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.

This function supports code generation (see Tips).

fstep is a string specifying the initial search direction for the next object pixel connected to P. You use strings such as 'N' for north, 'NE' for northeast, to specify the direction. 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

    Note:   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.


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Trace Boundary and Visualize Contours

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;

More About

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  • This function supports the generation of C code using MATLAB® Coder™. For more information, see Code Generation for Image Processing.

    When generating code, the dir, fstep, and conn arguments must be compile-time constants.

See Also


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