# public documents.sextractor_doc

## [/] [detect_filter.tex] - Diff between revs 25 and 29

Rev 25 Rev 29
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since filtering occurs on a background-subtracted \index{image} image. When weighting is applied
since filtering occurs on a background-subtracted \index{image} image. When weighting is applied
(\S\ref{chap:weight}), \index{bad pixel} \index{bad pixels} bad pixels (pixels with weight $<$ {\tt WEIGHT\_THRESH}) are interpolated
(\S\ref{chap:weight}), \index{bad pixel} \index{bad pixels} bad pixels (pixels with weight $<$ {\tt WEIGHT\_THRESH}) are interpolated
by default (\S\ref{chap:interp}) and should therefore not cause much trouble. It is recommended
by default (\S\ref{chap:interp}) and should therefore not cause much trouble. It is recommended
not to turn-off \index{interpolation} interpolation of \index{bad pixel} \index{bad pixels} bad pixels when filtering is on.
not to turn-off \index{interpolation} interpolation of \index{bad pixel} \index{bad pixels} bad pixels when filtering is on.
 
 
\subsection{Configuration parameters.}
\subsection{Configuration parameters}
Filtering is triggered when the {\tt FILTER} keyword is set to {\tt Y}. If active, a file with name
Filtering is triggered when the {\tt FILTER} keyword is set to {\tt Y}. If active, a file with name
specified by {\tt FILTER\_NAME} is searched for and loaded. Filtering with large retinas can be
specified by {\tt FILTER\_NAME} is searched for and loaded. Filtering with large retinas can be
extremely time consuming. In many cases, one is only interested in filtering pixels whose values
extremely time consuming. In many cases, one is only interested in filtering pixels whose values
stand out from the background noise. The {\tt FILTER\_THRESH keyword} can be given to specify the
stand out from the background noise. The {\tt FILTER\_THRESH keyword} can be given to specify the
range of pixel values within which retina-filtering will be applied, in units of background noise
range of pixel values within which retina-filtering will be applied, in units of background noise
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{\tt FILTER\_THRESH} has no effect on \index{convolution} convolution.
{\tt FILTER\_THRESH} has no effect on \index{convolution} convolution.
 
 
The result of the filtering process can be verified through a {\tt FILTERED} \index{check-image} check-image: see
The result of the filtering process can be verified through a {\tt FILTERED} \index{check-image} check-image: see
\S\ref{chap:check}.
\S\ref{chap:check}.
 
 
\subsection{CPU cost.}
\subsection{CPU cost}
The {\sc SExtractor} filtering routine is particularly optimized for small kernels. It thus
The {\sc SExtractor} filtering routine is particularly optimized for small kernels. It thus
provides a convenient way of filtering large \index{image} image data. On a 2GHz machine, a \index{convolution} convolution by a
provides a convenient way of filtering large \index{image} image data. On a 2GHz machine, a \index{convolution} convolution by a
$5\times5$ kernel will contribute less than 1 second to the processing time of a $2048\times4096$
$5\times5$ kernel will contribute less than 1 second to the processing time of a $2048\times4096$
\index{image} image. The numbers for non-linear (retina) filtering depend on the complexity of the neural
\index{image} image. The numbers for non-linear (retina) filtering depend on the complexity of the neural
network, but can be a hundred times larger.
network, but can be a hundred times larger.
\gam{Update time?}
\gam{Update time?}
 
 
\subsection{Filter file formats.}
\subsection{Filter file formats}
As described above, two kinds of filter
As described above, two kinds of filter
files are recognized by {\sc SExtractor}: \index{convolution} convolution files (traditionaly suffixed with
files are recognized by {\sc SExtractor}: \index{convolution} convolution files (traditionaly suffixed with
{\tt .conv}''), and retina'' files ({\tt .ret}'' extensions\footnote{In {\sc SExtractor},
{\tt .conv}''), and retina'' files ({\tt .ret}'' extensions\footnote{In {\sc SExtractor},
file name extensions are just conventions; they are not used by the software to distinguish
file name extensions are just conventions; they are not used by the software to distinguish
between different file formats.}).
between different file formats.}).