imbinarize

Binarize 2-D grayscale image or 3-D volume by thresholding

Syntax

BW = imbinarize(I)

BW = imbinarize(I,method)

BW = imbinarize(I,T)

BW = imbinarize(I,'adaptive',Name,Value)

Description

BW = imbinarize(I) creates a binary image from 2-D or 3-D grayscale image I by replacing all values above a globally determined threshold with 1s and setting all other values to 0s. By default, imbinarize uses Otsu's method, which chooses the threshold value to minimize the intraclass variance of the thresholded black and white pixels [1]. imbinarize uses a 256-bin image histogram to compute Otsu's threshold. To use a different histogram, see otsuthresh.

BW = imbinarize(I,method) creates a binary image from image I using the thresholding method specified by method: 'global' or 'adaptive'.

BW = imbinarize(I,T) creates a binary image from image I using the threshold value T. T can be a global image threshold, specified as a scalar luminance value, or a locally adaptive threshold, specified as a matrix of luminance values.

BW = imbinarize(I,'adaptive',Name,Value) creates a binary image from image I using name-value pairs to control aspects of adaptive thresholding.

Examples

Binarize Image Using Global Threshold

Read grayscale image into the workspace.

I = imread('coins.png');

Convert the image into a binary image.

BW = imbinarize(I);

Display the original image next to the binary version.

figure
imshowpair(I,BW,'montage')

Binarize Image Using Locally Adaptive Thresholding

Read grayscale image into workspace.

I = imread('rice.png');

Convert grayscale image to binary image.

BW = imbinarize(I, 'adaptive');

Display original image along side binary version.

figure
imshowpair(I,BW,'montage')

Binarize Images with Darker Foreground Than Background

Read a grayscale image into the workspace and display it.

I = imread('printedtext.png');
figure
imshow(I)
title('Original Image')

Convert the image to a binary image using adaptive thresholding. Use the ForegroundPolarity parameter to indicate that the foreground is darker than the background.

BW = imbinarize(I,'adaptive','ForegroundPolarity','dark','Sensitivity',0.4);

Display the binary version of the image.

figure
imshow(BW)
title('Binary Version of Image')

Binarize 3-D Volume Using Global Thresholding

Load 3-D grayscale intensity data into the workspace.

load mristack;
V = mristack;

View the 3-D volume.

figure
slice(double(V),size(V,2)/2,size(V,1)/2,size(V,3)/2)
colormap gray 
shading interp

Convert the intensity volume into a 3-D binary volume.

J = imbinarize(V);

View the 3-D binary volume.

figure
slice(double(J),size(J,2)/2,size(J,1)/2,size(J,3)/2)
colormap gray 
shading interp

Input Arguments

I — Input image

2-D grayscale image | 3-D grayscale volume

Input image, specified as a 2-D grayscale image or a 3-D grayscale volume. imbinarize expects pixel values of data type double and single to be in the range [0, 1]. You can use the rescale function to adjust pixel values to the expected range.

Note

imbinarize interprets an RGB image as a volumetric grayscale image and does not binarize each channel separately. To produce a binary image from an RGB image, first convert the image to a grayscale image using rgb2gray.

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

method — Method used to binarize image

'global' (default) | 'adaptive'

Method used to binarize image, specified as one of the following values.

Values	Meaning
'global'	Calculate global image threshold using Otsu's method. See graythresh for more information about Otsu’s method.
'adaptive'	Calculate locally adaptive image threshold chosen using local first-order image statistics around each pixel. See adaptthresh for details. If the image contains Infs or NaNs, the behavior of imbinarize for the 'adaptive' method is undefined. Propagation of Infs or NaNs might not be localized to the neighborhood around Inf and NaN pixels.

Data Types: char | string

T — Threshold

numeric scalar | numeric array

Threshold luminance value, specified as a numeric scalar or numeric array with values in the range [0, 1].

• If T is a numeric scalar, then imbinarize interprets T as a global image threshold. Use graythresh or otsuthresh to compute a global image threshold.

• If T is a numeric array, then imbinarize interprets T as a locally adaptive threshold. Use adaptthresh to compute a locally adaptive threshold.

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

Name-Value Pair Arguments

Specify optional comma-separated pairs of Name,Value arguments. Name is the argument name and Value is the corresponding value. Name must appear inside quotes. You can specify several name and value pair arguments in any order as Name1,Value1,...,NameN,ValueN.

Example: BW = imbinarize(I,'adaptive','Sensitivity',0.4);

'Sensitivity' — Sensitivity factor for adaptive thresholding

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

Sensitivity factor for adaptive thresholding, specified as the comma-separated pair consisting of 'Sensitivity' and a number in the range [0, 1]. A high sensitivity value leads to thresholding more pixels as foreground, at the risk of including some background pixels.

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

'ForegroundPolarity' — Determine which pixels are considered foreground pixels

'bright' (default) | 'dark'

Determine which pixels are considered foreground pixels for adaptive thresholding, specified as the comma-separated pair consisting of 'ForegroundPolarity' and one of the following values.

Value	Meaning
'bright'	The foreground is brighter than the background.
'dark'	The foreground is darker than the background

Data Types: char | string

Output Arguments

BW — Output binary image

logical matrix | logical array

Output binary image, returned as a logical matrix or logical array of the same size as I.

Data Types: logical

Tips

• To produce a binary image from an indexed image, first convert the image to a grayscale image using ind2gray.

Algorithms

The 'adaptive' method binarizes the image using a locally adaptive threshold. imbinarize computes a threshold for each pixel using the local mean intensity around the neighborhood of the pixel. This technique is also called Bradley's method [2]. The 'adaptive' method also uses a neighborhood size of approximately 1/8th of the size of the image (computed as 2*floor(size(I)/16)+1). To use a different first order local statistic or a different neighborhood size, see adaptthresh.

References

[1] Otsu, N., "A Threshold Selection Method from Gray-Level Histograms." IEEE Transactions on Systems, Man, and Cybernetics. Vol. 9, No. 1, 1979, pp. 62–66.

[2] Bradley, D., G. Roth, "Adapting Thresholding Using the Integral Image," Journal of Graphics Tools. Vol. 12, No. 2, 2007, pp.13–21.

Extended Capabilities

C/C++ Code Generation

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

Usage notes and limitations:

• imbinarize supports the generation of C code (requires MATLAB^® Coder™). Note that if you choose the generic MATLAB Host Computer target platform, imbinarize 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 Code Generation Using a Shared Library.

• When generating code, all character vector input arguments must be compile-time constants.

GPU Code Generation

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

Usage notes and limitations:

• When generating code, all character vector input arguments must be compile-time constants.