How does image noise affect which details the human eye can still recognize?
Asked 6/24/2022
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As noise increases in an image, fine details and low-contrast edges become harder to recognize, while only stronger edges remain visible at high noise levels. Is there established theory or data that relates noise level to what visual features humans can still identify, especially in the context of image or video denoising?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
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The relationship between signal and noise in the most general sense is full of math, and was fully developed by Claude Shannon, some 70 years ago.
In general, noise is characterized by a ratio between the signal, and the signal plus noise. As you surmise, as more noise is added, it becomes more difficult to detect the signal, but more importantly, the bandwidth that can be supported goes down. This means a noisy picture will look softer than one with much less noise.
It is not always a simple, linear equation. For example, adding small amounts of noise can actually increase detection of edges, through a process called stochastic resonance. This helps explain why photographs with a bit of noise added can look more "pleasing" than ones that are relatively noise-free.
So, there is no simple answer, but yes, there is a corpus of information knowledge theory that applies to photography. But it is complicated, and if you're looking for something simple, like "doubling the noise halves the image quality," you're going to be disappointed.
Originally by user8358. Source · Licensed CC BY-SA 4.0
user8358
4y ago
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Yes. In general this is described by signal-to-noise ratio (SNR) and broader signal-detection/information theory, including Shannon’s work. As noise rises relative to the signal, it becomes harder to distinguish real image structure from randomness. The practical result is that fine detail and low-contrast edges disappear first, while strong, high-contrast edges remain recognizable longer.
A useful way to think about it is that increasing noise reduces the effective bandwidth of the image: less subtle spatial information can be detected, so the image appears softer or less detailed. This is why denoising can make previously hidden detail recognizable again—if the method suppresses noise without destroying the underlying signal.
The relationship is not perfectly linear or simple. In some cases, a small amount of noise can actually help edge detection, a phenomenon called stochastic resonance. That helps explain why a little noise can sometimes make an image appear more natural or even slightly more perceptible.
So the short answer is: yes, there is theory, and the key concepts are SNR, detectability, contrast, and bandwidth. Exact visibility thresholds depend on the viewer, the image content, viewing conditions, and the type of noise.
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