im2gray

Convert RGB image to grayscale

Since R2020b

Syntax

I = im2gray(RGB)

Description

I = im2gray(RGB) converts the specified truecolor image RGB to a grayscale intensity image I. The im2gray function accepts grayscale images as inputs and returns them unmodified.

The im2gray function converts RGB images to grayscale by eliminating the hue and saturation information while retaining the luminance.

Examples

Convert Truecolor (RGB) Image to Grayscale Image

Read a truecolor (RGB) image into the workspace from a file and display it.

RGB = imread('example.tif');
imshow(RGB)

Convert the RGB image into a grayscale image.

I = im2gray(RGB);

Display the converted grayscale image.

imshow(I)

Input Arguments

RGB — Truecolor image

m-by-n-by-3 numeric array | m-by-n numeric array

Truecolor image, specified as an m-by-n-by-3 numeric array. im2gray also accepts m-by-n numeric arrays (grayscale images) and returns them unmodified.

The im2gray function expects truecolor images of data type double and single to have values in the range [0, 1]. If an image has values outside the range [0, 1], then you can rescale values to the expected range by using the rescale function.

If you have Parallel Computing Toolbox™ installed, RGB can also be a gpuArray object.

Data Types: single | double | uint8 | uint16

Output Arguments

I — Grayscale image

m-by-n numeric array

Grayscale image, returned as an m-by-n numeric array. If the input to im2gray is a grayscale image, the output image I is the same as the input image.

If you have Parallel Computing Toolbox installed, then I can also be a gpuArray object.

Tips

• The im2gray function is identical to rgb2gray except that it can accept grayscale images as inputs, returning them unmodified. The rgb2gray function returns an error if the input image is a grayscale image. If you use the im2gray function, code like this loop is no longer necessary.

  if ndims(I) == 3
      I = rgb2gray(I);
  end
  

• Unlike the rgb2gray function, the im2gray function does not accept colormaps as an input. To convert a colormap to grayscale, use the cmap2gray function.

Algorithms

The im2gray function converts RGB values to grayscale values by forming a weighted sum of the R, G, and B components:

0.298936021293775 * R + 0.587043074451121 * G + 0.114020904255103 * B 

The coefficients used to calculate grayscale values in the im2gray function are identical to those used to calculate luminance (E'y) in Rec.ITU-R BT.601-7 after rounding to three decimal places. Rec.ITU-R BT.601-7 calculates E'y using this formula:

0.299 * R + 0.587 * G + 0.114 * B

Extended Capabilities

C/C++ Code Generation

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

When generating code, if you choose the generic MATLAB Host Computer target platform, im2gray 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.

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.

GPU Arrays

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

The im2gray function fully supports GPU arrays. To run the function on a GPU, specify the input data as a gpuArray (Parallel Computing Toolbox). For more information, see Run MATLAB Functions on a GPU (Parallel Computing Toolbox).

Distributed Arrays

Partition large arrays across the combined memory of your cluster using Parallel Computing Toolbox™.

This function fully supports distributed arrays. For more information, see Run MATLAB Functions with Distributed Arrays (Parallel Computing Toolbox).

Version History

Introduced in R2020b