im2bw

(Not recommended) Convert image to binary image, based on threshold

im2bw is not recommended. Use imbinarize instead. For more information, see Version History.

Syntax

BW = im2bw(I,level)

BW = im2bw(X,cmap,level)

BW = im2bw(RGB,level)

Description

BW = im2bw(I,level) converts the grayscale image I to binary image BW, by replacing all pixels in the input image with luminance greater than level with the value 1 (white) and replacing all other pixels with the value 0 (black).

This range is relative to the signal levels possible for the image's class. Therefore, a level value of 0.5 corresponds to an intensity value halfway between the minimum and maximum value of the class.

BW = im2bw(X,cmap,level) converts the indexed image X with colormap cmap to a binary image.

BW = im2bw(RGB,level) converts the truecolor image RGB to a binary image.

Examples

Convert an Indexed Image to a Binary Image

load trees
BW = im2bw(X,map,0.4);
imshow(X,map), figure, imshow(BW)

Input Arguments

I — 2-D grayscale image

m-by-n numeric matrix

2-D grayscale image, specified as an m-by-n numeric matrix.

Data Types: single | double | int16 | uint8 | uint16

X — 2-D indexed image

m-by-n numeric matrix

2-D indexed image, specified as an m-by-n numeric matrix.

Data Types: single | double | int16 | uint8 | uint16

cmap — Colormap

c-by-3 numeric matrix

Colormap associated with indexed image X, specified as a c-by-3 numeric matrix with values in the range [0, 1]. Each row is a three-element RGB triplet that specifies the red, green, and blue components of a single color of the colormap.

Data Types: single | double | int16 | uint8 | uint16

RGB — 2-D RGB image

m-by-n-by-3 numeric matrix

2-D RGB image, specified as an m-by-n-by-3 numeric matrix.

Data Types: single | double | int16 | uint8 | uint16

level — Luminance threshold

0.5 (default) | number in the range [0, 1]

Luminance threshold, specified as a number in the range [0, 1]. To compute level, you can use the graythresh function.

Data Types: single | double | int16 | uint8 | uint16

Output Arguments

BW — Binary image

m-by-n logical matrix

Binary image, returned as an m-by-n logical matrix.

Data Types: logical

Algorithms

If the input image is not a grayscale image, im2bw converts the input image to grayscale using ind2gray or rgb2gray, and then converts this grayscale image to binary by thresholding.

Version History

Introduced before R2006a

R2016a: im2bw is not recommended

The default luminance threshold of im2bw is not optimal for most images. If you want to use a threshold appropriate for your image, you must compute the level using graythresh before calling im2bw.

In R2016a, the imbinarize function was introduced. This function computes the luminance threshold and performs binarization in one step. imbinarize has additional benefits, such as the ability to perform adaptive thresholding when the image has nonuniform shading. For more information, see Image Binarization - New 2016a Functions.

The table shows some typical usages of im2bw and how to update your code to use imbinarize instead.

Not Recommended	Recommended
BW = im2bw(I);	BW = imbinarize(I,0.5);
thresh = graythresh(I); BW = im2bw(I,thresh);	BW = imbinarize(I);

There are no plans to remove im2bw at this time.