imquantize

Quantize image using specified quantization levels and output values

Syntax

quant_A = imquantize(A,levels)

quant_A = imquantize(___,values)

[quant_A,index] = imquantize(___)

Description

quant_A = imquantize(A,levels) quantizes image A using specified quantization values contained in the N element vector levels. Output image quant_A is the same size as A and contains N + 1 discrete integer values in the range 1 to N + 1 which are determined by the following criteria:

• If A(k) ≤ levels(1), then quant_A(k) = 1.

• If levels(m-1) < A(k)  ≤  levels(m) , then quant_A(k) = m.

• If A(k) > levels(N), then quant_A(k) = N + 1.

Note that imquantize assigns values to the two implicitly defined end intervals:

• A(k) ≤ levels(1)

• A(k) > levels(N)

quant_A = imquantize(___,values) adds the N + 1 element vector values where N = length(levels). Each of the N + 1 elements of values specify the quantization value for one of the N + 1 discrete pixel values in quant_A.

• If A(k) ≤ levels(1), then quant_A(k) = values(1).

• If levels(m-1) < A(k)  ≤  levels(m) , then quant_A(k) = values(m).

• If A(k) > levels(N), then quant_A(k) = values(N + 1).

[quant_A,index] = imquantize(___) returns an array index such that:

quant_A = values(index)

Examples

Segment Image into Three Levels Using Two Thresholds

Read an image, convert it to grayscale, and display the result.

I = imread("foggysf2.jpg");
I = rgb2gray(I);
imshow(I)
title("Grayscale Image")

Calculate two threshold levels.

thresh = multithresh(I,2);

Segment the image into three levels using imquantize .

labels = imquantize(I,thresh);

Convert the segmented image into a color image using label2rgb and display it.

labelsRGB = label2rgb(labels);
imshow(labelsRGB)
title("Segmented Image")

Compare Thresholding Entire Image Versus Plane-by-Plane Thresholding

Read truecolor (RGB) image and display it.

I = imread('peppers.png');
imshow(I) 
axis off
title('RGB Image');

Generate thresholds for seven levels from the entire RGB image.

threshRGB = multithresh(I,7);

Generate thresholds for each plane of the RGB image.

threshForPlanes = zeros(3,7);			

for i = 1:3
    threshForPlanes(i,:) = multithresh(I(:,:,i),7);
end

Process the entire image with the set of threshold values computed from entire image.

value = [0 threshRGB(2:end) 255]; 
quantRGB = imquantize(I, threshRGB, value);

Process each RGB plane separately using the threshold vector computed from the given plane. Quantize each RGB plane using threshold vector generated for that plane.

quantPlane = zeros( size(I) );

for i = 1:3
    value = [0 threshForPlanes(i,2:end) 255]; 
    quantPlane(:,:,i) = imquantize(I(:,:,i),threshForPlanes(i,:),value);
end

quantPlane = uint8(quantPlane);

Display both posterized images and note the visual differences in the two thresholding schemes.

imshowpair(quantRGB,quantPlane,'montage') 
axis off
title('Full RGB Image Quantization        Plane-by-Plane Quantization')

To compare the results, calculate the number of unique RGB pixel vectors in each output image. Note that the plane-by-plane thresholding scheme yields about 23% more colors than the full RGB image scheme.

dim = size( quantRGB );
quantRGBmx3   = reshape(quantRGB,   prod(dim(1:2)), 3);
quantPlanemx3 = reshape(quantPlane, prod(dim(1:2)), 3);

colorsRGB   = unique(quantRGBmx3,   'rows' );
colorsPlane = unique(quantPlanemx3, 'rows' );

disp(['Unique colors in RGB image            : ' int2str(length(colorsRGB))]);

Unique colors in RGB image            : 188

disp(['Unique colors in Plane-by-Plane image : ' int2str(length(colorsPlane))]);

Unique colors in Plane-by-Plane image : 231

Threshold Grayscale Image from 256 to 8 Levels

Reduce the number of discrete levels in an image from 256 to 8. This example uses two different methods for assigning values to each of the eight output levels.

Read image and display it.

I = imread('coins.png');
imshow(I) 
axis off
title('Grayscale Image')

Split the image into eight levels by obtaining seven thresholds from the multithresh function.

thresh = multithresh(I,7);

Construct the valuesMax vector such that the maximum value in each quantization interval is assigned to the eight levels of the output image.

valuesMax = [thresh max(I(:))]

valuesMax = 1x8 uint8 row vector

    65    88   119   149   169   189   215   255

[quant8_I_max, index] = imquantize(I,thresh,valuesMax);

Similarly, construct the valuesMin vector such that the minimum value in each quantization interval is assigned to the eight levels of the output image. Instead of calling imquantize again with the vector valuesMin, use the output argument index to assign those values to the output image.

valuesMin = [min(I(:)) thresh]

valuesMin = 1x8 uint8 row vector

    23    65    88   119   149   169   189   215

quant8_I_min = valuesMin(index);

Display both eight-level output images side by side.

imshowpair(quant8_I_min,quant8_I_max,'montage') 
title('Minimum Interval Value           Maximum Interval Value')

Input Arguments

A — Input image

image

Input image, specified as a numeric array of any dimension.

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

levels — Quantization levels

vector

Quantization levels, specified as an N element vector. Values of the discrete quantization levels must be in monotonically increasing order.

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

values — Quantization values

vector

Quantization values, specified as an N+1 element vector.

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

Output Arguments

quant_A — Quantized output image

image

Quantized output image, returned as a numeric array the same size as A. If input argument values is specified, then quant_A is the same data type as values. If values is not specified, then quant_A is of class double.

index — Mapping array

array

Mapping array, returned as an array the same size as input image A. It contains integer indices which access values to construct the output image: quant_A = values(index). If input argument values is not defined, then index = quant_A.

Data Types: double

Extended Capabilities

C/C++ Code Generation

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

imquantize supports the generation of C code (requires MATLAB^® Coder™). For more information, see Code Generation for Image Processing.

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

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 in R2012b

R2021b: Support for thread-based environments

imquantize now supports thread-based environments.