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I am using the following formula to add the additive Gaussian noise. noise= (x/255)*(randn(size(image))), where x is the variable and randn is random value generator. Intensity range is $0-255$. But, what is the value of the standard deviation $\sigma$ here? Is it x or (x/255)?

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closed as unclear what you're asking by Matt L., Laurent Duval, MBaz, jojek May 15 '16 at 9:13

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randn gives you Gaussian noise with std. dev. of 1.

(x/255)*(randn(size(image))) gives you a noisy image of std. dev. (x/255)

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    $\begingroup$ And intensity range of this noise realization is unlikely to be in $[0,255]$, because there will be negative vaules $\endgroup$ – Laurent Duval May 14 '16 at 11:44

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