SAR AND FLIR IMAGE REGISTRATION METHOD

Abstract: There is disclosed herein, a method used to associate or correspond synthetic aperture radar (SAR) and forward looking infrared (FLIR) images. Based on feature points detectable from both images, a two stage approach is taken to address the issue of SAR and FLIR image registration: an initial registration stage where feature points detected from the FLIR image are transformed into the SAR image coordinates; and a residual registration stage where the SAR and FLIR feature points undergo a "Generalized Hough Transform" from which a maximal subset of matching feature points is obtained and the registration transformation can be derived. These two stages are separated into five steps which comprise the SAR and FLIR Image Registration Method: 1) extracting feature points from said SAR and FLIR images; 2) creating an initial registration of the FLIR image; 3) creating a two-dimensional residual registration utilizing a generalized Hough transform; 4) estimating the registration transformation; and, 5) verifying the registration. This method allows the residual registration to be done in a two-dimensional (rather than six) Hough transform, which results in fast and robust implementation as well as reduce the possibility of false registration.

Description: SAR AND FLIR IMAGE REGISTRATION METHOD FIELD OF THE INVENTION The present invention relates to a method of solving the registration of two images. In particular, this invention relates to a two-stage method of solving the registration of a synthetic aperture radar (SAR) image and of a forward-looking infrared (FLIR) image. BRLEF DESCRIPTION OF THE PRIOR ART Traditional image registration systems rely on similarity between a pair of images to be registered in spatial distribution of the gray-levels using a variant of many correlation techniques. These approaches do not work when spatial gray-level distribution of the image changes between images due to different phenomenology employed by the imaging sensors, such as SAR and FLIR. Feature-based image registration techniques rely on some similarity between the extracted features (other than points) from the two images. Individual feature segments of one image can be matched against those from another using these extracted features. However, these kinds of features either rarely exist in both SAR and FLIR, or are very difficult to extract. Point-based registration techniques, on the other hand, exploit the specific configuration of a set of points in the image that satisfy the imaging models and sensor configurations under perspective transformation. This usually involves searching for a set of parameters in a multi- dimensional space, involving high computational complexity. Registration is needed in a multi-sensor system (such as airborne sensors) to solve the correspondence or matching problem. Given a pair of SAR and FLIR images and the associated sensor models, at least 6 independent parameter values must be determined in order to register the two images. This is generally difficult without some prior knowledge about the sensor platforms (including the sensor models and the associated parameters, sensor positions and orientations). Theoretically, the two images can be registered if all the sensor parameters and sensor platform information are known. However in practice, even with the state-of-the-art sensor technology, registration is still needed due to errors and/or noise in the sensor systems or by the nature of their operation. For SAR and FLIR image registration, this poses a special challenge, since there are few features in the SAR and FLIR images that can be easily used to • correlate the two images. Therefore, it is desirable to provide a method by which the SAR and FLIR images can be registered with low computational complexity and high robustness. SUMMARY OF THE INVENTION This invention relates to a method of solving the registration of a synthetic aperture radar (SAR) image and of a forward-looking infrared (FLIR) image based on the feature points detectable from both images. The method is distinguished by two stages: an initial registration stage and a residual registration stage. In the initial registration, feature points detected from the FLIR image are transformed into the SAR image coordinates. In the residual registration stage, the set of feature points from SAR and that from FLIR undergo a generalized Hough transform (GHT) from which a maximal subset of matching points is obtained and the registration transformation can be derived. The method in the present invention allows the second stage residual registration, with the method used in the first stage, to be done in a two-dimension (2-D) GHT, rather than in higher dimensions. Using available sensor parameters and sensor location information, the registration task can be separated into 5 steps: (1) detecting feature points from both the SAR and the FLIR image; (2) transforming the feature points from the FLIR image into the SAR image using known sensor parameters and sensor platform information; (3) carrying out the GHT for the set of SAR feature points and the set of transformed FLIR feature points in 2-D translation domain, and select the maximal subset of the matching feature points from the GHT; (4) computing the updated registration transformation; and (5) evaluating the registration transformation. As a result, the present invention provides both an image registration transformation that aligns the SAR and the FLIR, and a set of corresponding point pairs between the SAR and the FLIR images. The reduction of registration search space involved in the registration is from six to two dimensions. Consequences of this reduction include a substantial reduction in the possibility of false registration and robust registration against noise and error introduced during data collection. This method also works in cases where the matching set of feature points constitute only a very small portion of the total feature point population in both SAR and FLIR images, with the rest being caused by random clutter in the scene. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram depicting the system diagram for the SAR and FLIR image registration method. DETAILED DESCRIPTION OF THE INVENTION This invention relates to a method for solving the registration of a SAR image and a FLIR image by using feature points extracted from both images. The method of this mvention is described below as a process involving five distinct steps. The necessary input to this invention lies in feature points described in Step 1 below, and the output of this invention includes 1) the correspondence of the feature points from SAR and FLIR images; and 2) a registration transformation for transforming any point in the FLIR image into the SAR image. Step 1: Feature Point Extraction Feature points are first extracted from both SAR and FLIR images by using one of the well- known algorithms such as the constant false-alarm rate (CFAR) detector. The requirement for feature points to be used in this invention is that they must represent certain points detectable from both the SAR and the FLIR, and can be located similarly in both images. An example of such feature points is the location of certain objects in the scene. In target detection/recognition applications, feature points can optionally have target-likeness measures attached. Step 1 assumes that the two images have overlap in the space they cover and that common feature points can be detected from both images. If these assumptions are not true, then the registration will fail and the situation can be detected in Step 3 or 5. There is a parameter in feature extraction called windows size w, which determines, among other things, how close two detected features can be (measured in image pixels). The window size is determined by the nature of the features in the image (such as object size). A window size of w=21 for SAR is used as a non- limiting example in the following description. A specific window size for FLIR is not required for the practice of the current invention. Step 2: Initial Registration Initial registration is the process of transforming the feature points detected in the FLIR image into the ground plane coordinate frame of the SAR image, using available (possibly with noise and error) sensor truth data. The sensor truth data includes sensor parameters (including, but not limited to, resolution for SAR and FLIR and the focal length for the FLIR), and the sensor position information (location of the sensors, plus azimuth, pitch, and roll angles for the FLIR). To transform the FLIR feature points into SAR, a FLIR sensor-imaging model is created in a homogeneous coordinate system: Eq. (1) where P is a three-dimensional (3-D) point in the camera coordinate frame representing the point being imaged, (X, Y) is the image of P, u is a scaling factor, and / is the camera focal length. P can be represented by its coordinates (x, y, zf in a reference frame, W, as follows: P = R d R a Eq. (2) where R r , R d and R a are 3 by 3 orthonormal rotation matrices representing the roll angle θ r , the depression angle θ d , and the azimuth angle θ a of the camera frame relative to the reference frame, and (t x , t y , tN is the position vector of the origin of the camera frame in the reference frame W. For simplicity, in the rest of this section, it is assumed that the reference frame is such that its XOY plane coincides with the SAR image ground plane, its X and Y axes coincide with the X and Y axes of the SAR image, respectively, and its Z axis points upwards. Assuming that all points of interest that the FLIR camera sees lie on a ground plane with z = h in the chosen reference frame, Equations (1) and (2) can be solved for x andy, resulting in a back-projection transformation from an image point (X, 7) to a 3-D point (x, y, z): A ! XA-A 2 Y+A 3 A 4 X+A 5 Y+A 6 y = - z = h Eq. (3) A 7 X+A 8 Y+A 9 A 7 X+A 8 Y+A 9 where J^ cos^ sm^- + ^-t.Xsin^ sin^ sin^^-cos^ cos^.)) A 2 =-cosφ d )cosφ -(h-t (smφ S mφ d )cos(θ r ) + co S φ a )sm(θ l .)) J 3 = f(__φ d )t x -smφ a ) C o S φ d )(h-t z )) A 4 = s(θ d ) S m(θ r )t y + (h- t z )(sm(θ a )cos(θ r ) + ∞&ψ a ) sin(0 sin(0 r )) A ; = -cosφ d ) C os(θ r )t y +(h-t 2 )( S φ a )smφ r )- C osφ a )sinφ d )cosφ r )) A 6 =f(s φ d )t y -∞sφ a )∞sφ d h-t t )) A j = cos(# rf )sin(0 r ) 4 = -cos(0 rf )cos(ι r ) A 9 = fs {θ d ) The back-projection transformation of Equation (3) is applied to each feature point in the FLIR image. Then these back-projected feature points can be simply scaled into the SAR image coordinates as follows: Eq. (4) where (x, yf is the back-projection of the FLIR feature point in the reference frame W, (xs, ys) is the back-projection in the SAR image, and c is a constant reflecting the resolution of the SAR image. In the remaining discussion, reference is made to the mathematical transformation involving both Equations (3) and (4) together as the "back-projection." Let {(xs^ysi)}, i = 1,..., N be the set of back-projected FLIR feature points, and {(xr t , yri)}, i = l,..., M be the set of SAR feature points. The registration problem now is to match a subset of {(xs;, ys t )} with a subset of { xri, yr t )} . If accurate measurements of the sensor truth are available, a subset of {(xs,, ysi)} will map exactly onto the corresponding subset of {(xr it yr,)}, assuming the detection locations are precise and the related assumptions mentioned before are true. In reality, however, sensor truth data is often not accurate due to noise and measurement errors. The noise and error in measurements can cause residual registration errors so that a SAR feature point and the back- projected FLIR feature point do not align exactly even if they correspond to the same feature point in the scene. Therefore, solving the general problem of matching such two sets of feature points involves searching in a parameter space of at least six dimensions (three for rotation and 3 for translation) to account for the measurement errors and noise. This is assuming the sensor parameters (sensor resolutions and focal length) can be accurately measured. Step 3: Residual Registration Parameter search space reduction Equation (3) represents a general formula that performs the back-projection of any image point into a 3-D point. An in-depth analysis of the relationship between changes in the sensor truth and the outcome of the back-projections of the FLIR feature points (xs, ys)' shows that, when the distance from the FLIR camera to the center of the FLIR field-of-view (FOV) is far larger relative to the size of the area covered by the FLIR FOV, the residual registration error is predominantly caused by changes in the depression angle θj and the azimuth angle θ a . Changes in other sensor truth values only cause relatively small changes in the back-projection and are negligible. For example, a small change in camera roll angle θ r causes skew in the resulting back-projection, and small changes in camera location cause small changes in translation in the back-projection. Furthermore, the change in location of the back-projection of the FLIR feature points due to inaccuracies in depression angle θ d and azimuth angle θ a can be modeled approximately as a pure translation. In other words, the mis-registration of the two sets of feature points can be solved in 2-D, because the two subsets of feature points (one from the SAR and one from the FLIR after back-projection) that correspond to the same physical feature points only differ in location by a translation. To solve this 2-D translation, only one pair of feature points is required: one each from the SAR and FLIR images. Unless the correspondence (which point in SAR corresponds to which point in FLIR) is known, this method cannot be used. Besides, replying on only one pair of points is neither accurate nor robust. All available feature points must be used. Both the correspondences of the feature points and the translation that will eliminate the residual registration error must be found. The GHT can help solve both problems at the same time. In the GHT implementation, a 2-D accumulation array is set up with array indices measuring the translation between the SAR feature points and the corresponding back-projected FLIR feature points. All possible pairs of feature points from SAR and FLIR are enumerated. For each such pair, the translation needed to bring them into registration is calculated, and one vote is cast in the GHT accumulation array using the translation as the index. In the end, the GHT accumulation array element with the highest score (which combines the number of votes and other factors. See below for definition.) represents the correct translation, and the set of feature points which voted for the element makes up the matching point set. The GHT is chosen to accomplish this task because it is a non- iterative approach, very simple to implement, and most importantly, robust against noise. The "noise" includes both the position errors in the detected feature points, and the undesired feature points due to clutter, which do not have correspondence in the other image. The GHT implementation is more thoroughly described below: Generalized Hough Transform (GHT) Size of the GHT accumulation array The array size is determined by the span of the back-projected FLIR feature points and the SAR feature points as shown in the following figure. Determining GHT Accumulation Array Size @ SAR feature points □ FLIR feature points X X... X... The GHT array will be of size (X ma χ-X m in) 7+l by (F mαϊ -F mi „)/^+l, where q is the quantization unit (that is, the cells in the accumulation array represent squares of a size q by q). The quantization unit q is determined according to the window size w (see above): q = [w/4 ~ ], where [x] is the smallest integer greater thanx. To increase memory efficiency in GHT, in case the groups of SAR feature points and the back- projected FLIR points are far from each other, one set of feature points (say that back-projected from FLIR) can be shifted before constructing the accumulation array so that the two sets occupy roughly the same space. This can be done, for example, by calculating the centers of gravity of both sets of points and forcing them to align with each other by shifting the back-projected FLIR points. This approach reduces total span of the feature points and therefore the size of the accumulation array. However, this extra shift must be taken into account when the actual translation is calculated in figuring out the residual registration. Let the total shift added to the back-projected FLIR points to accomplish this be denoted as (Δx g , Ay g ). The GHT Algorithm 1. Let C be a 2-D array of appropriate size. Each element in C represents a q by q square in the translation parameter space, where q is a quantization unit. Set all elements in C to zero (0). 2. Let {(xs t , ysi)}, i = l,..., N be the set of back-projected FLIR feature points, and {(xr,, yri)}, i = l,..., M be the set of SAR feature points. Perform the following iterations: for each (xs,-, ysi), i=\ to N for each (xr,-, yrj),j=l to M (dx, dy) <r ((xs l ~xr J )l q, (ys-yr^lq) C(Dx, Dy) r C(Dx, Dy)+\ C(Dx-\, Dy) <r C(Dx-\, Dy)Λ-\ C(Dx, Dy-\) <r C(Dx,Dy-l)+l C(Dx-\, Dy-l) r C(Dx-l, Dy-l)+l end end where d is the largest integer smaller than x. In this step, each pair of SAR and FLIR feature points cast votes in multiple adjacent cells. Therefore in effect the accumulation cells have overlapping coverage. This is also equivalent to convolving the resulting accumulation array, casting one vote for each pair, with a kernel of 2 by 2 containing all 1.0's. The use of overlapping cells prevents from the situation in which the true translation is located on or near the boundaries of two or more cells. This situation causes votes being split among the nearby cells and can result in a wrong solution. 3. Let L be a list of accumulation cells whose vote count is greater than zero and whose location standard deviation σ (defined below) is less than a set threshold T \ . Compute the scores for each cell in the list E. The score of each accumulation cell is a function of a) the number of votes; and b) the likelihood of the feature points (from both images) voting for the cell being from a set of objects with common features. For this invention, the score is defined as the sum of FLIR feature strength of all the pairs in the cell. The FLIR feature strength is in turn the average contrast of a FLIR feature against its background. The score function can also include feature strength from SAR. The purpose of this "scoring function" is to rank the results of the GHT. The location standard deviation σι is the geometric mean of the standard deviation of dx and dy for the votes in a given cell: σ / = σ x 2 + σ y , where σ x and σ y are the standard deviations of dx and dy, respectively. The threshold for the location standard deviation is set to 2 T \ = —q , where q is the cell quantization unit. The constant, 2/3, is used to maintain high confidence in the results obtained. Consequently, the use of location standard deviation effectively screens the results of GHT. 4. Let (dxm, dym) be the indices for the cell in L with the maximum score. 5. If the vote count K in C at (dxm, dym), K=C(dxm, dym), is less than 4 (i.e., C(dxm, dym)<4), then report failure and quit. Otherwise report success and let the set of pairs voted for the cell at (dxm, dym) constitute the matching solution. Step 4: Registration Transformation Estimation As the result of GHT in the last step, the set of matching pairs of feature points from SAR and FLIR is obtained. With this set of matching points the registration transformation can be estimated, and can be used to transform any point (not only the points in the matching set) in the FLIR image into the SAR image. Two methods for estimating the registration transformation are described. Least-Squares Method With a set of four or more pairs of matching feature points, it is easy to estimate the registration transformation between the SAR and FLIR images using a least-squares method. Substituting Equation (4) into (3), and normalizing the coefficients, the following equations for transforming the FLIR image points into the SAR images are obtained: xr = } yr_ Eq. (5) B 7 X+ B 8 Y+ l B 7 X+B s Y+ l where (X, Y) are the coordinates of the FLIR feature points, (xr,yr) are the coordinates of the corresponding SAR feature points, and where c and A„, n- 1 , ... ,8 are defined in Equations (4) and (3), respectively. After rearranging the terms in Equation (5), the following result is obtained: [ I B.1X + B 2 2Y +B 3 3 - xr(B η 7X + B s + 1) J = 0 E (7) [B 4 X + B 5 Y + B 6 - yr(B η X + BJ + 1) = 0 Each pair of matching points gives two simultaneous linear equations in the form of Equation (7). To solve for the eight parameters Bj to B 8 in Equation (7), at least 4 pairs of matching points are required. When there are 4 or more pairs of matching points, the least-squares solution for the parameters Bι to B 8 may be found, which minimizes the following error term: E = ∑( + ) Eq. (8) where K is the number of point pairs, and e xi and e yi are the error terms based on Equation (7): e 3d = B l X l +B 2 Y l +B i -xr l (R,X l +B t Y, +l) \ e yi = B 4 X, +BJ +B 6 -yr t (R,X, +B t +1)/' ' "*A ^ W Inverse Jacobian Method The least-squares method given above solves the 8 parameters of the transformation from scratch using a set of matching point pairs. Since the initial registration transformation is already given by Equation (3), the final registration transformation may also be found by updating the initial transformation using the set of matching point pairs found in the GHT. As discussed in "parameter search space reduction" in Step 3, the residual registration error is caused mainly by the errors in the depression angle θ d and the azimuth angle θ a . Therefore, it is possible to build a relationship between the residual registration error and the two angles involved. The following Jacobian matrix is defined: dxr dxr J _ "X* " ' Eq. (10) dyr dyr /is the Jacobian matrix of (xr, yr) in Equation (3) with respect to the two rotational parameters, the depression and the azimuth angles θ d and θ a , respectively. Updates to these two parameters are generated by use of the following: where (Δx, Ay) is the residual translational registration error, and J "1 is the inverse of the Jacobian matrix. (Δx, Ay) can be determined by averaging the dx's and dy's for all the votes in the cell (dxm, dym) from the result of GHT in Step 3: where K= C(dxm, dym) is the number of votes in the accumulation array at (dxm, dym), dxi and dyi , 1=1,..., K, are the components of translation for the pairs of feature points at (dxm, dym), and Ax g and Ay g are the global shift that was applied to the back-projected FLIR points (see page 8, line 13). After updating θ d and θ a in the sensor truth data according Equations (11) and (12), the transformation parameters Bj to B 8 can be recomputed by computing the parameters of A to Ag in Equation (3), plus z = h (where h is the ground plane elevation). In experiments, h = 0 was used. But h should be substituted with known terrain elevation for the imaged area. Once Aj to J have been computed, the coefficients Bj to B 8 for the transformation illustrated in Equation (5) can be derived according to Equation (6). Step 5: Registration Verification The registration verification step has two purposes. One is to ensure that the set of matches (the pairs of SAR and FLIR feature points) are indeed geometrically plausible. The second is to pick up additional possible matches or drop out some erroneous matches. The verification is done by first transforming all the FLIR feature points into the SAR image using the newly estimated parameters Bi to B 8 in Step 4 according to Equation (5). Then for each transformed FLIR point, a search is done for the closest SAR feature point within a distance in of 4 (pixels) in both X and Y directions. Again, the number 4 is preferably used to maintain high confidence. If such a SAR feature point is found, then the pair of FLIR and SAR feature points will be added to the new matched point list. Once the verification is done, the new matched point list is checked to determine whether it contains more than 4 points. Ifyes, then the registration is complete. If not, then the registration has failed.

Claims: CLAIMS What is claimed is: 1. A method for obtaining a set of matching feature points from SAR and FLIR images, comprising the following steps: extracting feature points from SAR and FLIR images; creating an initial registration of said FLIR image; creating a two-dimensional residual registration said FLIR image utilizing a generalized Hough transform; estimating a registration transformation; and then, verifying the registration. 2. A method for obtaining a set of matching feature points from SAR and FLIR images as recited in claim 1, wherein the extracting step comprises: using an algorithm such as a constant pulse-alarm rate (CFAR) detector. 3. A method for matching feature points from said SAR and FLIR images as recited in claim 1, wherein the SAR image has a ground plane and X, Y, and Z axes, wherein the FLIR image has a ground plane and X, Y, and Z axes, and wherein the step of creating an initial registration comprises: using a homogeneous coordinate system: uX 1 0 0 uY = 0 1 0 Eq. (1) u 0 0 Xlf where P is a three-dimensional (3-D) point in a camera coordinate frame having an origin and representing a point being imaged, (X, Y) is the image of P, w is a scaling factor, and / is a camera focal length; representing P by its coordinates (x, y, z)' in a reference frame, W, having a XOY plane, as follows: ■ where R r , R d and R a are 3 by 3 orthonormal rotation matrices representing a roll angle Θ r , a depression angle θ d , and an azimuth angle θ a of the camera coordinate frame relative to the reference frame, and (t x , t y , t z )' is a position vector representing the origin of the camera coordinate frame in the reference frame W; assuming that the reference frame is such that its XOY plane coincides with the SAR image ground plane, its X and Y axes coincide with X and Y axes of the SAR image, respectively, and its Z axis points upwards relative to the image ground plane, and that all points in the FLIR images lie on a ground plane with z = h representing a ground plane elevation relative to the reference frame W; solving for x and , resulting in a back-projection transformation from an image point (X, Y) to a 3-D point (x, y, z): x = - y = Eq. (3) A 7 X+A 8 Y+A 9 A 7 X+A 8 Y+A 9 where A l -= C o S (θ d ) S (θ + (h-t z )(s (Θ sin(θ d )s (θ r )-co S (θ a )co S (θ r )) A 2 -=-cos(θ d )cos(θ -(h-t z )(sin(Θ sm(θ d )cos(θ r ) + cos(θ a )sm(θ r )) A 3 = f(sin(θ d )t x -sm(θ a )co S (θ d )(h-t z )) A 4 = cos(θ d )sm(θ r )t y A-(h-t z )( S (θ a )cos(θ r ) + cos(Θ sm(θ d )sin(θ r ^ A, = -∞s(θ d )cos(θ r )t y +(h-t z )(sm(Θ sm(θ r )-cos(θ a )sia(θ d )cos(θ r )) A. _ ftsint/ Λt - nsrø r.nsrøθ d Λ)((hh --tt z Y))> = -cos(0 rf )cos(ι9 r ) = /sin(6> applying the back-projection transformation of Equation (3) for each feature point in the FLIR image; scaling the back-projected feature points into the SAR image coordinates as follows: where (x, y is the back-projection of the FLIR feature point in the reference frame W, (xs, ys) is the back-projection in the SAR image, and c is a constant reflecting the resolution of the SAR image; and then, generating {(xs,-, ysi)}, i = l,..., N as a set of back-projected FLIR feature points, and {(xr,-, yri)} , i = l,..., M as a set of SAR feature points. 4. A method for matching feature points from SAR and FLIR images as recited in claim 3, using a generalized Hough transform to create a two-dimensional residual registration, wherein the creation step comprises: determining an array size from the span of the back-projected FLIR feature points and the SAR feature points; having a Generalized Hough Transform array of size by (Y max - Y min )/q+l, where q represents a quantization unit; determining the quantization unit q according to a window size w: q = [ w/ 4 ~ ], where fxl is the smallest integer greater than x; and then, using the five-step Generalized Hough Transform (GHT) to generate a two-dimensional residual registration list. 5. A method for matching feature points from SAR and FLIR images as set forth in claim 1 , wherein the estimation step comprises: using a Least-Squares Method or, alternatively, an Inverse Jacobian Method; and then, updating said registration transformation using an Inverse Jacobian Method. 6. A method for matching feature points from SAR and FLIR images as set forth in claim 1, wherein the SAR images each comprise a plurality of feature points, with each feature point comprised of pixels, with each pixel represented by x and y coordinates, wherein the FLIR images each comprise a plurality of feature points, with each feature point comprised of pixels, with each pixel represented by x and y coordinates, and wherein the verification step comprises: transforming all of the FLIR feature points into SAR coordinates using a Least-Squares Method or, alternatively, an Inverse Jacobian Method to generate a set of newly estimated parameters Bi to B 8 ; performing a search for the closest SAR feature point to each transformed FLIR feature point within a distance of 4 pixels in both x and y coordinates; adding the pairs of FLIR and SAR feature points to the two-dimensional residual registration if such SAR feature points are found; and then, checking the two-dimensional residual registration, and determining the registration to be complete when it contains more than 4 points.

Inventors: CHEN YANG, CHEN, YANG
Applicants: CHEN YANG, HRL LAB LLC, CHEN, YANG, HRL LABORATORIES, LLC
Application year: 2001
Main classification: G06T7
IPC codes: G01S13, G06K9, G06T1, G06T7