uk.ac.starlink.fits
Class FitsConstants

java.lang.Object
  |
  +--uk.ac.starlink.fits.FitsConstants

public class FitsConstants
extends Object

Utility class providing some constants and static methods related to FITS file format and starlink classes.


Field Summary
static HeaderCard END_CARD
          Image of end-of-header card.
static int FITS_BLOCK
          Standard size of a FITS block in bytes.
static String NDARRAY_ORIGIN
          FITS header card for indicating NDArray origin values.
static String NDARRAY_PREFIX
          Prefix for NDArray-related FITS header cards.
static String NDX_PREFIX
          Prefix for NDX-related FITS header cards.
static String NDX_XML
          FITS header card for location (relative URL) of XML representation.
static String WCS_ENCODING
          Default encoding used for writing WCS into FITS headers.
 
Constructor Summary
FitsConstants()
           
 
Method Summary
static List defaultFitsExtensions()
          Gets the default permitted list of extensions which identify a FITS resource in the path part of a URL.
static long getDataSize(Header hdr)
          Utility function to find the number of bytes in the data segment of an HDU.
static ArrayDataInput getInputStream(DataSource datsrc)
          Returns an input stream which can be used with the various FITS classes based on a given DataSource object.
static ArrayDataInput getInputStreamStart(DataSource datsrc)
          Returns an input stream which can be used with the various FITS classes based on a given DataSource object, positioned at the start of the stream.
static boolean isMagic(byte[] buffer)
          Indicates whether the supplied buffer is the start of a FITS file.
static long positionStream(ArrayDataInput strm, String pos)
          Advances the position of an input stream according to a position string.
static int readHeader(Header hdr, ArrayDataInput strm)
          Populates a header from an input stream, reporting its length in bytes.
static long skipHDUs(ArrayDataInput stream, int nskip)
          Skips forward over a given number of HDUs in the supplied stream.
static void writeEmptyPrimary(DataOutput strm)
          Writes a null header representing an empty primary HDU to a stream.
static void writeHeader(DataOutput strm, Header hdr)
          Writes a header object to a DataOutput.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

NDARRAY_PREFIX

public static final String NDARRAY_PREFIX
Prefix for NDArray-related FITS header cards.

See Also:
Constant Field Values

END_CARD

public static final HeaderCard END_CARD
Image of end-of-header card.


NDARRAY_ORIGIN

public static final String NDARRAY_ORIGIN
FITS header card for indicating NDArray origin values. An integer (1, 2, ... ) is appended to this value for the different axes like for NAXIS.

See Also:
Constant Field Values

NDX_PREFIX

public static final String NDX_PREFIX
Prefix for NDX-related FITS header cards.

See Also:
Constant Field Values

NDX_XML

public static final String NDX_XML
FITS header card for location (relative URL) of XML representation.

See Also:
Constant Field Values

WCS_ENCODING

public static final String WCS_ENCODING
Default encoding used for writing WCS into FITS headers.

See Also:
Constant Field Values

FITS_BLOCK

public static final int FITS_BLOCK
Standard size of a FITS block in bytes.

See Also:
Constant Field Values
Constructor Detail

FitsConstants

public FitsConstants()
Method Detail

defaultFitsExtensions

public static List defaultFitsExtensions()
Gets the default permitted list of extensions which identify a FITS resource in the path part of a URL.

Returns:
unmodifiable list of default FITS extensions - ".fits", ".fit" etc

skipHDUs

public static long skipHDUs(ArrayDataInput stream,
                            int nskip)
                     throws IOException
Skips forward over a given number of HDUs in the supplied stream. If it reaches the end of the stream, it throws an IOException with a Cause of a TruncatedFileException.

Parameters:
stream - the stream to skip through
nskip - the number of HDUs to skip
Returns:
the number of bytes the stream was advanced
IOException

isMagic

public static boolean isMagic(byte[] buffer)
Indicates whether the supplied buffer is the start of a FITS file. Its contents is checked against the FITS 'magic number', which is the ASCII string "SIMPLE  =".

Parameters:
buffer - a byte buffer containing the start of a file to test
Returns:
true iff the bytes in buffer look like the start of a FITS file

getInputStreamStart

public static ArrayDataInput getInputStreamStart(DataSource datsrc)
                                          throws IOException
Returns an input stream which can be used with the various FITS classes based on a given DataSource object, positioned at the start of the stream.

Parameters:
datsrc - the DataSource pointing to the file/HDU required
Returns:
an ArrayDataInput acquired from datsrc,
IOException

positionStream

public static long positionStream(ArrayDataInput strm,
                                  String pos)
                           throws IOException
Advances the position of an input stream according to a position string. The string may contain a non-negative integer giving the number of HDUs to advance by. If it's "0" or null no advance will be made. The pos string will typically be the position attribute of a DataSource.

Parameters:
strm - the stream to advance
pos - number of HDUs to advance by (as a string)
Returns:
number of bytes the stream has been advanced
IOException

getInputStream

public static ArrayDataInput getInputStream(DataSource datsrc)
                                     throws IOException
Returns an input stream which can be used with the various FITS classes based on a given DataSource object. If the DataSource has a position attribute, it will be interpreted as the zero-based index of the HDU to start the stream at. Otherwise, the stream will start at the primary HDU (as if position="0").

Parameters:
datsrc - the DataSource pointing to the file/HDU required
Returns:
an ArrayDataInput acquired from datsrc, and positioned according to its position
IOException

readHeader

public static int readHeader(Header hdr,
                             ArrayDataInput strm)
                      throws TruncatedFileException,
                             IOException
Populates a header from an input stream, reporting its length in bytes. This does the same as Header.read(nom.tam.util.ArrayDataInput), but it returns the number of bytes read from the input stream in order to populate the header (including any padding bytes). There is no way to retrieve this information from the Header class in general; though Header.getSize() will sometimes give you the right answer, in the case of duplicated header keywords it can give an underestimate. This could be seen as a bug in nom.tam.fits classes, but there may be code somewhere which relies on that behaviour.

You can make a Header from scratch by doing

     Header hdr = new Header();
     int headsize = read( hdr, strm );
 
This method also differs from the Header implementation in that it does not print warnings to standard output about duplicate keywords.

Parameters:
hdr - the header to populate
strm - the input stream supplying the data
Returns:
the number of bytes in the FITS blocks which comprise the header content
TruncatedFileException
IOException
See Also:
Header.read(nom.tam.util.ArrayDataInput)

writeHeader

public static void writeHeader(DataOutput strm,
                               Header hdr)
                        throws IOException
Writes a header object to a DataOutput.

Parameters:
strm - destination stream
hdr - the header to write
IOException

writeEmptyPrimary

public static void writeEmptyPrimary(DataOutput strm)
                              throws IOException
Writes a null header representing an empty primary HDU to a stream.

Parameters:
strm - stream to write to
IOException

getDataSize

public static long getDataSize(Header hdr)
Utility function to find the number of bytes in the data segment of an HDU. As far as I can see, Header.getDataSize() ought to do this, but it doesn't seem to.

Parameters:
hdr - the Header
Returns:
the number of bytes in the data segment associated with hdr

Copyright © 2004 CLRC: Central Laboratory of the Research Councils. All rights reserved.