BLOCK

Smooths an NDF using an n-dimensional rectangular box filter.

Description:

This application smooths an n-dimensional NDF using a rectangular box filter, whose dimensionality is the same as that of the NDF being smoothed. Each output pixel is either the mean or the median of the input pixels within the filter box. The mean estimator provides one of the fastest methods of smoothing an image and is often useful as a general-purpose smoothing algorithm when the exact form of the smoothing point-spread function is not important.

It is possible to smooth in selected dimensions by setting the boxsize to 1 for the dimensions not requiring smoothing. For example you can apply two-dimensional smoothing to the planes of a three-dimensional NDF (see Parameter BOX). If it has three dimensions, then the filter is applied in turn to each plane in the cube and the result written to the corresponding plane in the output cube.

Usage:

block in out box [estimator]

Parameters:

BOX() = _INTEGER (Read)
The sizes (in pixels) of the rectangular box to be applied to smooth the data. These should be given in axis order. A value set to 1 indicates no smoothing along that axis. Thus, for example, BOX=[3,3,1] for a three-dimensional NDF would apply a 3x3-pixel filter to all its planes independently.

If fewer values are supplied than the number of dimensions of the NDF, then the final value will be duplicated for the missing dimensions.

The values given will be rounded up to positive odd integers, if necessary, to retain symmetry.

ESTIMATOR = LITERAL (Read)
The method to use for estimating the output pixel values. It can be either "Mean" or "Median". ["Mean"]
IN = NDF (Read)
The input NDF to which box smoothing is to be applied.
OUT = NDF (Write)
The output NDF which is to contain the smoothed data.
TITLE = LITERAL (Read)
The title for the output NDF. A null value will cause the title of the input NDF to be used. [!]
WLIM = _REAL (Read)
If the input image contains bad pixels, then this parameter may be used to determine the number of good pixels which must be present within the smoothing box before a valid output pixel is generated. It can be used, for example, to prevent output pixels from being generated in regions where there are relatively few good pixels to contribute to the smoothed result.

By default, a null (!) value is used for WLIM, which causes the pattern of bad pixels to be propagated from the input image to the output image unchanged. In this case, smoothed output values are only calculated for those pixels which are not bad in the input image.

If a numerical value is given for WLIM, then it specifies the minimum fraction of good pixels which must be present in the smoothing box in order to generate a good output pixel. If this specified minimum fraction of good input pixels is not present, then a bad output pixel will result, otherwise a smoothed output value will be calculated. The value of this parameter should lie between 0.0 and 1.0 (the actual number used will be rounded up if necessary to correspond to at least one pixel). [!]

Examples:

block aa bb 9
Smooths the two-dimensional image held in the NDF structure aa, writing the result into the structure bb. The smoothing box is 9 pixels square. If any pixels in the input image are bad, then the corresponding pixels in the output image will also be bad. Each output pixel is the mean of the corresponding input pixels.
block spectrum spectrums [5,1] median title="Smoothed spectrum"
Smooths the one-dimensional data in the NDF called spectrum using a box size of 5 pixels, and stores the result in the NDF structure spectrums. Each output pixel is the median of the corresponding input pixels. If any pixels in the input image are bad, then the corresponding pixels in the output image will also be bad. The output NDF has the title "Smoothed spectrum".
block ccdin(123,) ccdcol [1,9]
Smooths the 123rd column in the two-dimensional NDF called ccdin using a box size of 9 pixels, and stores the result in the NDF structure ccdcol. The first value of the smoothing box is ignored as the first dimension has only one element. Each output pixel is the mean of the corresponding input pixels.
block in=image1 out=image2 box=[5,7] estimator=median
Smooths the two-dimensional image held in the NDF structure image1 using a rectangular box of size 5×7 pixels. The smoothed image is written to the structure image2. Each output pixel is the median of the corresponding input pixels.
block etacar etacars box=[7,1] wlim=0.6
Smooths the specified image data using a rectangular box 7×1 pixels in size. Smoothed output values are generated only if at least 60% of the pixels in the smoothing box are good, otherwise the affected output pixel is bad.
block in=cubein out=cubeout box=[3,3,7]
Smooths the three-dimensional NDF called cubein using a box that has three elements along the first two axes and seven along the third. The smoothed cube is written to NDF cubeout.
block in=cubein out=cubeout box=[3,1,7]
As the previous example, except that planes comprising the first and third axes are smoothed independently for all lines.

Timing

When using the mean estimator, the execution time is approximately proportional to the number of pixels in the image to be smoothed and is largely independent of the smoothing box size. This makes the routine particularly suitable for applying heavy smoothing to an image. Execution time will be approximately doubled if a variance array is present in the input NDF.

The median estimator is much slower than the mean estimator, and is heavily dependent on the smoothing box size.

Related Applications

KAPPA: CONVOLVE, FFCLEAN, GAUSMOOTH, MEDIAN; FIGARO: ICONV3, ISMOOTH, IXSMOOTH, MEDFILT.

Implementation Status: