bwareafilt

Extract objects from binary image by size

Syntax

BW2 = bwareafilt(BW,range)

BW2 = bwareafilt(BW,n)

BW2 = bwareafilt(BW,n,keep)

BW2 = bwareafilt(___,conn)

Description

BW2 = bwareafilt(BW,range) extracts all connected components (objects) from the binary image BW, where the area of the objects is in the specified range, producing another binary image BW2. bwareafilt returns a binary image BW2 containing only those objects that meet the criteria.

BW2 = bwareafilt(BW,n) keeps the n largest objects. In the event of a tie for n-th place, only the first n objects are included in BW2.

BW2 = bwareafilt(BW,n,keep) specifies whether to keep the n largest objects or the n smallest objects.

BW2 = bwareafilt(___,conn) specifies the pixel connectivity that defines the objects.

Examples

Filter Binary Image by Area of Objects

Read image.

BW = imread('text.png');

Filter image, retaining only those objects with areas between 40 and 50.

BW2 = bwareafilt(BW,[40 50]);

Display the original image and filtered image side by side.

imshowpair(BW,BW2,'montage')

Filter Binary Image by Size of Objects

Read image.

BW = imread('text.png');

Filter image, retaining only the 5 objects with the largest areas.

BW2 = bwareafilt(BW,5);

Display the original image and the filtered image side by side.

imshowpair(BW,BW2,'montage')

Input Arguments

BW — Binary image to be filtered

logical matrix

Binary image to be filtered, specified as a logical matrix.

Data Types: logical

range — Minimum and maximum areas

2-element numeric vector

Minimum and maximum values of the area, specified as a 2-element numeric vector of the form [low high].

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

n — Number of objects to include

positive integer

Number of objects to include when filtering image objects by size, specified as a positive integer.

Data Types: double

keep — Size of objects to include

"largest" (default) | "smallest"

Size of objects to include in the output image, specified as "largest" or "smallest". In the event of a tie for n-th place, bwareafilt includes only the first n objects.

Data Types: char | string

conn — Pixel connectivity

8 (default) | 4 | 3-by-3 matrix of 0s and 1s

Pixel connectivity, specified as one of these values.

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.

Connectivity can also be defined in a more general way by specifying a 3-by-3 matrix of 0s and 1s. The 1-valued elements define neighborhood locations relative to the center element of conn. The matrix must be symmetric about its center element.

Data Types: double | logical

Output Arguments

BW2 — Filtered image

logical matrix

Filtered image, returned as a logical matrix of the same size as the input image BW.

Extended Capabilities

C/C++ Code Generation

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

Usage notes and limitations:

• bwareafilt supports the generation of C code (requires MATLAB^® Coder™). Note that if you choose the generic MATLAB Host Computer target platform, bwareafilt 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.

• The keep argument must be a compile-time constant.

Version History

Introduced in R2014b

R2024a: Generate C code using MATLAB Coder

bwareafilt now supports the generation of C code (requires MATLAB Coder).