### NDFCOMPRESS

Compresses an NDF so that it occupies less disk space

#### Description:

This application creates a copy of an NDF  that occupies less disk space. This compression does not affect the data values seen by subsequent application, since all applications will automatically uncompress the data.

Two compression methods are available: SCALE or DELTA (see Parameter METHOD).

#### Usage:

ndfcompress in out method

#### Parameters:

##### DSCALE = _DOUBLE (Read)
The scale factor to use for the Data component, when compressing with METHOD set to SCALE. If a null (!) value is supplied for DSCALE or DZERO, default values will be used for both that cause the scaled data values to occupy 96% of the available range of the data type selected using Parameter SCALEDTYPE. [!]
##### DZERO = _DOUBLE (Read)
The zero offset to use for the Data component, when compressing with METHOD set to SCALE. If a null (!) value is supplied for DSCALE or DZERO, default values will be used for both that cause the scaled data values to occupy 96% of the available range of the data type selected using Parameter SCALEDTYPE. [!]
The input NDF.
##### METHOD = LITERAL (Read)
The compression method to use. The options are as follows.
• "BOTH" –- A lossy compression scheme for all data types. It first creates an intermediate NDF from the supplied NDF using "SCALED" compression and then creates the final ouput NDF by applying "DELTA" compression to the intermediate NDF. The intermediate NDF is then deleted.

• "SCALED" –- A lossy compression scheme for all data types. See “Scaled Compression” below, and Parameters DSCALE, DZERO, VSCALE, VZERO, and SCALEDTYPE.

• "DELTA" –- A lossless compression scheme for integer data types. See “Delta Compression” below, and Parameters ZAXIS, ZMINRATIO, and ZTYPE.

The current value is the default, which is initially "DELTA". []
The output NDF.
##### SCALEDTYPE = LITERAL (Read)
The data type to use for the scaled data values. This is only used if METHOD is "SCALED". It can be one of the following options.
• "_INTEGER" –- four-byte signed integers

• "_WORD" –- two-byte signed integers

• "_UWORD" –- two-byte unsigned integers

• "_BYTE" –- one-byte signed integers

• "_UBYTE" –- one-byte unsigned integers

The same data type is used for both DATA and (if required) VARIANCE components of the output NDF. The initial default value is "_WORD". [current value]

##### VSCALE = _DOUBLE (Read)
The scale factor to use for the VARIANCE component, when compressing with METHOD set to SCALE. If a null (!) value is supplied for VSCALE or VZERO, default values will be used for both that cause the scaled variance values to occupy 96% of the available range of the data type selected using Parameter SCALEDTYPE. [!]
##### VZERO = _DOUBLE (Read)
The zero factor to use for the VARIANCE component, when compressing with METHOD set to SCALE. If a null (!) value is supplied for VSCALE or VZERO, default values will be used for both that cause the scaled variance values to occupy 96% of the available range of the data type selected using Parameter SCALEDTYPE. [!]
##### ZAXIS = _INTEGER (Read)
The index of the pixel axis along which differences are to be taken, when compressing with METHOD set to "DELTA". If this is zero, a default value will be selected that gives the greatest compression. [0]
##### ZMINRATIO = _REAL (Read)
The minimum allowed compression ratio for an array (the ratio of the supplied array size to the compressed array size), when compressing with METHOD set to "DELTA". If compressing an array results in a compression ratio smaller than or equal to ZMINRATIO, then the array is left uncompressed in the new NDF. If the supplied value is zero or negative, then each array will be compressed regardless of the compression ratio. [1.0]
##### ZTYPE = LITERAL (Read)
The data type to use for storing differences between adjacent uncompressed data values, when compressing with METHOD set to "DELTA". Must be one of _INTEGER, _WORD, _BYTE or blank. If a null (!) value or blank value is supplied, the data type that gives the best compression is determined and used. [!]

#### Examples:

ndfcompress infile outfile scale scaledtype=_uword
Copies the contents of the NDF structure infile to the new structure outfile, scaling the values so that they fit into unsigned two-byte integers. The scale and zero values used are chosen automatically.

#### Scaled Compression

The SCALE compression method scales the supplied data values using a linear transformation so that they fit into a smaller (integer) data type. A description of the scaling uses is stored with the output NDF so that later application can reconstruct the original unscaled values. This method is not lossless, due to the truncation involved in converting floating-point values to integers.

#### Delta Compression

DELTA compression is lossless, but can only be used on integer values. It assumes that adjacent integer values in the input tend to be close in value, and so differences between adjacent values can be represented in fewer bits than the absolute values themselves. The differences are taken along a nominated pixel axis within the supplied array (specified by Parameter ZAXIS). Any input value that differs from its earlier neighbour by more than the data range of the selected data type is stored explicitly using the data type of the input array.

Further compression is achieved by replacing runs of equal input values by a single occurrence of the value with a corresponding repetition count.

It should be noted that the degree of compression achieved is dependent on the nature of the data, and it is possible for a compressed array to occupy more space than the uncompressed array. The mean compression factor actually achieved is displayed (the ratio of the supplied NDF size to the compressed NDF size).

It is possible to delta compress an NDF that has already been scale compressed. This provides a means of further compressing floating-point arrays. However, note that the default values supplied for DSCALE, DZERO, VSCALE, and VZERO may not be appropriate as they are chosen to maximise the spread of the scaled integer values in order to minimise the integer truncation error, but delta compression works best on arrays of integers in which the spread of values is small.

If the input NDF is already DELTA compressed, it will be uncompressed and then recompressed using the supplied parameter values.

More details of delta compression can be found in SUN/11 (ARY - A Subroutine Library for Accessing ARRAY Data Structures), subsection Delta Compressed Array Form.

KAPPA: NDFCOPY.

#### Implementation Status:

The TITLE, LABEL, UNITS, DATA, VARIANCE, QUALITY, AXIS, WCS, and HISTORY components are copied by this routine, together with all extensions.