# Can we use standard deviation of color image as a contrast value?

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dila suay on 18 Mar 2023
Commented: Walter Roberson on 21 Mar 2023
Hello,
Is it possible to compare the contrasts of 2 image by comparing std2(rgbimage) or is it necessary to convert to gray scale? Can you describe what is the difference between them?
Thank you

Walter Roberson on 19 Mar 2023
That particular kind of contrast is called RMS contrast; there are other contrast measures as well.
c3 = std2(im2double(YourImage), [], [1 2]);
would give you the contrast of each color pane independently, giving you a 1 x 1 x 3 result.
But then what do you with the vector of 3 results? You could interpret them as an RGB color
image(c3)
but does it make sense for "contrast" to be interpreted as a color??
If you examine the other kinds of contrast measures, they mostly work with luminance -- requiring that you convert to grayscale or Lab or similiar first.
Walter Roberson on 21 Mar 2023
Thought experiment:
Are the contrasts of all of these the same, or are some different?
[255, 0, 0, 0, 0]
[255, 0, 255, 0, 0]
[255, 255, 0, 0, 0]
[0, 255, 255, 0, 0]
[0, 0, 255, 0, 0]

Sulaymon Eshkabilov on 18 Mar 2023
Numerically to find the differences of the two imaes (of the same size) and then compute the std of the computed differences are feasible, e.g.:
A = imread('A_RGB.png'); % Size is 1000-by-1200 RGB image
B = imread('B_RGB.png'); % Size is 1000-by-1200 RGB image
diff_AB = A-B;
std_diff=std2(diff_AB); % STD of the difference
dila suay on 21 Mar 2023
The problem with IM_contrast = max(IM(:))-min(IM(:)) is that even I have 1 pixel of black and white, it will give me 'full contrast'. Do you know how can I set a threshold for pixel count?