bwtraceboundary() - Image Processing

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 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.ValueMeaning
44-connected neighborhoodNote:  


With this connectivity, fstep is limited
to the following values: 'N', 'E', 'S',
and 'W'.
88-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:ValueMeaning
'clockwise'Search in a clockwise direction. This is the default.
'counterclockwise'Search in counterclockwise direction.
Code Generation support:
Yes. MATLAB® Function Block support: No.

Syntax

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

Example

Trace Boundary and Visualize ContoursOpen This Example
Read image and display it.BW = imread('blobs.png');
imshow(BW,[]);

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;
plot(contour(:,2),contour(:,1),'g','LineWidth',2);

Output / Return Value

Limitations

Alternatives / See Also

Reference