rgb2gray

Convert RGB image or colormap to grayscale

Syntax

I = rgb2gray(RGB)

newmap = rgb2gray(map)

Description

I = rgb2gray(RGB) converts the truecolor image RGB to the grayscale image I. The rgb2gray function converts RGB images to grayscale by eliminating the hue and saturation information while retaining the luminance. If you have Parallel Computing Toolbox™ installed, rgb2gray can perform this conversion on a GPU.

newmap = rgb2gray(map) returns a grayscale colormap equivalent to map.

Examples

Convert RGB Image to Grayscale Image

Read a sample RGB image, then display the image.

RGB = imread("peppers.png");
imshow(RGB)

Convert the RGB image to a grayscale image and display the result.

I = rgb2gray(RGB);
imshow(I)

Convert RGB Colormap to Grayscale Colormap

Read the sample file, corn.tif, which is an indexed image with an RGB colormap.

[X,map] = imread("corn.tif");

Display the indexed image with the RGB colormap.

imshow(X,map)

Convert the RGB colormap to a grayscale colormap.

newmap = rgb2gray(map);

Display the indexed image with the grayscale colormap.

imshow(X,newmap)

Input Arguments

RGB — Truecolor image

m-by-n-by-3 numeric array

Truecolor image, specified as an m-by-n-by-3 numeric array.

The rgb2gray 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

map — Colormap

c-by-3 numeric matrix

Colormap, specified as a c-by-3 numeric matrix with values in the range [0, 1]. Each row of map is a three-element RGB triplet that specifies the red, green, and blue components of a single color of the colormap.

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

Data Types: double

Output Arguments

I — Grayscale image

m-by-n numeric array

Grayscale image, returned as an m-by-n numeric array.

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

newmap — Grayscale colormap

c-by-3 numeric matrix

Grayscale colormap, returned as an c-by-3 numeric matrix with values in the range [0, 1]. The three columns of newmap are identical, so that each row of map specifies a single intensity value.

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

Data Types: double

Tips

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

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

Algorithms

rgb2gray 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 rgb2gray are identical to those used to calculate luminance (E'y) in Rec.ITU-R BT.601-7 after rounding to 3 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, rgb2gray 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.

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.

GPU Arrays

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

The rgb2gray 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 before R2006a

R2014b: Moved to MATLAB from Image Processing Toolbox

Previously, rgb2gray required Image Processing Toolbox™.