Why can a small amount of grain or noise make an image look sharper?
Asked 9/2/2019
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I’ve seen examples showing that adding a slight amount of grain or luminance noise to a soft image can make it appear to have more acutance, even though no real detail has been restored. Why does this happen? Is it mainly an optical/psychological effect, or is there an image-processing explanation as well?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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Unsharpness is the result of convolution with a kernel acting as a lowpass filter. The resulting image is characterized by a very low amount of high-frequency content (pixel colors changing at narrow distances). This is obviously desired for areas with smooth changes, like sky or flat surfaces. However, for items like bricks, we know that they contain a lot of pores and unevenness, and consequently should exhibit a high frequency of changes.
Adding noise (presumably mostly luminosity noise) brings the wall back to expectations since the kind of noise in images tends to be uncorrelated noise with the noise on each pixel independent from the noise on the next pixel. That gives noise an impact at all image frequencies, including the high image frequencies otherwise lacking in presence. That the additional "simulated" pores are at random locations not coinciding with where the "natural" pores blurred out would be is not apparent.
Of course, this trick does not work equally well for every kind of area.
Originally by user86723. Source · Licensed CC BY-SA 4.0
user86723
7y ago
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A small amount of grain/noise can make an image look sharper because acutance is about the perceived strength of edges and fine detail, not actual resolution.
When an image is blurred, it has lost high-frequency content: pixel values change more gradually, so textures and edges look soft. Random grain/noise adds rapid pixel-to-pixel variation across many frequencies, including high frequencies. That added fine variation can mimic the kind of micro-contrast we expect to see in textured subjects, so the image appears crisper even though no true detail has been recovered.
There is also a perceptual component: the visual system tends to interpret fine local variation as texture/detail and can “fill in” structure where little exists. So the effect is partly image-frequency related and partly psychological.
In short: slight noise can increase perceived sharpness by reintroducing high-frequency variation and by triggering the brain’s tendency to read that variation as detail. It improves apparent acutance, not real resolving power.
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