Not getting channel 1 in an Face images even though in Grey-colour

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Kartikeya
Kartikeya 2023-10-18
编辑： Harsha Vardhan 2023-10-18
Ran in:
originalImage = imread('greyface2.jpg');
% To get a 2-D grayscale image from the file, converting from RGB
% to gray scale if it's not already grayscale.
% Get the dimensions of the image.
% numberOfColorChannels should be = 1 for a gray scale image, and 3 for an RGB color image.
[rows, columns, numberOfColorChannels] = size(originalImage)
rows = 264
columns = 191
numberOfColorChannels = 3
if numberOfColorChannels > 1
% It's already RGB. Just put it into our rgbImage variable.
rgbImage = originalImage;
else
% It's not really RGB like we expected - it's grayscale (or indexed). We'll assume it's grayscale here.
fprintf('It is not really RGB like we expected - it is grayscale.\n');
% Concatenate the gray scale image to be all 3 color channels.
rgbImage = cat(3, originalImage, originalImage, originalImage);
end

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Harsha Vardhan
Harsha Vardhan 2023-10-18
编辑：Harsha Vardhan 2023-10-18
Hi,
I understand that you want to check if an image is a gray scale image.
In many cases, gray colored images are stored as RGB images. When the R, G and B values of a pixel are either equal or very close to each other, that pixel gets the appearance of grayscale. In such cases, it must not inferred that the image is a gray scale image. Such an image can still contain 3 channels of R, G and B.
When you load such an image in MATLAB using imread, it will still read it as a 3-channel RGB image, because that's how the image is stored. That's why you're seeing numberOfColorChannels as 3.
To understand further, please execute the below code and observe the output.
originalImage(1,1,:)
1×1×3 uint8 array
ans(:,:,1) =
197
ans(:,:,2) =
198
ans(:,:,3) =
200
It can be seen from the above output that the first pixel - originalImage(1,1,:) has 3 channels associated with it. And the values of each channel are very close to each other. This gives an impression of greyscale inspite of having 3 channels.
If you want to ensure you're working with a true single-channel grayscale image in MATLAB, you can convert it using:
grayImage = im2gray(originalImage);
This answer has more information about the values returned by the 'size' function in the context of images - https://www.mathworks.com/matlabcentral/answers/81089-finding-the-dimensions-of-an-image#answer_939490
Hope this helps in resolving your query!
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Harsha Vardhan
Harsha Vardhan 2023-10-18
编辑：Harsha Vardhan 2023-10-18
@Kartikeya
In the screenshot you attached, the colunm 'size' displays the size of the matrix. The 'grayImage'has 365x265 in the 'size' column. It means it can be identified as a 2-dimensional matrix since it has only rows X columns.

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