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Trace object in binary image


B = bwtraceboundary(BW,P,fstep)
B = bwtraceboundary(bw,P,fstep,conn)
B = bwtraceboundary(bw,P,fstep,conn,n,dir)


B = bwtraceboundary(BW,P,fstep) traces the outline of an object in binary image bw. Nonzero pixels belong to an object and 0 pixels constitute the background. P is a two-element vector specifying the row and column coordinates of the point on the object boundary where you want the tracing to begin.

fstep is a string or character vector specifying the initial search direction for the next object pixel connected to P. The following figure illustrates all the possible values for fstep.

bwtraceboundary returns B, a Q-by-2 matrix, where Q is the number of boundary pixels for the region. B holds the row and column coordinates of the boundary pixels.

B = bwtraceboundary(bw,P,fstep,conn) specifies the connectivity to use when tracing the boundary. conn can have either of the following scalar values.




4-connected neighborhood


With this connectivity, fstep is limited to the following values: 'N', 'E', 'S', and 'W'.


8-connected neighborhood. This is the default.

B = bwtraceboundary(bw,P,fstep,conn,n,dir) specifies n, the maximum number of boundary pixels to extract, and dir, the direction in which to trace the boundary. When n is set to Inf, the default value, the algorithm identifies all the pixels on the boundary. dir can have either of the following values:




Search in a clockwise direction. This is the default.


Search in counterclockwise direction.

Class Support

BW can be logical or numeric and it must be real, 2-D, and nonsparse. B, P, conn, and N are of class double. dir and fstep are strings or character vectors.


collapse all

Read image and display it.

BW = imread('blobs.png');

Pick an object in the image and trace the boundary. To select an object, specify a pixel on its boundary. This example uses the coordinates of a pixel on the boundary of the thick white circle, obtained through visual inspection using impixelinfo. The example specifies the initial search direction, the connectivity, how many boundary pixels should be returned, and the direction in which to perform the search.

r = 163;
c = 37;
contour = bwtraceboundary(BW,[r c],'W',8,Inf,'counterclockwise');

Plot the contour on the image.

hold on;


The bwtraceboundary function implements the Moore-Neighbor tracing algorithm modified by Jacob's stopping criteria. This function is based on the boundaries function presented in the first edition of Digital Image Processing Using MATLAB, by Gonzalez, R. C., R. E. Woods, and S. L. Eddins, New Jersey, Pearson Prentice Hall, 2004.

Extended Capabilities

Introduced before R2006a

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