How can i automatically convert the input image to gray scale

I am dealing with three image types: color, B&W and gray. i am reading a bunch of images and performing enhancement but i want all the images to be in gray scale before i proceed to enhance. But i do not know what type of an image i am reading at an instant. so if i am reading anything other than gray scale i want to convert it to gray and if am reading a gray input i want to proceed directly to enhancement. How can i automate this ??

Answer 1

[Image Analyst]

Well it's a little bit tricky to detect the most common types of images: grayscale, RGB, and indexed . Of course your "binary image" is most likely a grayscale uint8 image with gray levels of only 0 and either 1 or 25. See this snippet which handles all of those, and adapt as needed:

%=====================================================================
% Reads FullImageFileName from disk into the axesImage axes.
function imageArray = DisplayImage(handles, fullImageFileName)
% Read in image.
imageArray = []; % Initialize
try
  [imageArray, colorMap] = imread(fullImageFileName);
  % colorMap will have something for an indexed image (gray scale image with a stored colormap).
  % colorMap will be empty for a true color RGB image or a monochrome gray scale image.
catch ME
  % Will get here if imread() fails, like if the file is not an image file but a text file or Excel workbook or something.
  errorMessage = sprintf('Error in program %s, function %s(), at line %d.\n\nError Message:\n%s', ...
    mfilename, ME.stack(1).name, ME.stack(1).line, ME.message);
  uiwait(warndlg(errorMessage));
  return;  % Skip the rest of this function
end

try
  % Display image array in a window on the user interface.
  %axes(handles.axesImage);
  hold off;  % IMPORTANT NOTE: hold needs to be off in order for the "fit" feature to work correctly.
  
  % Here we actually display the image in the "axesImage" axes.
  imshow(imageArray, 'InitialMagnification', 'fit', 'parent', handles.axesImage);
  
  % Display a title above the image.
  [folder, basefilename, extension] = fileparts(fullImageFileName);
  extension = lower(extension);
  % Display the title.
  caption = [basefilename, extension];
  title(handles.axesImage, caption, 'Interpreter', 'none', 'FontSize', 14);
  
  [rows, columns, numberOfColorChannels] = size(imageArray);
  % Get the file date
  fileInfo = dir(fullImageFileName);
  txtInfo = sprintf('%s\n\n%d lines (rows) vertically\n%d columns across\n%d color channels\n', ...
    [basefilename extension], rows, columns, numberOfColorChannels);
  % Tell user the type of image it is.
  if numberOfColorChannels == 3
    colorbar 'off';  % get rid of colorbar.
    txtInfo = sprintf('%s\nThis is a true color, RGB image.', txtInfo);
  elseif numberOfColorChannels == 1 && isempty(colorMap)
    colorbar 'off';  % get rid of colorbar.
    txtInfo = sprintf('%s\nThis is a gray scale image, with no stored color map.', txtInfo);
  elseif numberOfColorChannels == 1 && ~isempty(colorMap)
    txtInfo = sprintf('%s\nThis is an indexed image.  It has one "value" channel with a stored color map that is used to pseudocolor it.', txtInfo);
    colormap(colorMap);
    whos colorMap;
    %fprintf('About to apply the colormap...\n');
    % Thanks to Jeff Mather at the Mathworks to helping to get this working for an indexed image.
    colorbar('peer', handles.axesImage);
    %fprintf('Done applying the colormap.\n');
  end
catch ME
  errorMessage = sprintf('Error in program %s, function %s(), at line %d.\n\nError Message:\n%s', ...
    mfilename, ME.stack(1).name, ME.stack(1).line, ME.message);
  uiwait(warngld(errorMessage));
end
uiwait(helpdlg(txtInfo));
return; % from DisplayImage

[Image Analyst]

To convert an indexed image to gray scale, you have to convert to RGB first, then use rgb2gray - you can't just take the indexed image and ignore the colormap because it will look like garbage.

rgbImage = ind2rgb(imageArray, colorMap);
grayImage = rgb2gray(rgbImage);

Answer 2

[Guillaume]

A black & white image is just a grey image with just two levels of grey, so I'm not sure what you want to do in that case. Possibly nothing. A colour image will be a 3d matrix, whereas a grey image will be 2d. That's easy to differentiate with ndims

if ndims(img) == 3
   %colour image, convert to greyscale any way you want
   img = rgb2gray(img); %note that there are many other ways to convert a colour image to grey
elseif isequal(logical(img), img) %one way of recognising a bw image
   %do whatever you want. Nothing?
end