Source: http://www.google.com/patents/US7479982?dq=7,403,220
Timestamp: 2015-04-26 01:32:39
Document Index: 664854544

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Patent US7479982 - Device and method of measuring data for calibration, program for measuring ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsA device for measuring data for calibration for obtaining data for calibration of a camera 2 capable of varying its optical conditions, wherein the data for calibration are obtained using a plurality of images of a calibration chart 1 having marks arranged thereon which were photographed with the camera...http://www.google.com/patents/US7479982?utm_source=gb-gplus-sharePatent US7479982 - Device and method of measuring data for calibration, program for measuring data for calibration, program recording medium readable with computer, and image data processing deviceAdvanced Patent SearchPublication numberUS7479982 B2Publication typeGrantApplication numberUS 10/612,404Publication dateJan 20, 2009Filing dateJul 3, 2003Priority dateJul 3, 2002Fee statusPaidAlso published asDE60317976D1, DE60317976T2, EP1378790A2, EP1378790A3, EP1378790B1, US20040066454Publication number10612404, 612404, US 7479982 B2, US 7479982B2, US-B2-7479982, US7479982 B2, US7479982B2InventorsHitoshi Otani, Nobuo Kochi, Takayuki NomaOriginal AssigneeTopcon CorporationExport CitationBiBTeX, EndNote, RefManPatent Citations (33), Non-Patent Citations (9), Referenced by (18), Classifications (16), Legal Events (3) External Links: USPTO, USPTO Assignment, EspacenetDevice and method of measuring data for calibration, program for measuring data for calibration, program recording medium readable with computer, and image data processing device
US 7479982 B2Abstract
FIG. 2 is a plan view, illustrating an example of a chart for calibration. The chart 1 is a flat sheet having first marks easy to see and a plurality of dots as the second marks printed thereon. Five first marks are provided on the chart 1. Each of the first marks is a rhombus with a mark which is the same as the second mark at the center. When the chart 1 is divided into four quadrants, each of the quadrants has one first mark. Namely, first marks 1 a, 1 b, 1 c and 1 d are located in the top left quadrant, top right quadrant, bottom left quadrant, and bottom right quadrant, respectively. A first mark 1 e is located at the point of origin. For example, the first marks 1 a, 1 b, 1 c and 1 d are located at the same distance �d� from the first mark 1 e. Supposing �h� and �l� be the vertical distance from the first marks 1 a or 1 b to the first mark 1 e, and the lateral distance from the first marks 1 c or 1 d to the first mark 1 e, respectively, the distance �d� from the first mark 1 e to the first mark 1 a, 1 b, 1 c, or 1 d satisfies the following relation:
d=(h 2 +l 2)1/2 (1)
Description will be made of the procedure for photographing the chart 1 with a camera 2 as an object of calibration. FIG. 5 is a perspective view illustrating the positions of a zoom camera in measuring the lens aberration at various focal lengths of the camera. Calibration can be performed when there are at least two images of the chart 1 photographed from different angles. When a flat chart printed on a sheet is used as the chart 1, the chart 1 is preferably photographed from at least three angles. Thereby, stable and reliable measurement of calibration elements, focal length, in particular, can be performed. FIG. 5 shows the procedure for photographing the chart 1 from the front (I), upper left side (II), upper right side (III), lower left side (IV) and lower right side (V). The incident angle of the optical axis of the camera 2 with respect to the flat chart is preferably in the range of 10 to 30 degrees when the depth accuracy on site is set to about 1 cm. In view of the fact that the distance range in which the camera can be focused is limited because of the focal depth of the lens, the incident angle is preferably in the range of 12 to 20 degrees. Typically, the incident angle is 15 degrees. The �various focal lengths� means the focal lengths equivalent to those of normal lens, wide-angle lens and telescopic lens in a single lens reflex camera.
(II): When the focal length of the zoom camera is equivalent to that of a telephoto lens or a normal lens, the camera is moved to a position at a distance about ⅓ the object distance H from the front position so that the first mark 1 a, for example, in the upper left quadrant of the chart 1 is located at the center of the image area (FIGS. 6(B), 6(F)). When the focal length of a zoom camera is equivalent to that of a wide-angle lens and the when the object distance H is within 1 m, the camera 2 is moved so that the target first mark is located in front of it. Then, the camera 2 is turned so that the first mark 1 e at the center of the chart 1 comes to the center of the image area with its position maintained (FIGS. 6(C), 6(G)). The camera 2 is then moved closer to the chart 1 so that the first and second marks fill the image area, and the image is photographed (FIGS. 6(D), 6(H)).
(III): The camera is moved so that the first mark 1 b in the top right quadrant of the chart 1 comes to the center of the image area. Then, the camera is turned so that the first mark 1 e at the center of the chart 1 comes to the center of the image area. The camera 2 is then moved closer to the chart 1 so that the first and second marks fill the image area, and the image is photographed.
(IV): The camera is moved so that the first mark 1 c in the bottom left quadrant of the chart 1 comes to the center of the image area. Then, the camera is turned so that the first mark 1 e at the center of the chart 1 comes to the center of the image area. The camera 2 is then moved closer to the chart 1 so that the first and second marks fill the image area, and the image is photographed.
(V): The camera is moved so that the first mark 1 d in the bottom right quadrant of the chart 1 comes to the center of the image area. Then, the camera is turned so that the first mark 1 e at the center of the chart 1 comes to the center of the image area. The camera 2 is then moved closer to the chart 1 so that the first and second marks fill the image area, and the image is photographed.
The distance H between the camera 2 and the chart 1 is obtained from the focal length f of a zoom camera. For example, when the focal length of a zoom camera is equivalent to that of a normal lens of a 35 mm camera, the object distance H is about 90 cm. The distance �d� between the first marks on the chart 1 is 20 cm, for example. Thus, when the photographing direction is inclined from the front position (I) to the upper left position (II) and so on, a photographing angle of about 10 degrees can be secured.
FIG. 7 is a view illustrating a camera distance in measuring the lens aberration when the focal length of a zoom camera is equivalent to that of a normal lens or a telephoto lens. When the focal length of a zoom camera is equivalent to that of a normal lens or a telephoto lens, the viewing angle to the photographing lens is narrow and the camera cannot be inclined very largely. Thus, when the photographing angle is inclined from the front position (I) to the top left position (II) and so on, a photographing angle of 10 degrees cannot be secured. This is because the distance H between the camera 2 and the chart 1 is 1 m or longer and the distance �d� between the first marks is about 20 cm when the focal length is long. Thus, the camera positions (II) and (IV) on the left side and the camera positions (III) and (V) on the right side are determined with respect to the front position (I). At this time, the camera is shifted by a distance of about one-third the object distance H from the front position (I). Then, photographing at the top left position (II), bottom left position (IV), top right position (III) and bottom right position (V) are performed. The optical axis of the camera is aligned with the normal line of the chart 1 or may be directed toward the chart 1.
In the first mark extraction process, in order to determine second-order equations for projection conversion of the plane coordinates of the chart 1 into image coordinates (camera side), the positions of at least three first marks out of the first marks on the plain coordinate system are measured on the image data. Here, since the first marks include the second marks therein, the positions of the first marks can be designated precisely by designating the positions of the second marks included in the first marks. In the first mark extraction process, the steps I-(1) to I-(4) are repeated for all the first marks. For example, in the case of the chart 1 shown in FIG. 2, the process is performed on the first marks 1 a, 1 b, 1 c and 1 d. I-(1): The operator points the cursor of the mouse to the second mark in the first mark to be detected on the entire image displayed on the display part 150 and clicks the mouse thereon to obtain the approximate position of the first mark.
X=(b1�x+b2�y+b3)/(b7�x+b8�y+1)
Y=(b4�x+b5�y+b6)/(b7�x+b8�y+1) (2)
ω=tan−1(C�b8)
φ=tan−1(−C�b7�cos ω)
κ=tan−1{−(A1�A3−A2�A4)/(A1�A2−A3�A4)} (φ≠0 and ω=0)
Z0=C�cos ω�{(A22+A32)/(A12+A42)}�+Zm
X0=b3−(tan ω�sin κ/cos φ−tan φ�cos κ)�(Zm−Z0)
Y0=b6−(tan ω�cos κ/cos φ−tan φ�sin κ)�(Zm−Z0) (3)
Wherein, A1=1+tan2 φ, A2=B1+B2�tan φ/sin ω, A3=B4+B5�tan φ/sin ω, A4=tan φ/(cos φ�tan ω), Zm is the average of the heights of the reference points 1 a, 1 b, 1 c and 1 d, and C is the focal length and corresponds to the screen distance. Here, the reference points 1 a, 1 b, 1 c and 1 d are on a plain coordinate system and thus assumed to form a uniform height plane.
( x p y p z p ) = ( 1 0 0 0 cos ω - sin ω 0 sin ω cos ω ) ( cos ϕ 0 sin ϕ 0 1 0 - sin ϕ 0 cos ϕ ) ( cos κ - sin κ 0 sin κ cos κ 0 0 0 1 ) ( X - X 0 Y - Y 0 Z - Z 0 ) = ( a 11 a 12 a 13 a 21 a 22 a 23 a 31 a 32 a 33 ) ( X - X 0 Y - Y 0 Z - Z 0 ) ( 4 ) wherein, (X0, Y0, Z0) are the ground coordinates of the projection center Oc as shown in FIG. 11(A).
x=−C�{a11(X−X0)+a12(X−X0)+a13(Z−Z0)}/{a31(X−X0)+a32(X−X0)+a33(Z−Z0)}
y=−C�{a21(X−X0)+a22(X−X0)+a23(Z−Z0)}/{a31(X−X0)+a32(X−X0)+a33(Z−Z0)} (5)
In the calculation of tan−1 in the equations (3), two solutions are obtained. Thus, each of the inclinations ω, φ and κ has two solutions. Here, all the solutions are calculated and correct ω, φ and κ are calculated by comparing the residuals between the image coordinates of the four first marks 1 a, 1 b, 1 c and 1 d and the image coordinates of the corresponding points obtained using the equations (5).
FIG. 11(B) is an explanatory view of a template image for normalized correlation and an object image for use in the target recognition. At first, an arbitrary target is selected from the gravity centers of the targets of the first marks such as the first marks 1 a, 1 b, 1 c and 1 d measured in the first mark extracting step (S206). The template image for normalized correlation is an M�M pixel image centered around the gravity center (image coordinates) of the target. The object image is an N�N pixel image centered around the approximate position (image coordinates) of the target calculated in the step of measuring the approximate positions of the second marks (S208).
A={M 2�Σ(Xi�Ti)−ΣXi�ΣTi}/[{M 2 �ΣXi 2−Σ(Xi)2 }�{M 2 �ΣTi 2−Σ(Ti)2}]1/2 (6)
Description will be made with reference again to FIG. 10. Sub-pixel edge detection is performed on the second marks (S304). The object image on which the sub-pixel edge detection of the second marks is performed is an N�N pixel image centered around the detecting point recognized as a target in step S62. Laplacian-Gaussian filter (LOG filter) as a quadratic differential of a Gauss function expressed by the equation (7) is applied to the brightness waveform in the object image and the two zero crossing points on a curve as a result of calculation, namely the edges, will be detected with sub-pixel accuracy:
∇2 �G(x)={(x 2−2σ2)/2πσ6}�exp(−x 2/2σ2) (7)
For the process for calculating the internal parameters of the camera in the internal parameter calculating part 134 is, �bundle adjustment with self-calibration� used in the field of photogrammetry is used. The �bundle adjustment� is a method in which an observation equation is set up for each of light bundles of each image based on the collinearity condition that light bundles connecting the object, lens and CCD surface should be on one line, and the position and the inclination of the camera (exterior orientation elements) and the coordinate positions of the second marks are simultaneously adjusted by a least square method. With the �bundle adjustment with self-calibration�, the calibration elements, namely the inner orientations of the camera (lens aberration, principle point position and focal length) can be also obtained. The collinearity condition basic equations of bundle adjustment with self-calibration (which will be hereinafter referred to as �bundle adjustment�) are the following equations (8) and (9):
The three-dimensional reference chart 20 shown in FIG. 14 has targets 20 a to 20 h whose positions have been three-dimensionally measured with precision. The number, heights, and plane coordinates are appropriately determined so that the chart 20 can be suitable for three-dimensional measurement. The camera 2 is used to photograph the three-dimensional reference chart 20 in stereo at a pair of right and left photographing positions 2L and 2R. The data of a pair of stereo images photographed with the camera 2 are sent to an image data storing part 110 via an image information recording medium or the like. The distance between the right and left photographing positions 2R and 2L, which is referred to as baseline distance, is measured precisely.
The mark extracting part 171 extracts targets 20 a to 20 h included in the stereo images of the three-dimensional chart 20 and measures the positions of the targets 20 a to 20 h on an image coordinate system. The internal parameter calculating part 173 calculates the internal parameters of the camera 2 as data for calibration using the data on the positions of the targets 20 a to 20 h measured in the mark extracting part 171 and the positions of the targets 20 a to 20 h stored in a three-dimensional reference chart target storing part 175, and adjusts the exterior orientation elements and the coordinates of objective points of the data of the paired right and left stereo images simultaneously. The positions of all the targets 20 a to 20 h of the three-dimensional chart 20 are stored in the three-dimensional reference chart target storing part 175. The internal parameters calculated in the internal parameter calculating part 173 are stored in the calculated internal parameter value storing part 177.
The mark extracting part 171 is preferably provided with a function of removing the stereo images of the three-dimensional reference chart 20 in which the targets 20 a to 20 h does not clearly appear. The internal parameters of the camera calculated in the internal parameter calculating part 173 are preferably stored in the calculated internal parameter value storing part 177 together with the focal lengths at which the three-dimensional reference chart was photographed.
Then, the mark extracting part 171 extracts marks formed on the chart 1 from the images taken from the chart (S105). The internal parameter calculating part 173 calculates the calibration elements of the camera 2 (S106). The operation of the component parts of the calibration element calculating part 170 (mark extracting part 171 and the internal parameter calculating part 173) is as follows. The mark extracting part 171 extracts the targets 20 a to 20 h photographed in the stereo images of the three-dimensional reference chart 20 and measured the positions of the targets 20 a to 20 h on an image coordinate system. Then, the internal parameter calculating part 173 calculates the internal parameters of the camera 2 as data for calibration using the data on the positions of the targets 20 a to 20 h measured in the mark extracting part 171 and the positions of the targets 20 a to 20 h stored in a three-dimensional reference chart target storing part 175. The internal parameters of the camera calculated in the internal parameter calculating part 173 are stored in the calculated internal parameter value storing part 177 together with the focal lengths at which the three-dimensional reference chart was photographed.
Description will be made of an example in which stereo image measurement is performed in a real field using the image data processing device 200. FIG. 18 shows a stone wall as an example of the field for stereo image measurement. A stone wall has a three-dimensional configuration similar to a real field such as a historical site or a civil engineering site and thus is suitable for a field for experiment. In the field for stereo image measurement shown in FIG. 18, there are 49 control points represented by the white dots, and their positions have been measured with a depth accuracy of �1 mm. In the image data processing device 200, eight points selected from the 49 control points are used for calculation of the focal length of the camera as control points for use in the bundle adjustment with self-calibration. The other 41 control points are used for measurement of depth accuracy by stereo image measurement.
FIG. 19 is a view illustrating the photographing conditions of the camera, and shows the object distance H and the photographing baseline length B corresponding to the focal lengths equivalent to those of wide, intermediate and telephoto lenses. The focal length of the camera 2 (approximate value) is set to 9 mm in case 1 (wide), 30 mm in case 2 (normal), and 42 mm in case 3 (telephoto). The size of the photographed area of the stone wall is 2 m�2 m.
σxy =[H/f]�σ p (14)σz =[H/f]�[H/B]�σ p (15)
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