃｘ，ｃｙ：光軸中心であり、検出された前記格子点の座標（ｘｄ，ｙｄ）は、前記レンズ歪みによって前記レンズ歪みの無い理想状態における前記格子点の座標（ｕ，ｖ）とは異なり、下記の式（２）により示され、

ここで、ｋ１，ｋ２，ｋ３：半径方向の歪み係数、ｐ１，ｐ２：円周方向の歪み係数であり、前記式（１）を前記式（２）に代入した式に、検出された前記格子点の座標（ｘｄ，ｙｄ）の値を代入し、前記半径方向の歪み係数及び前記円周方向の歪み係数を算出し、算出された前記半径方向の歪み係数及び前記円周方向の歪み係数を代入した前記式（２）に、前記画像データの各格子点の座標を代入して算出された座標（ｕ，ｖ）に、検出された前記格子点の座標（ｘｄ，ｙｄ）を移動させ、前記レンズ歪みが補正された前記画像データを射影変換し、射影変換された前記画像データから、テストチャート内の各パッチの領域及びリファレンスチャート内の各パッチの領域を検出し、検出されたテストチャート内の各パッチの領域及びリファレンスチャート内の各パッチの領域から色値を特定し、特定されたテストチャート内の各パッチの色値及びリファレンスチャート内の各パッチの色値により前記画像形成手段の階調特性に対する階調補正データを作成することを特徴とする。
An image forming apparatus according to the present invention includes a test chart output unit that outputs a test chart including a plurality of color patches to the image forming unit in an image forming apparatus that performs gradation correction on the gradation characteristics of the image forming unit; Image data acquisition means for acquiring image data obtained by simultaneously imaging a test chart output by the test chart output means and a reference chart including a plurality of color patches with known color densities, and the image data acquisition means and distortion correcting means for correcting the lens distortion contained in the acquired image data, a projective transformation means for projective transformation of the image data to which the lens distortion is corrected by strained correcting means, projective transformation by projective transformation means From the image data thus obtained, each patch area in the test chart and each patch in the reference chart. Patch area detecting means for detecting the H area, color value specifying means for specifying a color value from each patch area in the test chart and each patch area in the reference chart detected by the patch area detecting means, Tone correction data creation for creating tone correction data for the tone characteristics of the image forming means based on the color value of each patch in the test chart specified by the color value specifying means and the color value of each patch in the reference chart The test chart output means outputs a grid-like ruled line surrounding each patch to the test chart, and the distortion correction means is a grid point that is an intersection of the grid-like ruled lines included in the image data. And the coordinates of the lattice points in the ideal state without lens distortion are (u, v), the (u, v) can be expressed by the following equation (1). In is shown,

Here, fx, fy: focal length, cx, cy: optical axis center, and the coordinates (xd, yd) of the detected lattice point are the lattice points in the ideal state without the lens distortion due to the lens distortion. Unlike the coordinates (u, v), the following equation (2) shows:

Here, k1, k2, k3: distortion coefficients in the radial direction, p1, p2: distortion coefficients in the circumferential direction, and the detected lattice in the expression obtained by substituting the expression (1) into the expression (2). Substituting the value of the coordinate (xd, yd) of the point, calculating the radial distortion coefficient and the circumferential distortion coefficient, and calculating the calculated radial distortion coefficient and the circumferential distortion coefficient. The coordinates (xd, yd) of the detected grid points are moved to the coordinates (u, v) calculated by substituting the coordinates of the grid points of the image data into the substituted equation (2). It is characterized by.
In the image forming apparatus of the present invention, the test chart output unit outputs a marker indicating the position of the test chart, and the projective conversion unit detects the marker from the image data, and the image data is detected based on the position of the marker. Projective transformation.
In the image forming apparatus of the present invention, the test chart output means outputs the size of each patch in the test chart to the same size as each patch in the reference chart, and arranges the reference chart side by side on the test chart. A position designation image is output.
The image forming apparatus of the present invention, the test chart output means, for each of CMYK patch array, and outputs a patch of rectangular shape corresponding to the color values of the reference chart and the same row, the color values specifying means, the CMYK For each patch row, a table of values for linearizing the output from the image forming means by multiplying the color value of the patch in the test chart by the color value of the patch of the reference chart of the corresponding density is provided. It is calculated as tone correction data.
The image forming method of the present invention is an image forming method by an image forming apparatus that performs gradation correction on the gradation characteristics of the image forming means, wherein the image forming apparatus uses a test chart including a plurality of color patches as the image forming means. The test chart is output to the image forming means by outputting a grid-like ruled line surrounding each patch to the test chart and outputting the test chart and a plurality of color patches with known color densities. The reference chart including the image data obtained simultaneously is acquired, the lens distortion included in the acquired image data is corrected, and the intersection of the grid-like ruled lines included in the image data in the correction of the lens distortion If the coordinates of the lattice points are detected and the coordinates of the lattice points in the ideal state without the lens distortion are (u, v), the (u, v) is Represented by the following formula (1),

Here, fx, fy: focal length, cx, cy: optical axis center, and the coordinates (xd, yd) of the detected lattice point are the lattice points in the ideal state without the lens distortion due to the lens distortion. Unlike the coordinates (u, v), the following equation (2) shows:

Here, k1, k2, k3: distortion coefficients in the radial direction, p1, p2: distortion coefficients in the circumferential direction, and the detected lattice in the expression obtained by substituting the expression (1) into the expression (2). Substituting the value of the coordinate (xd, yd) of the point, calculating the radial distortion coefficient and the circumferential distortion coefficient, and calculating the calculated radial distortion coefficient and the circumferential distortion coefficient. The coordinates (xd, yd) of the detected grid points are moved to the coordinates (u, v) calculated by substituting the coordinates of the grid points of the image data into the substituted equation (2), the lens distortion and projection transformation of the image data corrected, the projective transformed the image data, detects the area of each patch in the region and in the reference chart of each patch in the test chart, the detected test chart Each patch area and referrer A color value is specified from the area of each patch in the process chart, and tone correction is performed for the tone characteristics of the image forming unit based on the color value of each patch in the specified test chart and the color value of each patch in the reference chart It is characterized by creating data.

  According to the present invention, it is possible to provide an image forming apparatus capable of performing gradation correction with high accuracy by correcting lens distortion included in captured image data.

1 is a system configuration diagram according to an embodiment of an image forming apparatus of the present invention. FIG. 2 is a block diagram illustrating a control configuration of the image forming apparatus illustrated in FIG. 1. FIG. 2 is a schematic diagram of the image forming apparatus illustrated in FIG. 1. It is a flowchart of the test chart output and imaging process which concern on embodiment of this invention. It is a conceptual diagram of the test chart output and imaging process shown in FIG. It is a flowchart of the gradation correction process which concerns on embodiment of this invention. It is a flowchart which shows the detail of the distortion correction process shown in FIG. It is a conceptual diagram of the ruled line recognition process shown in FIG. It is a conceptual diagram of the distortion correction application process shown in FIG. It is a flowchart which shows the detail of the projective transformation process shown in FIG. It is a conceptual diagram of the marker recognition process and projective transformation application process shown in FIG. It is a flowchart which shows the detail of the gradation correction data creation process shown in FIG. It is a conceptual diagram of the gradation correction data calculation process shown in FIG.

<Embodiment>
[System Configuration of Image Forming Apparatus 1]
Referring to FIG. 1, the image forming apparatus 1 includes an image processing unit 11, a conveyance unit (a paper feed roller 422, a conveyance roller 44, and a discharge roller 45), a network transmission / reception unit 15, an operation panel unit 16, and an image formation unit. 17 (image forming means), a storage unit 19 and the like are connected to the control unit 10. Further, an external device connection unit 20 for connecting the external device 2 is connected to the control unit 10.
The operation of each unit is controlled by the control unit 10.

The control unit 10 includes a general purpose processor (GPP), a central processing unit (CPU), a micro processing unit (MPU), a digital signal processor (DSP), a graphics processing unit (GPU), and an application specific processor (ASIC). Information processing means such as a processor for a specific application)
The control unit 10 reads out a control program stored in the ROM or HDD of the storage unit 19, develops the control program in the RAM, and executes it to operate as each unit of a functional block described later. Further, the control unit 10 controls the entire apparatus according to predetermined instruction information input from the operation panel unit 16.

The image processing unit 11 is control arithmetic means such as a DSP (Digital Signal Processor) or a GPU (Graphics Processing Unit). The image processing unit 11 is a unit that performs predetermined image processing on image data, and performs various types of image processing such as enlargement / reduction processing, density adjustment, gradation adjustment, and image improvement processing.
The image processing unit 11 can also convert image data received from the external device 2 or an external network into a file unit in a format such as PDF or TIFF, and store the converted data in the storage unit 19 as print data.

The image forming unit 17 is a unit that is obtained from a user terminal (not shown) and stored in the storage unit 19 according to a user output instruction, or causes image formation on recording paper from data captured by the external device 2 It is.
The transport unit transports the recording paper from the paper feed cassette 421 (FIG. 3), causes the image forming unit 17 to form an image, and then transports it to the tray unit 50.
The operations of the transport unit and the image forming unit 17 will be described later.

The network transmission / reception unit 15 is a network connection unit including a LAN board and a wireless transceiver for connecting to an external network. The external network is a wired or wireless Internet, an intranet, a mobile phone network, or the like.
The network transmission / reception unit 15 transmits / receives data on a data communication line and transmits / receives voice signals on a voice telephone line.

  The operation panel unit 16 acquires instructions for various jobs of the image forming apparatus 1 by the user. It is also possible to input and change information of each user according to user instructions acquired from the operation panel unit 16.

The storage unit 19 is a storage unit that uses a semiconductor memory such as a ROM (Read Only Memory) or a RAM (Random Access Memory) or a recording medium such as an HDD (Hard Disk Drive).
The ROM and HDD of the storage unit 19 store a control program for controlling the operation of the image forming apparatus 1 and various data to be described later.
The storage unit 19 includes a temporary file storage area and the like. The storage unit 19 also stores print data, thumbnail images, data of various other files, and the like transmitted from a user terminal (not shown) of the control unit 10.

The external device connection unit 20 is connected to the external device 2. The external device connection unit 20 includes various USB and USB 1394 connectors such as USB 1.1, 2.0 and 3.0, connectors for HDMI (registered trademark), color difference input and video input, wireless LAN and NFC. (Near Field Communication) or Bluetooth (registered trademark) connection circuit, an antenna, a connection control circuit, and the like. The connection control circuit of the external device connection unit 20 includes a host unit that controls access to the external device 2, an N / W interface unit that controls communication with the external device, an interface unit, a video capture unit, and the like.
The external device 2 is a digital camera, video camera, mobile phone, smartphone, tablet, PDA (Personal Data Assistant), web camera, network camera, USB memory, various flash memory cards such as SD card / xD / memory stick, USB connected HDD NFC, RFID (Radio Frequency IDentification) card reader, and the like.

When the external device 2 connected to the external device connection unit 20 has a built-in camera having an imaging device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) imaging sensor, an image is displayed. It functions as an imaging means for imaging and creating image data 500 (FIG. 2).
When the storage device is built in, the external device 2 functions as a storage unit that stores the image data 500 that has already been acquired by a digital camera or the like, like the storage unit 19.

In the above example, the external device connection unit 20 is described as one. However, the number of external device connection units 20 of the image forming apparatus 1 is arbitrary. A configuration in which the external device 2 is connected to a port such as an external USB hub connected to the USB port is also possible.
In the image forming apparatus 1, the control unit 10 and the image processing unit 11 may be integrally formed, such as a CPU with a built-in GPU or a chip-on-module package.
The control unit 10 and the image processing unit 11 may include a RAM, a ROM, a flash memory, and the like.
Further, the image forming apparatus 1 may include a FAX transmission / reception unit that performs facsimile transmission / reception.

[Control Configuration of Image Forming Apparatus 1]
With reference to FIG. 2, the control configuration of the image forming apparatus 1 of the present embodiment will be described based on functional blocks.
The image forming apparatus 1 includes a test chart output unit 100 (test chart output unit), an image data acquisition unit 110 (image data acquisition unit), a distortion correction unit 120 (distortion correction unit), a projection conversion unit 130 (projection conversion unit), A patch area detecting unit 140 (patch area detecting unit), a color value specifying unit 150 (color value specifying unit), and a gradation correction data creating unit 160 (gradation correction data creating unit) are provided.
Further, the storage unit 19 stores image data 500, gradation correction data 510, test chart data 520, and marker data 530.

The test chart output unit 100 causes the image forming unit 17 to output test chart data 520 including a plurality of color patches as a test chart.
Further, the test chart output unit 100 causes the test chart data 520 to output a grid-like ruled line surrounding each patch on the test chart.
In addition, the test chart output unit 100 causes the marker data 530 to output a marker indicating the position of the test chart.
Further, the test chart output unit 100 outputs the size of each patch in the test chart to the same size as each patch in the reference chart based on the test chart data 520. Further, the test chart output unit 100 outputs a position designation image for arranging and arranging the reference chart on the test chart based on the test chart data 520.

  The image data acquisition unit 110 receives, from the external device 2, image data 500 in which the test chart output by the test chart output unit 100 and the reference chart including a plurality of color patches with known color densities are simultaneously captured. get.

The distortion correction unit 120 corrects lens distortion included in the image data 500 acquired by the image data acquisition unit 110.
The distortion correction unit 120 recognizes a grid-like ruled line included in the image data 500, calculates the position of the intersection of the ruled line, and corrects lens distortion.

The projective conversion unit 130 performs projective conversion on the image data 500 whose lens distortion has been corrected by the distortion correction unit 120.
The projective conversion unit 130 detects a marker from the image data 500 and performs projective conversion on the image data 500 based on the position of the marker.

The patch area detection unit 140 detects the area of each patch in the test chart and the area of each patch in the reference chart from the image data 500 subjected to the projective conversion by the projective conversion unit 130.
For example, the patch area detection unit 140 recognizes a shape such as a colored rectangle inside the grid-like ruled line included in the image data 500, and detects the area of each patch from this.

  The color value specifying unit 150 specifies a color value from each patch area in the test chart and each patch area in the reference chart detected by the patch area detection unit 140.

  The gradation correction data creation unit 160 performs gradation correction on the gradation characteristics of the image forming unit 17 based on the color value of each patch in the test chart specified by the color value specifying unit 150 and the color value of each patch in the reference chart. Data 510 is created.

The image data 500 includes a test chart output to the image forming unit 17 by the test chart output unit 100, a reference chart including a plurality of color patches whose colors and densities (hereinafter referred to as “color densities”) are known. Is data of an image simultaneously captured by a digital camera or the like.
The image data 500 may be, for example, an RAW format image format such as a JPEG format or a bitmap of a predetermined format. The image data 500 can acquire the RGB value of each pixel.

The gradation correction data 510 is, for example, a table for correcting the input value of the density output to the image forming unit 17 and the density actually output to the recording paper so as to be substantially linear. is there. The tone correction data 510 includes input / output characteristic correction values for each color of C (Cyan), M (Magenta), Y (Yellow), and K (Key Plate, blacK), for example. The gradation correction data 510 is a correction value for each color. For example, if it is 8 bits, the output value of the density formed by the image forming unit 17 is multiplied by the input value of each density of 0 to 255. This value is included as a table.
Note that the gradation correction data 510 may be in the form of a curve that is complemented at intervals of predetermined input values.

The test chart data 520 is bitmap image data or PDL (Page Description Language) data for drawing a test chart including a plurality of color patches of CMYK colors.
The test chart data 520 includes data indicating the position and color type of each patch. The size of the plurality of color patches in the test chart data 520 is configured to be output to the same size as each patch in the reference chart.
Further, the test chart data 520 includes data of a position designation image for drawing a reference chart frame 650 (FIG. 5) for arranging and arranging the reference chart on the test chart.

The marker data 530 is bitmap image data or PDL data of a marker drawn on the test chart.
The marker data 530 also includes data on the number and position of markers on the test chart. For example, the marker data 530 includes the patch group 640 (FIG. 5), position data of the marker 630 drawn at predetermined positions at the four corners outside the reference chart frame 650, and the like.

Here, the control unit 10 and the image processing unit 11 of the image forming apparatus 1 execute the control program stored in the storage unit 19, so that the test chart output unit 100, the image data acquisition unit 110, the distortion correction unit 120, It functions as a projection conversion unit 130, a patch area detection unit 140, a color value identification unit 150, and a gradation correction data creation unit 160.
Each unit of the image forming apparatus 1 is a hardware resource for executing the image forming method of the present invention.

[Operation of Image Forming Apparatus 1]
The operation of the image forming apparatus 1 according to the embodiment of the present invention will be described with reference to FIG.
The tray unit 50 is disposed on the discharge port 41 side of the recording sheet on which recording is performed by the image forming unit 17, and the operation panel unit 16 is disposed on the front side of the image forming apparatus 1.

The main body 14 includes the image forming unit 17, and includes a paper feeding unit 42, a paper conveyance path 43, a conveyance roller 44, and a discharge roller 45. The paper feed unit 42 includes a plurality of paper feed cassettes 421 that store recording papers of different sizes or orientations, and a paper feed roller 422 that feeds the recording papers one by one from the paper feed cassette 421 to the paper transport path 43. Yes. The paper feed roller 422, the transport roller 44, and the discharge roller 45 function as a transport unit. The recording paper is conveyed by this conveyance unit.
The recording paper fed to the paper transport path 43 by the paper feed roller 422 is transported to the image forming unit 17 by the transport roller 44. Then, the recording paper on which recording has been performed by the image forming unit 17 is discharged to the tray unit 50 by the discharge roller 45.

  The image forming unit 17 includes a photosensitive drum 17a, an exposure unit 17b, a developing unit 17c, a transfer unit 17d, and a fixing unit 17e. The exposure unit 17b is an optical unit including a laser device, a mirror, a lens, an LED array, and the like. The exposure unit 17b outputs light or the like based on image data to expose the photosensitive drum 17a, and the surface of the photosensitive drum 17a is statically exposed. An electrostatic latent image is formed. The developing unit 17c is a developing unit that develops the electrostatic latent image formed on the photosensitive drum 17a using toner, and forms a toner image based on the electrostatic latent image on the photosensitive drum 17a. The transfer unit 17d transfers the toner image formed on the photosensitive drum 17a by the developing unit 17c to a recording sheet. The fixing unit 17e heats the recording paper on which the toner image is transferred by the transfer unit 17d to fix the toner image on the recording paper.

[Test Chart Output and Imaging Process by Image Forming Apparatus 1]
With reference to FIG.4 and FIG.5, the test chart output and imaging process which concern on embodiment of the image forming apparatus 1 of this invention are demonstrated.
In this processing, a test chart including a plurality of color patches is output to the image forming unit 17. Then, a reference chart including a plurality of color patches with known color densities is arranged on the output test chart, and images are simultaneously captured by the external device 2 or another digital camera.
In the processing related to the test chart output, the control unit 10 mainly executes the program stored in the storage unit 19 using hardware resources in cooperation with each unit.
Further, the processing related to imaging is executed by the external device 2 or other digital camera or the like using hardware resources in cooperation with each unit according to a user instruction.

(Step S101)
First, the control unit 10 performs a test chart output process using the test chart output unit 100.
Referring to FIG. 5A, the control unit 10 draws (rasterizes) the image data 500 and creates bitmap data for drawing in the storage unit 19 as a temporary file.
For example, based on the test chart data 520, the control unit 10 draws a ruled line for drawing a patch group 640 including a patch row corresponding to each color of CMYK and a reference chart frame 650. In addition, the control unit 10 draws, for example, a predetermined number of rectangular patches for each density in the patch group 640 for each of CMYK.

(Step S102)
Next, the control unit 10 causes the test chart output unit 100 to perform marker output processing. For example, based on the marker data 530, the control unit 10 draws the marker 630 at a predetermined position outside the patch group 640 and the reference chart frame 650 in the bitmap data for drawing in the storage unit 19. At this time, the control unit 10 may draw a frame line or the like outside each marker.
In addition, the control part 10 can also show the direction of a test chart by drawing the marker 630 of the marker 630 of a different shape in either of four corners.
The control unit 10 may be configured to draw data in which the test chart data 520 and the marker data 530 are collected.

(Step S103)
Next, the control unit 10 performs an image forming process using the test chart output unit 100 and the image forming unit 17.
The control unit 10 causes the image forming unit 17 to form an image after correcting the gradation of the drawing bitmap data stored in the storage unit 19 using the gradation correction data 510. Thereby, the test chart 620 is printed on the recording paper.
At this time, the input value of the density of each patch input to the image forming unit 17 and the density actually output to the recording sheet are different because the gradation correction data 510 is not adjusted.

(Step S104)
Next, the external device 2 or other digital camera performs an imaging process.
5A and 5B, a user such as a service person places the reference chart 700 in the reference chart frame 650 of the test chart 620 printed on the recording paper. The reference chart 700 is, for example, a printed matter in which patches of known color density are offset printed together with ruled lines so as to fit within the reference chart frame 650, and is held by the user.
In this state, the external device 2 or other digital camera or the like acquires a user instruction and performs imaging. As a result, the test chart 620 and the reference chart 700 are captured as one image data 500 at the same time.
In order to avoid the shadows of the user from appearing during the imaging, the test chart 620 placed on a horizontal table can also be imaged from an oblique direction. In addition, in order to capture as large an image as possible, it is also possible to capture with the zoom lens on the zoom side.
The external device 2 or other digital camera or the like converts the captured image into a predetermined format and stores it as image data 500 in a recording medium such as a RAM, flash memory, or HDD.
The test chart output and imaging process according to the embodiment of the present invention is thus completed.

[Tone correction processing by the image forming apparatus 1]
Next, tone correction processing according to the embodiment of the image forming apparatus 1 of the present invention will be described with reference to FIG.
The image forming apparatus 1 acquires the image data 500 captured by the above-described test chart output and imaging processing, corrects distortions derived from the curved surface of the camera lens in the image data 500, and performs projective transformation (projection transformation). After that, each patch area and its color value of the test chart and reference chart are calculated, and tone correction data 510 is created based on this color value.
In this process, the control unit 10 mainly executes a program stored in the storage unit 19 using hardware resources in cooperation with each unit.
Hereinafter, the outline of the gradation correction processing will be described step by step with reference to the flowchart of FIG.

(Step S201)
First, the control unit 10 performs an image data acquisition process using the image data acquisition unit 110.
When the external device 2 is a digital camera or the like and functions as an imaging unit, the control unit 10 acquires the image data 500 captured by the imaging unit and recorded on the recording medium, and stores the image data 500 in the storage unit 19. In addition, the control unit 10 may acquire a captured image by video capture or the like and directly store the acquired image data 500 in the storage unit 19.
In addition, when the external device 2 is a recording medium such as a USB memory, the control unit 10 separately acquires image data 500 that has been captured by a digital camera or the like and stored in the external device 2, and is stored in the storage unit 19. Remember.

(Step S202)
Next, the control unit 10 performs a distortion correction process using the distortion correction unit 120.
The control unit 10 performs processing for correcting lens distortion included in the image data 500. As a result, significant distortion is eliminated at the edge of the image data 500, particularly at the end of the image data 500, and the test chart and the reference chart are in an inclined state.
Details of this processing will be described later.

(Step S203)
Next, the control unit 10 performs projection conversion processing by the projection conversion unit 130.
The control unit 10 performs a projective transformation on the image data 500 whose lens distortion has been corrected. Thereby, the inclination of the image data 500 is eliminated, and the test chart and the reference chart can be easily analyzed.
Details of this processing will also be described later.

(Step S204)
Next, the control unit 10 performs tone correction data creation processing using the patch area detection unit 140, the color value identification unit 150, and the tone correction data creation unit 160.
The control unit 10 performs image analysis of the image data 500 that has undergone the projective transformation, detects each patch area in the test chart and each patch area in the reference chart, and determines the color value of each corresponding patch area. The gradation correction data 510 is created by specifying the color values of the corresponding patches. Thereby, accurate gradation correction can be performed.
Details of this processing will also be described later.
This completes the gradation correction processing according to the embodiment of the present invention.

[Distortion Correction Processing by Image Forming Apparatus 1]
Next, details of the distortion correction processing in step S202 of FIG. 6 will be described with reference to FIGS.
This process is mainly performed by the distortion correction unit 120. Hereinafter, the details of the distortion correction processing will be described step by step with reference to the flowchart of FIG.

(Step S221)
First, the control unit 10 performs ruled line recognition processing.
The control unit 10 recognizes an image of grid-like ruled lines surrounding each patch from the image data 500. In the example of FIG. 5, the ruled lines of the patch group 640 are recognized.
The controller 10 recognizes the ruled line by, for example, detecting an edge from the image data 500 using a Sobel filter or the like, and recognizes a line segment having a predetermined length as a ruled line. At this time, the ruled line is recognized as a curved line due to lens distortion.
Note that the control unit 10 may also recognize the reference chart frame 650 and the ruled lines in the reference chart 700 in the example of FIG.
As for image recognition, the calculation may be speeded up by using a function such as GPU of the image processing unit 11. The same applies to the following processing.

(Step S222)
Next, the control unit 10 performs lattice point coordinate detection processing.
The control unit 10 recognizes the coordinates of the lattice point from the recognized ruled line. Here, the position (coordinate) of the lattice point that is the intersection of the ruled lines recognized as curves is detected. Here, for example, the control unit 10 calculates an intersection point of a curved ruled line and sets it as a detected grid point.
Note that the control unit 10 may also perform the detection of the coordinates of the lattice points on the ruled lines of the reference chart 700 in the example of FIG.

(Step S223)
Next, the control unit 10 performs a distortion correction count calculation process.
Referring to FIG. 8, the lens distortion is a phenomenon in which a straight line does not appear as a straight line because a camera lens such as a digital camera has a curved surface, and is particularly noticeable at the periphery of the image. As the types of lens distortion, there are barrel distortion shown in FIG. 8A and pincushion distortion shown in FIG. 8B. Further, as shown in FIG. 8C, there is also a composite type distortion in which a barrel type distortion and a pincushion type distortion are combined.

  On the image data 500 captured through a lens such as a digital camera, the control unit 10 represents m, which is the coordinates of the lattice points (intersection coordinates on the image plane), by the following equation (1):

In Equation (1), m: intersection coordinates on the image plane, s: scale factor, M: world coordinate system indicating the coordinates of the original lattice points, A: internal camera parameters (3 × 3 matrix), [R | t]: External parameter.

Also, equation (1) can be expressed as the following equation (2):

In Expression (2), fx, fy: focal length, cx, cy: optical axis center are shown.

Here, when the coordinates of the lattice points in an ideal state without distortion are (u, v), the above-described equation (2) can be transformed as the following equation (3).

In practice, due to lens distortion, the lattice points appear at different locations from the ideal state coordinates. That is, the coordinates (xd, yd) of the lattice points acquired by the above-described lattice point coordinate detection process are actually coordinates that deviate from lens distortion.
A lens distortion model including the coordinates (xd, yd) of the lattice points actually acquired in this way is expressed by the following equation (4).

In equation (4), k1, k2, k3: radial distortion coefficients, p1, p2: circumferential distortion coefficients.

Here, the control unit 10 can calculate the distortion coefficients by substituting the values of the coordinates (xd, yd) of the acquired lattice points into the equation obtained by substituting the equation (3) into the equation (4). it can.
Note that the control unit 10 may estimate the distortion coefficient by a method of fitting a ruled line recognized as a curve in the image data 500 to a straight line to be originally projected.

(Step S224)
Next, the control unit 10 performs distortion correction application processing.
The control unit 10 applies the calculated distortion coefficient to the image data 500 and deforms it.
The control unit 10 substitutes the coordinates of each pixel of the image data 500 in the equation (4) in which the calculated distortion coefficient is substituted, and moves this pixel to the calculated position (u, v). Let At this time, the control unit 10 may perform processing such as smoothing.
Referring to FIG. 9, FIG. 9A, which is an image before lens distortion correction, is corrected as shown in FIG. 9B by the calculated distortion coefficient, for example.
Thus, the process according to the embodiment of the present invention is completed.

[Projection Conversion Processing by Image Forming Apparatus 1]
Next, details of the projective transformation process in step S203 of FIG. 6 will be described with reference to FIGS.
This process is mainly performed by the projective transformation unit 130. Hereinafter, the projective transformation process will be described step by step with reference to the flowchart of FIG.

(Step S231)
First, the control unit 10 performs a marker detection process.
The control unit 10 detects a marker indicating the position of the test chart from the image data 500.
Referring to FIG. 11A, the control unit 10 detects the positions of the markers 630 printed at the four corners. Referring to FIG. 11B, the projective conversion unit 130 recognizes the shape of the marker described in the marker data 530 from the image data 500a before the projective conversion in order to detect the marker.
When the shape of the marker or the like is recognized as an image, the control unit 10 calculates the center coordinates of the shape or the like on the image data 500a as the position of the marker 630, for example. At this time, the control unit 10 may also detect a frame line printed outside each marker. The control unit 10 may also recognize the orientation of the test chart 620 based on the recognized marker 630, the position of the frame line, and the like.

(Step S232)
Next, the control unit 10 performs projective transformation matrix calculation processing.
The control unit 10 compares the coordinates of the four corner markers on the image data 500 with the coordinates of the four corner markers from which the marker data 530 is output on the test chart data 520, and calculates a projective transformation matrix.
Here, the coordinates (x ′, y ′) before the projective transformation and the coordinates (x, y) after the projective transformation are obtained by using the projective transformation matrix H having matrix elements a to h, It can be expressed by a relationship such as Equation (6).

The control unit 10 uses the coordinates of the four corner markers detected from the image data 500a before projective transformation in FIG. 11A as coordinates (x1 ′, y1 ′) to (x4 ′, y4 ′) before projective transformation, respectively. Get as. Furthermore, the control unit 10 acquires the coordinates of the four corner marker data 530 on the test chart data 520 in FIG. 11B as the coordinates (x1, y1) to (x4, y4) after the projective transformation.
The control unit 10 substitutes (x1 ′, y1 ′) to (x4 ′, y4 ′) and (x1, y1) to (x4, y4) into the equation (5) or the equation (6), Eight simultaneous equations shown in 7) are solved to calculate matrix elements a to h of the projective transformation matrix H. As described above, by using the coordinates of the markers at the four corners recognized from the image data 500, it is possible to calculate an accurate value of the projective transformation matrix H.

(Step S233)
Next, the control unit 10 performs projective transformation application processing.
The control unit 10 performs actual projective transformation on the image data 500.
Here, the projective transformation matrix H calculated in the above-described step is a matrix for performing projective transformation on the coordinates (x, y) after the projective transformation to the coordinates (x ′, y ′) before the projective transformation. Therefore, the control unit 10 calculates an inverse matrix H ′ of the projective transformation matrix H in order to convert the coordinates (x ′, y ′) before the projective transformation into the coordinates (x, y) after the projective transformation.
The control unit 10 performs projective transformation on each pixel of the image data 500 using the calculated inverse matrix H ′. At this time, the control unit 10 may perform smoothing or the like. In the example of FIG. 11, the control unit 10 can projectively convert the image data 500a of FIG. 11A to obtain an image like the image data 500b of FIG. 11C.
In the example of FIG. 11, for ease of explanation, although the reference chart 700 is not arranged, the control unit 10 performs projective transformation on the image data 500 captured in the actually arranged state. Do.
This completes the projective transformation process according to the embodiment of the present invention.

[Tone Correction Data Creation Processing by Image Forming Apparatus 1]
Next, details of the tone correction data creation processing in step S204 of FIG. 6 will be described with reference to FIGS.
Hereinafter, the gradation correction data creation processing will be described step by step with reference to the flowchart of FIG.

(Step S241)
First, the control unit 10 uses the patch region detection unit 140 to perform patch region detection processing.
The control unit 10 detects each patch area in the test chart and each patch area in the reference chart from the image data 500 subjected to the projective transformation.
The control unit 10 recognizes the image data 500 and detects a patch area as coordinate data indicated by, for example, an approximate rectangle, at a position corresponding to the ruled line of the test chart data 520.
In addition, the control unit 10 recognizes the image data 500 and detects a region of each patch of the reference chart arranged side by side on the test chart. Here, according to the example of FIG. 5, the reference chart 700 may be disposed slightly obliquely in the reference chart frame 650, so the control unit 10 converts the ruled line of the reference chart 700 into a Hough transform. For example, the image may be recognized and rotated, and the area of each patch may be detected.
After detecting the patch areas of the test chart and the reference chart, the control unit 10 temporarily stores them in the storage unit 19 in association with the color densities in the CMYK patch row of the test chart data 520.

(Step S242)
Next, the control unit 10 causes the color value specifying unit 150 to perform color value specifying processing.
The control unit 10 specifies a color value from each patch area in the test chart and each patch area in the reference chart. Specifically, the control unit 10 calculates an average value for each of the RGB components of each pixel in the patch area of the image data 500 and specifies this as a color value.
Note that the control unit 10 can calculate a median or the like from a histogram of RGB components instead of a simple average value, and specify a color value based on the median. Further, the control unit 10 may not use a predetermined ratio of pixels near the boundary of the patch area for calculating the average value. Thereby, an accurate color value can be calculated while suppressing an error near the boundary.

(Step S243)
Next, the control unit 10 performs gradation correction data calculation processing by using the gradation correction data creation unit 160.
Referring to FIG. 13, the control unit 10 determines gradation correction data for the gradation characteristics of the image forming unit 17 based on the color value of each patch in the specified test chart and the color value of each patch in the reference chart. 510 is created.
For each CMYK patch row, the control unit 10 multiplies the color value of the patch in the test chart by the color value of the patch in the reference chart of the corresponding density to linearize the output from the image forming unit 17. A value table is calculated as the gradation correction data 510.

FIG. 13A is a graph showing the concept of the pre-correction input / output characteristic value 810 indicating the characteristic of the color value observed when an image is formed on the recording paper from the image forming unit 17 without being corrected by each density. is there. In each graph of FIG. 13, the horizontal axis represents density and the vertical axis represents color value.
FIG. 13B shows tone correction data 510 calculated by comparing with the patches in the reference chart.
In FIG. 13C, the input / output characteristic value 810 before correction in FIG. 13A is corrected by multiplying it with the calculated gradation correction data 510, and an image is formed on the recording paper from the image forming unit 17. 6 is a graph showing a concept of a corrected input / output characteristic value 820 indicating the characteristic of a color value observed in a case. Thus, the calculated gradation correction data 510 provides input / output density characteristics close to linear.
Thus, the gradation correction data creation process according to the embodiment of the present invention is completed.

With the configuration described above, the following effects can be obtained.
Conventionally, in the technique of performing gradation correction from image data captured by an external device such as a digital camera as described in Patent Document 1, consideration is given to imaging conditions and image distortion derived from the lens of the imaging device. Was not made. For this reason, the accuracy in analyzing the image is lowered, and it has not been possible to perform practically accurate gradation correction.
In contrast, the image forming apparatus 1 of the present invention performs a tone correction on the tone characteristics of the image forming unit 17 and outputs a test chart including a plurality of color patches to the image forming unit 17. 100, an image data acquisition unit 110 that acquires image data 500 obtained by simultaneously capturing a test chart output by the test chart output unit 100 and a reference chart including a plurality of color patches with known color densities; A distortion correction unit 120 that corrects lens distortion included in the image data 500 acquired by the image data acquisition unit 110; a projective conversion unit 130 that performs projective conversion on the image data 500 whose lens distortion has been corrected by the distortion correction unit 120; From the image data 500 subjected to the projective transformation by the projective transformation unit 130, the area of each patch in the test chart and The patch area detection unit 140 that detects the area of each patch in the reference chart, the color value is specified from the area of each patch in the test chart and the area of each patch in the reference chart detected by the patch area detection unit 140 The color value specifying unit 150 and the tone correction data 510 for the tone characteristics of the image forming unit 17 based on the color value of each patch in the test chart specified by the color value specifying unit 150 and the color value of each patch in the reference chart. And a gradation correction data creating unit 160 for creating the image data. With such a configuration, it is possible to correct the lens distortion and projective transformation on the captured image data 500 and accurately detect the patch area. Therefore, the image data 500 can be converted to a state close to the test chart data 520 and compared with the reference chart, and accurate color values of each patch of the test chart and each patch of the reference chart can be calculated. it can. Thereby, even with the image data 500 captured by a digital camera or the like, accurate gradation correction data 510 can be created, and practical gradation correction can be performed. In addition, even when the image data 500 is captured by the external device 2 having an imaging unit with poor gradation characteristics and lens performance, such as a PDA or a smartphone, robust and accurate gradation correction can be performed.
This eliminates the need to manually check the printed patches, measure them with a colorimeter or densitometer, and manually input the tone correction parameters to perform tone correction. , User's trouble can be reduced.
In addition, even when the input / output characteristics of the image forming apparatus 1 change due to aging or the like, gradation correction can be performed easily and accurately. Thus, when it is desired to faithfully output a halftone image such as a photograph, the digital camera or the like can be used for immediate calibration.

Conventionally, there has also been a method of performing tone correction of input / output characteristics by printing a test chart including a patch on a recording sheet and measuring the color value of the patch using a reading device such as a scanner. Although this gradation correction method has high accuracy, it cannot be used in a printer without a reading device.
On the other hand, the image forming apparatus 1 of the present invention can perform gradation correction with high accuracy from the image data 500 captured by an external digital camera or the like even with a printer or the like without a reading device. Further, since a reading device is not required separately, the cost can be reduced.

Conventionally, when measuring lens distortion, a check pattern print is prepared, and distortion is determined from the positions (coordinates) of lattice points in a photographed image.
On the other hand, in the image forming apparatus 1 of the present invention, the test chart output unit 100 causes the test chart to output grid-like ruled lines surrounding each patch, and the distortion correction unit 120 includes the grid included in the image data 500. The lens distortion is corrected by the position of the ruled line.
By adopting such a test chart configuration, (1) separately preparing a check pattern print and measuring lens distortion, and (2) imaging a test chart on which a tone correction patch is printed. It is possible to save time and effort. In other words, gradation correction can be performed from image data 500 obtained by simultaneously capturing gradation correction patches and lens distortion correction images, and the number of imaging can be reduced and calibration can be performed with a simple procedure. It becomes. For this reason, user convenience can be improved.
In addition, by arranging a grid ruled line for distortion correction on the test chart, correction of lens distortion, detection of a patch area, and the like can be performed with the same image data 500. Therefore, high-precision gradation correction data 510 Can be created.

Further, in the image forming apparatus 1 of the present invention, the test chart output unit 100 outputs a marker indicating the position of the test chart, and the projective conversion unit 130 detects the marker from the image data 500, and determines the marker according to the position of the marker. The image data 500 is projectively transformed.
With this configuration, when the image data 500 is imaged, accurate gradation correction can be performed even if the test chart and the reference chart are imaged obliquely from above so that no shadow appears in the test chart.
Further, by performing the transformation so as to be substantially flat by projective transformation, the area of each patch area can be made uniform, and the accuracy of gradation correction can be improved.
Also, by performing projective transformation from the image data 500 after correcting lens distortion, accurate projective transformation can be performed, and the accuracy of gradation correction can be improved compared to performing projective conversion without correcting lens distortion.

In the image forming apparatus 1 of the present invention, the test chart output unit 100 outputs the size of each patch in the test chart to the same size as each patch in the reference chart, and arranges the reference chart side by side on the test chart. With this configuration, the area of each patch area of the test chart and the area of each patch area of the reference chart are unified, and each of the imaging data when the image data 500 is captured is output. The effect of pixel error is reduced. For this reason, the accuracy of gradation correction can be improved.
In addition, since each patch of the reference chart and each patch of the test chart are arranged, each patch can be easily associated with the density. Therefore, the processing cost for gradation correction can be reduced.

[Other Embodiments]
In the above-described embodiment, although it has been described that gradation correction of each color of CMYK colors is performed, it is also possible to perform gradation correction of a printer for monochrome output. It is also possible to perform gradation correction corresponding to any toner of CMYK or a combination of each toner. The gradation correction method of the present embodiment can also be applied to an image forming apparatus that includes a reading device such as a scanner.
In the above-described embodiment, although the patch area is detected after performing the projective transformation, the patch area can be detected without performing the projective transformation. That is, the patch area can be acquired as a deformed rectangle, and the color value can be specified from this area.
Further, when calculating the color value for the patch area calculated from the image data 500 that has been subjected to the lens distortion removal and the projective transformation, the control unit 10 determines the coordinates before performing the projective transformation and the lens distortion removal for each pixel. It is also possible to employ a configuration in which the average value is calculated from the RGB color values of the pixels of the image data 500 before the conversion at this coordinate by inverse conversion or the like. Thereby, it is possible to suppress the influence of image conversion and smoothing, and to create gradation correction data 510 with higher accuracy.
In addition, the control unit 10 further corrects the generated gradation correction data 510 based on information such as the arrangement of RGB pixels of a CCD or CMOS element of a digital camera or the like, white balance included in JPEG exif data, and the like. Also good.
In addition, when the external device 2 is a web camera or the like, the control unit 10 acquires image data of a predetermined format captured by a CCD or CMOS sensor by video capture or the like, and directly stores the image data 500 in the storage unit 19. It may be configured to obtain as That is, even when the external device 2 is a monitoring camera or the like, gradation correction can be performed from the image of the monitoring camera.

  The present invention can also be applied to an information processing apparatus having a USB other than the image forming apparatus. That is, a configuration using a network scanner, a server in which the scanner is separately connected via USB, or the like may be used.

  Further, the configuration and operation of the above-described embodiment are examples, and it goes without saying that they can be appropriately modified and executed without departing from the gist of the present invention.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 External apparatus 10 Control part 11 Image processing part 14 Main body part 15 Network transmission / reception part 16 Operation panel part 17 Image formation part 17a Photosensitive drum 17b Exposure part 17c Development part 17d Transfer part 17e Fixing part 19 Storage part 20 External Device connection section 41 Discharge port 42 Paper feed section 43 Paper transport path 44 Transport roller 45 Discharge roller 50 Tray section 100 Test chart output section 110 Image data acquisition section 120 Distortion correction section 130 Projection conversion section 140 Patch area detection section 150 Color value specification Unit 160 Gradation correction data creation unit 421 Paper feed cassette 422 Paper feed rollers 500, 500a, 500b Image data 510 Gradation correction data 520 Test chart data 530 Marker data 620 Test chart 630 Marker 640 Patch group 650 Reference chart 700 reference chart 810 pre-correction input-output characteristic value 820 correction after input-output characteristic value

Claims (5)

In an image forming apparatus that performs gradation correction on the gradation characteristics of an image forming unit,
Test chart output means for causing the image forming means to output a test chart including a plurality of color patches;
Image data acquisition means for acquiring image data obtained by simultaneously imaging a test chart output by the test chart output means and a reference chart including a plurality of color patches with known color densities;
Distortion correction means for correcting lens distortion included in the image data acquired by the image data acquisition means;
A projective transformation means the lens distortion is projective transformation of the image data corrected by the strained correction means,
Patch area detecting means for detecting the area of each patch in the test chart and the area of each patch in the reference chart from the image data subjected to the projective transformation by the projective transformation means;
Color value specifying means for specifying a color value from the area of each patch in the test chart and the area of each patch in the reference chart detected by the patch area detecting means;
Tone correction data creation for creating tone correction data for the tone characteristics of the image forming means based on the color value of each patch in the test chart specified by the color value specifying means and the color value of each patch in the reference chart and means,
The test chart output means causes the test chart to output a grid-like ruled line surrounding each patch,
The distortion correction means includes
Detecting coordinates of grid points that are intersections of grid-like ruled lines included in the image data;
When the coordinates of the lattice points in the ideal state without lens distortion are (u, v), the (u, v) is expressed by the following equation (1):

Here, fx, fy: focal length, cx, cy: optical axis center,
The coordinates (xd, yd) of the detected lattice points are different from the coordinates (u, v) of the lattice points in the ideal state without the lens distortion due to the lens distortion, and are expressed by the following equation (2). ,

Where k1, k2, k3: radial distortion coefficients, p1, p2: circumferential distortion coefficients,
Substituting the values of the coordinates (xd, yd) of the detected lattice points into the equation obtained by substituting the equation (1) into the equation (2), the radial distortion coefficient and the circumferential distortion coefficient. And the coordinates (u) calculated by substituting the coordinates of each grid point of the image data into the equation (2) in which the calculated radial distortion coefficient and the circumferential distortion coefficient are substituted. , V), the coordinates (xd, yd) of the detected grid point are moved.
An image forming apparatus. The test chart output means outputs a marker indicating the position of the test chart,
The image forming apparatus according to claim 1, wherein the projective conversion unit detects the marker from the image data and performs projective conversion on the image data based on a position of the marker. The test chart output means outputs the size of each patch in the test chart to the same size as each patch in the reference chart, and outputs a position designation image for arranging and arranging the reference chart on the test chart. the image forming apparatus according to claim 1 or 2, characterized. The test chart output means includes
  For each CMYK patch column, output a rectangular patch corresponding to the color value in the same row as the reference chart,
  The color value specifying means includes:
  For each CMYK patch row, a table of values for multiplying the color value of the patch in the test chart by the color value of the patch of the reference chart of the corresponding density and making the output from the image forming means linear, Calculate as the gradation correction data
  The image forming apparatus according to claim 3. In the image forming method by the image forming apparatus that performs gradation correction on the gradation characteristics of the image forming unit, the image forming apparatus includes:
Causing the image forming means to output a test chart including a plurality of color patches;
The output of the test chart to the image forming means is to output a grid-like ruled line surrounding each patch to the test chart,
Obtain image data obtained by simultaneously imaging the output test chart and a reference chart including a plurality of color patches with known color densities,
Correct lens distortion included in the acquired image data,
In correcting the lens distortion,
Detecting coordinates of grid points that are intersections of grid-like ruled lines included in the image data;
When the coordinates of the lattice points in the ideal state without lens distortion are (u, v), the (u, v) is expressed by the following equation (1):

Here, fx, fy: focal length, cx, cy: optical axis center,
The coordinates (xd, yd) of the detected lattice points are different from the coordinates (u, v) of the lattice points in the ideal state without the lens distortion due to the lens distortion, and are expressed by the following equation (2). ,

Where k1, k2, k3: radial distortion coefficients, p1, p2: circumferential distortion coefficients,
Substituting the values of the coordinates (xd, yd) of the detected lattice points into the equation obtained by substituting the equation (1) into the equation (2), the radial distortion coefficient and the circumferential distortion coefficient. And the coordinates (u) calculated by substituting the coordinates of each grid point of the image data into the equation (2) in which the calculated radial distortion coefficient and the circumferential distortion coefficient are substituted. , V), the coordinates (xd, yd) of the detected lattice points are moved,
The image data which the lens distortion is corrected by projective transformation,
From the image data subjected to projective transformation, each patch area in the test chart and each patch area in the reference chart are detected,
Identify the color value from each patch area in the detected test chart and each patch area in the reference chart,
An image forming method, wherein gradation correction data for gradation characteristics of the image forming means is created based on the color value of each patch in the specified test chart and the color value of each patch in the reference chart.

Images