mip1_05_image_enhancement_090519_1462589

5 eventsinterval 19052009 dr pierre elbischger

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Unformatted text preview: ld use as model a Poisson distribution with λ = 10/4 = 2.5 events/interval. 19.05.2009 Dr. Pierre Elbischger - MIP1/ISAP'SS09 k 36 Signal-to-noise ratio (SNR) The SNR is a ratio between a signal that contains meaningful information and noise that is also present in the signal and obscures the meaningful information. Because many signals have a very wide dynamic range, SNRs are usually expressed in terms of the logarithmic decibel scale. Ps... Pn... 19.05.2009 signal power noise power Dr. Pierre Elbischger - MIP1/ISAP'SS09 37 Signal-to-noise ratio (SNR) – Quantization Noise E11 sine wave sawtooth signal The error of the sine wave is approximately uniformly distributed. Signal Signal power of a sine wave with amplitude A Quantization error Assuming a uniform distribution of input signal values, the quantization noise is a uniformlydistributed random signal with a peak-to-peak amplitude of one quantization level, The equidistant quantization step for a n-bit resolution is given by The resulting quantization error is uniformly distributed with and its power corresponds to the variance of the distribution 19.05.2009 Dr. Pierre Elbischger - MIP1/ISAP'SS09 38 Contrast in images An object is visible in an image because it has a different brightness than its surroundings. The human eye can detect a minimum contrast c of about 0.5 to 5%, depending on the observation conditions. 100% contrast is the difference between pure black and pure white. In other words, humans can distinguish about 20 to 200 shades of gray between the blackest black and the whitest white c = 0 pure black c = 1 pure white Squares with diffeent contrast to the background (c is given in percent) 19.05.2009 Dr. Pierre Elbischger - MIP1/ISAP'SS09 39 SNR/CNR in images An object is visible in an image only if its contrast is large enough to overcome the random image noise. In general, trouble begins when the SNR falls below about 1.0. Sometimes the signal power in the SNR calculation is replaced by the contrast c of the image resulting in the socal...
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