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bwperim

Find perimeter of objects in binary image

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Syntax

BW2 = bwperim(BW)
BW2 = bwperim(BW,conn)
bwperim(___)

Description

BW2 = bwperim(BW) returns a binary image that contains only the perimeter pixels of objects in the input image BW. A pixel is part of the perimeter if it is nonzero and it is connected to at least one zero-valued pixel.

example

BW2 = bwperim(BW,conn) also specifies the pixel connectivity, conn.

bwperim(___) without output arguments displays the binary image of the perimeter in a new figure window. To use this syntax, BW must be a 2-D binary image.

Examples

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Open Live Script

Read binary image into workspace.

BW = imread('circles.png');

Calculate the perimeters of objects in the image.

BW2 = bwperim(BW,8);

Display the original image and the perimeters side-by-side.

imshowpair(BW,BW2,'montage')

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

Open Live Script

This example shows how to find the perimeter pixels in a binary image using the bwperim function.

Read a binary image into the workspace.

BW1 = imread('circbw.tif');

Find the perimeters of objects in the image.

BW2 = bwperim(BW1);

Display the original image and the image showing perimeters side-by-side.

montage({BW1,BW2},'BackgroundColor','blue','BorderSize',5)

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

Input Arguments

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Input binary image, specified as a numeric or logical array of any dimension. 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

Pixel connectivity, specified as one of the values in this table. The default connectivity is 4 for 2-D images, and 6 for 3-D images.

Value

Meaning

Two-Dimensional Connectivities

4

Pixels are connected if their edges touch. Two adjoining pixels are part of the same object if they are both on and are connected along the horizontal or vertical direction.

Center pixel connected to four pixels

Current pixel is shown in gray.

8

Pixels are connected if their edges or corners touch. Two adjoining pixels are part of the same object if they are both on and are connected along the horizontal, vertical, or diagonal direction.

Center pixel connected to eight pixels

Current pixel is shown in gray.

Three-Dimensional Connectivities

6

Pixels are connected if their faces touch. Two adjoining pixels are part of the same object if they are both on and are connected in:

  • One of these directions: in, out, left, right, up, and down

Center pixel connected to the faces of 6 pixels

Current pixel is center of cube.

18

Pixels are connected if their faces or edges touch. Two adjoining pixels are part of the same object if they are both on and are connected in:

  • One of these directions: in, out, left, right, up, and down

  • A combination of two directions, such as right-down or in-up

Center pixel connected to the faces of 6 pixels and the edges of 12 pixels

Current pixel is center of cube.

26

Pixels are connected if their faces, edges, or corners touch. Two adjoining pixels are part of the same object if they are both on and are connected in:

  • One of these directions: in, out, left, right, up, and down

  • A combination of two directions, such as right-down or in-up

  • A combination of three directions, such as in-right-up or in-left-down

Center pixel connected to the faces of 6 pixels, the edges of 12 pixels, and the corners of 8 pixels

Current pixel is center of cube.

For higher dimensions, bwperim uses the default value conndef(ndims(BW),"minimal").

Connectivity can also be defined in a more general way for any dimension by specifying a 3-by-3-by- ... -by-3 matrix of 0s and 1s. The 1-valued elements define neighborhood locations relative to the center element of conn. Note that conn must be symmetric about its center element. See Specifying Custom Connectivities for more information.

Data Types: double | logical

Output Arguments

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Output image containing only perimeter pixels of objects, returned as a logical array.

Data Types: logical

Extended Capabilities

Version History

Introduced before R2006a

See Also

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