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 0
s and 1
s
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-connected | 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. | |
8-connected | 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. | |
Three-Dimensional Connectivities |
6-connected | 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
| |
18-connected | 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
| |
26-connected | 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
| |
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 0
s and 1
s. 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
-
Determine the connected components:
CC = bwconncomp(BW, conn);
-
Compute the area of each component:
S = regionprops(CC, 'Area');
-
Remove small objects:
L = labelmatrix(CC);
BW2 = ismember(L, find([S.Area] >= P));
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