According to the literature, zero mean noises are those whose expected value is zero. But what does this mean in the context of images. Let us consider grayscale for now. The intensity values ranging from 0 to 255. The noise will also have values between 0 and 255. How can then the mean be zero?
Answer
The image pixel values will be between 0 and 255 for 8 bit values.
There is nothing stopping the additive noise from increasing or decreasing the pixel values. Obviously you can't increase the pixel values above 255 or reduce them below 0, but it still allows the noise to be zero mean.
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