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Convert RGB image to grayscale

Since R2020b



    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.


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

    RGB = imread('example.tif');

    Convert the RGB image into a grayscale image.

    I = im2gray(RGB);

    Display the converted grayscale image.


    Input Arguments

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    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

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    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.


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


    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 + 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

    Version History

    Introduced in R2020b

    See Also

    (Image Processing Toolbox) | | (Image Processing Toolbox) | (Image Processing Toolbox) | | (Image Processing Toolbox) |