How to convert multiple images from RGB to Grayscale or Index, Grayscale to RGB or Index and Index to RGB orGrayscale having a single function in use?

I want to create a function which can convert multiple images into any desired image types .

[DGM]

Look up function, imread, imwrite, im2gray, repmat, rgb2ind, ind2rgb

Also see:

https://www.mathworks.com/matlabcentral/answers/812105-grayscale-to-rgb-in-matlab

https://www.mathworks.com/matlabcentral/answers/99136-how-do-i-convert-my-rgb-image-to-grayscale-without-using-the-image-processing-toolbox#answer_950190

It's up to you to decide how your function needs to work, because it's up to you to determine what you want. You'll have to determine what the expected inputs and outputs should be and how its behavior should be controlled. It may be more complicated than you expect to write something that can seamlessly handle any and all sorts of image files. Consider that not all images are grayscale/RGB/indexed. That's just a thought.

[Muhammad]

I am a beginer, i don't know much about functions in MATLAB.

[Muhammad]

Can you plz help me code this scenario?

[DGM]

Like I said, that starts with a concrete description of what you want.

[Muhammad]

I want a function which may converts image types of "n" number of images from one type to another.

Answer 1

[DGM]

Since it's up to me to decide what you want, this must be it.

pathlist = {'peppers.png'; 'cameraman.tif'; 'canoe.tif'};
outputprefix = 'outputfile';
outputsuffix = '.png';
outputcolortype = 'indexed'; % 'rgb','indexed','gray'

% process the specified files
blindfileconverter(pathlist,outputprefix,outputsuffix,outputcolortype)


% a totally half-baked function that will probably break
function blindfileconverter(pathlist,outputprefix,outputsuffix,outputcolortype)
% BLINDFILECONVERTER(PATHLIST, PREFIX, SUFFIX, COLORTYPE)
%  Convert a bunch of files from color type to another.
%  Write all the files using a common filetype and naming scheme.
%  This will strip alpha content from anything with it.
%  Not all color options work with all output filetypes.
%
%  PATHLIST is a cell array of char vectors specifying the files to load
%  PREFIX is the output filename prefix
%     (e.g. '/thisisthe/placeiwant/myimage')
%  SUFFIX is the output filename suffix
%     (e.g. '.png')
%  COLORTYPE is the color type of the output file
%     Supported: 'gray', 'rgb', or 'indexed'

defaultctlength = 256; % for indexed outputs

for k = 1:numel(pathlist)
    % read the file, discarding unattached alpha
    [inpict,map,~] = imread(pathlist{k});
    
    % skip any multiframe images
    if size(inpict,4) ~= 1
        warning('image %s is multiframe; skipping',pathlist{k})
        continue; 
    end
    
    % if alpha is present, it may be attached or separate
    % since there's no universal way to handle it for all possible outputs
    % discard alpha if attached, ignore separate alpha
    if any(size(inpict,3) == [2 4])
        inpict = inpict(:,:,1:end-1);
    end
    
    % image depth should now be either 1 or 3
    if ~any(size(inpict,3) == [1 3])
        warning('image %s is not gray, indexed, or RGB; skipping',pathlist{k})
        continue; 
    end
    
    % determine if image is indexed
    isindexed = ~isempty(map);
    
    switch lower(outputcolortype)
        case 'gray'
            % if image is indexed
            if isindexed
                inpict = ind2rgb(inpict,map);
            end
            
            % if image is RGB
            if size(inpict,3) == 3
                inpict = rgb2gray(inpict);
            end
            
        case 'rgb'
            % if image is indexed
            if isindexed
                inpict = ind2rgb(inpict,map);
            end
            
            % if image is gray
            if size(inpict,3) == 1
                inpict = repmat(inpict,[1 1 3]);
            end
            
        case 'indexed'
            % if image is gray
            if ~isindexed && (size(inpict,3) == 1)
                inpict = repmat(inpict,[1 1 3]);
            end
            
            % if image is RGB
            if size(inpict,3) == 3
                [inpict map] = rgb2ind(inpict,defaultctlength);
            end
            
        otherwise
            error('unknown output color type %s',outputcolortype)
    end
    
    if strcmp(outputcolortype,'indexed')
        imwrite(inpict,map,sprintf('%s_%04d%s',outputprefix,k,outputsuffix))
    else
        imwrite(inpict,sprintf('%s_%04d%s',outputprefix,k,outputsuffix))
    end
end
end

[Muhammad]

Thank you for your help, But this is so much complex and causing so many errors.

[DGM]

Since you won't say what you want or how you intend to use it in any detail, that's a predictable outcome. Note that you still aren't saying how you tried to use it or what errors you got.

[Muhammad]

I want a function which can change image types in matlab. The number of images can be n, assume the number of images is 50 and i want to change their types all together. Just like we change types using, rgb2gray or ind2rgb.

[DGM]

Let's say you have N = 50.

Which 50 images get used?

Is it possible that any of the images could be multiframe? Might any of them have alpha content?

When the conversion is done, where do the 50 new images go?

[Muhammad]

The images can be of any type, i.e: rgb, grayscale or indexed. Yes it is possible that images could be of multiframes. When all the images had been processed they should go to a new folder.

[DGM]

Again, you haven't said which files are being selected. N just specifies how many files to select. Are these some sort of numbered image sequences and N just selects the first N files in the sequence? If so, what's the naming convention?

So inputs may be multiframe. imread() supports multiframe inputs for TIFF, GIF, PNM (PBM/PGM/PPM), CUR, ICO, HDF, and SVS files, but the means by which multiframe images are identified and read differs between those formats and is currently not correctly functional for GIF except in rare cases which cannot be detected from within MATLAB. Can you rely on all your files having the same format, or is this another required addition to complexity?

If images can be multiframe, then how do you intend to specify the output filetype? Is the output for each file supposed to be inherited from the input file, or is the output filetype explicitly specified? I hope that either way the answer is TIFF, because TIFF is the only supported filetype that can handle all the requirements. The only filetypes for which imwrite() supports multiframe output are GIF, TIF, and HDF. Even if multiframe support were dropped, PNG, TIFF and RAS are the only formats that would work. I don't know how much external support you'll find for RAS. While there are multiframe specifications for PNG, imwrite() doesn't support it.

% test these file formats
suffixes = {'gif','pgm','pbm','ppm','tif','hdf', ...
    'png','jpg','jp2','bmp','ras','xwd'};

A = imread('peppers.png');
[Aind map] = rgb2ind(A,256);
Agray = im2gray(A);

nsuf = numel(suffixes);
supportsIDX = false(nsuf,1);
supportsGRY = false(nsuf,1);
supportsRGB = false(nsuf,1);

for k = 1:nsuf
    fname = sprintf('test.%s',suffixes{k});
    
    % test gray
    try
        imwrite(Agray,fname)
        [Atest maptest] = imread(fname);
        supportsGRY(k) = isempty(maptest) && size(Atest,3)==1; 
    catch
        supportsGRY(k) = false;
    end
    
    % test RGB
    try
        imwrite(A,fname)
        [Atest maptest] = imread(fname);
        supportsRGB(k) = isempty(maptest) && size(Atest,3)==3; 
    catch
        supportsRGB(k) = false;
    end
    
    % test indexed
    try
        imwrite(Aind,map,fname)
        [Atest maptest] = imread(fname);
        supportsIDX(k) = ~isempty(maptest);
    catch
        supportsIDX(k) = false;
    end
end

supportsALL = supportsGRY & supportsRGB & supportsIDX;
table(suffixes',supportsGRY,supportsRGB,supportsIDX,supportsALL)

ans = 12×5 table
     Var1      supportsGRY    supportsRGB    supportsIDX    supportsALL
    _______    ___________    ___________    ___________    ___________

    {'gif'}       false          false          true           false   
    {'pgm'}       true           false          false          false   
    {'pbm'}       true           false          false          false   
    {'ppm'}       false          true           false          false   
    {'tif'}       true           true           true           true    
    {'hdf'}       false          true           true           false   
    {'png'}       true           true           true           true    
    {'jpg'}       true           true           false          false   
    {'jp2'}       true           true           false          false   
    {'bmp'}       false          true           true           false   
    {'ras'}       true           true           true           true    
    {'xwd'}       false          false          true           false   

So should all output be TIFF-only? If output format is inherited, how would you convert a GIF file to RGB?

As far as complexity goes, if you're asking for multiframe support and generalized handling of numbered file sequences, I hope you realize that the code will only become more complex, not less.

Answer 2

[Image Analyst]

See the attached demo. You can specify the input extension and output extension on lines 13 and 14 to be whatever you want. You can optionally delete the input image, if you want, so that only the desired output extension version of the image remains.