grayslice

Convert grayscale image to indexed image using multilevel thresholding

Syntax

X = grayslice(I,N)

X = grayslice(I,thresholds)

Description

X = grayslice(I,N) converts a grayscale image to an indexed image by using multilevel thresholding. The function automatically calculates the threshold values based on the number of threshold values, N. To learn more about the threshold calculation, see Algorithms.

X = grayslice(I,thresholds) returns an indexed image by multilevel thresholding of input image using a specified set of thresholds.

Examples

Convert Grayscale Image to Indexed Image Using Thresholding

Read grayscale image into the workspace.

I = imread('snowflakes.png');

Threshold the intensity image, returning an indexed image.

X = grayslice(I,16);

Display the original image and the indexed image, using one of the standard colormaps.

imshow(I)

figure
imshow(X,jet(16))

Convert Grayscale Image to Indexed Image Using Multilevel Thresholds

Read a grayscale image into the workspace. Display the image.

I = imread('coins.png');
imshow(I)

Specify the threshold values for multilevel thresholding.

thresholds = [45 65 84 108 134 157 174 189 206 228];

Convert the input grayscale image to an indexed image.

X = grayslice(I,thresholds);

Display the indexed image. Set the colormap of the indexed image to jet. The length of colormap, m, is the maximum intensity value in the indexed image.

m = double(max(X(:)));

figure
imshow(X,colormap(jet(m)))

Input Arguments

I — Input grayscale image

m-by-n numeric matrix

Input grayscale image, specified as an m-by-n numeric matrix. The grayslice function expects images of data type double and single to have values in the range [0, 1]. If I has values outside the range [0, 1], then you can rescale values to the expected range by using the rescale function.

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

N — Number of threshold values

positive scalar

Number of threshold values, specified as a positive scalar. The value represents the total number of thresholds to be used for multilevel thresholding.

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

thresholds — Set of thresholds

numeric vector

Set of thresholds, specified as a numeric vector. The number of threshold values to be used for multilevel thresholding is equal to length(thresholds).

Image Data Type	Range of Threshold Values
uint8	[0, 255]
int16 or uint16	[0, 65535]
single or double	[0, 1]

Note

Before thresholding an image of data type int16, the grayslice function converts the image to uint16 by adding 32,768 to each pixel. Consider this additive offset when specifying thresholds for input images of data type int16.

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

Output Arguments

X — Output indexed image

m-by-n matrix

Output indexed image, returned as a m-by-n matrix of the same size as the input grayscale image. The data type of X depends on the number of threshold values used for multilevel thresholding.

• If the number of threshold values is less than 256, then X is of data type uint8. In this case, the range of intensity values in X is either [0, N-1] or [0, length(thresholds)].

• If the number of threshold values is greater than or equal to 256, then X is of data type double. In this case, the range of intensity values in X is either [1, N] or [1, length(thresholds)+1].

Data Types: uint8 | double

Tips

• You can view the thresholded image using imshow(X,map) with a colormap of appropriate length.

Algorithms

The function performs multilevel thresholding of the input grayscale image and returns an indexed image as the output. If you specify the number of thresholds N, then grayslice assigns pixels to N indices according to these thresholds.

• The first index in X consists of the grayscale pixels in the range $$max\_intensity\times \left[0,\frac{1}{N}\right)$$

• The k-th index in X consists of the grayscale pixels in the range $$max\_intensity\times \left[\frac{k-1}{N},\frac{k}{N}\right)$$

• The last index in X consists of the grayscale pixels in the range $$max\_intensity\times \left[\frac{N-1}{N},1\right]$$.

max_intensity depends on the data type of the input image.

Image Data Type	max_intensity
uint8	255
int16 or uint16	65535
single or double	1

Note

Before thresholding an image of data type int16, the grayslice function converts the image to uint16 by adding 32,768 to each pixel.

Extended Capabilities

Thread-Based Environment

Run code in the background using MATLAB® backgroundPool or accelerate code with Parallel Computing Toolbox™ ThreadPool.

This function fully supports thread-based environments. For more information, see Run MATLAB Functions in Thread-Based Environment.

Version History

Introduced before R2006a

R2021b: Support for thread-based environments

grayslice now supports thread-based environments.