Method of bridging between contour elements in an image
BACKGROUND OF THE INVENTION 
     The present invention relates to a method or process of bridging between contour elements in an image. It applies more particularly to the recognition of vessels in digital angiography but may also apply to the analysis of aerial views for the recognition of rivers and roads. 
     Angiography is a process permitting visual analysis of blood vessels. It principally consists in X-raying the zone of observation before then after injection of a product opaque to the X-rays. The logarithmic difference of these two images, if there is no movement between the two images, allows a well contrasted image to be obtained of the zones of the vascular network where theproduct has diffused. The support for the image is formed, in digital angiography, by a digital image memory allowing the observed zones of the vascular network to be analysed practically in real time. Only the contour lines of the vascular network are restored discontinuously because of the thresholds used in the contour extraction methods. It is consequently necessary to determine whether these discontinuities are due to a real absence of contours or if, on the contrary, they are caused by a local minimum of the luminance gradient. 
     Known processes are generally based on considerations of proximity or orientation at the ends of the contour lines likely to be bridged, but these processes are still imprecise and give no information with sufficient certainty about the real presence or absence of discontinuities in the vascular network. 
     SUMMARY OF THE INVENTION 
     The aim of the invention is to overcome the above drawbacks. 
     For this, the invention provides a method of bridging between disjointed contour elements in an image by searching for an optimum bridging path between the facing ends of the disjointed contour elements. The method consists of the steps of defining a search window between each of the facing ends of the disjointed contour elements, considering in the window the different image points as nodes on a graph, determining the elementary cost associated with each path connecting each node to its adjacent nodes from amplitude and orientation information of the luminance function used for detecting the contours, and determining the optimum path by following, from the costs obtained, a line for which the luminance gradient of the detected points appears to be a maximum. 
     The invention has the principle advantage that it allows at the same time a factor of confidence to be associated with each bridging effected and favors a particular form of path by adjusting the cost functions associated with each arc of the graph. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will appear from the following description with reference to the accompanying drawings in which: 
     FIG. 1 shows a research window defined from two ends of a contour to be bridged; 
     FIGS. 2a and 2b show graphs formed of image points disposed inside the search windows; 
     FIG. 3 shows a flow chart retracing the steps of the method used by the invention for calculating an optimum path between points of the graph to be bridged; and 
     FIG. 4 shows one example of applying the bridging method of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In digital image formation, the methods of detecting contours generally comprise two main processing phases, one consisting in defining, from a digital image in which each image point is coded over 8 bits, for example, another image more characteristic of the desired contours from a filtering procedure, the other consisting in adopting thresholds for the image obtained and effecting binary operations on the image thus thresholded for detecting the associated zones, thinning them down and obtaining segments by linear approximation of the contours. 
     However, the contours thus obtained and stored in digital image memory have discontinuities and it may be desirable to fill these in when the real contour data is missing or non-existent. To fill in a discontinuity, bridging may, for example, be provided directly on the binary image stored in the memory by taking into consideration certain criteria such as the length thereof and/or the angles formed by the direction of the contour lines situated on each side thereof. 
     The drawback of these methods is that they carry out processing on reduced binary data, and much information is lost which would be useful for helping in making a correct decision. 
     Another method described in an article of the CGIPI on pages 169 to 182, published in 1972 and entitled &#34;Martelli Edge Detection Using Heuristic Search Methods&#34;, is based on a heuristic procedure of searching for paths in a luminance image, and the method allows contours to be detected in given image zones. The bridging method of the present invention takes up again certain principles of the method described in this article. However, the present invention is essentially distinguished therefrom because of its adaptation to the specific bridging problem of the invention and by the fact that it uses as starting data that data representing the grey levels of the image of the amplitudes and/or orientations of the gradients which are already calculated for elaborating the image of the contours. This data better represents the contours than the initial luminance data. The present invention also uses a search algorithm known under the name of Moore-Dijkstra algorithm which seems better adapted for providing a solution to one of the specific problems raised by the invention, this method consisting of searching for the existence of a contour passing through two points A and B, bringing this problem down to that already solved by this algorithm and for finding the shortest path in a graph between two tops. 
     According to the invention, if the coding cost of the shortest path is less than a given threshold, this path is considered as the corresponding to the desired bridging, if not, there is no bridging possible between the two points. The search space in the image memory is defined in the way shown in FIG. 1, where there is inserted between two points A and B, marking the ends of a discontinuity in a contour C of the image, a square of side D equal to the distance separating the two points A and B and oriented in the plane so that points A and B are disposed on two opposite sides of the square in the middle thereof. The search space F thus defined forms a search window which is intersected in FIG. 2A by lines and columns, each node placed at the intersection of a line and a column representing an image point. In FIG. 2A, the square search window is divided into three lines and three columns, and each node P i  inside the search window F is connected to each of its eight neighbors P j  by an arc (i,j) to which is assigned a cost M i  =C ij  corresponding to the cost associated with the passage from a point P i  to an adjacent point P j . Thus, by defining the characteristic costs for each arc subtended between two adjacent nodes, an overall cost may be calculated for determining the shortest path for going from point A to point B. 
     Searching for the shortest path is effected by applying the teaching of the &#34;Moore Dijkstra&#34; algorithm in the way described hereafter with reference to the flow chart in FIG. 3, as applied to a graph represented by an assembly of apices (A, 2, . . . , N) and an assembly of arcs (i,j) to which a cost C ij  is assigned. The execution of this algorithm begins by an initialization phase and continues with a phase for selecting the points of the graph of minimum cost, followed by a phase for updating the costs for each minimum cost point selected inside the graph. The initialization phase shown at 1 in FIG. 3 causes the search to start at apex S=A by considering also the assembly S=(2, . . . , N) of the pixels or remaining image points of the window. 
     In FIG. 3 M(A)=0 designates the cost associated with the starting point A, M i  =C Ai  designates the coding cost for travelling over the path separating point A from a point i adjacent point A and P.sub.(i) a vector which is formed at each selected point of the optimum path for reconstituting this path when all the steps of the algorithm are finished. During the initialization phase, the vector P.sub.(i) (predecessor of i) is placed equal to P.sub.(A) for designating the starting apex A. 
     The phase for selecting the minimum costs is carried out in steps 2 to 5. Step 2 consists in choosing the apex j belonging to the assembly S so that the cost M(j) is equal to the minimum of the costs (M(i)) whatever the apex i belong to the assembly S. If the minimum of the costs obtained reveals that the minimum cost apex j corresponds to the apex B (step 3), the procedure stops at step 4 and the minimum cost path is obtained from the vector P.sub.(i). On the other hand, if the minimum cost apex j does not correspond to apex B, the procedure continues to step 5 by updating the whole of the apices S by removing the apex j from this assembly and updating the costs in step 6, the starting cost M i  being updated possibly by a cost M j  +C ji  equal to the cost of point j increased by the cost C ji  of the path connecting the preceding point i to point j. Since this updating corresponds to the notation in FIG. 3, M.sub.(i) ←min (M.sub.(i), M.sub.(j) +C ji ), allows a test to be carried out in step 7 on the new cost obtained, and if the new cost obtained is different from the preceding cost M.sub.(i), then the vector P i  is updated for designating in step 8 the apex j as predecessor of apex i. If not, the processing returns to the execution of step 2. 
     The elementary costs C ij  depend on the gradient extraction method selected as well as on the constraints imposed on the shape of the path. In the preferred embodiment of the invention, the gradient extraction method used is derived from that which is proposed in the article by NEVATA and BABU entitled &#34;Linear picture edges extraction and description&#34; published in volume no. 13 of 1980 in the &#34;Review of Computer Graphics and Images Processing&#34;. This method uses several different masks by characterizing each contour element by an amplitude and a local orientation thereof. It gives good results for the specific problems of digital angiography. However, other methods of extracting contours giving an image with a grey level characteristic of these contours could also be applied to the bridging method of the invention which has just been described. 
     The elementary costs C ij  may then also depend on the constraints which may be imposed on the shape of the path. For example, in a first case, if it is desired to follow the line of maximum amplitude of the gradient so as to determine the shape of the path, elementary costs of the type c ij  =Max-a j  may be determined, where 
     C ij  designates the elementary cost for going from apex i to apex j; 
     Max designates the maximum amplitude of the gradient in window F; and 
     a j  designates the amplitude of the gradient at apex j. 
     In a second case, so as to take into account more particularly the shape of the path, it may be profitable to use the local orientation data so that the desired shape of the path intervenes in the cost. For example, for favoring a rectilinear path, elementary costs of the type: 
     
         C.sub.ij =Max-a.sub.j +K.sub.1.|θ.sub.J -θmean| 
    
     or C ij  =Max-a j  /(K 2  ·Q) may be used with Q=1/K 2  if θ j  =θ mean or Q=|θ j  -θ mean | if θ j  is different from mean θ mean. In these formulae θ j  designates the local orientation at apex j and mean θ designates the mean of the local orientation at apices A and B. 
     Similarly, for defining a regular contour, without a sudden change of orientation (i.e., the local orientation varies progressively, an elementary cost of the type: 
     
         C.sub.ij =Max-a.sub.j +K.sub.1 ·|θ.sub.j -θ.sub.i | 
    
     or else 
     
         C.sub.ij =Max-a.sub.i /(K.sub.2 ·Q) 
    
     may be used with Q=1/K 2  if θ j  =Q i  or else Q=|θ i  -θ j  | if θ j  is different from θ i . 
     Once the optimum path has been found, it must then be decided whether its character is acceptable or not. A first method consists in only selecting the path if the cost with respect to the number of points of the path is less than a predetermined threshold. This method is valid but determination of the threshold remains delicate. It seems much preferable to calculate along a path L a relative criterion of quality of the form: ##EQU1## where A i  =the amplitude of the gradient at the point i of the path corrected by the orientation: (Max-C ij ); 
     N=the number of points on the path; 
     a max  =maximum amplitude on the path, including end points. 
     Thus the bridging will only be effected if C r  is greater than a given threshold expressed by a percentage. 
     The example which has just been given of the preferred embodiment of the invention is not limitative, for other variants are also possible which will depend for the most part on the approach used for defining the system for processing the image without them departing from the scope of the invention claimed. In the case, for example, of an approach with the aim of extracting the maximum of data from the image without taking into account the results of the high level processing steps, it may be desirable to bridge all the ends of the contour lines which satisfy certain criteria of proximity. A corresponding algorithm may in this case by of the type shown in FIG. 4 in steps 10 to 13, beginning at step 10 by collecting the initial data forming the images of the contours, the amplitudes and the directions of the gradient, for selecting in step 11, the ends of the candidate contour lines if these distances are less than a threshold and if they have a closely related orientation. In this case, the bridging method which has just been described may be applied to the selected ends at step 12, and if the test of accessibility is verified the bridge obtained will be selected or rejected in the opposite case at step 13. It will be noted that this strategy may be further used in two different ways, either such as it is or else iteratively. In this latter case, the holes will be stopped up first of all with one point, then two points, etc. . . The most interesting method will depend on the type of image and on the time available. 
     Other approaches may also be directed not from data but from a concept of the type known as &#34;Top. Down&#34;, for example. In fact, the formulation of a hypothesis about the existence of an object may lead to forming a hypothesis about the continuity of a contour. This continuity may consequently be verified by the bridging process which has just been described so as to confirm or invalidate the hypothesis. In this case it will be sufficient to simply apply the method of the invention to the ends designated A and B and not to the whole image as in the preceding approach.