Pixel density - can the playing field be leveled???

Started Jun 6, 2009 | Discussions thread
ejmartin Veteran Member • Posts: 6,274
Re: The answer of your question is right here

bushi wrote:

Graystar wrote:

What you’re not realizing is that you are averaging out signal as well as noise, and so the signal to noise ratio stays the same. That’s why noise isn’t reduced.

I think I realize this, but again, noise is random in nature, signal is not. Thus averaging image pixels (each being sum of signal + noise) yields averaged pixel with canceled noise (well, it is simplified statement to exaggerate my point)

ejmartin wrote:

There is signal (the brightness of the red patch), I think your point more precisely is that there is no high frequency signal. Noise has a spectrum that spans all spatial frequencies. When an image is downsampled, one removes high frequency signal (detail) as well as the high frequency noise. The remaining low frequency signal and noise are the very much the same (assuming the downsampling was done properly) as they were before downsampling,

yes and no, because in lower frequencies S/N ratio is higher, so yes, there still is noise on lower freq, but there is also easier to distinguish signal here.

What I am saying is that pixel std dev integrates (adds up) noise over all spatial frequencies. A good downsampling filter removes the portion of the frequency spectrum between the pixel scale of the downsampled image and the pixel scale of the original; it does very little to the spectrum of either noise or signal at spatial frequencies below the pixel scale of the downsampled image. So, the signal and noise of the downsampled image were present before downsampling, and any statement made about S/N at lower frequencies based on an examination of the downsampled image, is true of the image before downsampling as well as after. All that downsampling does is to get the high-frequency signal and noise out of the way so that all that is available for you to look at is lower-frequency signal and noise that was there all along.

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