KR101056995B1 - 3D shape inspection method - Google Patents

Abstract

The three-dimensional shape inspection method for a predetermined device formed on a printed circuit board includes generating a shape template that abstracts the shape of the device, irradiating a grid image light to the measurement object in at least two directions, and thereby each pixel of the measurement object. Obtaining the star height information, generating a contrast map in which the contrast is set corresponding to the height information for each pixel, and comparing the contrast map and the shape template of the measurement object to determine the shape template at the measurement object. Obtaining information of a device corresponding to the. Therefore, it is possible to accurately extract the elements on the printed circuit board.

Description

3D shape inspection method {METHOD OF INSPECTING THREE-DIMENSIONAL SHAPE}

The present invention relates to a three-dimensional shape inspection method, and more particularly to a three-dimensional shape inspection method for a device on a printed circuit board.

In general, at least one printed circuit board (PCB) is provided in an electronic device, and includes a device such as a chip on the printed circuit board.

Extracting an element such as the chip from the printed circuit board is necessary to determine whether the element mounted on the printed circuit board is defective or whether the pad connected to the element is defective.

Conventionally, the image taken by taking a two-dimensional image for the above extraction operation has been used. However, the task of extracting a specific device from a two-dimensional image is sensitive to the color or illumination of the device, making it difficult to distinguish the device from the surroundings, and it is difficult to distinguish the device even when the dimension of the device is changed. In addition, when there is noise in an image, for example, when a pattern or silk is formed on a substrate other than the device, it may be difficult to distinguish the device, and noise may be generated by a camera inside the device. It can also be confused with adjacent parts. In particular, the conventional method has been extracting the fillet portion of the 2D image of the device, but when the fillet of the device is small, there is a limit in extracting the device using the fillet.

Therefore, a three-dimensional shape inspection method using an element extraction method that can prevent the above problems is required.

Therefore, the problem to be solved by the present invention is to provide a three-dimensional shape inspection method that can accurately extract the desired device.

According to an exemplary embodiment of the present invention, a three-dimensional shape inspection method may include generating a shape template that abstracts a shape of a predetermined device, irradiating a grid image light to a measurement object in at least two directions. Acquiring height information for each pixel of the measurement object; generating a contrast map in which a contrast is set corresponding to the height information for each pixel; and And comparing the contrast map and the shape template to obtain information of the device corresponding to the shape template in the measurement object.

In an embodiment, the obtaining of information of the device corresponding to the shape template from the measurement object may include determining whether the device corresponding to the shape template is present in the measurement object and the device may be used. If present in the object to be measured may include obtaining the size, position and rotation angle information of the device.

For example, determining whether the element corresponding to the shape template exists in the measurement object may include setting a predetermined inspection area on a printed circuit board on which the element is formed, and determining the position of the shape template. Comparing with the contrast map while sequentially moving from the initial position. At this time, the step of comparing the contrast map with the contrast map while sequentially moving the position of the shape template from the initial position, a portion overlapping the contrast map with values set to 0, 1 according to the pixel coordinates on the shape template Multiplying the contrast value of and multiplying each other, setting the position representing the maximum value according to the sequential movement of the position of the shape template as a preliminary position in which the device exists, and if the maximum value is equal to or greater than the reference value, And determining that the device corresponds to the shape template.

The method may further include determining whether the device corresponding to the shape template is defective after obtaining the information on the device corresponding to the shape template from the measurement object. For example, when the device includes a chip formed on a printed circuit board, the determining of whether the device corresponding to the shape template is defective may include extracting a chip body that is the body of the chip. Removing the chip body information of the chip body from the chip information of the chip; and determining whether the chip formed on the printed circuit board is defective from the chip information from which the chip body information is removed. It may include.

In one embodiment, the three-dimensional shape inspection method may further comprise the step of obtaining the visibility information of each pixel of the measurement object by irradiating the grid image light in at least two directions. The contrast per pixel may be defined as a value obtained by multiplying the height information by the visibility information.

For example, the shape template may be defined by a template determinant including the planar size of the device. In this case, the contrast map and the shape template may be compared within a predetermined tolerance of the template determinant.

According to the present invention, since the desired device is extracted using the contrast map according to the height, the device is not sensitive to the color or illumination of the device as compared with the case of extracting the device using the 2D image, and is easy even when the dimension of the device is changed. The device can be discriminated.

In addition, the image may not be affected by noise such as a pattern or silk around the device, or noise caused by a camera inside the device, and compared with the template even when there are other devices around the device such as pad areas. Since the device is discriminated, the device can be extracted accurately.

In addition, even when the fillet of the device is small, the device is extracted using a contrast map according to the height without extracting the device using the fillet, so that the device can be extracted more accurately.

As the inventive concept allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to the specific disclosed form, it should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention.

Terms such as first and second may be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the first component may be referred to as a second component, and similarly, the second component may also be referred to as a first component.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "comprise" or "having" are intended to indicate that there is a feature, number, step, action, component, part, or combination thereof described in the specification, and that one or more other features It should be understood that it does not exclude in advance the possibility of the presence or addition of numbers, steps, actions, components, parts or combinations thereof.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art.

Terms such as those defined in the commonly used dictionaries should be construed as having meanings consistent with the meanings in the context of the related art, and shall not be construed in ideal or excessively formal meanings unless expressly defined in this application. I do not.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

1 is a conceptual diagram illustrating an exemplary three-dimensional shape measuring apparatus used in the three-dimensional shape measuring method according to an embodiment of the present invention.

Referring to FIG. 1, the three-dimensional shape measuring apparatus used in the three-dimensional shape measuring method according to the present embodiment includes a measurement stage unit 100, an image photographing unit 200, first and second lighting units 300 and 400, The image acquirer 500, the module controller 600, and the central controller 700 may be included.

The measurement stage unit 100 may include a stage 110 for supporting the measurement object 10 and a stage transfer unit 120 for transferring the stage 110. In the present exemplary embodiment, the measurement object 10 is moved by the stage 110 with respect to the image capturing unit 200 and the first and second lighting units 300 and 400. The measuring position at can be changed.

The image capturing unit 200 is disposed above the stage 110 and receives the light reflected from the measurement object 10 to measure an image of the measurement object 10. That is, the image capturing unit 200 receives the light emitted from the first and second lighting units 300 and 400 and reflected from the measuring object 10 to take a plane image of the measuring object 10. .

The image capturing unit 200 may include a camera 210, an imaging lens 220, a filter 230, and a lamp 240. The camera 210 receives the light reflected from the measurement object 10 to take a planar image of the measurement object 10. For example, one of a CCD camera and a CMOS camera may be employed. The imaging lens 220 is disposed under the camera 210 to form light reflected from the measurement object 10 in the camera 210. The filter 230 is disposed below the imaging lens 220 to filter the light reflected from the measurement object 10 to provide the imaging lens 220, and for example, a frequency filter, a color filter, and light. It may be made of any one of the intensity control filter. The lamp 240 is disposed below the filter 230 in a circular shape, for example, to provide light to the measurement object 10 to capture a specific image such as a two-dimensional shape of the measurement object 10. can do.

The first lighting unit 300 may be disposed to be inclined with respect to the stage 110 supporting the measurement object 10 on the right side of the image capturing unit 200, for example. The first lighting unit 300 may include a first lighting unit 310, a first grating unit 320, a first grating transfer unit 330, and a first condensing lens 340. The first lighting unit 310 is composed of an illumination source and at least one lens to generate light, the first grating unit 320 is disposed below the first lighting unit 310 to the first illumination The light generated in the unit 310 is changed into the first lattice pattern light having the lattice pattern. The first grating transfer unit 330 is connected to the first grating unit 320 to transfer the first grating unit 320, for example, one of the PZT (Piezoelectric) transfer unit or fine linear transfer unit It can be adopted. The first condenser lens 340 is disposed under the first grating unit 320 to condense the first grating pattern light emitted from the first grating unit 320 to the measurement object 10.

The second lighting unit 400 may be disposed to be inclined with respect to the stage 110 supporting the measurement object 10 on the left side of the image capturing unit 200, for example. The second lighting unit 400 may include a second lighting unit 410, a second grating unit 420, a second grating transfer unit 430, and a second condensing lens 440. Since the second lighting unit 400 is substantially the same as the first lighting unit 300 described above, detailed descriptions thereof will be omitted.

The first lighting unit 300 irradiates the N first grating pattern lights to the measurement target 10 while the first grating transfer unit 330 moves the first grating unit 320 N times in sequence. In this case, the image capturing unit 200 may photograph the N first pattern images by sequentially receiving the N first grating pattern lights reflected from the measurement object 10. In addition, the second lighting unit 400 moves N second grid pattern lights to the measurement target 10 while the second grid transfer unit 430 moves the second grid unit 420 sequentially N times. When irradiating, the image capturing unit 200 may photograph the N second pattern images by sequentially applying the N second grid pattern lights reflected from the measurement object 10. Here, N is a natural number, for example, may be 3 or 4.

Meanwhile, in the present exemplary embodiment, only the first and second lighting units 300 and 400 are described as an illumination device for generating the first and second grid pattern lights. Alternatively, the number of the lighting units may be three or more. That is, the grid pattern light irradiated to the measurement object 10 may be irradiated from various directions, and various kinds of pattern images may be photographed. For example, when three lighting units are arranged in an equilateral triangle shape around the image capturing unit 200, three grid pattern lights may be applied to the measurement object 10 in different directions, and four lighting units may be applied. When they are arranged in a square shape around the image capturing unit 200, four grid pattern lights may be applied to the measurement object 10 in different directions.

The image acquisition unit 500 is electrically connected to the camera 210 of the image capturing unit 200 to obtain and store the pattern images from the camera 210. For example, the image acquisition unit 500 includes an image system that receives and stores the N first pattern images and the N second pattern images photographed by the camera 210.

The module controller 600 is electrically connected to and controlled by the measurement stage unit 100, the image capturing unit 200, the first lighting unit 300, and the second lighting unit 400. The module controller 600 includes, for example, a lighting controller, a grid controller, and a stage controller. The lighting controller generates light by controlling the first and second lighting units 310 and 410, respectively, and the grid controller controls the first and second grid transfer units 330 and 430, respectively. The second grid units 320 and 420 are moved. The stage controller may control the stage transfer unit 120 to move the stage 110 up, down, left, and right.

The central control unit 700 is electrically connected to the image acquisition unit 500 and the module control unit 600 to control each. Specifically, the central control unit 700 receives the N first pattern images and the N second pattern images from the image system of the image acquisition unit 500, processes them, and processes the three-dimensional image of the object to be measured. The shape can be measured. In addition, the central controller 700 may control the lighting controller, the grid controller, and the stage controller of the module controller 600, respectively. As such, the central control unit may include an image processing board, a control board, and an interface board.

Hereinafter, a method of inspecting a predetermined device mounted on a printed circuit board employed as the measurement target 10 using the above-described three-dimensional shape measuring apparatus will be described in more detail.

2 is a flowchart showing a three-dimensional shape inspection method according to an embodiment of the present invention, Figure 3 is a schematic diagram showing an example of a shape template.

2 and 3, in order to inspect a predetermined device mounted on a printed circuit board, first, a shape template 800 that abstracts the shape of the device is generated (S110). The abstracted device 810 may include, for example, a hexahedral chip.

For example, the shape template 800 abstracts the shape of the device 810 as shown in FIG. 3 so that a part corresponding to the device 810 is expressed in white, and a part corresponding to the device is expressed in black in advance. Can be set. In FIG. 3, the hatched part shows the part corresponding to an element. In this case, the shape template 800 may be formed as a digital image and set to have a value of 1 in the case of white and 0 in the case of black.

The shape template 800 may be defined by a template determinant. That is, the template determinant may determine the shape template 800. For example, when the device 810 is a hexahedral chip, the template determinant may include the planar size of the chip. Specifically, the template determinant may include a horizontal length (X) and a vertical length (Y) of the chip, and the shape template 800 is formed by a template determinant including the horizontal and vertical lengths. Can be defined.

Subsequently, the grid image light is irradiated to the measurement object in at least two directions to obtain height information for each pixel of the measurement object (S120).

Height information for each pixel of the measurement object may be easily obtained from data obtained by measuring the measurement object by the three-dimensional shape measuring apparatus shown in FIG. 1.

Next, a contrast map in which contrast is set corresponding to the height information for each pixel is generated (S130). The comparison target element (hereinafter referred to as a "target element") located on the measurement object is set to have a higher value contrast as the height is higher according to the height information for each pixel, and thus the contrast map. According to the shape of the target element is a place where the target element is located can be made dark where the target element is not located.

In this case, since the contrast map is prepared according to the height information, it is independent of the color of the target element, the character or the figure printed on the target element, and the like, and is also independent of the color, print shape, etc. of the surrounding of the target element. . That is, since the target device displays only a light and dark degree depending on the height, it is possible to determine the shape of the target device more clearly than a general two-dimensional image.

Meanwhile, in order to more clearly inspect the target device, visibility information for each pixel of the measurement object may be obtained and used.

The Visionary Stability is according to the brightness signal of the video it means a ratio of the amplitude average _{(A i (x, y)} ) of _{(B i (x, y)} ) , and there is generally a tendency to increase as the reflectance increases. The Visionary Stability _{(V i (x, y)} ) is defined as follows:

V _i (x, y) = B _i (x, y) / A _i (x, y)

Lattice pattern light may be irradiated onto the printed circuit board in various directions to capture various types of pattern images. As illustrated in FIG. 1, N patterns of the image acquisition unit 500 photographed by the camera 210 are photographed. From the images, N brightness degrees I ⁱ ₁ , I ⁱ ₂ , ..., I ⁱ _N at each position i (x, y) of the XY coordinate system are extracted, and the N-bucket algorithm (N The average brightness (A _i (x, y)) and viability (V _i (x, y)) are calculated using the -bucket algorithm.

For example, when N = 3 and when N = 4, viability can be calculated as follows, respectively.

In other words, when N = 3,

Can be calculated as

If N = 4,

It can be calculated as

The visibility information may be obtained by irradiating the measurement object with the grating image light in at least two directions, in the same manner as in operation S120 of obtaining height information for each pixel of the measurement object. That is, the visibility information for each pixel can also be easily obtained from the data measured by the measurement object by the three-dimensional shape measuring apparatus shown in FIG. 1 as an example.

The pixel-by-pixel contrast may be defined as a value obtained by multiplying the height information by the obtained visibility information. In general, in the case of a device whose reflectance is larger than the surroundings, the visibility is much larger than the surroundings. Therefore, when the visibility information is reflected in the contrast map, the presence of the device is more emphasized than when the height information is set according to the height information. Can be.

Referring back to FIG. 2, next, the contrast map of the measurement object and the shape template 800 are compared to obtain information on the element 810 corresponding to the shape template 800 in the measurement object. Acquire (S140). The information on the device 810 may include whether the device 810 exists, an actual size and an arrangement state of the device 810.

FIG. 4 is a flowchart illustrating an embodiment of a method of obtaining information of a device in FIG. 2.

Referring to FIG. 4, in order to obtain information of the device 810 corresponding to the shape template 800 from the measurement object, the device 810 corresponding to the shape template 800 is first measured by the measurement object. It can be determined whether or not present (S142).

For example, a predetermined inspection region (or region of interest) is set to check whether there is a target element, and then the existence of the target element is checked. In this case, the inspection area may be set by using CAD information that records the shape of the measurement object. The CAD information includes design information of the measurement object. In another embodiment, the inspection area may be set using learning information obtained by the learning mode. The learning mode is to obtain the design reference information of the printed circuit board by learning the bare board of the printed circuit board, it can be used to set the inspection area by obtaining the learning information through the learning mode.

FIG. 5 is a conceptual diagram illustrating an embodiment of a method of comparing whether a target device corresponds to a shape template.

Referring to FIG. 5, in order to compare whether a target element 820 is an element 810 corresponding to the shape template 800, first, a predetermined inspection area ROI is set on the printed circuit board. The position of the shape template 800 may be compared with the contrast map while sequentially moving from the initial position 800a.

For the comparison, first, values multiplied with the contrast value of the portion overlapping the contrast map by adding a value set to 0, 1 according to pixel coordinates, for example, are added to each other on the shape template 800. Subsequently, the position representing the maximum value may be determined as a preliminary position where the device 810 is present. Next, if the maximum value is greater than or equal to the reference value, the target element 820 may be determined to be an element 810 corresponding to the shape template 800.

The element 810 corresponding to the shape template 800 has a predetermined size. The target element 820 of the measurement object may have a different size and may be disposed to be rotated. Accordingly, a predetermined tolerance for determining the device 810 and the target device 820 recorded in the shape template 800 as the same device may be set, and the contrast map and the shape template ( 800 may be compared within a predetermined allowance of the template determinant. For example, the predetermined tolerance may range from 50% to 150% of the size of the device 810 corresponding to the shape template 800. In addition, a predetermined rotation angle for determining the device 810 corresponding to the shape template 800 and the target device 820 as the same device may be set, and the contrast map and the shape template 800 may be It can be compared while rotating at the predetermined rotation angle.

Referring back to FIG. 4, when the device 810 is present in the measurement object, information on the size, position, and rotation angle of the device, that is, the target device 820 may be obtained (S144). Such information can be easily obtained using a contrast map.

On the other hand, after obtaining the information of the device corresponding to the shape template 800 from the measurement object (S140), the information of the device may be utilized in various ways in the three-dimensional shape inspection method.

For example, whether the device corresponding to the shape template 800 is defective may be determined using the information of the device (S150). That is, it is possible to determine whether the device is defective by checking whether the device is properly disposed on the measurement object using the size information, the rotation information, and the like. Alternatively, the information of another device or part may be obtained by excluding the information about the device from the information of the measurement object by using the information of the device.

Meanwhile, a part of the device may be extracted and removed from the information of the device, and the remaining information of the removed device may be used to determine whether the device is defective.

For example, when the device is a chip formed on a printed circuit board, information of a terminal extending from the chip body except for the body of the chip and information of a pad connected to the terminal of the chip may be obtained without noise. It may be. Therefore, it is also possible to determine whether these parts are defective by using the information thus obtained.

In an embodiment, when the device is a chip formed on a printed circuit board, first, a chip body which is a body of the chip is extracted, and then chip body information for the chip body is removed from chip information for the chip. Next, from the chip information from which the chip body information is removed, it is possible to determine whether the chip formed on the printed circuit board is defective, that is, a connection state between the terminal of the chip and the pad.

Hereinafter, as an example, a contrast map reflecting a two-dimensional image and three-dimensional height information is created for a chip, and examples of the two-dimensional image and the contrast map are compared with reference to the drawings.

6 is an image obtained by obtaining a 2D image as an example of the case where the device is a chip.

Referring to FIG. 6, the shape of the chip body shown in the image may be estimated as a rectangular shape. However, it is difficult to clearly check the outline of the chip body, and in particular, it is not easy to identify where the top, bottom, left, and right sides of the chip body correspond to the chip body, and where and where are the solder, pads, terminals, and the like. .

In addition, since the image is a two-dimensional image, it can be seen that portions where the numbers or colors displayed on the chip body are different.

FIG. 7 is an image of a chip of FIG. 6 as a contrast map. FIG.

Referring to Figure 7, it can be clearly seen that the shape of the chip body shown in the image is rectangular. Since the image is a three-dimensional image set to have a large value of contrast according to the height information of each pixel, the chip body having the same height and protruding is brighter and protrudes to a lower height than the chip body. The terminal and the like appear darker than the chip body. Therefore, the part corresponding to the chip body can be easily identified.

In addition, since the image is a three-dimensional image reflecting the height information, it can be seen that the color of the chip body or the number displayed on the chip body does not appear in the image. Accordingly, the color or the displayed characters do not cause any confusion in confirming the shape of the chip body, and in particular, even in the case of a chip body having a particularly complicated color or character, the shape of the chip body can be easily formed from the contrast map. Can be obtained.

FIG. 8 is an image showing the chip of FIG. 6 as a contrast map reflecting visibility information.

Referring to FIG. 8, the shape of the chip body shown in the image may be more clearly confirmed. Since the image is a contrast map reflecting visibility information, the chip body portion is more highlighted and brighter, and the other portion is darker. Therefore, the presence of the chip body is further emphasized, so that the shape of the chip body can be easily obtained.

As described above, according to the present invention, since the desired device is extracted using the contrast map according to the height, the device is not sensitive to the color or illumination of the device as compared with the case of extracting the device using the 2D image, and the dimension of the device is changed. The device can be easily identified.

In addition, the image may not be affected by noise such as a pattern or silk around the device, or noise caused by a camera inside the device, and compared with the template even when there are other devices around the device such as pad areas. Since the device is discriminated, the device can be extracted accurately.

In addition, even when the fillet of the device is small, the device is extracted using a contrast map according to the height without extracting the device using the fillet, so that the device can be extracted more accurately.

While the present invention has been described in connection with what is presently considered to be practical and exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention. Therefore, the above description and the drawings below should be construed as illustrating the present invention, not limiting the technical spirit of the present invention.

1 is a conceptual diagram illustrating an exemplary three-dimensional shape measuring apparatus used in the three-dimensional shape measuring method according to an embodiment of the present invention.

2 is a flowchart illustrating a three-dimensional shape inspection method according to an embodiment of the present invention.

3 is a schematic diagram illustrating an example of a shape template.

FIG. 4 is a flowchart illustrating an embodiment of a method of obtaining information of a device in FIG. 2.

FIG. 5 is a conceptual diagram illustrating an embodiment of a method of comparing whether a target device corresponds to a shape template.

6 is an image obtained by obtaining a 2D image as an example of the case where the device is a chip.

FIG. 7 is an image of a chip of FIG. 6 as a contrast map. FIG.

FIG. 8 is an image showing the chip of FIG. 6 as a contrast map reflecting visibility information.

<Short Description of Main Drawing Numbers>

10: measuring object 100: measuring stage part

200: image capturing unit 300: first lighting unit

400: second lighting unit 500: image acquisition unit

600: module control unit 700: central control unit

800: shape template 810: device

820: target element ROI: inspection area

Claims (8)

Generating a shape template that abstracts a shape of a predetermined device; Irradiating grating image light on at least two directions to a measurement object to obtain height information for each pixel of the measurement object; Generating a contrast map in which a contrast is set corresponding to the height information for each pixel; And And comparing the contrast map of the measurement object with the shape template to obtain information of the device corresponding to the shape template in the measurement object. The method of claim 1, wherein the obtaining of the device information corresponding to the shape template from the measurement object comprises: Determining whether the element corresponding to the shape template exists in the measurement object; And And obtaining the size, position, and rotation angle information of the device when the device is present in the measurement object. The method of claim 2, wherein the determining of whether the device corresponding to the shape template exists in the object to be measured comprises: Setting a predetermined inspection area on a printed circuit board on which the device is formed; And Comparing with the contrast map while moving the position of the shape template from the initial position in sequence. The method of claim 3, wherein the comparing of the shape template with the contrast map while sequentially moving the position of the shape template from the initial position comprises: Adding values obtained by multiplying the values set to 0 and 1 according to pixel coordinates with the contrast values of the portions overlapping the contrast map on the shape template; Determining a position indicating a maximum value as a preliminary position where the device exists according to the sequential movement of the position of the shape template; And And determining that the device corresponds to the shape template when the maximum value is equal to or greater than a reference value. The method of claim 1, wherein after obtaining information of the device corresponding to the shape template from the measurement object, 3. The method of claim 3, further comprising determining whether the device corresponding to the shape template is defective. The device of claim 5, wherein the device comprises a chip formed on a printed circuit board. Determining whether the device corresponding to the shape template is defective, Extracting a chip body which is a body of the chip; Removing chip body information for the chip body from chip information for the chip; And And determining whether the chip formed on the printed circuit board is defective formed from the chip information from which the chip body information is removed. The method of claim 1, Illuminating the measurement object with the grid image light in at least two directions to obtain visibility information for each pixel of the measurement object, The pixel-by-pixel contrast is defined as a value obtained by multiplying the height information by the visibility information. The method of claim 1, The shape template is defined by a template determinant comprising the planar size of the device, And the contrast map and the shape template are compared within a predetermined tolerance of the template determinant.

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