### COMPAVE

Reduces the size of an NDF by averaging values in rectangular boxes

#### Description:

This application takes an NDF  data structure and reduces it in size by integer factors along each dimension. The compression is achieved by averaging the input NDF within non-overlapping ‘rectangular’ boxes whose dimensions are the compression factors. The averages may be weighted when there is a variance array present. The exact placement of the boxes can be controlled using Parameter ALIGN.

#### Usage:

compave in out compress [wlim]

#### Parameters:

This parameter controls the placement of the compression boxes within the input NDF (also see Parameter TRIM). It can take any of the following values:
• "ORIGIN" — The compression boxes are placed so that the origin of the pixel co-ordinate Frame  (i.e. pixel co-ordinates (0,0)) in the input NDF corresponds to a corner of a compression box. This results in the pixel origin being retain in the output NDF. For instance, if a pair of two-dimensional images which have previously been aligned in pixel co-ordinates are compressed, then using this option ensures that the compressed images will also be aligned in pixel co-ordinates.

• "FIRST" — The compression boxes are placed so that the first pixel in the input NDF (for instance, the bottom-left pixel in a two-dimensional image) corresponds to the first pixel in a compression box. This can result in the pixel origin being shifted by up to one compression box in the output image. Thus, images which were previously aligned in pixel co-ordinates may not be aligned after compression. You may want to use this option if you are using a very large box to reduce the number of dimensions in the data (for instance averaging across the entire width of an image to produce a one-dimensional array).

• "LAST" — The compression boxes are placed so that the last pixel in the input NDF (for instance, the top-right pixel in a two-dimensional image) corresponds to the last pixel in a compression box. See the "FIRST" option above for further comments. ["ORIGIN"]

When there is an AXIS variance array present in the NDF and AXWEIGHT=TRUE the application forms weighted averages of the axis centres using the variance. For all other conditions the non-bad axis centres are given equal weight during the averaging to form the output axis centres. [FALSE]
##### COMPRESS( ) = _INTEGER (Read)
Linear compression factors to be used to create the output NDF. There should be one for each dimension of the NDF. If fewer are supplied the last value in the list of compression factors is given to the remaining dimensions. Thus if a uniform compression is required in all dimensions, just one value need be entered. The suggested default is the current value.
The NDF structure to be reduced in size.
##### OUT = NDF (Write)
NDF structure to contain compressed version of the input NDF.
If the input data type is to be preserved on output then this parameter should be set TRUE. However, this will probably result in a loss of precision. If this parameter is set FALSE then the output data type will be one of _REAL or _DOUBLE, depending on the input type. [FALSE]
Title for the output NDF structure. A null value (!) propagates the title from the input NDF to the output NDF. [!]
If Parameter TRIM is set TRUE, the output NDF only contains data for compression boxes which are entirely contained within the input NDF. Any pixels around the edge of the input NDF which are not contained within a compression box are ignored. If TRIM is set FALSE, the output NDF contains data for all compression boxes which have any overlap with the input NDF. All pixels outside the bounds of the NDF are assumed to be bad. That is, any boxes which extend beyond the bounds of the input NDF are padded with bad pixels. See also Parameter ALIGN. [current value]
When there is a variance array present in the NDF and WEIGHT=TRUE the application forms weighted averages of the data array using the variance. For all other conditions the non-bad pixels are given equal weight during averaging. [FALSE]
If the input NDF contains bad pixels, then this parameter may be used to determine the number of good pixels which must be present within the averaging 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.

WLIM specifies the minimum fraction of good pixels which must be present in the averaging 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 an averaged 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). [0.3]

#### Examples:

compave cosmos galaxy 4
This compresses the NDF called cosmos averaging four times in each dimension, and stores the reduced data in the NDF called galaxy. Thus if cosmos is two-dimensional, this command would result in a sixteen-fold reduction in the array components.
compave cosmos profile [10000,1] wlim=0 align=first trim=no
This compresses the two-dimensional NDF called cosmos to produce a one-dimensional NDF called profile. This is done using a compression box which is 1 pixel high, but which is wider than the whole input image. Each pixel in the output NDF thus corresponds to the average of the corresponding row in the input image. WLIM is set to zero to ensure that bad pixels are ignored. ALIGN is set to "FIRST" so that each compression box is flush with the left edge of the input image. TRIM is set to NO so that compression boxes which extend outside the bounds of the input image (which will be all of them if the input image is narrower than 10000 pixels) are retained in the output NDF.
compave cosmos galaxy 4 wlim=1.0
This compresses the NDF called cosmos averaging four times in each dimension, and stores the reduced data in the NDF called galaxy. Thus if cosmos is two-dimensional, this command would result in a sixteen-fold reduction in the array components. If an averaging box contains any bad pixels, the output pixel is set to bad.
compave cosmos galaxy 4 0.0 preserve
As above except that an averaging box need only contains a single non-bad pixels for the output pixel to be good, and galaxy’s array components will have the same as those in cosmos.
compave cosmos galaxy [4,3] weight title="COSMOS compressed"
This compresses the NDF called cosmos averaging four times in the first dimension and three times in higher dimensions, and stores the reduced data in the NDF called galaxy. Thus if cosmos is two-dimensional, this command would result in a twelve-fold reduction in the array components. Also, if there is a variance array present it is used to form weighted means of the data array. The title of the output NDF is "COSMOS compressed".
compave in=arp244 compress=[1,1,3] out=arp244cs
Suppose arp244 is a huge NDF storing a spectral-line data cube, with the third dimension being the spectral axis. This command compresses arp244 in the spectral dimension, averaging every three pixels to form the NDF called arp244cs.

#### Notes:

• The axis centres and variances are averaged, whilst the widths are summed and always normalised for bad values.

#### Related Applications

KAPPA: BLOCK, COMPADD, COMPICK, PIXDUPE, SQORST, REGRID; FIGARO: ISTRETCH.