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bwmorph

Morphological operations on binary images

Description

example

BW2 = bwmorph(BW,operation) applies a specific morphological operation to the binary image BW.

Note

To perform morphological operations on a 3-D volumetric image, use bwmorph3.

BW2 = bwmorph(BW,operation,n) applies the operation n times. n can be Inf, in which case the operation is repeated until the image no longer changes.

Examples

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Read binary image and display it.

BW = imread('circles.png');
imshow(BW);

Figure contains an axes object. The axes object contains an object of type image.

Remove interior pixels to leave an outline of the shapes.

BW2 = bwmorph(BW,'remove');
figure
imshow(BW2)

Figure contains an axes object. The axes object contains an object of type image.

Get the image skeleton.

BW3 = bwmorph(BW,'skel',Inf);
figure
imshow(BW3)

Figure contains an axes object. The axes object contains an object of type image.

Input Arguments

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Binary image, specified as a 2-D numeric matrix or 2-D logical matrix. For numeric input, any nonzero pixels are considered to be 1 (true).

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | logical

Morphological operation to perform, specified as one of the following.

Operation

Description

'bothat'

Perform the morphological bottom hat operation, returning the image minus the morphological closing of the image.

The bwmorph function performs morphological closing using the neighborhood ones(3). If you want to perform a morphological bottom hat operation with a different neighborhood, then use the imbothat function.

'branchpoints'

Find branch points of skeleton. For example:

0  0  1  0  0           0  0  0  0  0
0  0  1  0  0           0  0  0  0  0
1  1  1  1  1  becomes  0  0  1  0  0
0  0  1  0  0           0  0  0  0  0
0  0  1  0  0           0  0  0  0  0

Note: To find branch points, the image must be skeletonized. To create a skeletonized image, use bwmorph(BW,'skel').

'bridge'

Bridge unconnected pixels, that is, sets 0-valued pixels to 1 if they have two nonzero neighbors that are not connected. For example:

1  0  0           1  1  0 
1  0  1  becomes  1  1  1 
0  0  1           0  1  1

'clean'

Remove isolated pixels (individual 1s that are surrounded by 0s), such as the center pixel in this pattern.

0  0  0 
0  1  0 
0  0  0

'close'

Perform morphological closing (dilation followed by erosion).

The bwmorph function performs morphological closing using the neighborhood ones(3). If you want to perform a morphological closing operation with a different neighborhood, then use the imclose function.

'diag'

Use diagonal fill to eliminate 8-connectivity of the background. For example:

0  1  0           0  1  0 
1  0  0  becomes  1  1  0 
0  0  0           0  0  0

'endpoints'

Find end points of skeleton. For example:

1  0  0  0           1  0  0  0
0  1  0  0  becomes  0  0  0  0
0  0  1  0           0  0  1  0
0  0  0  0           0  0  0  0

Note: To find end points, the image must be skeletonized. To create a skeletonized image, use bwmorph(BW,'skel').

'fill'

Fill isolated interior pixels (individual 0s that are surrounded by 1s), such as the center pixel in this pattern.

1  1  1 
1  0  1 
1  1  1

'hbreak'

Remove H-connected pixels. For example:

1  1  1           1  1  1 
0  1  0  becomes  0  0  0 
1  1  1           1  1  1

'majority'

Set a pixel to 1 if five or more pixels in its 3-by-3 neighborhood are 1; otherwise, set the pixel to 0.

'open'

Perform morphological opening (erosion followed by dilation).

The bwmorph function performs morphological opening using the neighborhood ones(3). If you want to perform a morphological opening operation with a different neighborhood, then use the imopen function.

'remove'

Remove interior pixels. This option sets a pixel to 0 if all its 4-connected neighbors are 1, thus leaving only the boundary pixels on.

'shrink'

With n = Inf, shrink objects to points by removing pixels from the boundaries of objects. Objects without holes shrink to a point, and objects with holes shrink to a connected ring halfway between each hole and the outer boundary. This option preserves the Euler number (also known as the Euler characteristic).

'skel'

With n = Inf, remove pixels on the boundaries of objects without allowing objects to break apart. The pixels remaining make up the image skeleton. This option preserves the Euler number.

When working with 3-D volumes, or when you want to prune a skeleton, use the bwskel function.

'spur'

Remove spur pixels. For example:

0  0  0  0           0  0  0  0
0  0  0  0           0  0  0  0
0  0  1  0  becomes  0  0  0  0
0  1  0  0           0  1  0  0
1  1  0  0           1  1  0  0

'thicken'

With n = Inf, thicken objects by adding pixels to the exterior of objects until doing so would result in previously unconnected objects being 8-connected. This option preserves the Euler number.

'thin'

With n = Inf, thin objects to lines by removing pixels from the boundary of objects. An object without holes shrinks to a minimally connected stroke, and an object with holes shrinks to a connected ring halfway between each hole and the outer boundary. This option preserves the Euler number. See Algorithms for more detail.

'tophat'

Perform the morphological top hat operation, returning the image minus the morphological opening of the image.

The bwmorph function performs morphological opening using the neighborhood ones(3). If you want to perform a morphological top hat operation with a different neighborhood, then use the imtophat function.

Tip

To perform morphological erosion or dilation, use the imerode or imdilate function, respectively. If you want to replicate the dilation or erosion performed by the bwmorph function, then specify the neighborhood as ones(3).

Data Types: char | string

Number of times to perform the operation, specified as a positive integer or Inf. When you specify n as Inf, the bwmorph function repeats the operation until the image no longer changes.

Example: 100

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64

Output Arguments

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Image after morphological operations, returned as a 2-D logical matrix.

Data Types: logical

Algorithms

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When used with the 'thin' option, bwmorph uses the following algorithm [3]:

  1. In the first subiteration, delete pixel p if and only if the conditions G1, G2, and G3 are all satisfied.

  2. In the second subiteration, delete pixel p if and only if the conditions G1, G2, and G3 are all satisfied.

Condition G1:

XH(p)=1

where

XH(p)=i=14bi

bi={1, if x2i1=0 and (x2i=1 or x2i+1=1)0, otherwise                                          

x1, x2, ..., x8 are the values of the eight neighbors of p, starting with the east neighbor and numbered in counter-clockwise order.

Condition G2:

2min{n1(p),n2(p)}3

where

n1(p)=k=14x2k1x2k

n2(p)=k=14x2kx2k+1

Condition G3:

(x2x3x¯8)x1=0

Condition G3':

(x6x7x¯4)x5=0

The two subiterations together make up one iteration of the thinning algorithm. When the user specifies an infinite number of iterations (n=Inf), the iterations are repeated until the image stops changing. The conditions are all tested using applylut with precomputed lookup tables.

References

[1] Haralick, Robert M., and Linda G. Shapiro, Computer and Robot Vision, Vol. 1, Addison-Wesley, 1992.

[2] Kong, T. Yung and Azriel Rosenfeld, Topological Algorithms for Digital Image Processing, Elsevier Science, Inc., 1996.

[3] Lam, L., Seong-Whan Lee, and Ching Y. Suen, "Thinning Methodologies-A Comprehensive Survey," IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol 14, No. 9, September 1992, page 879, bottom of first column through top of second column.

[4] Pratt, William K., Digital Image Processing, John Wiley & Sons, Inc., 1991.

Extended Capabilities

Introduced before R2006a