added todos to all chapters
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@@ -5,6 +5,7 @@
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% Repetitive Box filter to approx Gauss
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% Simple multipass, n/m approach, extended box filter
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\todo{normalisierungsfaktor, sigma vs. h beschreiben}
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The Gaussian filter is a widely used smoothing filter.
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It is defined as the convolution of an input signal and the Gaussian function
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\begin{equation}
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@@ -13,26 +14,27 @@ g(x) = \frac{1}{\sigma \sqrt{2\pi}} \expp{-\frac{x^2}{2\sigma^2}} \text{,}
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\end{equation}
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where $\sigma$ is a smoothing parameter called standard deviation.
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In the discrete case the Gaussian filter is easily computed with the sliding window algorithm in time domain.
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It is easily extended to multi-dimensional signals, as convolution is separable if the filter kernel is separable, i.e. multidimensional convolution can be computed as individual one-dimensional convolutions with a one-dimensional kernel.
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Because of $\operatorname{e}^{x^2+y^2} = \operatorname{e}^{x^2}\cdot\operatorname{e}^{y^2}$ the Gaussian filter is separable and can be easily applied to multi-dimensional signals.
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%In the discrete case the Gaussian filter is easily computed with the sliding window algorithm in time domain.
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If the filter kernel is separable, the convolution is also separable i.e. multi-dimensional convolution can be computed as individual one-dimensional convolutions with a one-dimensional kernel.
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Because of $\operatorname{e}^{x^2+y^2} = \operatorname{e}^{x^2}\cdot\operatorname{e}^{y^2}$ the Gaussian filter is separable and can be easily applied to multi-dimensional signals. \todo{quelle}
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% TODO ähnlichkeit Gauss und KDE -> schneller Gaus = schnelle KDE
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Computation of a filter using the a naive implementation of the sliding window algorithm yields $\landau{NK}$, where $N$ is the length of the input signal and $K$ is the size of the filter kernel.
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Computation of a filter using the a naive implementation of the discrete convolution algorithm yields $\landau{NK}$, where $N$ is the length of the input signal and $K$ is the size of the filter kernel.
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Note that in the case of the Gaussian filter $K$ depends on $\sigma$.
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In order to capture all significant values of the Gaussian function the kernel size $K$ must be adopted to the standard deviation of the Gaussian.
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A popular approach to efficiently compute a filter result is the FFT-convoultion algorithm which is $\landau{N\log(N)}$.
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For large values of $\sigma$ the computation time of the Gaussian filter might be reduced by applying the filter in frequency domain.
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In order to do so, both signals are transformed into frequency domain using the FFT.
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The convoluted time signal is equal to the point-wise multiplication of the signals in frequency domain.
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In case of the Gaussian filter the computation of the Fourier transform of the kernel can be saved, as the Gaussian is a eigenfunction for the Fourier transform \cite{?}.
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%A popular approach to efficiently compute a filter result is the FFT-convoultion algorithm which is $\landau{N\log(N)}$.
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%For large values of $\sigma$ the computation time of the Gaussian filter might be reduced by applying the filter in frequency domain.
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%In order to do so, both signals are transformed into frequency domain using the FFT.
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%The convoluted time signal is equal to the point-wise multiplication of the signals in frequency domain.
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%In case of the Gaussian filter the computation of the Fourier transform of the kernel can be saved, as the Gaussian is a eigenfunction for the Fourier transform \cite{?}.
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While the FFT-convolution algorithm poses an efficient algorithm for large signals, it adds an noticeable overhead for small signals.
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%While the FFT-convolution algorithm poses an efficient algorithm for large signals, it adds an noticeable overhead for small signals.
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While the above mentions algorithms poses efficient computations schemes to compute an exact filter result, approximative algorithms can further speed up the computation.
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%While the above mentions algorithms poses efficient computations schemes to compute an exact filter result, approximative algorithms can further speed up the computation.
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\todo{o(nk) ist scheiße und wir wollen o(n) haben, deshalb box filter boy}
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A well-known rapid approximation of the Guassian filter is given by the moving average filter.
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\subsection{Moving Average Filter}
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