An image affected by periodic noise will look like a repeating pattern has been added on top of the original image. Periodic noise Ī common source of periodic noise in an image is from electrical interference during the image capturing process. For example, image sensors are sometimes subject to row noise or column noise. Some noise sources show up with a significant orientation in images. Its effect is made worse by the distribution of silver halide grains in the film also being random. įilm grain is usually regarded as a nearly isotropic (non-oriented) noise source. A simple Gaussian distribution is often used as an adequately accurate model. In areas where the probability is low, this distribution will be close to the classic Poisson distribution of shot noise.
If film grains are uniformly distributed (equal number per area), and if each grain has an equal and independent probability of developing to a dark silver grain after absorbing photons, then the number of such dark grains in an area will be random with a binomial distribution.
The grain of photographic film is a signal-dependent noise, with similar statistical distribution to shot noise. Though it can be signal dependent, it will be signal independent if other noise sources are big enough to cause dithering, or if dithering is explicitly applied. It has an approximately uniform distribution. The noise caused by quantizing the pixels of a sensed image to a number of discrete levels is known as quantization noise. If dark-frame subtraction is not done, or if the exposure time is long enough that the hot pixel charge exceeds the linear charge capacity, the noise will be more than just shot noise, and hot pixels appear as salt-and-pepper noise. The variable dark charge of normal and hot pixels can be subtracted off (using "dark frame subtraction"), leaving only the shot noise, or random component, of the leakage. Dark current is greatest at "hot pixels" within the image sensor. In addition to photon shot noise, there can be additional shot noise from the dark leakage current in the image sensor this noise is sometimes known as "dark shot noise" or "dark-current shot noise". Shot noise has a standard deviation proportional to the square root of the image intensity, and the noise at different pixels are independent of one another. Shot noise follows a Poisson distribution, which can be approximated by a Gaussian distribution for large image intensity. This noise is known as photon shot noise. The dominant noise in the brighter parts of an image from an image sensor is typically that caused by statistical quantum fluctuations, that is, variation in the number of photons sensed at a given exposure level. Also, there are many Gaussian denoising algorithms. At higher exposures, however, image sensor noise is dominated by shot noise, which is not Gaussian and not independent of signal intensity. In color cameras where more amplification is used in the blue color channel than in the green or red channel, there can be more noise in the blue channel. Amplifier noise is a major part of the "read noise" of an image sensor, that is, of the constant noise level in dark areas of the image. Ī typical model of image noise is Gaussian, additive, independent at each pixel, and independent of the signal intensity, caused primarily by Johnson–Nyquist noise (thermal noise), including that which comes from the reset noise of capacitors ("kTC noise"). The sensor has inherent noise due to the level of illumination and its own temperature, and the electronic circuits connected to the sensor inject their own share of electronic circuit noise. Principal sources of Gaussian noise in digital images arise during acquisition. Such a noise level would be unacceptable in a photograph since it would be impossible even to determine the subject. Image noise can range from almost imperceptible specks on a digital photograph taken in good light, to optical and radioastronomical images that are almost entirely noise, from which a small amount of information can be derived by sophisticated processing. By analogy, unwanted electrical fluctuations are also called "noise". The original meaning of "noise" was "unwanted signal" unwanted electrical fluctuations in signals received by AM radios caused audible acoustic noise ("static"). Typically the term “image noise” is used to refer to noise in 2D images, not 3D images. Image noise is an undesirable by-product of image capture that obscures the desired information. Image noise can also originate in film grain and in the unavoidable shot noise of an ideal photon detector.
It can be produced by the image sensor and circuitry of a scanner or digital camera. Image noise is random variation of brightness or color information in images, and is usually an aspect of electronic noise. Noise clearly visible in an image from a digital camera
0 kommentar(er)
