bwperim

Find perimeter of objects in binary image

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.

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

Find Perimeter of Objects in Binary Image

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')

Find Perimeter Pixels in Binary Image

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)

Input Arguments

BW — Input binary image

numeric array | logical array

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

conn — Pixel connectivity

4 | 8 | 6 | 18 | 26 | 3-by-3-by- ... -by-3 matrix of 0s and 1s

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.	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.	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	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	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	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

BW2 — Output binary image containing only perimeter pixels of objects

logical array

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

Data Types: logical

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• bwperim supports the generation of C code (requires MATLAB^® Coder™). Note that if you choose the generic MATLAB Host Computer target platform, bwperim generates code that uses a precompiled, platform-specific shared library. Use of a shared library preserves performance optimizations but limits the target platforms for which code can be generated. For more information, see Types of Code Generation Support in Image Processing Toolbox.

• bwperim supports only 2-D images.

• bwperim does not support a no-output-argument syntax.

• The connectivity matrix input argument, conn, must be a constant.

GPU Code Generation

Generate CUDA® code for NVIDIA® GPUs using GPU Coder™.

Usage notes and limitations:

• bwperim supports only 2-D images.

• bwperim does not support a no-output-argument syntax.

• The connectivity matrix input argument, conn, must be a constant.

Version History

Introduced before R2006a