JP2019194577A - Mtf measurement device and program - Google Patents

Abstract

To provide an MTF measurement device capable of enhancing MTF measurement accuracy in a slanted-edge method.SOLUTION: An MTF measurement device 1 includes: phase-based edge projection information generation means 14a for generating phase-based edge projection information obtained by projecting each pixel of an ROI image to a plurality of projection axes obtained by shifting the phase of a bin of a subpixel interval along the inclination of an edge of an ROI; phase-based MTF calculation means 14b for calculating a phase-based MTF by using the phase-based edge projection information; averaging means 16 for averaging phase-based MTFs; phase selection means 17 for selecting a phase being the most approximate to the averaged MTF among the phase-based MTFs; and selection phase MTF calculation means 18a for calculating an MTF of the selected phase in an ROI image sequentially extracted from a chart image caught by an imaging system 2 by using edge projection information of the selected phase in the phase-based edge projection information.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an MTF measuring apparatus for measuring an MTF (Modulation Transfer Function) indicating a spatial frequency characteristic of a camera and a program thereof.

The biggest feature of high-resolution television is high spatial resolution, and the resolution characteristics of the camera are important. As a resolution measurement of the current high-definition camera, a method using a test chart in which rectangular waves having a plurality of spatial frequencies are spatially arranged is known. In general, a test chart provided by the Institute of Image Information and Television Engineers (ITE) (ITE high-definition megacycle chart: see FIG. 10) is used for this test chart. It is done.
As shown in FIG. 10, the Inmega cycle chart is a rectangle in which black and white vertical stripes corresponding to video frequencies from 1 MHz to 36 MHz are arranged, and 800 TVL / ph (27.5 MHz) vertical stripes are arranged in the center of the chart and four corner portions. Has a wave pattern.
In the method using the inmega cycle chart, it is common for a measurer to read a CTF (Contrast Transfer Function) representing the spatial frequency characteristics of the modulation degree of a rectangular wave from a waveform monitor.

This method using the inmegacycle chart is easy to read the CTF visually from the waveform monitor, but has the ambiguity that the amplitude of the waveform fluctuates due to the influence of the sampling phase and camera noise. In addition, since the resolution characteristics of the method using the inmega cycle chart are different between the center and the periphery of the lens to be measured, only the rectangular wave response at the center of the chart corresponding to 800 TVL / ph is currently measured.
Further, the technique using the inmega cycle chart needs to accurately frame the imaging angle of view to the chart size in order to obtain a desired spatial frequency characteristic. However, for example, since a wide-angle lens is often used in a 4K / 8K camera, an in-megacycle chart having a large size is required, which is unrealistic.

  Therefore, in recent years, the Slanted-edge method (tilted edge method) has been proposed in place of the method using the inmega cycle chart, which has a relatively small chart size and does not require framing. In the Slanted-edge method, a black and white pattern in which a boundary inclined by several degrees from a vertical (or horizontal) direction with respect to an image sensor is a straight line is captured by a camera, and a predetermined region (ROI: Region Of Interest) ]) (Frequency characteristic (MTF)) using the image (ROI image) (see Patent Documents 1 and 2 and Non-Patent Document 1).

Here, with reference to FIG. 11, the MTF measurement by the conventional Slanted-edge method is demonstrated.
First, in the Slanted-edge method, a region of interest (ROI) R including an edge as shown in FIG. 11A is selected from a chart image (not shown) obtained by imaging a chart.
Next, in the Slanted-edge method, an edge is detected from the region of interest R as shown in FIG. In the Slanted-edge method, each pixel of the region of interest R is projected along the edge e onto a projection axis (x axis) in which bins divided at equal intervals are arranged. At this time, the bin width is set to a sub-pixel width such as 1/4 or 1/8 of the width of one pixel of the region of interest R.

In the Slanted-edge method, the pixel values projected on the bins of the projection axis are averaged in each bin, thereby oversampling as shown in FIG. 11C (4 times oversampling in the case of ISO12233). The obtained edge profile (edge spread function) is obtained.
Further, the Slanted-edge method obtains a line spread function (LSF) as shown in FIG. 11D by differentiating the edge profile.
Finally, the Slanted-edge method obtains the MTF as shown in FIG. 11E by Fourier-transforming the LSF and taking the absolute value.

Japanese Patent Laying-Open No. 2015-94701 JP-A-2018-13416

ISO 12233: 2017, “Photography-Electronic still picture imaging-Resolution and spatial frequency responses”

The discrete system is not a shift-invariant system whose output does not depend on the sampling position, but the Slanted-edge method assumes that the phase of the edge is uniformly distributed with respect to the sampling position, and achieves a super solution with a sufficiently high oversampling ratio. The MTF is calculated from the edge response obtained by imaging.
However, depending on the inclination angle of the edge, when calculating the edge response by oversampling, there is a problem that the measurement accuracy is lowered due to variations in the number of pixels projected on the bin.

  The present invention has been made in view of such a problem, and provides an MTF measuring apparatus and a program thereof capable of increasing MTF measurement accuracy when measuring MTF by the Slanted-edge method. Let it be an issue.

  In order to solve the above-described problem, an MTF measurement apparatus according to the present invention is an MTF measurement apparatus that measures MTFs representing spatial frequency characteristics of resolution of an imaging system using charts having different contrasts at boundaries, and extracts ROI images. And a phase-specific edge projection information generating unit, a phase-specific MTF calculating unit, an averaging unit, a phase selecting unit, and a selected phase MTF calculating unit.

In such a configuration, the MTF measuring apparatus extracts an ROI image that is an image including a boundary from a chart image obtained by capturing an image of the chart by the imaging system by the ROI image extracting unit.
Then, the MTF measurement device uses the phase-specific edge projection information generation unit to shift each pixel of the ROI image along the gradient of the edge that is the boundary to a plurality of projection axes in which the phase (binning phase) of the bins at the subpixel interval is shifted. To generate edge projection information for each phase. As a result, the bin position is associated with each pixel position of the ROI image for each bin phase. In this way, by generating edge projection information with different phases, the number of pixels projected on the bin changes for each phase, and various projection patterns can be reflected.

Then, the MTF measurement apparatus calculates the MTF for each phase by the Slanted-edge method using the edge projection information for each phase by the MTF calculation unit for each phase. As a result, an MTF reflecting various projection patterns can be generated.
Then, the MTF measuring apparatus averages the MTFs for each phase by the averaging means, and selects the phase that most closely approximates the MTF averaged among the MTFs for each phase by the phase selection means. As a result, an optimum phase with the smallest MTF error is selected from among a plurality of phases.

Then, the MTF measuring apparatus assumes that the edge position does not shift in the subsequent MTF measurement, and the phase of the phase selected by the phase selecting unit in the edge projection information for each phase by the selected phase MTF calculating unit. Using the edge projection information, the MTF of the phase selected by the phase selection means is calculated in the ROI image sequentially extracted from the chart image captured by the imaging system by the Slanted-edge method.
As a result, the MTF measurement apparatus can calculate the MTF at high speed by omitting the generation of edge projection information for the chart image input in time series from the imaging system.

  In order to solve the above problem, an MTF measuring apparatus according to the present invention is an MTF measuring apparatus that measures MTFs representing spatial frequency characteristics of resolution of an imaging system using charts having different contrasts at boundaries, An image extraction unit, a phase-specific edge projection information generation unit, a phase-specific MTF calculation unit, and an averaging unit are provided.

In such a configuration, the MTF measuring apparatus extracts an ROI image that is an image including a boundary from a chart image obtained by capturing an image of the chart by the imaging system by the ROI image extracting unit.
Then, the MTF measurement device uses the phase-specific edge projection information generation unit to shift each pixel of the ROI image along the gradient of the edge that is the boundary to a plurality of projection axes in which the phase (binning phase) of the bins at the subpixel interval is shifted. To generate edge projection information for each phase.

Then, the MTF measurement apparatus calculates the MTF for each phase by the Slanted-edge method using the edge projection information for each phase by the MTF calculation unit for each phase.
Further, the MTF measuring device averages the MTFs for each phase by the averaging means.
Thereby, the MTF measuring apparatus can calculate the MTF for the chart image input as a still image from the imaging system or the chart image at a certain point in time of the chart image input in time series.

  Note that the MTF measurement apparatus can be operated by an MTF measurement program for causing a computer to function as each of the means described above.

The present invention has the following excellent effects.
According to the present invention, the MTF measurement accuracy can be improved by using the average of the MTF calculated by shifting the phase of the bin projected on the projection axis or the phase closest to the average.

It is a block block diagram which shows the structure of the MTF measuring apparatus which concerns on embodiment of this invention. It is a figure for demonstrating ROI in a captured image, Comprising: (a) is vertical ROI (vertical edge image), (b) is horizontal ROI (horizontal edge image). It is explanatory drawing for demonstrating a response | compatibility with each pixel position of a ROI image, and the bin of a projection axis. It is explanatory drawing for demonstrating the state which shifted the phase of the bin of the projection axis. It is explanatory drawing for demonstrating the method of producing | generating an edge profile. It is a graph figure of several MTF for demonstrating the average of MTF. It is a flowchart which shows the real-time measurement operation | movement of the MTF measuring apparatus which concerns on embodiment of this invention. It is a flowchart which shows the non-real-time measurement operation | movement of the MTF measuring apparatus which concerns on embodiment of this invention. It is a figure which shows the example of the chart containing a multi-directional edge. It is an in megacycle chart which is an example of the conventional chart. It is explanatory drawing explaining the MTF measurement procedure of the conventional Slanted-edge method by (a)-(e).

Embodiments of the present invention will be described below with reference to the drawings.
[Configuration of MTF measuring device]
Initially, with reference to FIG. 1, the structure of the MTF measuring apparatus 1 which concerns on embodiment of this invention is demonstrated.

The MTF measuring device 1 measures MTF representing the spatial frequency characteristics of the imaging system 2. Here, the MTF measurement device 1 connects the imaging system 2 and the display device 3.
The imaging system 2 is a video camera or a still camera that is an object to be measured by the MTF, a camera including a lens that is an object to be measured by the MTF, or the like. The imaging system 2 may be a video processing device (for example, a down converter or an up converter [super-resolution]) that affects the MTF characteristics. Further, the measurement target may be an image in which the MTF is changed by the DETAIL (detail) control of the camera.
The imaging system 2 outputs an image (moving image or still image) obtained by imaging the chart CH to the MTF measuring apparatus 1.

  The chart (MTF measurement chart) CH is a chart having different contrasts at the boundaries, and is a monochrome pattern in which the boundaries tilted at a predetermined angle from the vertical direction or the horizontal direction with respect to the image sensor (not shown) of the imaging system 2 are straight It is a chart used by the Slanted-edge method which has. The imaging system 2 captures an image in a state where the black-and-white boundary line of the chart CH is inclined at a predetermined angle (about several degrees).

Here, when measuring the frequency characteristics in the horizontal direction, the imaging system 2 images the chart CH in a state where the boundary line is in an obliquely vertical direction as shown in FIG. When measuring the frequency characteristics in the vertical direction, the imaging system 2 images the chart CH with the boundary line in an oblique horizontal direction, as shown in FIG.
Note that the boundary line does not need to be positioned at the center portion of the chart CH, and is an imaging region in which the MTF of the imaging system 2 is to be measured, for example, the center right (left, top, bottom) portion, right (left) diagonally top Part, right (left) diagonally lower part, etc.

The display device 3 provides a user interface for operating the MTF measurement device 1, and displays a chart image captured by the imaging system 2, a graph as a measurement result, and the like. For example, the display device 3 is a liquid crystal display, an organic EL display, or the like.
Note that the display device 3 may be provided with a display device that displays the chart image captured by the imaging system 2 and a display device that displays the measurement result.
Hereinafter, the configuration of the MTF measuring apparatus 1 that measures the MTF of the imaging system 2 from the image captured by the imaging system 2 will be described in detail.

  As shown in FIG. 1, the MTF measuring apparatus 1 includes a chart image storage unit 10, an ROI setting unit 11, a measurement instruction unit 12, an ROI image extraction unit 13, a first MTF calculation unit 14, and an edge projection information storage. Means 15, averaging means 16, phase selection means 17, second MTF calculation means 18, and MTF output means 19 are provided.

The chart image storage means 10 stores an image (chart image) obtained by imaging the chart CH for MTF measurement by the imaging system 2. If the output of the imaging system 2 is a moving image, the chart image storage means 10 inputs the chart images sequentially from the imaging system 2 as frame images in time series via a video input means (not shown). The stored chart image is overwritten and stored. 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. 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 output to the display device 3 through a video output unit (not shown) and read out by the ROI image extraction unit 13.

  The ROI setting unit 11 sets a region of interest (ROI) including a boundary (edge) in the chart image captured by the imaging system 2. For example, the ROI setting unit 11 designates a rectangular area with a pointing device (not shown) operated by the measurer in the chart image displayed on the display device 3, so that the ROI area (position and size) is specified. Etc.) are set as ROI information.

Here, when measuring the frequency characteristics in the horizontal direction, the ROI setting unit 11 designates a vertically long rectangular region (ROI) from the chart image obtained by imaging the chart CH as shown in FIG. As a result, ROI information is set. In this case, an ROI image (vertical edge image) R having different brightness in the horizontal direction is specified by the ROI.
When measuring the frequency characteristics in the vertical direction, the ROI setting means 11 is designated by the measurer as a horizontally long rectangular area (ROI) from the chart image obtained by imaging the chart CH as shown in FIG. Thus, the ROI information is set. In this case, an ROI image (horizontal edge image) R having different brightness in the vertical direction is specified by the ROI.

The ROI setting by the ROI setting unit 11 may be performed once if it is not necessary to change the ROI in the sequentially input chart images. In addition, the ROI set by the ROI setting unit 11 is not necessarily rectangular if the shape can be specified.
The ROI setting unit 11 outputs ROI information indicating the set ROI region (position, size, etc.) to the ROI image extraction unit 13.

  The measurement instruction means 12 receives an instruction to start MTF measurement. For example, an instruction may be received using a switch (not shown), an instruction screen may be displayed on the display device 3, and the instruction may be received by operating a mouse or the like. The measurement instructing unit 12 receives the operation mode together with the MTF measurement instruction.

The operation mode defines the operation of the MTF measuring apparatus 1 when performing MTF measurement. Here, the MTF measuring apparatus 1 operates in a real time mode and a non-real time mode as operation modes.
The real-time mode is a mode in which the MTF is sequentially measured (real-time measurement) for each frame image with respect to the image (moving image) input from the imaging system 2.
In this real-time mode, the optimum association between each pixel position of the ROI image and the bin position of the projection axis in the Slanted-edge method that is first performed by the first MTF calculating unit 14 is used in the processing by the second MTF calculating unit 18. Then, by projecting the ROI pixel to the bin position based on the correspondence, the MTF is measured from the chart images sequentially input.

  In the real-time mode, it is assumed that the edge position does not shift during measurement. That is, in the real-time mode, even if the focus of the imaging system 2 changes, the boundary of the edge only blurs, and the inclination and position of the edge basically do not change. Therefore, the MTF measuring apparatus 1 can omit the association between each pixel position of the ROI image and the position of the bin of the projection axis in the second MTF calculating unit 18.

The non-real time mode is a mode for measuring the MTF for one frame image with respect to the image input from the imaging system 2. When the non-real time mode is designated, the MTF measuring apparatus 1 measures the MTF using the chart image stored in the chart image storage means 10 at that timing.
The measurement instruction unit 12 outputs the designated operation mode to the ROI image extraction unit 13 and the averaging unit 16 at the stage of receiving the instruction.

The ROI image extraction unit 13 extracts an image of the ROI region (position, size, etc.) indicated by the ROI information set by the ROI setting unit 11 as an ROI image from the chart image stored in the chart image storage unit 10. To do.
When the real time mode is designated as the operation mode from the measurement instruction unit 12, the ROI image extraction unit 13 outputs the extracted ROI image to the first MTF calculation unit 14 and then stores the chart in the chart image storage unit 10. Each time an image is input, an ROI image is extracted, and the extracted ROI image is output to the second MTF calculating means 18.
In addition, when the non-real time mode is designated as the operation mode from the measurement instruction unit 12, the ROI image extraction unit 13 outputs the extracted ROI image to the first MTF calculation unit 14.

The first MTF calculating unit 14 calculates MTF from the ROI image extracted by the ROI image extracting unit 13. The first MTF calculating unit 14 calculates the MTF for each phase by changing the bin phase (binning phase) when associating each pixel position of the ROI image with the bin position of the projection axis in the Slanted-edge method.
Here, the first MTF calculating unit 14 includes a phase-specific edge projection information generating unit 14a and a phase-specific MTF calculating unit 14b.

  The phase-specific edge projection information generation unit 14 a generates edge projection information for each phase of the projection axis bin from the ROI image extracted by the ROI image extraction unit 13. The edge projection information is information associating the ROI pixel with the position where the pixel is projected onto the projection axis along the inclination of the edge with the contrast boundary of the ROI image as an edge.

Specifically, the phase-specific edge projection information generating unit 14a first obtains the inclination of the edge in a coordinate system (xy coordinate system) having two horizontal and vertical axes in the ROI image. The inclination of the edge can be obtained by a general method. For example, as shown in the following reference, it can be obtained by fitting a normal cumulative density function or the like to the ROI image.
(Reference) K. Masaoka et al., "Modified slanted-edge method and multidirectional modulation transfer function estimation", Optics Express, 22 (5), 2014
Then, the phase-specific edge projection information generating unit 14a associates each pixel position of the ROI image with a position where each pixel is projected on the projection axis (x-axis or y-axis) along the inclination of the edge. Is generated.

Here, when obtaining the horizontal frequency characteristics as the MTF from the ROI image (vertical edge image) R specified by the ROI in FIG. 2A, the phase-specific edge projection information generating means 14a is as shown in FIG. The pixels of the ROI image are projected on the horizontal axis (x axis) at the same angle as the inclination θe of the edge e.
When obtaining the vertical frequency characteristics as the MTF from the ROI image (horizontal edge image) R specified by the ROI in FIG. 2B, the phase-specific edge projection information generation unit 14a has the same angle as the edge inclination. The pixels of the ROI image are projected onto the vertical axis (y axis: not shown). Of course, the phase-specific edge projection information generation unit 14a may perform the same processing as the vertical edge image by rotating the ROI image (horizontal edge image) R by 90 degrees to the left, for example.
The coordinate width (bin width) of the projection axis is set to a sub-pixel unit smaller than the pixel, and is, for example, a width of 1/4 of one pixel (four times oversampling).

  The phase-specific edge projection information generation unit 14a generates edge projection information in which the pixels of the ROI image are associated with the positions of the bins arranged on the projection axes divided in subpixel units. Further, the phase-specific edge projection information generating unit 14a generates edge projection information with a plurality of phases by shifting the phase of the bin of the projection axis.

Here, with reference to FIG. 4, the phase of the projection axis bin (binning phase) will be described.
In FIG. 4, sub-pixels x _1,0 , x _1,1 , x _1,2 , x _1,3 , x obtained by sub-pixelizing pixels x ₁ , x ₂ ,..., X _M on the projection axis (x-axis). ₂ , ₀ ,..., _XM , ₃ correspond to bin positions.
As shown in FIG. 4, the phase-specific edge projection information generation unit 14a changes the position of the bin in the axial direction with a predetermined phase shift with respect to the projection axis (x-axis) shown in the uppermost stage. Edge projection information is generated for each phase. When the position of the bin is sequentially changed in the axial direction, the sub-pixels x _{0 and 3} of the pixel x ₀ adjacent to the pixel x ₁ project the pixels of the ROI image as in the projection axis shown in the lowermost stage. It will be a new bottle.

Here, when the phase for one bin is 2π and the uppermost phase is “0”, the phase-specific edge projection information generating unit 14a determines the 2π / N, ( 2π × 2) / N,..., (2π × (N−1)) / N, and for each of N projection axes (x-axis) whose phases are shifted in the axial direction by 2π / N, Corresponds to the position of the projection axis bin.
Here, N is a predetermined number of phases (positive integer), for example, N = 16, N = 32, and the like.
Thereby, the pixels of the ROI image are projected onto the bin of the projection axis in various patterns for each phase of the bin.
Returning to FIG. 1, the description of the configuration of the MTF measuring apparatus 1 will be continued.

The phase-specific edge projection information generation unit 14a stores 15 edge projection information in the edge projection information storage unit for each phase of the bin.
The phase-specific edge projection information generation means 14a notifies the phase-specific MTF calculation means 14b that generation of edge projection information has been completed at the stage of generating edge projection information for a predetermined number of phases.

The MTF calculating unit 14b for each phase calculates the MTF for each phase of the bin from the ROI image extracted by the ROI image extracting unit 13.
This phase-specific MTF calculation means 14b calculates the MTF by the Slanted-edge method, and the correspondence between each pixel of the ROI image and the coordinate position of the sub-pixel unit of the projection axis is related to the phase-specific edge projection information generation means 14a. Is generated, and the edge projection information for each phase stored in the edge projection information storage unit 15 is referred to.

Specifically, when obtaining the horizontal frequency characteristics as the MTF from the ROI image (vertical edge image) R specified by the ROI in FIG. 2A, the phase-specific MTF calculating means 14b is as shown in FIG. As shown in FIG. 5, each pixel of the ROI image is projected onto the projection axis (x-axis) by the associated edge projection information, and the pixel value is averaged for each bin to show the edge characteristics. Generate an edge profile.
When obtaining the vertical frequency characteristics as MTF from the ROI image (horizontal edge image) R specified by the ROI in FIG. 2B, the phase-specific MTF calculating means 14b uses the edge projection information to determine the ROI image. The pixel value of each pixel is projected on the vertical axis (y-axis: not shown), and averaged for each sub-pixel unit, thereby generating an edge profile indicating edge characteristics.

When the phase-specific edge projection information generation unit 14a rotates the horizontal edge image 90 degrees to the left and generates the same edge projection information as the vertical edge image, the phase-specific MTF calculation unit 14b Is rotated 90 degrees to the left, and an edge profile may be generated in the same manner as the vertical edge image.
The phase-specific MTF calculation unit 14b generates an edge profile for each phase-specific edge projection information.
Then, the phase-specific MTF calculating unit 14b obtains a phase-specific line spread function (LSF) by differentiating the phase-specific edge profiles, and obtains the MTF by Fourier transforming the LSF.
Thus, the phase-specific MTF calculating unit 14b can calculate a plurality of MTFs for each phase of the projection axis bin.

It should be noted that in the phase-specific edge projection information generation unit 14a and the phase-specific MTF calculation unit 14b, whether to obtain the horizontal frequency characteristic or the vertical frequency characteristic as the MTF may be set in advance. The determination may be made depending on whether the ROI image is vertically long or horizontally long.
The phase-specific MTF calculation means 14 b outputs the calculated phase-specific MTF to the averaging means 16 and the phase selection means 17.

The averaging unit 16 has different functions depending on the operation mode notified from the measurement instruction unit 12.
When the operation mode is the real-time mode, the averaging unit 16 uses the MTF corresponding to a predetermined frequency (reference frequency) for the plurality of MTFs for each phase calculated by the MTF calculation unit 14b for each phase of the first MTF calculation unit 14. An average value of values, an average value of frequencies corresponding to a predetermined MTF value (reference MTF value), or an average value of MTF areas in a predetermined frequency range is calculated.
For example, as shown in FIG. 6, in a plurality of MTFs (number of phases N) for each phase calculated by the MTF calculation unit 14b for each phase, a typical frequency (for example, 0.37 cycles) in the imaging system evaluation is used as a reference frequency. The average value of the MTF values at the reference frequency (0.37 cycles / pixel) is calculated by dividing the sum of the MTF values corresponding to / pixel) by the number of phases (N).
Since the MTF calculated by the phase-specific MTF calculating means 14b is discretely represented by a spatial frequency at equal intervals, the MTF value at the reference frequency is linearly interpolated from the MTF value obtained at a frequency near the reference frequency. It can be obtained by

  Further, for example, in the plurality of MTFs for each phase calculated by the MTF calculation unit for each phase 14b, the reference MTF value is obtained by dividing the sum of the frequencies corresponding to 50% by the number of phases (N) as the reference MTF value. The average value of the frequency at (50%) is calculated. The frequency in the reference MTF value may be obtained by linear interpolation from the frequency obtained from the MTF value near the reference MTF value.

Further, for example, in a plurality of MTFs for each phase calculated by the MTF calculation unit for each phase 14b, the area of the MTF in a predetermined frequency range (for example, 0 to 0.5) is obtained for each phase, and the sum is calculated as the number of phases. By dividing by (N), the average value of the area of the MTF is calculated. The area of the MTF can be obtained by adding MTF values for each predetermined frequency unit (for example, 0.01) by a predetermined frequency range. The MTF value for each predetermined frequency unit may be obtained by linear interpolation from the MTF values obtained at frequencies near the corresponding frequency in the frequency unit.
Since this averaging means 16 calculates which average value (MTF value, frequency or area), it is assumed to be predetermined.
In the real-time mode, the averaging means 16 outputs the calculated average value (MTF value, frequency or area) to the phase selection means 17.

The averaging means 16 averages the plurality of MTFs for each phase calculated by the MTF calculation means 14b for each phase of the first MTF calculation means 14 when the operation mode is the non-real time mode.
The averaging means 16 generates an averaged MTF by averaging a plurality of MTF values calculated for each phase for each same frequency.
In the non-real time mode, the averaging means 16 outputs the averaged MTF to the MTF output means 19.

  When the averaging means 16 averages a plurality of MTFs for each phase, the length (number of bins) of bins onto which pixels are projected may differ depending on the phase. In that case, the averaging means 16 performs averaging after complementing each MTF so that the pitches of the spatial frequencies are equal. Alternatively, the MTF calculation unit 14b for each phase may calculate the MTF by aligning the bin lengths of each phase in advance.

The phase selection unit 17 selects the MTF value, frequency, or area closest to the average value calculated by the averaging unit 16 among the MTFs by phase calculated by the phase-specific MTF calculation unit 14b of the first MTF calculation unit 14. The phase of the bin to be selected is selected.
When the reference for calculating the average value by the averaging unit 16 is a reference frequency (for example, 0.37), the phase selection unit 17 selects the phase of the bin having the closest MTF value at the same frequency. Further, when the reference for calculating the average value by the averaging unit 16 is a reference MTF value (for example, 50%), the phase selection unit 17 selects the phase of the bin having the closest frequency at the same MTF value.
The phase selection unit 17 outputs the selected phase to the second MTF calculation unit 18.

The second MTF calculating unit 18 calculates MTF from the ROI images sequentially extracted by the ROI image extracting unit 13. The second MTF calculating means 18 includes a selected phase MTF calculating means 18a.
The selected phase MTF calculating unit 18 a calculates MTF from the ROI image extracted by the ROI image extracting unit 13 with reference to the edge projection information stored in the edge projection information storage unit 15.
In addition, the edge projection information corresponding to the phase selected by the phase selection unit 17 in the edge projection information stored in the edge projection information storage unit 15 is referred to.
The selected phase MTF calculating unit 18a calculates the MTF by the same process as the phase-specific MTF calculating unit 14b except that the edge projection information corresponding to the selected phase is referred to.
The selected phase MTF calculation unit 18 a outputs the calculated MTF to the MTF output unit 19.

The MTF output means 19 outputs the MTF as a measurement result to the outside (for example, the display device 3).
The MTF output means 19 may output an MTF value for each frequency, or may output only an MTF value of a representative frequency (for example, 0.37 cycles / pixel) in imaging system evaluation.

  By configuring the MTF measurement device 1 as described above, the MTF measurement device 1 can obtain an average MTF value or frequency in a plurality of MTFs generated by changing the phase of the bin of the projection axis in the Slanted-edge method. By calculating the MTF using the phase of the bin closest to, the MTF measurement accuracy can be improved.

[Operation of MTF measuring device]
Next, the operation of the MTF measuring apparatus 1 according to the embodiment of the present invention will be described. Here, the real-time measurement operation and the non-real-time measurement operation will be described separately.

(Real-time measurement operation)
First, the real-time measurement operation of the MTF measurement apparatus 1 will be described with reference to FIG.
Here, the MTF measuring apparatus 1 sequentially stores the chart image input from the imaging system 2 and captured the chart CH in the chart image storage unit 10 and displays the chart image on the display apparatus 3. Hereafter, the operation | movement which measures MTF from the chart image which the MTF measuring apparatus 1 inputs continuously is demonstrated in detail.

  In step S1, the ROI setting means 11 sets the ROI area (position, size, etc.) as ROI information. For example, the ROI setting unit 11 sets ROI information by designating a rectangular area with a pointing device (not shown) operated by the measurer in the chart image displayed on the display device 3.

In step S2, the MTF measuring apparatus 1 stands by until the measurement instructing unit 12 instructs the start of MTF measurement (No in step S2).
When the start of MTF measurement is instructed in the real-time mode (Yes in step S2), in step S3, the ROI image extraction unit 13 is set in step S1 from the chart image stored in the chart image storage unit 10. The ROI image indicated by the ROI information is extracted.

  In step S4, the phase-specific edge projection information generation unit 14a of the first MTF calculation unit 14 generates phase-specific edge projection information from the ROI image extracted in step S3. That is, the phase-specific edge projection information generation unit 14a obtains the slope of the edge from the ROI image, and projects each pixel position on the ROI image and the projection axis in which the bin phase is sequentially shifted along the edge slope. The edge projection information for each phase is generated in association with the position.

In step S5, the phase-specific edge projection information generation unit 14a stores the phase-specific edge projection information generated in step S4 in the edge projection information storage unit 15.
In step S6, the MTF calculating unit 14b for each phase of the first MTF calculating unit 14 calculates the MTF for each phase by the Slanted-edge method from the ROI image extracted in step S3. That is, the phase-specific MTF calculation unit 14b refers to the phase-specific edge projection information stored in the edge projection information storage unit 15 in step S5, and determines the pixel value of each pixel of the ROI image as the phase-specific projection axis. The edge profile which shows the characteristic of an edge is produced | generated by projecting and averaging for every sub-pixel unit. Then, the MTF calculating unit 14b for each phase obtains a line broadening function (LSF) by differentiating the edge profile, and calculates the MTF for each phase by Fourier transforming the LSF.

In step S7, the averaging means 16 sets the average value of the MTF values corresponding to the predetermined frequency (reference frequency) and the predetermined MTF value (reference MTF value) for the MTF for each phase calculated in step S6. The average value of the corresponding frequencies or the average value of the MTF areas in a predetermined frequency range is calculated.
In step S8, the phase selection unit 17 selects the phase of the bin having the MTF value, frequency, or area closest to the average value calculated in step S7 among the MTFs for each phase calculated in step S6.

  In step S9, the ROI image extraction unit 13 extracts the ROI image indicated by the ROI information set in step S1 from the chart image stored in the chart image storage unit 10.

  In step S10, the selected phase MTF calculation unit 18a of the second MTF calculation unit 18 calculates the MTF from the ROI image extracted in step S9 by the Slanted-edge method. That is, the selected phase MTF calculating unit 18a refers to the edge projection information corresponding to the phase selected in step S8 among the edge projection information stored in the edge projection information storage unit 15 in step S5, and returns the ROI. The pixel value of each pixel of the image is projected onto the projection axis of the selected phase, and averaged for each sub-pixel unit, thereby generating an edge profile indicating edge characteristics. Then, the selected phase MTF calculating means 18a obtains a line spread function (LSF) by differentiating the edge profile, and calculates MTF by Fourier transforming the LSF.

In step S11, the MTF output means 19 outputs the MTF calculated in step S10.
In step S <b> 12, the ROI image extraction unit 13 determines the end of input of the chart image depending on whether or not a new chart image is stored in the chart image storage unit 10. Here, when a new chart image is input (No in step S12), the MTF measurement device 1 returns to step S9 and continues the operation.
On the other hand, when a new chart image is not input (Yes in step S12), the MTF measuring apparatus 1 ends the operation. This end determination may be made by instructing the measurement instruction means 12 to end.
With the above operation, the MTF measuring apparatus 1 can measure the MTF in real time in the chart images that are sequentially input.

(Non-real-time measurement operation)
Next, the non-real time measurement operation of the MTF measuring apparatus 1 will be described with reference to FIG.
Here, the MTF measuring apparatus 1 stores a chart image obtained by imaging the chart CH, which is input from the imaging system 2, in the chart image storage unit 10 and displays the chart image on the display device 3. Hereinafter, an operation for measuring the MTF from the chart image input as a still image by the MTF measuring apparatus 1 or the chart image selected at an arbitrary timing of the frame images continuously input will be described in detail.

  In step S1, the ROI setting means 11 sets the ROI area (position, size, etc.) as ROI information. In addition, this step S1 is the same process as step S1 demonstrated in FIG.

In step S2A, the MTF measuring apparatus 1 waits until the measurement instructing unit 12 instructs the start of non-real time measurement of MTF (No in step S2A).
When the start of MTF measurement is instructed in the non-real-time mode (Yes in step S2A), the operations after step S3 are performed.
Since the operation from step S3 to step S6 is the same as the real-time mode operation described in FIG. 7, the same step number is assigned and the description is omitted.

After step S6, in step S20, the averaging means 16 averages the MTF for each phase calculated in step S6.
In step S21, the MTF output means 19 outputs the MTF averaged in step S20.
With the above operation, the MTF measuring apparatus 1 can measure the MTF in non-real time.

As mentioned above, although embodiment of this invention was described, this invention is not limited to this embodiment.
Here, a horizontal edge image and a vertical edge pixel have been described as examples of ROI images (edge images).
However, the inclination of the ROI image is arbitrary as long as the edge is included.
For example, the MTF measuring apparatus 1 may use, as the chart CH, a chart CH _{m in} which the slope of the edge exists in multiple directions (24 directions in FIG. 9) as shown in FIG. In this case, the ROI setting unit 11 sets 24 ROIs with the center C of the chart CH _m as a point-symmetrical center by setting one ROI including an edge in the horizontal direction or the vertical direction. You can do that.
At this time, the first MTF calculating unit 14 and the second MTF calculating unit 18 rotate the image for the ROI other than the reference ROI (vertical edge image), for example, as in the reference ROI. Or MTF may be calculated.

Here, the MTF output means 19 outputs the measured MTF as it is. However, the MTF output means 19 may output the MTF as a graph.
For example, the MTF output unit 19 generates a graph that can be visualized by plotting the MTF value for each spatial frequency on the coordinates with the frequency on the horizontal axis and the MTF value on the vertical axis. The MTF output unit 19 outputs the generated graph to the display device 3 so that the measurer can visually recognize the analysis result of the MTF of the imaging system 2.

Here, the averaging means 16 is configured to have different functions depending on the operation mode (real time mode, non-real time mode).
However, the averaging means 16 may operate in a limited manner in any operation mode. For example, when the MTF measurement device 1 is operated only in the non-real time mode, the phase selection unit 17 and the second MTF calculation unit 18 may be omitted from the configuration.

Further, here, as the real-time mode, the second MTF calculating unit 18 optimally associates each pixel position of the ROI image with the bin position of the projection axis in the Slanted-edge method first performed by the first MTF calculating unit 14. In this process, the MTF is measured from the chart image that is sequentially input by projecting the ROI pixel to the bin position in accordance with the correspondence.
However, even in the real-time mode, as in the non-real-time mode, for each frame of the moving image, the first MTF calculating unit 14 calculates the MTF for each phase, and the averaging unit 16 averages the plurality of MTFs for each phase. It is good also as making it.
In this case, for example, if the CPU performance is high, the MTF measurement apparatus 1 can measure MTF with high accuracy even for a moving image. Further, even when the CPU performance is low and the MTF cannot be measured in every frame, the MTF measuring apparatus 1 can measure the MTF with high accuracy even once per second (once every 60 frames), for example.

DESCRIPTION OF SYMBOLS 1 MTF measuring apparatus 10 Chart image storage means 11 ROI setting means 12 Measurement instruction means 13 ROI image extraction means 14 First MTF calculation means 14a Phase-specific edge projection information generation means 14b Phase-specific MTF calculation means 15 Edge projection information storage means 16 Averaging Means 17 Phase selection means 18 Second MTF calculation means 18a Selection phase MTF calculation means 19 MTF output means 2 Imaging system 3 Display device CH Chart image

Claims (6)

An MTF measurement device that measures MTFs representing spatial frequency characteristics of resolution of an imaging system using charts having different contrasts at boundaries,
ROI image extraction means for extracting an ROI image that is an image including the boundary from a chart image obtained by imaging the chart by the imaging system;
Phase-specific edge projection information generation for generating phase-specific edge projection information by projecting each pixel of the ROI image along the inclination of the edge that is the boundary on a plurality of projection axes in which the phase of bins of subpixel intervals is shifted Means,
A phase-specific MTF calculating means for calculating a phase-specific MTF by the Slanted-edge method using the phase-specific edge projection information;
Averaging means for averaging the MTFs for each phase;
Phase selection means for selecting a phase closest to the MTF averaged by the averaging means among the MTFs for each phase;
Among the edge projection information for each phase, the edge projection information of the phase selected by the phase selection means is used to sequentially extract the chart image captured by the imaging system by the Slanted-edge method. A selected phase MTF calculating means for calculating the MTF of the phase selected by the phase selecting means in the ROI image;
An MTF measuring apparatus comprising: The averaging means averages MTF values of a predetermined reference frequency in the MTF for each phase,
2. The MTF measurement apparatus according to claim 1, wherein the phase selection unit selects the phase of the bin having the MTF value averaged by the averaging unit and the MTF value closest to the reference frequency. The averaging means averages the frequency of a predetermined reference MTF value in the MTF for each phase,
2. The MTF measuring apparatus according to claim 1, wherein the phase selecting unit selects a phase of the bin having a frequency averaged by the averaging unit and a frequency closest to the reference MTF value. The averaging means averages the area of the MTF in a predetermined frequency range in the MTF for each phase,
The MTF measuring apparatus according to claim 1, wherein the phase selecting unit selects a phase of the bin having an area closest to an area averaged by the averaging unit. An MTF measurement device that measures MTFs representing spatial frequency characteristics of resolution of an imaging system using charts having different contrasts at boundaries,
ROI image extraction means for extracting an ROI image that is an image including the boundary from a chart image obtained by imaging the chart by the imaging system;
Phase-specific edge projection information generation for generating phase-specific edge projection information by projecting each pixel of the ROI image along the inclination of the edge that is the boundary on a plurality of projection axes in which the phase of bins of subpixel intervals is shifted Means,
A phase-specific MTF calculating means for calculating a phase-specific MTF by the Slanted-edge method using the phase-specific edge projection information;
Averaging means for averaging the MTFs for each phase;
An MTF measuring apparatus comprising:   An MTF measurement program for causing a computer to function as the MTF measurement device according to any one of claims 1 to 5.