rgb2gray - Convert RGB image or colormap to grayscale - MATLAB (original) (raw)

Convert RGB image or colormap to grayscale

Syntax

Description

[I](#buiz8mj-1-I) = rgb2gray([RGB](#buiz8mj-1-RGB)) converts the truecolor image RGB to the grayscale imageI. 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](#buiz8mj-1-newmap) = rgb2gray([map](#buiz8mj-1-map)) returns a grayscale colormap equivalent to map.

example

Examples

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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)

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.

Convert the RGB colormap to a grayscale colormap.

Display the indexed image with the grayscale colormap.

Input Arguments

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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 agpuArray object.

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 agpuArray object.

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 agpuArray object.

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

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

Data Types: double

Tips

• If the input image is a grayscale image, then thergb2gray 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 theim2gray 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

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When generating code, if you choose the genericMATLAB 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.

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).

Version History

Introduced before R2006a

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Previously, rgb2gray required Image Processing Toolbox™.