im2gray not working

I am trying to read my image and to make it as an array. I want to use im2gray command but it is not working. However the same code with same image is working in matlab online and other systems. I have image processing software insatlled in the system. Please help me with this.

[KSSV]

The error says im1 (image) is of class uint8 and im2gray is not working on that.

Try:

im2 = im2gray(double(im1)) ;

What version of MATLAB you are using? Actually im2gray should work for uint8 as well.

[Preeti Sharma]

I am using version 2020a. I tried and mat2gray worked perfectly. im2gray for the same image worked in version 2020b.

Answer 1

[DGM]

im2gray() was introduced in R2020b. You're using R2020a.

Depending on what you need, you might be able to do it with rgb2gray() and some array size checks.

switch size(inpict,3)
    case 3
        outpict = rgb2gray(inpict);
    case 1
        outpict = inpict;
    otherwise
        % i'm assuming you don't want to support IA/RGBA images
        error('a wild error message appears')
end

If you know all your images are RGB and aren't worried about running into ones that aren't, you can just use rgb2gray().

[Stephen23]

Note that a single page may be an indexed image, so case 1 is latent buggy.

https://www.mathworks.com/help/matlab/creating_plots/image-types.html

[DGM]

In the same way, any 3-channel image may be HSV, LAB, or any other format, so every function in MATLAB/IPT that handles RGB images is latent buggy too.

https://blogs.mathworks.com/steve/2007/03/09/what-happened-to-isgray-isind-isbw-and-isrgb/

FWIW, im2gray() makes no attempt to discern whether the input is grayscale or indexed. It accepts any image so long as it has:

• ndims<3
• ndims==3 and has 3 channels

So im2gray() has the exact same problem.

Of course, if you accept that all these tools are flawed in that they're susceptible to ambiguities that need to be resolved at a higher level, you could always just use MIMT mono() and get support for I/IA/RGB/RGBA inputs and other conversions as well.

[A,~,alph] = imread('sources/standardmods/cmanrgba.png');
A = joinalpha(A,alph); % an RGBA image

B = mono(A,'y601'); % an IA image

Now you can be susceptible to mistakenly being fed CMYK images too!

[Stephen23]

"so every function in MATLAB/IPT that handles RGB images is latent buggy too."

Of course, the user must know what image type they have. My comment was more for the reader.

Ultimately an array of numbers is just that: any possible meaning it has can only be resolved at a higher level, as you very neatly put it.

[DGM]

Apologies if I interpreted your comment as a message to me and not the reader/OP.

Either way, that blog post from Steve is good.

Answer 2

[Image Analyst]

im2 = im1; % Initialize.
if ndims(im2) == 3
    % If it's color, convert to gray scale.
    im2 = rgb2gray(im2);
end

[DGM]

rgb2gray() accepts only mxnx3 arrays, but not all 3D images have 3 channels.

[Image Analyst]

@DGM correct. It assumes that the image im2 is a gray scale or RGB image. If im2 is a hyperspectral image or a volumetric image (like from CT MRI) then one should not use rgb2gray() or im2gray(). If you do an error will likely be thrown to alert you of that.