Dear Matlab community,
so after several trial and error using imnoise and rethinking my orginal question over again, i think the function imnoise is not suitable when your image includes negative values which you would like to "preserve" after noise addition.
For example if you would use a imnoise with a 'gaussian' with a mean value close to zero, say 0.01, and then use a sigma of 0.1 , pixel values that lie below zero will be truncated to zero. This means that after you scale back your image you would have a trend toward higher pixel values since the "negative truncated pixels are not there". That is why in this case the mean gray value of the noisy image will be larger than the one in the noiseless image.
Regards,
Jaime
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