JP2014092501A - Device, method and program for measuring reflectance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device, method, and program for measuring BRDFs at spatially different points on a measurement object surface without arranging a plurality of measuring instruments or moving a measuring instrument.SOLUTION: The device for measuring reflectance is provided for measuring a bidirectional reflectance distribution function BRDF at each point of an object surface in a plane area of the object surface. The device for measuring reflectance includes: means for illuminating a surface of an object; means for moving and rotating the illuminating means; means which has a prescribed aperture and shields reflected light from the object in different direction, in a part other than the aperture; means for changing the shape and size of the prescribed aperture; means for condensing the reflected light passing the aperture; means for measuring irradiance of condensed light; and means which fits a prescribed BRDF model to the value of irradiance obtained by the measurement to acquire a BRDF model parameter.

Description

  The present invention relates to an apparatus for measuring the reflectance of a measurement target object surface.

  Digital archives require that object information be recorded accurately. By recording information such as the color, gloss characteristics, and shape of the target object surface, the appearance of the target object in a desired illumination / observation environment can be imaged. The color and gloss characteristics depend on the illumination / observation direction, and are represented by a bidirectional reflectance distribution function (hereinafter referred to as BRDF) at each point on the surface of the target object. In Non-Patent Document 1, a plurality of light sources and cameras are arranged at arbitrary positions, a target object is illuminated from each light source position, and captured by each camera, so that each point on the surface of the target object is determined from the pixel values of the image group. Obtained BRDF.

`` Time-varying surface appearance: acquisition, modeling and rendering '', Gu, Jinwei and Tu, Chien-I and Ramamoorthi, Ravi and Belhumeur, Peter and Matusik, Wojciech and Nayar, Shree, ACM Trans.Graph., Vol. 25, No. 3 (2006) 762-771

  However, the method of Non-Patent Document 1 requires a means for arranging a plurality of cameras at different positions, or a means for moving using a single camera, and there is a problem that the apparatus becomes complicated.

  The present invention has been made in view of the above-described problems, and an apparatus for measuring BRDF at spatially different points on a measurement target object plane without arranging a plurality of measurement devices or moving the measurement devices, An object is to provide a method and a program.

In order to achieve the above object in the present invention, first, the invention of claim 1
An apparatus for measuring a bi-directional reflectance distribution function (hereinafter referred to as BRDF) of each point on the surface of the target object in a planar region of the target object surface,
Means for illuminating the surface of the target object;
Means for moving and rotating the illumination means;
Means having a predetermined opening, and shielding light reflected from the target object in different directions by a portion other than the opening;
Means for changing the shape and size of the predetermined opening;
Means for collecting the reflected light that has passed through the opening;
Means for measuring the irradiance of the condensed light;
And a means for fitting a predetermined BRDF model to the irradiance value obtained by the measurement to obtain a BRDF model parameter.

The invention of claim 2
A BRDF model parameter is obtained using the reflectance measuring apparatus according to claim 1.

The invention of claim 3
A reflectance measurement program that is provided in the reflectance measurement apparatus according to claim 1 and that acquires a BRDF model parameter.

  According to the present invention, it is possible to measure the BRDF on the surface of the measurement target object without arranging a plurality of measurement devices or moving the measurement devices.

The figure which shows an example of schematic structure of the reflectance measuring device of this invention. The figure which shows typically an example of the reflectance measuring apparatus of this invention. The figure which shows the shape of the opening part of the light-shielding plate which concerns on this invention. (A) is a circle (b) is a square (c) is a rectangle (d) is a ring. The step figure which shows the flow of a process of the reflectance measuring device of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram illustrating an example of a schematic configuration of a reflectance measurement apparatus according to an embodiment of the present invention. The reflectance measurement apparatus of the present invention includes a light illumination unit 103 that is a means for illuminating the surface of an object 101 (hereinafter, the same as a measurement target object), and a light illumination unit movement that is a means for moving and rotating the illumination means. Part 102, light shielding plate 104 which has a predetermined opening and shields reflected light in a different direction from the target object at a part other than the opening, and the shape and size of the predetermined opening An opening shape changing unit 104a that is a means for changing, a reflected light collecting unit 105 that is a means for collecting the reflected light that has passed through the opening, and a reflected light that is a means for measuring the irradiance of the collected light A measurement unit 106 and a computer 107 which is a means for fitting a predetermined BRDF model to the irradiance value obtained by the measurement unit 106 and acquiring a BRDF model parameter.

  The light illumination unit 103 is a means for illuminating the surface of the measurement target object 101 and may be any illumination as long as it can irradiate the measurement target object 101 with parallel light. If parallel light cannot be irradiated, the direction of incident light to the BRDF measurement point and the radiance are known and light does not have to be incident simultaneously from a plurality of directions.

  The light illumination unit moving unit 102 is a means for moving and rotating the light illumination unit 103 to illuminate the measurement target object 101 from a predetermined direction. The light illumination unit moving unit 102 includes a moving mechanism, a rotary motor, and the like.

  The light shielding plate 104 transmits light having a different reflection direction from the reflected light from the measurement target object 101 through the opening and shields light other than the opening.

  The reflected light condensing unit 105 is a means for condensing the reflected light from the measurement target object 101 and includes a lens or the like.

  The reflected light measuring unit 106 is a means for measuring the irradiance of light obtained by collecting reflected light from the measurement target object 101 using the reflected light condensing unit 105, and includes a photoelectric conversion element and the like.

  A computer 107 is a computer including a recording unit 108, a calculation unit 109, and a control unit 110. The computer 107 is provided with a program for fitting a predetermined BRDF model to the irradiance value obtained by the reflected light measuring unit 106 and acquiring a BRDF model parameter.

  The recording unit 108 is provided in the computer 107 and is means for recording the measurement result by the reflected light measurement unit 106 and the calculation result by the calculation unit 109. The recording unit 108 includes a hard disk, a flash memory, and the like for recording data of measurement results and calculation results.

  The calculation unit 109 is provided in the computer 107 and measures the BRDF of the measurement target object 101 from the measurement value data recorded in the recording unit 108. A method of measuring the BRDF of the measurement target object from the measurement value will be described later.

  The control unit 110 is provided in the computer 107 and controls operations of the light illumination unit moving unit 102, the light illumination unit 103, the reflected light measurement unit 106, the recording unit 108, and the calculation unit 109. The control unit 110 includes a ROM that stores various control programs.

  The input unit 111 is a means for a user to send an instruction to the control unit 110. The input unit 111 includes a mouse, a keyboard, and the like.

  The output unit 112 outputs data from the calculation unit 109 and the recording unit 108. The output unit 112 includes a monitor, a printer, and the like for displaying the data of the calculation unit 109 and the recording unit 108.

  FIG. 2 is a diagram schematically showing an example of the reflectance measuring apparatus of the present invention. 201 is a planar area on the surface of the measurement target object, 202 is a condensing optical element, 203 is a light shielding plate, and 204 is a photoelectric conversion element group.

  In the BRDF measurement, the planar region 201, the condensing optical element 202, and the photoelectric conversion element group 204 are installed so as to be parallel to each other.

  In the present invention, the shape 203a of the opening of the light shielding plate 203 is not limited. FIG. 3 is a diagram showing the shape of the opening of the light shielding plate according to the present invention. If light having different reflection directions from the planar region 201 can be partially transmitted, a circular shape (FIG. 3A), a square shape (FIG. 3B), a rectangular shape (FIG. 3C), and a ring shape (FIG. 3D) Any of)) can be used. In addition, the installation position of the light shielding plate 203 may be in front of or behind the condensing optical element 202.

  The photoelectric conversion element group 204 is an element that receives the light collected by the condensing optical element 202 and converts it into a signal value. A CCD element used for a photodiode or an imaging camera corresponds to this.

  Xo is an arbitrary point in the planar region 201 on the surface of the measurement target object, and its position is represented by (Equation 1). The condensing optical element 202 condenses the light incident on the condensing optical element 202 out of the reflected light from the point Xo to the point Xp. The point Xp is the position of the photoelectric conversion element that receives the collected light, and the position (Equation 2) depends on the position (Equation 1).

  Next, the flow of processing of the reflectance measurement apparatus according to this embodiment will be described with reference to FIG. FIG. 4 is a step diagram showing the flow of processing of the reflectance measuring apparatus in the present embodiment.

(Step S300) In step S300, the user designates the direction in which the planar area 201 on the surface of the measurement target object is illuminated (used in step S301) and the shape of the opening of the light shielding plate 203 (used in step S303). The shape of the opening and the illumination direction will be described later. The aperture shape and the illumination direction index are i and j, respectively, and are initialized to 1.

(Step S301) In step S301, the light illumination unit moving unit 102 moves and rotates the light illumination unit 103 in order to perform illumination from the direction designated by the user in the jth.

(Step S302) In step S302, the light illumination unit 103 illuminates the planar area 201. Illumination is performed from step S302 to step S306.

  The direction of the incident light to the position (Equation 1) in the plane area 201 in illumination from the direction designated by the user is assumed to be (Equation 3). Further, the direction of reflected light by illumination is assumed to be (Equation 4). Let radiance of incident light be (Equation 5), and radiance of reflected light in the (Equation 4) direction in illumination from the (Equation 3) direction is (Equation 6). The radiance of the reflected light is expressed by (Equation 6).

  Where (Equation 7) is a hemispherical direction region representing the incident light direction component with respect to the position (Equation 1), (Equation 8) is the BRDF at the position (Equation 1), and (Equation 9) is the method at the position (Equation 1). Is a line.

  (Step S303) In step S303, the light having a different reflection direction (Equation 4) in the shape of the i-th opening designated by the user is partially transmitted / shielded using the light shielding plate 203.

  (Step S304) In step S304, the irradiance of the light condensed by the condensing optical element 202 out of the transmitted reflected light is measured by the photoelectric conversion element at the point Xp. Condensing by the condensing optical element 202 is the direction integration of light, and the irradiance of the measured light is expressed by (Equation 10).

However, (Equation 11) is the direction area of the light collected from the position (Equation 1) without being blocked, and (Equation 12) is reflected from the position (Equation 1) to the (Equation 4) direction. The incident direction of the light point Xp to the photoelectric conversion element, (Equation 13) is a normal line at the position (Equation 2).

  (Step S305) In step S305, i is incremented, and if i is equal to or less than the number N of opening shapes designated by the user, step S303 is performed. If i is larger than N (YES in S305), step S306 is performed.

  (Step S306) In step S306, j is incremented, and if j is not more than the number M of illumination directions designated by the user (NO in S306), step S301 is performed. If j is larger than M (YES in S306), step S307 is performed.

  As described above, the radiance information of the reflected light in the predetermined direction is acquired. In order to acquire radiance information of reflected light in Non-Patent Document 1, a plurality of condensing optical elements 202 and photoelectric conversion element groups 204 in the measurement apparatus of the present invention must be installed or moved. In contrast, in the reflectance measuring apparatus of the present invention, it is possible to acquire radiance information of reflected light in different directions only by changing the shape of the opening of the light shielding plate 203.

  (Step S307) In step S307, the BRDF model is fitted to the measured value, and the BRDF model parameter is acquired. By using the obtained BRDF model parameters, the BRDF at the position (Equation 1) can be expressed.

  Assuming that the BRDF at the position (Equation 1) can be approximated by the BRDF model (Equation 15) with S model parameters (Equation 14) as arguments, the reflected light when the radiance of the incident light is (Equation 5) The radiance is expressed by (Equation 16).

  Here, the irradiance of light measured by the photoelectric conversion element at the point Xp without being shielded by the light shielding plate 203 is calculated by (Equation 17).

However, (Equation 18) is the reflected light direction region in which (Equation 11) is evenly sampled, (Equation 19) is the k-th sampled direction, and _Wi is the solid angle of (Equation 11).

  Here, the parameter A of the BRDF model (Equation 15) at the position (Equation 1) is obtained by (Equation 20).

  However, c is a conversion coefficient (Equation 21) from the irradiance to the signal value at the photoelectric conversion element at the point Xp, the signal value of the irradiance (Equation 22) of the light to the photoelectric conversion element at the point Xp, and || A function that outputs an absolute value, (Equation 23) is a function that outputs a parameter A that minimizes an argument using a non-linear optimization method such as the Levenberg-Marquardt method.

  Examples of means for obtaining the conversion coefficient c include a method of measuring an object whose BRDF is known with a predetermined geometry and calculating a ratio between the irradiance of reflected light and a signal value.

(Equation 15) may be any BRDF model as long as the BRDF is determined by parameters such as a Torrance-Sparrow model and a Ward model.

  As shown in (Equation 10), the measuring device measures the radiance of all the light in an arbitrary direction region as one signal value by the photoelectric conversion element at the point Xp. After the measurement, in (Equation 20), the BRDF model parameter at the position (Equation 1) is obtained from a plurality of signal values based on the irradiance of light. When the number of signal values is smaller than the number of model parameters, the optimization of (Expression 20) may become unstable. In order to optimize more stably, the opening size of the light shielding plate 203 may be reduced or the number of light transmission patterns may be increased.

  By performing the following procedure in the fitting instead of reducing the opening size of the light shielding plate 203, information at a smaller opening size can be obtained. In the case of (Equation 24), the difference between the signal values obtained by measurement in each direction region is calculated by (Equation 25), and fitting is performed on the signal value.

  However, xor is an exclusive OR.

  The difference is a signal value with respect to the irradiance of (Equation 27), and is a value obtained when the direction area (Equation 26) is measured.

  The BRDF of the position (Equation 1) of the planar region 201 on the surface of the measurement target object can be acquired by the processing from step S300 to step S307.

<Implementation procedure>
Example 1
A plane object as a subject, an imaging camera capable of adjusting the aperture to F4, F5.6, F8, and F11, and a xenon lamp are prepared.

  F is a value representing the aperture size. The larger the F value, the smaller the aperture. In this case, the diaphragm is an opening shape changing portion 104a which is a means for changing the size of the pattern having the same shape.

  Place the subject so that it is parallel to the sensor surface of the camera.

  Specify the direction to illuminate the subject, and install the xenon lamp according to the illumination direction. Illuminate the subject with a xenon lamp. Be careful not to install a xenon lamp between the subject and the camera. Also, nothing is installed between the camera and the subject.

  The subject is photographed with the aperture set to F4.

  The aperture is changed to F5.6, and the subject is photographed without changing settings other than the aperture. Hereinafter, similarly, the aperture values are set to F8 and F11, and the respective images are taken without changing settings other than the aperture.

  Move the xenon lamp to change the illumination direction from the xenon lamp to the subject.

  Shooting (four times in total) with the aperture set to F4, F5.6, F8, and F11 is performed.

  Move and shoot the xenon lamp until illumination from all specified directions is complete.

  Using the measured values obtained from the captured image, BRDF fitting is performed to obtain model parameters. The BRDF model used for fitting is a Torrance-Sparrow model.

  As described above, according to the reflectance measuring device, the reflectance measuring method, and the reflectance measuring program according to the present invention, the space on the measurement target object plane can be obtained without arranging a plurality of measuring devices or moving the measuring devices. It is possible to measure BRDF at different points.

DESCRIPTION OF SYMBOLS 101 ... Measurement object 102 ... Light illumination part moving part 103 ... Light illumination part 104 ... Light-shielding plate 104a ... Opening shape change part 105 ... Reflected light condensing part 106 ... Reflected light measuring part 107 ... Computer 108 ... Recording part 109 ... Calculation unit 110 ... control unit 111 ... input unit 112 ... output unit 201 ... measurement target object surface 202 ... condensing optical element 203 ... light shielding plate 203a ... opening 204 ... photoelectric conversion element group Xo ... one point Xp on the measurement target object surface ... Photoelectric conversion element

Claims (3)

An apparatus for measuring a bi-directional reflectance distribution function (hereinafter referred to as BRDF) of each point on the surface of the target object in a planar region of the target object surface,
Means for illuminating the surface of the target object;
Means for moving and rotating the illumination means;
Means having a predetermined opening, and shielding light reflected from the target object in different directions by a portion other than the opening;
Means for changing the shape and size of the predetermined opening;
Means for collecting the reflected light that has passed through the opening;
Means for measuring the irradiance of the condensed light;
And a means for fitting a predetermined BRDF model to the irradiance value obtained by the measurement and obtaining a BRDF model parameter.   A BRDF model parameter is acquired using the reflectance measuring apparatus according to claim 1.   A reflectance measurement program that is provided in the reflectance measurement device according to claim 1 and that acquires a BRDF model parameter.

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