How to show noise spectrum from a grayimage

Hello, please help me solve this problem. I have a grayimage with lots of noise (don't know the type of noise). I want to draw noise spectrum. So how can i do?

Answer 1

[Image Analyst]

First get the noise image alone. To get this you'll have to know/have the noise-free image. Then call fft2(). If you don't have a noise-free image, then the best you can do is to get the spectrum of the noise+(true image) together, rather than the noise alone.

[Helen Sky]

I draw 12 ROIs then calculate the mean value of these 12 ROIs then have the average figure. But now, i want to separate the noise to plot noise spectrum. I don't know what type of noise is it. My image is like this:

[Image Analyst]

How do you know that's not true signal? Of course we know that all images have noise. If you want, you can synthesize an image where you have uniform gray levels in the regions, then subtract that "perfect" image from your actual image. Then call fft2()

noiseImage = double(actualImage) - double(perfectImage);
fftImage = fft2(noiseImage);
magSpectrum = log(abs(fftImage));
imshow(magSpectrum, []);

[Helen Sky]

Thank you very much. Now i understand what to do.