Matlab: Check all images in a directory for grayscale? - matlab

Question

I want to check all jpg-images in the current directory if they are grayscale or contain coloured pixels... I tried:
figdirectory = pwd;
fullpath = sprintf('%s/*.jpg', figdirectory);
d = dir(fullpath);
% Loop
pages = [];
for i = 1:length(d)
f = d(i).name;
fname_input = sprintf('%s/%s', figdirectory, f);
A = imread(fname_input);
B = rgb2gray(A);
if(A-B == 0)
hascolor = 0;
else
hascolor = 1;
end
pages = [pages; hascolor];
end
but this gives me an error about the matrix dimensions of A and B. Why has A a third dimension?
Thanks!

Answer 1

To do that:
0) Assuming your input is purely an 'image' and not a 'volume'.
1) [r c d] = size(im);
2) if d is larger than 1, then it is a colored image
3) other wise, it has to be a gray-scale image.
EDITS:
You can add one more condition to solidly distinguish grey scale img from color ones. Assuming ur color imgs are of 3 channels,
if d is equal to 3 then check if all 3 channels are equal
im(:,:,1)==im(:,:,2) && im(:,:,1)==im(:,:,3)
Then you have an grayscale image otherwise color img

Answer 2

Here is one simple solution based on IMFINFO:
%# some test images shipped with the Image Processing Toolbox
fNames = {
'circles.png' %# binary
'shadow.tif' %# indexed color
'coins.png' %# gray
'peppers.png' %# RGB
};
isGrayscale = false(size(fNames));
for i=1:numel(fNames)
imgInfo = imfinfo(fNames{i});
if strcmp(imgInfo.ColorType,'truecolor')
isGrayscale(i) = false;
elseif strcmp(imgInfo.ColorType,'grayscale')
isGrayscale(i) = true;
elseif strcmp(imgInfo.ColorType,'indexed')
%# indexed images colormap (the three channels should be all same)
isGrayscale(i) = all(all( diff(imgInfo.Colormap,[],2)==0 ,2),1);
end
end
The first part in your case can be:
dirName = 'C:\path\to\myimages';
files = dir( fullfile(dirName,'*.jpg') );
fNames = {files.name}';
EDIT:
#Adrian: With regards to the image you posed, as far as the image format saved, it IS a color image. Now the fact that all R/G/B channels are all the same is simply a special case...
Anyway if you want to be able to detect such cases, change the 'truecolor' part of the above code to:
#% ...
if strcmp(imgInfo.ColorType,'truecolor')
img = imread(fNames{i});
isGrayscale(i) = isequal(img(:,:,1),img(:,:,2),img(:,:,3));
elseif strcmp(imgInfo.ColorType,'grayscale')
#% ...

Answer 3

You can use IMFINFO for this task, so that you won't have to load the image into memory.
figdirectory = pwd;
fullpath = sprintf('%s/*.jpg', figdirectory);
d = dir(fullpath);
nImages = length(d);
%# imageType is a cell array with either 'grayscale', 'truecolor', or 'indexed',
%# depending on the kind of image you have.
imageType = cell(nImages,1);
for iImg = 1:nImages
info = imfinfo(d(iImg).name);
imageType{iImg} = info.ColorType;
end
%# isGrayscale is true for grayscale images, false otherwise
%# (though note that there might be mapped images that map to a grayscale colormap).
isGrayscale = cellfun(#(x)strcmp(x,'grayscale'),imageType);
EDIT
%# the indexed images have to be loaded in order for you to check
%# for grayscale maps
indexedIdx = find(cellfun(#(x)strcmp(x,'indexed'),imageType));
for iImg = indexedIdx(:)'
[~,map] = imread(fullfile(figDirectory,d(iImg).name));
%# It's a grayscale image if rgb map values are all equal
isGrayscale(iImg) = all(all(bsxfun(#eq,map,map(:,1)),2),1);
end
%# finally, it is possible that the images are *stored* as truecolor
%# but containing, in fact, a grayscale image
truecolorIdx = find(cellfun(#(x)strcmp(x,'truecolor'),imageType));
for iImg = truecolorIdx(:)'
img = imread(fullfile(figDirectory,d(iImg).name));
%# It's a grayscale image if rgb map values are all equal
isGrayscale(iImg) = all(all(all(bsxfun(#eq,img(:,:,1),img),1),2),3);
end

Answer 4

From the manual for imread http://www.mathworks.co.uk/help/techdoc/ref/imread.html
If the file contains a grayscale image, A is an M-by-N array. If the
file contains a truecolor image, A is an M-by-N-by-3 array.
So A will have a third dimension for the coloured images.
If you want to test for greyscale then you could convert the image to hsv.
B = rgb2hsv(A)
from the manual, http://www.mathworks.co.uk/help/techdoc/ref/hsv2rgb.html
When B(:,2) is 0, the colors are unsaturated (i.e., shades of gray).
In the case of a true colour image like the one posted it will be if unique(B(:,:,2)) is zero, hence
A = imread(fname_input);
B = rgb2hsv(A);
%# if saturation levels are zero then it's a greyscale image
if(unique(B(:,:,2)) == 0)
hascolor = 0;
else
hascolor = 1;
end