bwtraceboundary
Trace object in binary image
Syntax
Description
B = bwtraceboundary(BW,P,fstep)BW. Nonzero
pixels belong to an object and zero-valued pixels constitute the background.
P specifies the row and column coordinates of the point on
the object boundary where you want the tracing to begin. fstep
specifies the initial search direction for the next object pixel connected to
P. B holds the row and column
coordinates of the boundary pixels for the region.
Examples
Trace Boundary and Visualize Contours
Read an 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. By default, bwtraceboundary identifies all pixels on the boundary.
r1 = 163;
c1 = 37;
contour = bwtraceboundary(BW,[r1 c1],'W');Plot the contour on the image.
hold on plot(contour(:,2),contour(:,1),'g','LineWidth',2)
Pick a point on the boundary of a second object. This example uses the coordinates of a pixel near the upper-left corner of the largest rectangle. Trace the first fifty boundary pixels in the clockwise direction.
r2 = 68; c2 = 95; contourCW = bwtraceboundary(BW,[r2 c2],'W',8,50,'clockwise');
Starting at the same point on the second object boundary, trace the first fifty boundary pixels in the counterclockwise direction.
contourCCW = bwtraceboundary(BW,[r2 c2],'W',8,50,'counterclockwise');
Plot the clockwise contour on the image in red. Plot the counterclockwise contour on the image in blue.
plot(contourCW(:,2),contourCW(:,1),'r','LineWidth',2) plot(contourCCW(:,2),contourCCW(:,1),'b','LineWidth',2)
Input Arguments
BW — Binary image
2-D numeric matrix | 2-D logical matrix
Binary image, specified as a 2-D numeric or logical matrix.
Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | logical
P — Coordinates of starting point
2-element vector
Coordinates of starting point on the object boundary where you want the
tracing to begin, specified as a 2-element vector of the format
[row column].
Data Types: double
fstep — Initial search direction
'N' | 'NE' | 'E' | 'SE' | 'S' | 'SW' | 'W' | 'NW'
conn — Pixel connectivity
8 (default) | 4
Pixel connectivity, specified as 8 or
4.
Value | Meaning | |
|---|---|---|
Two-Dimensional Connectivities | ||
4-connected | Pixels are connected if their edges touch. The neighborhood of a pixel are the adjacent pixels in the horizontal or vertical direction. |
|
8-connected | Pixels are connected if their edges or corners touch. The neighborhood of a pixel are the adjacent pixels in the horizontal, vertical, or diagonal direction. |
|
Data Types: double
m — Maximum number of boundary pixels to extract
Inf (default) | positive integer
Maximum number of boundary pixels to extract, specified as a positive
integer. By default, m is Inf and
bwtraceboundary identifies all the pixels on the
boundary.
Data Types: double
dir — Direction in which to trace boundary
'clockwise' (default) | 'counterclockwise'
Direction in which to trace boundary, specified as
'clockwise' or
'counterclockwise'.
Data Types: char | string
Output Arguments
Algorithms
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.
References
[1] Gonzalez, R. C., R. E. Woods, and S. L. Eddins, Digital Image Processing Using MATLAB, New Jersey, Pearson Prentice Hall, 2004.