In theory of noise,Typically, the standard deviation of noise has 1 and mean has 0. I think that the reason of noise mean has 0 that we can assume that all noise signal go to zero when we sum it all. But I can't understand standard deviation of noise has 1 in image noise. They are always 0~255. There is no negative value.
Especially, but I have got an idea as following. I can't catch quite a bit. So would you please let me know how to illustrate following answer?
"for an image intensity which is always non-negative, you can simply modify your added N(0,1) standard gaussian noise in 3 steps: 1-scale it so that your noise has whatever std you wish. 2-add whatever proper mean you need such as 128. 3-clip the noise for below zero and above 255 samples. This is simple bt It will help you get what you need at least initially."