rgb2gray

Convert RGB image or colormap to grayscale

Syntax

I = rgb2gray(RGB)
newmap = rgb2gray(map)

Description

example

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.

example

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

Examples

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Read and display an RGB image, and then convert it to grayscale.

Read the sample file, peppers.png, and display the RGB image.

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

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

I = rgb2gray(RGB);
figure
imshow(I)

Read an indexed image with an RGB colormap. Then, convert the colormap to grayscale.

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

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

Display the image.

imshow(X,map)

Convert the RGB colormap to a grayscale colormap and redisplay the image.

newmap = rgb2gray(map);
imshow(X,newmap)

Input Arguments

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

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

Data Types: single | double | uint8 | uint16

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.

Data Types: double

Output Arguments

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

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

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.

Data Types: double

Tips

  • rgb2gray supports the generation of C code using MATLAB® Coder™.

Algorithms

rgb2gray converts RGB values to grayscale values by forming a weighted sum of the R, G, and B components:

0.2989 * R + 0.5870 * G + 0.1140 * B 

These are the same weights used by the rgb2ntsc function to compute the Y component.

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 the following formula:

0.299 * R + 0.587 * G + 0.114 * B

Extended Capabilities