imresize

Resize image

Syntax

J = imresize(I,scale)

J = imresize(I,[numrows numcols])

[Y,newmap] = imresize(X,map,___)

___ = imresize(___,method)

___ = imresize(___,Name,Value)

Description

J = imresize(I,scale) returns image J that is scale times the size of I. The input image I can be a grayscale, RGB, binary, or categorical image.

You optionally can resize images using a GPU (requires Parallel Computing Toolbox™).

J = imresize(I,[numrows numcols]) returns image J that has the number of rows and columns specified by the vector [numrows numcols].

[Y,newmap] = imresize(X,map,___) resizes the indexed image X with colormap map. By default, imresize returns an optimized colormap, newmap, with the resized indexed image. To return a colormap that is the same as the original colormap, use the Colormap name-value pair argument.

This syntax is not supported on a GPU.

___ = imresize(___,method) specifies the interpolation method used.

___ = imresize(___,Name,Value) uses name-value pair arguments to control various aspects of the resizing operation.

This syntax is not supported on a GPU.

Examples

Resize Image Specifying Scale Factor

Read image into the workspace.

I = imread('rice.png');

Resize the image, specifying scale factor and using default interpolation method and antialiasing.

J = imresize(I, 0.5);

Display the original and the resized image.

figure
imshow(I)
title('Original Image')

figure
imshow(J)
title('Resized Image')

Resize Image Specifying Scale Factor and Interpolation Method

Read image into the workspace.

I = imread('rice.png');

Resize the image, specifying scale factor and the interpolation method.

J = imresize(I, 0.5, 'nearest');

Display the original and the resized image.

figure
imshow(I)
title('Original Image')

figure
imshow(J)
title('Resized Image Using Nearest-Neighbor')

Resize Indexed Image

Read image into the workspace.

[X, map] = imread('trees.tif');

Resize the image, specifying a scale factor. By default, imresize returns an optimized color map with the resized indexed image.

[Y, newmap] = imresize(X, map, 0.5);

Display the original image and the resized image.

figure
imshow(X,map)
title('Original Image')

figure
imshow(Y,newmap)
title('Resized Image')

Resize RGB Image Specifying Size of Output Image

Read image into the workspace.

RGB = imread('peppers.png');

Resize the image, specifying that the output image have 64 rows. Let imresize calculate the number of columns necessary to preserve the aspect ratio.

RGB2 = imresize(RGB, [64 NaN]);

Display the original image and the resized image.

figure
imshow(RGB)
title('Original Image')

figure
imshow(RGB2)
title('Resized Image')

Input Arguments

I — Image to be resized

numeric array | logical array | categorical array

Image to be resized, specified as a numeric, logical, or categorical array of any dimension. If I has more than two dimensions, then imresize only resizes the first two dimensions.

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

scale — Resize factor

positive number

Resize factor, specified as a positive number.

• If scale is less than 1, then the output image is smaller than the input image.

• If scale is greater than 1, then the output image is larger than the input image.

imresize applies the scale factor to each dimension in the image. To apply a different resize factor to each dimension, use the Scale name-value pair argument.

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

[numrows numcols] — Row and column dimensions of output image

2-element vector of positive integers

Row and column dimensions of the output image, specified as a 2-element vector of positive integers. You can specify the value NaN for either numrows or numcols. In this case, imresize computes the number of rows or columns for that dimension automatically, preserving the aspect ratio of the image.

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

X — Indexed image to be resized

numeric array

Indexed image to be resized, specified as a numeric array.

Data Types: double | uint8 | uint16

map — 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: double

method — Interpolation method or kernel

string scalar | character vector | 2-element cell array

Interpolation method or kernel, specified as a string scalar, character vector, or 2-element cell array.

When method is a string scalar or character vector, it identifies a particular method or named interpolation kernel, listed in the following table.

Method	Description
'nearest'	Nearest-neighbor interpolation; the output pixel is assigned the value of the pixel that the point falls within. No other pixels are considered. Nearest-neighbor interpolation is the only interpolation method supported for categorical images and it is the default method for images of this type.
'bilinear'	Bilinear interpolation; the output pixel value is a weighted average of pixels in the nearest 2-by-2 neighborhood.
'bicubic'	Bicubic interpolation; the output pixel value is a weighted average of pixels in the nearest 4-by-4 neighborhood. Bicubic interpolation is the default method for numeric and logical images. Note Bicubic interpolation can produce pixel values outside the original range.
Interpolation Kernel	Description
'box'	Box-shaped kernel The box-shaped kernel is the only interpolation kernel supported for categorical images.
'triangle'	Triangular kernel (equivalent to 'bilinear')
'cubic'	Cubic kernel (equivalent to 'bicubic')
'lanczos2'	Lanczos-2 kernel
'lanczos3'	Lanczos-3 kernel

When method is a 2-element cell array, it defines a custom interpolation kernel. The cell array has the form {f,w}, where f is a function handle for a custom interpolation kernel and w is the width of the custom kernel. f(x) must be zero outside the interval -w/2 <= x < w/2. The function handle f can be called with a scalar or a vector input. For user-specified interpolation kernels, the output image can have some values slightly outside the range of pixel values in the input image.

Data Types: char | string | cell

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: I2 = imresize(I,0.5,'Antialiasing',false);

'Antialiasing' — Perform antialiasing when shrinking an image

true | false

Perform antialiasing when shrinking an image, specified as the comma-separated pair consisting of 'Antialiasing' and true or false.

• If method is 'nearest', then the default value of 'Antialiasing' is false.

• If the interpolation method is the 'box' interpolation kernel and the input image is categorical, then the default value of 'Antialiasing' is false.

• For all other interpolation methods, the default is true.

Data Types: logical

'Colormap' — Return optimized or original colormap

'optimized' (default) | 'original'

Return optimized or original colormap for indexed image, specified as the comma-separated pair consisting of 'Colormap' and one of the following.

Value	Description
'original'	The output colormap newmap is the same as the input colormap map.
'optimized'	imresize returns a new optimized colormap.

The 'Colormap' argument is valid only when resizing indexed images.

Data Types: char | string

'Dither' — Perform color dithering

true (default) | false

Perform color dithering, specified as the comma-separated pair consisting of 'Dither' and true or false. In dithering, you apply a form of noise to the image to randomize quantization error and prevent large-scale patterns.

The 'Dither' argument is valid only when resizing indexed images.

Data Types: logical

'Method' — Interpolation method

string scalar | character vector | 2-element cell array

Interpolation method, specified as the comma-separated pair consisting of 'Method' and a scalar string, character vector, or 2-element cell array. For details, see method.

Data Types: char | string | cell

'OutputSize' — Size of output image

2-element vector of positive integers

Size of the output image, specified as the comma-separated pair consisting of 'OutputSize' and a 2-element vector of positive integers of the form [numrows numcols].

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

'Scale' — Resize scale factor

positive number | 2-element vector of positive numbers

Resize scale factor, specified as the comma-separated pair consisting of 'Scale' and a positive number or 2-element vector of positive numbers. If you specify a scalar, then imresize applies the same scale factor to each dimension in the image. If you specify a 2-element vector, then imresize applies a different scale value to each dimension.

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

Output Arguments

J — Resized image

numeric array | logical array | categorical array

Resized image, returned as a numeric, logical, or categorical array of the same class as the input image, I.

Y — Resized indexed image

numeric array

Resized indexed image, returned as a numeric array of the same class as the input indexed image, X.

newmap — Optimized colormap

c-by-3 numeric array

Optimized colormap, returned 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.

Tips

• The function imresize changed in version 5.4 (R2007a). Previous versions of the Image Processing Toolbox™ used a different algorithm by default. If you need the same results produced by the previous implementation, use the function imresize_old.

• There is a slight numerical difference between the results of imresize on the CPU and the GPU. These differences occur on the right and bottom borders of the image and are barely noticeable to the naked eye.

• If the size of the output image is not an integer, then imresize does not use the scale specified. imresize uses ceil when calculating the output image size.

Extended Capabilities

C/C++ Code Generation

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

Usage notes and limitations:

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

• Syntaxes that support indexed images are not supported, including the named parameters 'Colormap' and 'Dither'.

• Custom interpolation kernels are not supported.

• All name-value pairs must be compile-time constants.

GPU Code Generation

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

Usage notes and limitations:

• 'Colormap' and 'Dither' name-value pair arguments are not supported.

• Indexed images are not supported.

• Custom interpolation kernels are not supported.

• All name-value pairs must be compile-time constants.

• For certain interpolation kernels, there can be a small numerical mismatch between the results in MATLAB and the generated code.

GPU Arrays

Accelerate code by running on a graphics processing unit (GPU) using Parallel Computing Toolbox™.

Usage notes and limitations:

• gpuArray input must be of type single or double.

• Only bicubic interpolation is supported on GPU and the function always performs antialiasing.

For more information, see Image Processing on a GPU.