bwareaopen

Remove small objects from binary image

Syntax

BW2 = bwareaopen(BW,P)

BW2 = bwareaopen(BW,P,conn)

Description

BW2 = bwareaopen(BW,P) removes all connected components (objects) that have fewer than P pixels from the binary image BW, producing another binary image, BW2. This operation is known as an area opening.

BW2 = bwareaopen(BW,P,conn) removes all connected components, where conn specifies the desired connectivity.

Examples

Remove Objects in Image Containing Fewer Than 50 Pixels

Read binary image.

BW = imread('text.png');

Remove objects containing fewer than 50 pixels using bwareaopen function.

BW2 = bwareaopen(BW, 50);

Display original image next to morphologically opened image.

imshowpair(BW,BW2,'montage')

Input Arguments

BW — Binary image

logical array | numeric array

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

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

P — Maximum number of pixels in objects

nonnegative integer

Maximum number of pixels in objects, specified as a nonnegative integer.

Example: 50

Data Types: double

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 8 for 2-D images, and 26 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, bwareaopen uses the default value conndef(ndims(BW),'maximal').

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 — Area-opened image

logical array

Area-opened image, returned as a logical array of the same size as BW.

Algorithms

The basic steps are

1. Determine the connected components:

   CC = bwconncomp(BW, conn);

2. Compute the area of each component:

   S = regionprops(CC, 'Area');

3. Remove small objects:

   L = labelmatrix(CC);
   BW2 = ismember(L, find([S.Area] >= P));
   

Extended Capabilities

C/C++ Code Generation

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

Usage notes and limitations:

• bwareaopen supports the generation of C code (requires MATLAB^® Coder™). For more information, see Code Generation for Image Processing.

• BW must be a 2-D binary image. N-D arrays are not supported.

• conn can only one of the two-dimensional connectivities (4 or 8) or a 3-by-3 matrix. The 3-D connectivities (6, 18, and 26) are not supported. Matrices of size 3-by-3-by-...-by-3 are not supported.

• conn must be a compile-time constant.

GPU Code Generation

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

Usage notes and limitations:

• BW must be a 2-D binary image. N-D arrays are not supported.

• conn must be one of the two-dimensional connectivities (4 or 8) or a 3-by-3 matrix. The 3-D connectivities (6, 18, and 26) are not supported. Matrices of size 3-by-3-by-...-by-3 are not supported.

• conn must be a compile-time constant.

Version History

Introduced before R2006a