JP6742180B2 - MTF measuring device and its program - Google Patents

Description

The present invention relates to an MTF measuring device for measuring an MTF representing a spatial frequency characteristic of resolution of an image pickup system and a program thereof.

The greatest feature of high-resolution television is high spatial resolution, and the resolution characteristics of the camera are important. As a resolution measurement of a current high-definition camera, as shown in FIG. 6, a method using an inmega cycle chart in which rectangular waves having a plurality of spatial frequencies are spatially arranged is known.

In the method using the inmega cycle chart, a CTF (Contrast Transfer Function) representing the spatial frequency characteristic of the modulation degree of a rectangular wave is read from the waveform monitor. The method using the inmegacycle chart is easy to read visually from the waveform monitor, but the waveform amplitude fluctuates due to the sampling phase and camera noise. Further, in the method using the Inmega cycle chart, since the resolution characteristics of the camera are different between the center and the periphery of the lens, only the rectangular wave response at the center of the chart corresponding to 800 TVL/ph is often measured. Further, in the method using the Inmega cycle chart, it is necessary to accurately frame the imaging angle of view to the chart size in order to obtain a desired spatial frequency characteristic, but since a 4K/8K camera often uses a wide-angle lens, It is unrealistic because it requires a large Inmega chart.

Therefore, the slanted-edge method has been proposed instead of the in-megacycle chart (Patent Documents 1 and 2, Non-Patent Documents 1 and 2). This slanted-edge method is a method in which framing is unnecessary because the chart size is comparatively small, and a slightly tilted edge image is captured, and the spatial frequency characteristic of the modulation degree of a sine wave is represented from the spread of the edge. Calculate MTF (Modulation Transfer Function).

First, in the slanted-edge method, a rectangular region of interest (ROI, FIG. 7) including an edge is selected from a chart image obtained by capturing a chart. Next, in the slanted-edge method, an edge is detected from the region of interest using an algorithm conforming to ISO 12233 (Non-Patent Document 2) or a more accurate algorithm (Patent Document 1, Non-Patent Document 1). Next, in the slanted-edge method, each pixel of the region of interest is projected on a horizontal axis divided at equal intervals by subpixels along the edge inclination. Then, in the slanted-edge method, an average value of pixel values of a plurality of pixels projected in each division is obtained, and an oversampled (in the case of ISO 12233, four-fold oversampling) edge spread function is obtained. Further, in the Slanted-edge method, the line spread function is calculated by differentiating the edge spread function, and the line spread function is Fourier transformed to obtain the absolute value, thereby obtaining the MTF in the band exceeding the sampling frequency from the DC component. ..

JP, 2005-94701, A JP, 2010-237177, A (patent No. 5193113).

K. Masaoka et al., "Modified slanted-edge method and multidirectional modulation transfer function estimation", Optics Express, 22(5), 2014 ISO 12233:2000, "Photography-Electronic Still-picture Cameras-Resolution Measurements"

The spatial frequency characteristic of the resolution of the camera cannot be accurately measured even with a slight focus shift. Therefore, in the measurement of the MTF, it is necessary to finely adjust the focus, repeat the ROI designation and the edge inclination detection at that time, and search for the best spatial frequency characteristic. As described above, in the slanted-edge method, it takes time to specify the ROI and detect the edge inclination, and there is a problem that the MTF measurement becomes complicated.

Therefore, an object of the present invention is to provide an MTF measuring device and a program therefor capable of measuring MTF at high speed.

In view of the above-mentioned problems, the MTF measuring device according to the present invention is an MTF measuring device that measures MTFs representing spatial frequency characteristics of resolution of an imaging system by using charts having different contrasts at boundaries, and ROI image extraction A means, a chart image designating means, an edge inclination detecting means, and an MTF measuring means are provided.

According to this structure, the MTF measuring device extracts the ROI image including the boundary as an edge from each of the plurality of chart images in which the imaging system images the chart with different focus values by the ROI image extracting means. ..
In the MTF measuring device, the chart image designating unit designates a detection target chart image, which is a chart image for which the edge inclination is to be detected, among the plurality of chart images.

The MTF measuring device detects the edge inclination from the ROI image included in the detection target chart image by the edge inclination detecting means.
The MTF measuring device uses the MTF measuring means to generate edge tilt information corresponding to the detected edge tilt, applies the edge tilt information to ROI images included in the plurality of chart images, and uses the slanted-edge method to perform the edge tilt information. Measure MTF.

Here, while finely adjusting the focus, the position and inclination of the edge can be regarded as invariant. Therefore, the MTF measuring device specifies the detection target chart image that is the target of the edge tilt detection when the image is focused to some extent, and detects the edge tilt from the ROI image included in the detection target chart image. On the other hand, the MTF measuring device omits the ROI designation and the edge inclination detection in the chart images other than the detection target chart image.

According to the present invention, the following excellent effects are exhibited.
Since the MTF measuring device according to the present invention omits the time-consuming ROI designation and the edge tilt detection when the edge position and tilt can be regarded as invariant, the MTF can be measured at high speed.

It is a block diagram showing the composition of the MTF measuring device concerning an embodiment. FIG. 6 is an explanatory diagram illustrating a method of generating projection information and an edge profile in the embodiment. FIG. 3 is a diagram showing an example of an edge profile in the embodiment. FIG. 3 is a diagram showing an example of a graph in the embodiment. 3 is a flowchart showing the operation of the MTF measuring device of FIG. 1. It is explanatory drawing explaining the conventional in-megacycle chart. It is explanatory drawing explaining the area of interest in the conventional Slanted-edge method.

(Embodiment)
[Configuration of MTF measuring device]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.
An MTF measuring device 1 according to the embodiment will be described with reference to FIG. 1.

The MTF measuring device 1 measures the MTF representing the spatial frequency characteristic of the resolution of the imaging system, using the chart CH having different contrasts at the boundaries. As shown in FIG. 1, the MTF measuring device 1 includes a chart image storage unit 10, an ROI image extraction unit 20, a chart image designation unit 30, a calculation unit 40, an edge inclination information storage unit 50, and a graph generation unit. And 70. Here, the MTF measuring device 1 connects the imaging system 2 and the display device 3.

The imaging system 2 is, for example, a lens or a camera to be measured.
The display device 3 provides a user interface for operating the MTF measuring device 1 and displays the chart image captured by the imaging system 2. For example, the display device 3 is a display such as a CRT or FPD.

<Capture of chart image and measurement of MTF>
Imaging of a chart image by the imaging system 2 and measurement of MTF by the MTF measuring device 1 will be described.

First, the operator arranges the imaging system 2 and the chart CH so that the chart CH can be imaged in a state in which the chart CH is inclined at a predetermined angle with respect to the direction in which the boundary is orthogonal to the measurement direction of the MTF. For example, when measuring the MTF in the vertical direction, the operator sets the boundary at a predetermined angle from the horizontal direction. On the other hand, when measuring the MTF in the horizontal direction, the operator sets the boundary at a predetermined angle from the vertical direction. This predetermined angle is, for example, about several degrees. In this embodiment, the MTF in the horizontal direction is measured.

Here, the operator may incline the chart CH by a predetermined angle and attach it to the wall surface, or may incline the imaging system 2 by a predetermined angle. Further, the operator may color the black and white pattern on the chart CH by inclining it by a predetermined angle and attach the chart CH to the wall surface without inclining it.

At this time, the operator wants to measure the MTF of the imaging system 2, for example, a central portion, a central right (left, upper, lower) portion, a right (left) diagonally upper portion, a right (left) diagonally lower portion, and the like. The imaging system 2 or the chart CH is arranged so that the black-and-white pattern is located at.

Next, the operator finely adjusts the focus of the image pickup system 2, and the image pickup system 2 takes an image of the chart CH. Then, the MTF measuring device 1 stores the chart image captured by the operator and displays the stored chart image on the display device 3.

Here, the operator designates the chart image when the imaging system 2 is focused on the chart CH to some extent as the detection target chart image to the MTF measuring device 1. For example, the operator can designate the chart image displayed on the display device 3 at that timing as the detection target chart image by pressing the designation button of the display device 3 with a pointing device (not shown) such as a touch pen.

The operator can arbitrarily specify the detection target chart image used for detecting the edge inclination from the chart images included in the captured image. For example, the operator can specify the chart image when the focus is achieved to some extent as the detection target chart image. Further, the operator may specify the chart image of the first frame of the captured video as the detection target chart image.

Subsequently, the MTF measuring device 1 detects the edge inclination from the ROI image extracted from the detection target chart image. Then, the MTF measuring device 1 generates and stores edge tilt information indicating the detected edge tilt. Further, the MTF measuring device 1 measures the MTF using the generated edge inclination information and presents the operator with a graph showing the measurement result of the MTF.

Next, the operator finely adjusts the focus of the image pickup system 2, and the image pickup system 2 takes an image of the chart CH. Then, the MTF measuring device 1 overwrites and stores the newly captured chart image, and displays the overwritten and stored chart image on the display device 3.

Subsequently, the MTF measuring device 1 similarly extracts the ROI image from the newly captured chart image. At this time, the MTF measuring device 1 measures the MTF by utilizing the already stored edge tilt information without detecting the edge tilt from the newly captured ROI image. Then, the MTF measuring device 1 presents a graph showing the measurement result of the MTF to the operator. That is, the MTF measuring device 1 does not detect the edge inclination from all ROI images.

The operator repeats the imaging of the chart CH and the measurement of the MTF while referring to the presented graph until the spatial frequency characteristic of the resolution of the imaging system 2 becomes the best. That is, the operator repeats the imaging of the chart CH and the measurement of the MTF while finely adjusting the focus of the imaging system 2 to search for the best spatial frequency characteristic. Then, when the operator determines that the spatial frequency characteristic is the best, the operator ends the imaging of the chart CH and the measurement of the MTF.
It should be noted that the operator can end the imaging at any timing.

The configuration of the MTF measuring device 1 will be described below.
The chart image storage means 10 stores an image (chart image) obtained by imaging the chart CH for MTF measurement in the imaging system 2 to be measured. The chart image storage means 10 sequentially receives the chart images captured with different focus values from the imaging system 2, and overwrites and stores the input chart images. That is, when a new chart image is input from the imaging system 2, the chart image storage unit 10 overwrites the already stored chart image with the new chart image. For example, the chart image storage means 10 is a general storage device such as a hard disk or a memory.

The chart image stored in the chart image storage unit 10 is displayed by the display device 3 and read by the ROI extraction unit 23.

The ROI image extraction means 20 extracts an ROI image including a boundary as an edge from the chart image stored in the chart image storage means 10. The ROI image extracting means 20 includes an ROI specifying means 21 and an ROI extracting means 23.

The ROI designating means 21 designates the ROI in the chart image captured by the image capturing system 2. For example, the operator designates a rectangular area with the pointing device in the chart image displayed by the display device 3, and the ROI designation means 21 acquires the position and size of the ROI. Here, the operator can specify the ROI in any chart image, for example, the ROI in the detection target chart image. Then, the ROI designation means 21 generates ROI designation information indicating the position and size of the acquired ROI, and outputs the generated ROI designation information to the ROI extraction means 23.
As described above, the MTF measuring apparatus 1 can specify the ROI once in any of the chart images and does not need to specify the ROI in the other chart images. Therefore, the labor of the operator can be reduced.

The ROI extraction unit 23 extracts the ROI image from the chart image stored in the chart image storage unit 10. Here, the ROI extraction means 23 reads the chart image from the chart image storage means 10 and extracts the ROI image from the read chart image. For example, the ROI extraction unit 23 extracts the ROI image at the position and size indicated by the ROI designation information input from the ROI designation unit 21. Then, the ROI extraction means 23 outputs the extracted ROI image to the edge inclination detection means 41 and the edge profile generation means 61.
The ROI extraction means 23 uses the already-input ROI designation information until new ROI designation information is input from the ROI designation means 21.

The chart image designating unit 30 designates a detection target chart image, which is a chart image that is a target of edge tilt detection, among the plurality of chart images captured by the imaging system 2. For example, the chart image designating unit 30 designates the chart image displayed by the display device 3 when the designation button is pressed as the detection target chart image. Then, the chart image designating unit 30 notifies the edge inclination detecting unit 41 that the chart image to be detected is designated (detection target chart image designation notification).

The calculation unit 40 performs a calculation required for MTF measurement, and includes an edge inclination detection unit 41 and an MTF measurement unit 60.

The edge tilt detection means 41 detects the edge tilt from the ROI image included in the detection target chart image, and generates edge tilt information corresponding to the detected edge tilt. That is, the edge tilt detection means 41 detects the edge tilt from the ROI image input from the ROI extraction means 23 in response to the detection target chart image designation notification input from the chart image designation means 30. Then, the edge inclination detecting means 41 outputs the detected edge inclination to the edge profile generating means 61.

For example, the edge tilt detection unit 41 calculates the edge tilt from the xy coordinate value of the ROI and its pixel value in a coordinate system (xy coordinate system) having two axes in the horizontal and vertical directions in the chart image. This edge slope can be obtained by, for example, fitting with a normal cumulative density function (see Non-Patent Document 1). Since this fitting is performed to obtain the edge inclination, it is not necessary to fit the image completely. In addition, the fitting can be performed in any pixel structure.

The edge tilt information storage means 50 stores the edge tilt information generated by the edge profile generation means 61 described later. For example, the edge inclination information storage unit 50 is a general storage device such as a hard disk or a memory.

In this way, the MTF measuring device 1 generates edge tilt information only from the ROI image included in the detection target chart image designated by the operator. Then, the MTF measuring device 1 stores the detected edge tilt information, and applies the stored edge tilt information when measuring the MTF on another chart image.

The MTF measuring unit 60 applies the edge tilt information of the edge tilt information storage unit 50 to the ROI image input from the ROI extracting unit 23, and measures the MTF by the slanted-edge method. The MTF measuring means 60 includes an edge profile generating means 61 and an MTF calculating means 63.

The edge profile generation unit 61 generates projection information (edge tilt information) representing the sub-pixel position where each pixel of the ROI image is projected on the coordinate axis along the edge tilt input from the edge tilt detection unit 41. Then, the edge profile generation means 61 writes the generated projection information in the edge inclination information storage means 50. Further, the edge profile generation unit 61 projects the pixel value of each pixel of the ROI images included in the plurality of chart images on the coordinate axis based on the projection information, thereby generating an edge profile that is a function representing the response to the edge. To do.

Here, the edge profile generation means 61 generates projection information only from the ROI image included in the detection target chart image. That is, the edge profile generation means 61 does not generate projection information from ROI images included in other chart images other than the detection target chart image.
Further, the edge profile generation means 61 applies the projection information stored in the edge inclination information storage means 50 not only to the detection target chart image but also to the ROI images included in other chart images to generate the edge profile.
After that, the edge profile generation means 61 outputs the generated edge profile to the MTF calculation means 63.

<Generation of projection information and edge profile>
Generation of projection information and an edge profile will be described with reference to FIGS. 2 and 3 (see FIG. 1 as needed).

First, the generation of projection information will be described. If the position (coordinates) of each pixel of the ROI image of FIG. 2A and the edge inclination θe of FIG. 2B are known, the pixel value of each pixel of the ROI image is projected on the x-axis along the edge e. it can. Therefore, the edge profile generation means 61 obtains a position where the center position of each pixel of the ROI image is projected on the x axis along the edge inclination θe. Then, the edge profile generation unit 61 generates projection information (lookup table) in which the center position of each pixel of the ROI image and the position on the x-axis after projection are associated with each other, and the edge inclination information storage unit 50 stores the projection information. Remember.

Here, since the edge inclination θe does not change in other chart images other than the detection target chart image, the projection information can also be applied to ROI images included in other chart images. Therefore, the edge profile generation means 61 needs to generate the projection information only once when measuring the MTF.

Next, generation of the edge profile will be described. As shown in FIG. 2B, the edge profile generation unit 61 refers to the projection information of the edge inclination information storage unit 50, and sets the ROI at the position on the x-axis corresponding to the center position of each pixel of the ROI image. Project the pixel value of each pixel of the image. Here, the edge profile generation means 61 refers to the projection information stored in the edge inclination information storage means 50 when performing projection with ROI images included in other chart images as well as the detection target chart image.

Then, as shown in FIG. 3, the edge profile generation means 61 averages the pixel values for each x-coordinate bin and generates an edge profile from each ROI image.
The size of the x-axis bin to be averaged is smaller than the pixel. For example, the width of 1/4 of one pixel is the size of the bin.

Returning to FIG. 1, the description of the configuration of the MTF measuring device 1 will be continued.
The MTF calculation means 63 calculates the MTF from the edge profile input from the edge profile generation means 61. Here, the MTF calculating means 63 calculates the MTF for each ROI image using a general Slanted-edge method. That is, the MTF calculating means 63 calculates the MTF by performing a discrete Fourier transform after obtaining the line spread function (LSF) by sequentially differentiating the edge profile.

In this way, the MTF calculation means 63 can obtain the horizontal MTF from the ROI image of FIG. After that, the MTF calculation means 63 outputs the calculated MTF to the graph generation means 70.

The graph generation means 70 graphs the MTF calculated by the MTF calculation means 63. For example, as shown in FIG. 4, the graph generation means 70 plots the MTF at the coordinates where the horizontal axis represents frequency and the vertical axis represents MTF.
After that, the graph generating means 70 outputs the generated graph to the display device 3.

[Operation of MTF measuring device]
The operation of the MTF measuring device 1 will be described with reference to FIG. 5 (see FIG. 1 as needed).
Here, the operator finely adjusts the focus of the imaging system 2, and the imaging system 2 images the chart CH. Then, the MTF measuring device 1 stores the chart image captured by the operator and displays the stored chart image on the display device 3.

The MTF measuring device 1 reads the chart image from the chart image storage means 10 (step S1).

The ROI designating means 21 receives the ROI designation from the operator in the chart image read in step S1. For example, the operator specifies the ROI with the pointing device in the chart image displayed on the display device 3. Then, the ROI designation means 21 generates ROI designation information indicating the position and size of the ROI designated by the operator (step S2).

The ROI extraction means 23 extracts the ROI image having the position and size indicated by the ROI designation information from the chart image read in step S1 (step S3).

The chart image designating unit 30 receives designation of a detection target chart image from the operator. For example, the operator designates the chart image displayed on the display device 3 when the designation button of the display device 3 is pressed, as the detection target chart image (step S4).

The edge tilt detection means 41 detects the edge tilt from the ROI image included in the detection target chart image (step S5).
The edge profile generation means 61 generates edge tilt information (projection information) corresponding to the detected edge tilt and stores the generated projection information in the edge tilt information storage means 50 (step S6).

The edge profile generator 61 refers to the edge inclination information (projection information) stored in step S6 to generate an edge profile. For example, the edge profile generation means 61 averages the pixel value of each pixel of the ROI image extracted from the chart image in each section divided into subpixel intervals on the coordinate axis that is the projection destination (step S7).

The MTF calculation means 63 calculates the MTF from the edge profile generated by the edge profile generation means 61 by the slanted-edge method. For example, the MTF calculating unit 63 calculates the MTF by performing the discrete Fourier transform after obtaining the line spread function (LSF) by sequentially differentiating the edge profile (step S8).

The graph generation means 70 graphs the MTF calculated by the MTF calculation means 63. For example, the graph generating means 70 plots the MTF at the coordinates where the horizontal axis is the frequency and the vertical axis is the MTF (step S9).

The ROI extraction means 23 determines whether or not to end the MTF measurement for all chart images. For example, the ROI extraction means 23 determines whether or not to end the MTF measurement depending on whether or not the end button of the display device 3 has been pressed (step S10).

When the MTF measurement is completed for all the chart images (Yes in step S10), the MTF measurement device 1 ends the process.
When the MTF measurement is not completed for all the chart images (No in step S10), the MTF measuring device 1 proceeds to the process of step S11.

The MTF measuring device 1 reads a new chart image from the chart image storage means 10 (step S11).
The ROI extraction means 23 extracts the ROI image of the position and size indicated by the ROI designation information from the chart image read in step S11 (step S12), and returns to the process of step S7.

With the above operation, the MTF measuring device 1 detects the edge inclination only from the ROI image included in the detection target chart image, and diverts the detected edge inclination when measuring the MTF in another chart image. The detection of the required edge inclination can be omitted. Further, the MTF measuring device 1 does not need to specify the ROI in another chart image once the ROI is specified in any of the chart images.

[Action/effect]
Since the MTF measuring device 1 can omit the time-consuming ROI specification and the detection of the edge inclination when the edge position and the inclination can be regarded as unchanged, the MTF can be measured at high speed.
Furthermore, since the MTF measuring device 1 performs projection using projection information (look-up table), it is possible to quickly generate an edge profile and measure MTF at high speed.

(Modification)
Although the respective embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and includes design changes and the like within a range not departing from the gist of the present invention.

Although the above-described embodiment has been described as displaying the measurement result of the MTF as a graph, the present invention is not limited to this. For example, the MTF measuring device may present the MTF of a representative spatial frequency (for example, 0.37 cycles/pixel) to the operator. Furthermore, the MTF measuring device may output the MTF measurement result as text data.

Although the MTF measuring device is described as independent hardware in the above-described embodiment, the present invention is not limited to this.
For example, by mounting the MTF measuring device on a waveform monitor and connecting a broadcast video camera to the waveform monitor, the MTF can be calculated in almost real time.
In addition, for example, hardware resources such as a CPU, a memory, and a hard disk included in the computer can be realized by an MTF measurement program that causes the MTF measurement device to cooperate with each other. This program may be distributed via a communication line, or may be written and distributed in a recording medium such as a CD-ROM or a flash memory. Then, by connecting a digital camera to this computer and sequentially capturing images taken by the digital camera from the computer, MTF can be measured at high speed.

In the embodiment described above, the detection target chart image is designated by pressing the designation button of the display device, but the present invention is not limited to this. For example, the operator may specify the detection target chart image by operating an operation means such as a button or a switch attached to the imaging system.

Although the above embodiment has been described as measuring the MTF in the horizontal direction, the present invention can also measure MTF in other than the horizontal direction (for example, in the vertical direction).
Furthermore, the present invention can be applied not only to a single direction such as a horizontal direction and a vertical direction but also to a method of measuring MTF in multiple directions as in JP 2010-237177 A, and when there are a plurality of fixed edges, Be more effective.

DESCRIPTION OF SYMBOLS 1 MTF measuring device 10 Chart image storage means 20 ROI image extraction means 21 ROI designation means 23 ROI extraction means 30 Chart image designation means 40 Calculation means 41 Edge tilt detection means 50 Edge tilt information storage means 60 MTF measurement means 61 Edge profile generation means 63 MTF calculation means 70 Graph generation means

Claims (3)

An MTF measuring device for measuring an MTF representing a spatial frequency characteristic of a resolution of an imaging system using a chart having different contrasts at boundaries.
ROI image extraction means for extracting an ROI image including the boundary as an edge from each of a plurality of chart images in which the imaging system images the chart with different focus values,
Of the plurality of chart images, a chart image designating unit that designates a detection target chart image that is a chart image that is a target of detecting edge inclination,
Edge inclination detecting means for detecting the edge inclination from the ROI image included in the detection target chart image;
MTF measuring means for generating edge tilt information corresponding to the detected edge tilt, applying the edge tilt information to ROI images included in the plurality of chart images, and measuring the MTF by the slanted-edge method,
An MTF measuring device comprising: The MTF measuring means is
As the edge tilt information, projection information representing a sub-pixel position where each pixel of the ROI image is projected on a coordinate axis along the detected edge tilt is generated, and the plurality of chart images are generated based on the generated projection information. Edge profile generation means for generating an edge profile that is a function representing the response to the edge by projecting the pixel value of each pixel of the ROI image included in the above onto the coordinate axis;
The MTF measuring device according to claim 1, further comprising: an MTF calculating unit that calculates the MTF from the edge profile. An MTF measurement program for causing a computer to function as the MTF measurement device according to claim 1.