MATLAB: ??? Index exceeds matrix dimensions

rgb values of image

why do I get the error ??? Index exceeds matrix dimensions.
Error in ==> ImagExperiment at 38 g=rgb{N}(:,:,2); when trying to run the code:
A=cell(1,Num);
rgb=cell(1,Num);
for N=1:Num
  rgb{N} = imread(sprintf('test_image_%d.png', N));
  r=rgb{N}(:,:,1);
  g=rgb{N}(:,:,2);
  b=rgb{N}(:,:,3);
end
How can I solve this problem?
Your support will be well-appreciated. Thanks.

Best Answer

The error appears, if the corresponding picture is a grayscale image. So test this at first:
aRGB = imread(sprintf('test_image_%d.png', N));
if ndims(aRGB) == 3
  r = aRGB(:, :, 1);
  g = aRGB(:, :, 2);
  b = aRGB(:, :, 3);
else
  ... any code to catch gray scale images, which are matrices
end
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