Patent Application: US-201214234907-A

Abstract:
the present invention relates to a high precision method , model and apparatus for calibrating , determining the rotation of the lens scope around its symmetry axis , updating the projection model accordingly , and correcting the image radial distortion in real - time using parallel processing for best image quality . the solution provided herein relies on a complete geometric calibration of optical devices , such as cameras commonly used in medicine and in industry in general , and subsequent rendering of perspective correct image in real - time . the calibration consists on the determination of the parameters of a suitable mapping function that assigns each pixel to the 3d direction of the corresponding incident light . the practical implementation of such solution is very straightforward , requiring the camera to capture only a single view of a readily available calibration target , that may be assembled inside a specially designed calibration apparatus , and a computer implemented processing pipeline that runs in real time using the parallel execution capabilities of the computational platform .

Description:
we propose a complete solution for the calibration , online updating of the projection parameters in case of lens rotation , and radial distortion correction in real - time . the solution comprises the modules and blocks shown in the scheme in fig3 and the apparatus of fig1 , being that the operator starts by acquiring a single image of a checkerboard pattern using the setup in fig2 . the system herein presented was originally meant for medical endoscopy that typically employs a boroscopic lens probe mounted on a standard ccd camera . however , our solution is extendable to other application domains using cameras with significant radial distortion that might benefit from accurate geometric calibration and real - time image correction . examples of these domains include surveillance , industrial inspection , automotive , robot navigation , etc , that often rely in images / video acquired by cameras equipped fish - eye lenses , wide angular lenses , mini lenses , or low quality optics . the apparatus that is disclosed in this patent application consists of a calibration box ( fig1 ) used to acquire a single image of a known pattern . the acquired image is used as input to the novel calibration algorithm ( fig1 ), that will provide the necessary camera parameters to perform a real - time and lens rotation independent image distortion correction . the calibration images are acquired using a box built using opaque materials for avoiding the entrance of light from the exterior ( fig1 ). the box has one or more openings for giving access to the lens probe ( fig1 - 3 ), and each opening is surrounded by a rubber membrane that minimizes the entrance of light from the exterior ( fig1 - 2 ). the box is divided in two compartments by a separator in transparent or translucent material ( fig1 - 6 ). a planar grid with a known pattern ( fig1 - 7 ), made of transparent or translucent material , is placed in the upper compartment ( fig1 - 1 ) on the top of the separator . a light source , for diffuse back - illumination of the planar grid , is placed in the lower compartment ( fig1 - 5 ). the upper compartment can be filled with a fluid for better replicating the mean where the visual inspection will be carried . the apparatus comprises a computer unit , with sequential and parallel processing capabilities , such as a central processing unit and a graphics processing unit , which is connected to a camera for image acquisition ( fig1 - 4 ). a display and an interface device connected to the computer unit are also required for visualizing and entering user commands . the method for automatic camera calibration , on - line update of the projection parameters in case of lens rotation , and real - time image rectification / correction using parallel processing , is summarized as follows : the corners in the calibration frame are detected , and both the camera intrinsic k 0 and the radial distortion ξ are estimated without further user intervention . after this brief initialization step the processing pipeline on the right of fig3 is executed for each acquired image . at each frame time instant i we detect the boundary contour ω i , as well as the position of the triangular mark p i . the detection results are used as input in an estimation filter which , given the boundary ω 0 and marker position p 0 in the calibration frame , then estimates the relative image rotation due to the possible motion of the lens probe with respect to the camera head . this 2d rotation is parameterized by the angle αi and the fixed point qi that serve as input for updating the camera calibration based on a new adaptive projection model . finally , the current geometric calibration k i , ξ is used for warping the input frame and correct the radial distortion . this processing pipeline runs in real time with computationally intensive tasks , like the image warping and the boundary detection , being efficiently implemented using the parallel execution capabilities of the gpu . user intervention is limited to the acquisition of one or more calibration images using the calibration box of fig2 ( b ), which is detailed in fig1 . the pose of the camera with respect to the checkerboard plane is arbitrary , and the box of fig1 is meant for controlling the illumination conditions for the automatic detection of image corners . the image acquired is similar to fig2 ( a ). b . detection of grid corners in the calibration image and estabilishment of image - plane correspondances it is important to note that locating corners in an image with strong radial distortion can be highly problematic because of the bending of the straight lines . after the frame acquisition the processing steps are : 1 ) localization of a minimum of 12 corners in the center image region where the distortion is less pronounced . this operation is carried out by a heuristic algorithm that uses standard image processing techniques . the consistent correspondence between image corners x and grid points g is accomplished by counting the squares of the checkerboard pattern . 2 ) the image - plane correspondences are used for estimating the function that maps points in planar grid coordinates into points in image coordinates , this is , estimate ĥ 6 × 6 using a dlt - like approach ( equation 7 in section b ). 3 ) the checkerboard pattern is projected onto the image plane using the homography for generating corner hypotheses in the image periphery . these hypotheses are confirmed and refined by applying a standard image corner finder . 4 ) the steps 2 ) and 3 ) are iterated until the number of established correspondences is considered satisfactory . c . determination of the camera intrinsic parameters using the image - plane correspondences in at least one calibration image . the purpose of the initialization procedure in fig3 is to determine the intrinsic calibration matrix k 0 and the radial distortion ξ when the lens probe is at a reference position ω 0 , p 0 . camera calibration is a well - studied topic with hundreds of references in the literature . the most widely used software is probably bouguet &# 39 ; s toolbox [ 16 ] that implements zhang &# 39 ; s method [ 17 ] for calibrating a generic camera from a minimum of 3 images of a planar grid . unfortunately , the bouguet toolbox does not meet the usability requirements of our application . in practice the number of input images for achieving accurate results is way above 3 , and the detection of grid corners in images with rd needs substantial user intervention . our method works with a minimum of one image . several authors have addressed the specific problem of intrinsic calibration of medical endoscopes [ 9 ]-[ 11 ], [ 18 ], [ 19 ]. however , these methods are either impractical for use in the or or they employ circular dot patterns to enable the automatic detection of calibration points , which undermines the accuracy of the results [ 20 ]. a camera equipped with a borescope is a compound system with a complex optical arrangement . the projection is central [ 5 ] and the image distortion is described well by the so - called division model [ 21 ], [ 14 ], [ 13 ]. barreto et al . show that a camera following the projection model described above can be calibrated from a single image of a planar checkerboard pattern acquired from an arbitrary position [ 21 ]. let g be a point in the checkerboard expressed in plane homogeneous coordinates , x is the corresponding point in the image plane and h is the homography encoding the relative pose between plane and camera [ 8 ]. a point g in the checkerboard plane is mapped into the image through the following function : where γ ξ is a nonlinear projective function that accounts for the image radial distortion [ 21 ], and k 0 is the matrix of intrinsic parameters with the following structure : where f , a , and s , stand respectively for the focal length , aspect ratio , and skew , and c =( c x , c y ) t are the non - homogeneous coordinates of the image principal point . let ĝ and { circumflex over ( x )} be homogeneous vectors with dimension 6 corresponding to the lifted representation of points g and x according to a second order veronese map [ 13 ]. it can be proved that with ĥ 6 × 6 being a 6 × 6 matrix . since each image - plane correspondence imposes 3 linear constraints on the lifted homography , the matrix ĥ 6 × 6 can be estimated from a minimum of 12 point correspondences ( x , g ) using a direct linear transformation ( dlt )- like approach . given ĥ 6 × 6 , the matrix of intrinsic parameters k 0 , the distortion parameter ξ , and the original homography h , can be factorized in a straightforward manner . this initial camera calibration can be further refined using standard iterative non - linear optimization . finding the image contour that separates circular and frame regions ( see fig1 ( b )) is important not only to delimit meaningful visual contents , but also to infer the rotation of the lens probe with respect to the camera head . the proposed approach for tracking the boundary contour and the triangular mark across successive frames is related to work by fukuda et al . [ 4 ] and stehle et al . [ 15 ]. the first infers the lens rotation in oblique viewing endoscopes by extracting the triangular mark using conventional image processing techniques . the method assumes that the position of the circular region does not change during operation , which is in general not true , and it is unclear if it can run in real time . stehle et al . proposes tracking the boundary contour across frames by fitting a conic curve to edge points detected in radial directions . the main difference from our algorithm is that we use a hybrid serial + parallel implementation for rendering a polar image and carry out the search along horizontal lines . this strategy makes it possible to reconcile robustness and accuracy with low computational cost . since the lens probe moves with respect to the camera head the contour position changes across frames , which precludes using an initial off - line estimation . the boundary detection must be performed at each frame time instant , which imposes constraints on the computational complexity of the chosen algorithm . several issues preclude the use of naive approaches for segmenting the circular region : the light often spreads to the frame region ; the circular image can have dark areas , depending on the imaged scene and lighting conditions ; and there are often highlights , specularity , and saturation that affect the segmentation performance . it is reasonable to assume that the curve to be determined is always an ellipse ω with 5 degrees of freedom ( dof ) [ 8 ]. thus , we propose to track the boundary across frames using this shape prior to achieve robustness and quasi - deterministic runtime . let ω i - 1 be the curve estimate at the frame time instant i − 1 as shown in fig4 ( a ). the boundary contour for the current frame i is updated as follows : 1 ) definition of a ring area in the original image ( fig4 ( b )), partially or totally containing the last estimate of the boundary . 2 ) rendering of a new stripe image where the previous boundary estimate is mapped into the central vertical line ( fig4 ( c )). it is well known that there is always an affine transformation that maps an arbitrary ellipse into a unitary circle whose center is in the origin [ 8 ]. such transformation s is given by : where r is the ratio between the minor and major axis of ω i - 1 , φ is the angle between the major axis and the horizontal direction , and ( w i - 1 , x , w i - 1 , y ) are the non - homogeneous coordinates of the conic center w i - 1 . the transformation s is used to generate the intermediate result of fig4 ( b ), and the polar image ( fig4 ( c )) is obtained by applying a change from cartesian to spherical coordinates . 3 ) detection of boundary points in the stripe image using image processing techniques . the edge points in the stripe image of fig4 ( c ) are detected by applying a 1 - dimensional edge filter ( such as laplacian of gaussian ) and scanning the horizontal lines from the right to the left to find the transition . 4 ) the edge points , which are expressed in spherical coordinates x =( ρ , θ ), are mapped back into the original image points by the function f s of equation 1 . the current conic boundary qi is finally estimated using the robust conic fitting [ 23 ] and using ransac [ 24 ] to avoid the pernicious effects of possible outliers . 5 ) the steps from 1 ) to 4 ) are repeated until the boundary contour estimate converges . the steps presented to estimate the boundary contour of the circular image are accelerated using the parallel processing capabilities of the computational unit . e . estimation of the angular displacement of the lens probe relatively to the camera when the lens undergoes rotation motion after correct convergence of the method described in section d , the boundary contour is mapped into the central vertical line of the polar image ( fig4 ( c )), enabling the robust detection of the triangular mark by scanning an auxiliary vertical line slightly deviated to the right , and selecting the pixel location that has maximum intensity . the position of the triangular mark is fed into a stochastic filter that infers the current angular displacement relatively to the calibration position at each frame . f . update of the projection model for the current frame based on the calibration and angular displacement of the lens probe the relative motion between lens and camera head causes changes in the calibration parameters that prevent the use of a constant model for correcting the distortion [ 5 ]. there is a handful of works proposing solutions for this problem [ 2 ]-[ 4 ], [ 15 ], [ 22 ], but most of them have the drawback of requiring additional instrumentation for determining the lens rotation [ 2 ], [ 3 ], [ 22 ]. the few methods relying only on image information for inferring the relative motion either lack robustness [ 4 ] or are unable to update the full set of camera parameters [ 15 ]. this section proposes the new intrinsic camera model for oblique - viewing endoscopes that is driven simultaneously by experiment and by the conceptual understanding of the optics arrangement . it is assumed that the probe rotates around an axis that is orthogonal to the image plane but not necessarily coincident with the optical axis . the parameters of the lens rotation required to update the camera model are estimated by a robust ekf that receives image information about the boundary contour and triangular mark as input . our dynamic calibration scheme has two important advantages with respect to [ 15 ]: ( i ) the entire projection model is updated as a function of the lens rotation , and not only the rd profile curve ; and ( ii ) the rotation of the lens can still be estimated in the absence of the triangular mark ( see fig1 ( b )). convincing experimental results in re - projecting 3d points in the scene validates the approach . in order to assess effect of the relative rotation between lens and camera - head in the camera model we acquired 10 calibration images while rotating the lens probe for a full turn . the camera calibration was estimated for each angular position using the methodology in section c , and both the boundary ω and the triangular mark were located as described in section d and e . fig8 ( a ) plots the results for the principal point c , the boundary center w , and the lens mark p . since the three parameters describe almost perfect concentric trajectories it seems reasonable to model the effect of the lens rotation on the camera intrinsics by means of a rotation around an axis orthogonal to the image plane . this idea has already been advanced by wu et al . [ 3 ], but they consider that the axis always goes through the principal point , an assumption that in general does not hold , as shown by our experiment . the scheme in fig5 ( b ) and ( c ) aims to give the idea of the proposed model for describing the effect of the lens rotation in the camera intrinsics . let us assume that the borescopic lens projects a virtual image onto a plane l ′ placed at the far end . we can think of l ′ as the image that would be seen by looking directly through a camera eye - piece . k c is the intrinsic matrix of this virtual projection , and c ′ is the point location where the optical axis meets the plane . now assume that a camera head is connected to the eye - piece , such that the ccd plane l is perfectly parallel to l ′ and orthogonal to the optical axis . the projection onto i has intrinsic k h , with the principal point c being the image of c ′. so , if k h is the intrinsic matrix estimate with f c being the focal length of the borescope lens , and f h being the focal length of the camera head that converts metric units into pixels . let us now consider that the lens probe is rotated around an axis l by an angle α ( fig8 ( c )). l is assumed to be orthogonal to the virtual plane i ′, but not necessarily coincident with the lens axis . in this case the point c ′ describes an arc of circle with amplitude α and , since l and l ′ are parallel , the same happens with its image c . the intrinsic matrix of the compound optical system formed by the camera head and the rotated borescope becomes with r α , q ′ being a plane rotation by α and around the point q ′, where the axis 1 , intersects l ′. the position of q ′ is obviously unchanged by the rotation , and the same is true of its image q ˜ k h q ′. taking into account the particular structure of k h , we can re - write equation 8 in the following manner we have just derived a projection model for the endoscopic camera that accommodates the rotation of the lens probe and is consistent with the observations of fig8 ( a ). the initialization procedure estimates the camera calibration k 0 , ξ at an arbitrary reference position ( α = 0 ). at a certain frame time instant i , the matrix of intrinsic parameters becomes where α i is the relative angular displacement of the lens , and qi is the image point that remains fixed during the rotation . since the radial distortion is a characteristic of the lens , the parameter ξ is unaffected by the relative motion with respect to the camera - head . thus , from equation 2 , it follows that a generic 3d point x represented in the camera coordinate frame is imaged at : the update of the matrix of intrinsic parameters at each frame time instant requires knowing the relative angular displacement α i and the image rotation center q i . we now describe how these parameters can be inferred from the position of the boundary contour ω and the triangular mark p ( section iii ). let w i and w 0 be respectively the center of the boundary contours ω i and ω 0 in the current and reference frames . likewise , p i and p 0 are the positions of the triangular markers in the two images . we assume that both w i , w 0 and p i , p 0 are related by the plane rotation r α i , α i whose parameters we aim to estimate . this situation is illustrated in fig6 ( a ) where it can be easily seen that the rotation center qi must be the intersection of the bisectors of the line segments defined by w i , w 0 and p i , p 0 . once q i is known the estimation of the rotation angle α i is trivial . whenever the triangular mark is unknown ( if it does not exist or cannot be detected ), the estimation of qi requires a minimum of three distinct boundary contours ( fig6 ( b )). in order to avoid under - constrained situations and increase the robustness to errors in measuring w and p , we use a stochastic ekf [ 12 ] for estimating the rotation parameters . the state transition assumes a constant velocity model for the motion and stationary rotation center . the equation is linear on the state variables the measurement equation is nonlinear in α i and q i with the two last equations being discarded whenever the detection of the triangular mark fails . the proposed model was validated by re - projecting grid corners onto images of the checkerboard pattern acquired for different angles α . the sic calibration [ 21 ] was performed for the reference position ( α = 0 ), enabling determination of the matrix k0 , the rd parameter ξ , and the 3d coordinates of the grid points . then , the camera head was carefully rotated without moving the lens probe in order to keep the relative pose with respect to the calibration pattern . the image rotation center q i and the angular displacement α i were estimated for each frame using the geometry in fig6 . finally , the 3d grid points were projected onto the frame using equation 11 , and the error distance to the actual image corner locations was measured . fig7 plots the rms of the reprojection error for different angular displacements α . the values vary between 2 and 5 pixels , but no systematic behavior can be observed . we only focused on the intrinsic parameters , while [ 2 ] and [ 3 ] consider both intrinsic and extrinsic calibration and employ additional instrumentation . although no direct comparison can be made , it is worth mentioning that our reprojection error is smaller than [ 3 ] and equivalent to [ 2 ], where only points close to the image center were considered . from the above , and despite all conjectures , the results of the experiment clearly validate the proposed model . g . real time correction of the radial distortion through image warping this section discusses the rendering of the correct perspective images that are the final output of the visualization system . as pointed out in [ 7 ], the efficient warping of an image by a particular transformation should be performed using the inverse mapping method . thus , we must derive the function f that maps points y in the desired undistorted image into points x in the original distorted frame . from equation 11 , it follows that f ( y )˜ k i γ ξ ( r − α i mq i ″ k y − 1 y ). k y specifies certain characteristics of the undistorted image ( e . g . center , resolution ), r − αi , q ″ rotates the warping result back to the original orientation , and q ″ is the back - projection of the rotation center q i q i ″˜( q i , x ″ q i , y ″ 1 ) t ˜ γ ξ − 1 ( k i − 1 q i ). in order to preserve object &# 39 ; s scale in the center region we expand the image periphery and keep the size of the undistorted center region . this is done by computing the size u of the warped image from the radius of the boundary contour of section d . let rd be the distance between the origin and the point k i - 1 p 0 ( the distorted radius ). the desired image size u is given by u = f r u , where f is the camera focal length , and r u is the undistorted radius . accordingly , the matrix k y must be with the center of the warped image being the locus where the image rotation center q i is mapped . fig8 shows the rd correction results for some frames of an endoscopic video sequence . the examples clearly show the improvements in the scene &# 39 ; s perception , and the importance of taking into account the lens probe rotation during the correction of the image geometry ( fig8 ( b )). the rendering of the corrected images requires high performance computational resources to process data in real - time . we propose to parallelize parts of our algorithms for correcting the rd on the a parallel processing unit ( in this case the gpu ). we evaluate the impact of our hybrid serial + parallel solution by comparing 4 approaches : ( ii ) a hypothetical cpu version using openmp2directives [ 6 ], which can be used for shared - memory architectures , such as conventional cots multicore cpus of the x86 family . the experimental setup of the complete running system consists of a workstation equipped with an intel ® coretm2 quad cpu at 2 . 40 ghz , 4 gbyte of ram and a geforce 8800 gtx gpu . the gpu has 128 stream processors ( sps ), each running at 1 . 35 ghz , and a total video ram of 768 mbyte . the source code was compiled with gcc version 4 . 3 . 4 and cuda version 3 . 0 was used . fig9 compares the execution times ( required to process a single frame ) achieved with the solutions mentioned above . the speedup table , a measure of how much faster the gpu is than the cpu , presents the ratio between the naive cpu and the optimized hybrid cpu + gpu execution times . the comparison given in fig9 shows that the cpu is not able to handle large warping operations ( the most time consuming task of the algorithms ) as efficiently as the gpu . the table shows that for images with output sizes above 3000 × 3000 pixels not only the gpu accelerates execution , but also the cpu performance degrades due to the increase of cache miss penalties . fig1 presents the hybrid serial + parallel implementation steps of the radial distortion correction algorithm and the details of the implementation strategy adopted in the development of the hybrid program to execute on the cpu and gpu . the parallelization of the algorithms on the gpu is divided into three main steps : ( i ) image conversions : the image is divided into its rgb channels and a grayscale conversion is performed . ( ii ) boundary estimation : the grayscale image is bound to the parallel processing unit memory and the mapping to spherical coordinates ( equation 1 ) along with the contour enhancement kernels are launched . ( iii ) radial distortion correction : the 3 image channels , from the image conversions stage , are bound to the parallel processing unit memory and the rd correction kernel is launched . there is an intermediary step ( isolated in fig1 ) that is computed in the cpu . each line of the polar image is scanned for the contour points to which the ellipse is fitted . the ellipse parameters are fed to the ekf discussed in section f and the calibration parameters are updated and passed as arguments to the distortion correction kernel . this step is implemented on the cpu rather than the gpu because of the serial nature of the processes involved . the optimized hybrid cpu + gpu solution relies on a data pre - alignment procedure , that allows to perform a single memory access per group of threads , which is known by coalescence [ 1 ]. if the data to be processed by the gpu is misaligned instead , no coalesced accesses are performed and several memory transactions occur for each group of threads , reducing significantly the performance of the kernel under execution . although the alpha channel of the image is not being used , it is necessary to fulfill the memory layout requirement for performing fully coalesced accesses . an increase in the amount of data to be transferred introduces a penalty of 10 . 6 % in the transfer time while the coalescence achieved reduces the kernels execution time by 66 . 1 %. in sum , the coalesced implementation saves 38 . 7 % of computational time relatively to the unoptimized hybrid cpu + gpu implementation . the table of fig1 presents the time taken by each step in fig1 while processing a 150 frame video sequence ( the times represent the mean time value of all frames processed for each resolution ). the image conversions ( i . c .) time also includes the transfer time of the input image from the host to the device &# 39 ; s global memory . the boundary estimation ( b . e .) time also considers the contour points &# 39 ; extraction , the ellipse fitting and the ekf estimation processes that are performed on the cpu ( purple block in fig1 ). note that the mapping of the input image to the spherical space and the contour enhancement filter , both computed on the gpu , consume 1 to 3 milliseconds of the boundary estimation computational time represented in table ii . the radial distortion correction ( r . d . c .) time also considers the transfer time from the device &# 39 ; s global memory to the host . 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