Sigma Noise Comparison: Not Fair part 2

Started Nov 27, 2002 | Discussions thread
Kok Chen Contributing Member • Posts: 740
Re: Sigma Noise Comparison: Not Fair part 2

By upsampling an image, the signal-to-noise ratio should not appreciably change.

It is true however, that when you do this, the noise turns from the pleasing high frequency noise you see in the example Phil showed, to a more low frequency noise that is more visually objectionable. Perhaps this is what you are alluding to.

The fact that the eye prefers higher frequency noise is pretty well known. The lower frequency noise is sometimes referred to as "pink noise" in the literature (pink has to do with spatial frequency, not the spectral color). And the higher frequency noise is known as "blue noise." That fact that "blue noise" is more pleasing to the human visual system has been used advantageously in halftoning techniques in printing (for example, the Blue Noise Mask). Most of the ink jet printers used in home photo-printing tends to have blue noise characteristics, whether they are directly using the Blue Noise Mask, or whether they are using error diffusion techniques.

Noise from Bayer cameras tend to be "pink" since the samples you see have been interpolated from a sparse array. The interpolation process by necessity is a low pass process, turning what was white noise into pink noise.

  • kc

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