From: https://ww2.mathworks.cn/help/images/ref/bwareaopen.html

bwareaopen

Remove small objects from binary image

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Syntax

BW2 = bwareaopen(BW,P)

BW2 = bwareaopen(BW,P,conn)

Description

example

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

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Remove Objects in Image Containing Fewer Than 50 Pixels

Try This Example

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

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BW — Binary image
real, nonsparse, logical or numeric array of any dimension

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

Example: BW2 = bwareaopen(BW, 50);

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

P — Maximum number of pixels in objects
nonnegative, integer-valued, numeric scalar

Maximum number of pixels in objects, specified as a nonnegative, integer-valued, numeric scalar.

Example: BW2 = bwareaopen(BW, 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.

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

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BW2 — Image that has been morphologically opened
nonsparse, logical array of any dimension

Image that has been morphologically opened, specified as a nonsparse, logical array 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));