2002.09.11 14:21 "Re: Tags modifcation", by Frank Warmerdam

In trying to use the tag extension schemes in "./contrib/tags" I came across a not so obvious limitation in the lines

#define FIELD_SETLONGS  3
#define        FIELD_LAST                      (32*FIELD_SETLONGS-1)
u_long     td_fieldsset[FIELD_SETLONGS];

in tif_dir.h.

Here FIELD_LAST indicates the maximum number of tags you may define in libtiff. If, for instance, you want to read Exif files it seems you need to increase FIELD_SETLONGS to accomodate the large number of tags.

Gerben,

I would like to take this opportunity to suggest that the libtiff 3.5.x mechanism for extending the set of tags (as in contrib/tags) is flawed in a number of ways, including the one you mention, and I would encourage you to use the new (CVS only) mechanism for extending tags.

The key item is that there is now a TIFFMergeFieldInfo() function which you can call to register new tags, and there is a field value "FIELD_CUSTOM" which avoids the need to assign specific field values for new tags.

I have attached the xtiff.c for libgeotiff using the new mechanism as an example.

Best regards,

---------------------------------------+--------------------------------------
I set the clouds in motion - turn up | Frank Warmerdam, warmerdam@pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush | Geospatial Programmer for Rent

/*
* xtiff.c
*
* Extended TIFF Directory GEO Tag Support.
*
* You may use this file as a template to add your own
* extended tags to the library. Only the parts of the code
* marked with "XXX" require modification.
*
* Author: Niles D. Ritter
*
* Revisions:
* 18 Sep 1995 -- Deprecated Integraph Matrix tag with new one.
* Backward compatible support provided. --NDR.
*/

#include "xtiffio.h"
#include <stdio.h>

/* Tiff info structure.
*
* Entry format:
* { TAGNUMBER, ReadCount, WriteCount, DataType, FIELDNUM,
* OkToChange, PassDirCountOnSet, AsciiName }
*
* For ReadCount, WriteCount, -1 = unknown.
*/

static const TIFFFieldInfo xtiffFieldInfo[] = {

 /* XXX Insert Your tags here */
 { TIFFTAG_GEOPIXELSCALE, -1,-1, TIFF_DOUBLE, FIELD_CUSTOM,
 TRUE, TRUE, "GeoPixelScale" },
 { TIFFTAG_INTERGRAPH_MATRIX,-1,-1, TIFF_DOUBLE, FIELD_CUSTOM,
 TRUE, TRUE, "Intergraph TransformationMatrix" },
 { TIFFTAG_GEOTRANSMATRIX, -1,-1, TIFF_DOUBLE, FIELD_CUSTOM,
 TRUE, TRUE, "GeoTransformationMatrix" },
 { TIFFTAG_GEOTIEPOINTS, -1,-1, TIFF_DOUBLE, FIELD_CUSTOM,
 TRUE, TRUE, "GeoTiePoints" },
 { TIFFTAG_GEOKEYDIRECTORY,-1,-1, TIFF_SHORT, FIELD_CUSTOM,
 TRUE, TRUE, "GeoKeyDirectory" },
 { TIFFTAG_GEODOUBLEPARAMS, -1,-1, TIFF_DOUBLE, FIELD_CUSTOM,
 TRUE, TRUE, "GeoDoubleParams" },
 { TIFFTAG_GEOASCIIPARAMS, -1,-1, TIFF_ASCII, FIELD_CUSTOM,
 TRUE, FALSE, "GeoASCIIParams" },
#ifdef JPL_TAG_SUPPORT
 { TIFFTAG_JPL_CARTO_IFD, 1, 1, TIFF_LONG, FIELD_CUSTOM,
 TRUE, TRUE, "JPL Carto IFD offset" }, /** Don't use this! **/
#endif
};

#define N(a) (sizeof (a) / sizeof (a[0]))
static void _XTIFFLocalDefaultDirectory(TIFF *tif)
{
 /* Install the extended Tag field info */
 TIFFMergeFieldInfo(tif, xtiffFieldInfo, N(xtiffFieldInfo));
}

/**********************************************************************
* Nothing below this line should need to be changed.
**********************************************************************/

static TIFFExtendProc _ParentExtender;

/*
* This is the callback procedure, and is
* called by the DefaultDirectory method
* every time a new TIFF directory is opened.
*/

static void
_XTIFFDefaultDirectory(TIFF *tif)
{
 /* set up our own defaults */
 _XTIFFLocalDefaultDirectory(tif);

 /* Since an XTIFF client module may have overridden
 * the default directory method, we call it now to
 * allow it to set up the rest of its own methods.
 */

 if (_ParentExtender)
 (*_ParentExtender)(tif);
}

/*
* XTIFF Initializer -- sets up the callback
* procedure for the TIFF module.
*/

static
void _XTIFFInitialize(void)
{
 static int first_time=1;

 if (! first_time) return; /* Been there. Done that. */
 first_time = 0;

 /* Grab the inherited method and install */
 _ParentExtender = TIFFSetTagExtender(_XTIFFDefaultDirectory);
}

/**
* GeoTIFF compatible TIFF file open function.
*
* @param name The filename of a TIFF file to open.
* @param mode The open mode ("r", "w" or "a").
*
* @return a TIFF * for the file, or NULL if the open failed.
*
This function is used to open GeoTIFF files instead of TIFFOpen() from
libtiff. Internally it calls TIFFOpen(), but sets up some extra hooks
so that GeoTIFF tags can be extracted from the file. If XTIFFOpen() isn't
used, GTIFNew() won't work properly. Files opened
with XTIFFOpen() should be closed with XTIFFClose().

The name of the file to be opened should be passed as name, and an
opening mode ("r", "w" or "a") acceptable to TIFFOpen() should be passed as the
mode.

If XTIFFOpen() fails it will return NULL. Otherwise, normal TIFFOpen()
error reporting steps will have already taken place.
*/

TIFF*
XTIFFOpen(const char* name, const char* mode)
{
 TIFF *tif;

 /* Set up the callback */
 _XTIFFInitialize();

 /* Open the file; the callback will set everything up
 */
 tif = TIFFOpen(name, mode);
 if (!tif) return tif;

 return tif;
}

TIFF*
XTIFFFdOpen(int fd, const char* name, const char* mode)
{
 TIFF *tif;

 /* Set up the callback */
 _XTIFFInitialize();

 /* Open the file; the callback will set everything up
 */
 tif = TIFFFdOpen(fd, name, mode);
 if (!tif) return tif;

 return tif;
}

/**
* Close a file opened with XTIFFOpen().
*
* @param tif The file handle returned by XTIFFOpen().
*
* If a GTIF structure was created with GTIFNew()
* for this file, it should be freed with GTIFFree()
* before calling XTIFFClose().
*/

void
XTIFFClose(TIFF *tif)
{
 TIFFClose(tif);
}