Defining New TIFF Tags¶

Libtiff has built-in knowledge of all the standard TIFF tags, as well as extensions. The following describes how to add knowledge of new tags as builtins to libtiff, or how to application specific tags can be used by applications without modifying libtiff.

TIFFFieldInfo¶

How libtiff manages specific tags is primarily controlled by the definition for that tag value stored internally as a TIFFFieldInfo structure. This structure looks like this:

typedef struct {
  uint32_t       field_tag;        /* field's tag */
  short          field_readcount;  /* read count / TIFF_VARIABLE / TIFF_VARIABLE2 / TIFF_SPP */
  short          field_writecount; /* write count / TIFF_VARIABLE / TIFF_VARIABLE2 */
  TIFFDataType   field_type;       /* type of associated data */
  unsigned short field_bit;        /* bit in fieldsset bit vector */
  unsigned char  field_oktochange; /* if true, can change while writing */
  unsigned char  field_passcount;  /* if true, pass dir count on set */
  char          *field_name;       /* ASCII name */
} TIFFFieldInfo;

uint32_t TIFFFieldInfo.field_tag¶: The tag number. For instance 277 for the SamplesPerPixel tag. Builtin tags will generally have a #define in tiff.h for each known tag.

short TIFFFieldInfo.field_readcount¶: The number of values which should be read. The special value TIFF_VARIABLE (-1) indicates that a variable number of values may be read. The special value TIFFTAG_SPP (-2) indicates that there should be one value for each sample as defined by TIFFTAG_SAMPLESPERPIXEL. The special value TIFF_VARIABLE2 (-3) is similar to TIFF_VARIABLE but the required TIFFGetField() count value must be uint32_t* instead of uint16_t* as for TIFF_VARIABLE (-1). For ASCII fields with variable length, this field is TIFF_VARIABLE.

short TIFFFieldInfo.field_writecount¶: The number of values which should be written. Generally the same as field_readcount. A few built-in exceptions exist, but I haven't analysed why they differ.

TIFFDataType TIFFFieldInfo.field_type¶: Type of the field. One of TIFF_BYTE, TIFF_ASCII, TIFF_SHORT, TIFF_LONG, TIFF_RATIONAL, TIFF_SBYTE, TIFF_UNDEFINED, TIFF_SSHORT, TIFF_SLONG, TIFF_SRATIONAL, TIFF_FLOAT, TIFF_DOUBLE or TIFF_IFD. And for BigTIFF TIFF_LONG8, TIFF_SLONG8 and TIFF_IFD8, which are automatically reverted to 4 byte fields in ClassicTIFF.

unsigned short TIFFFieldInfo.field_bit¶: Built-in tags stored in special fields in the TIFF structure have assigned field numbers to distinguish them (e.g. FIELD_SAMPLESPERPIXEL). New tags should generally just use FIELD_CUSTOM indicating they are stored in the generic tag list.

unsigned char TIFFFieldInfo.field_oktochange¶: TRUE if it is OK to change this tag value while an image is being written. FALSE for stuff that must be set once and then left unchanged (like ImageWidth, or PhotometricInterpretation for instance).

unsigned char TIFFFieldInfo.field_passcount¶: If TRUE, then the count value must be passed in TIFFSetField(), and TIFFGetField(), otherwise the count is not required. This should generally be TRUE for non-ascii variable count tags unless the count is implicit (such as with the colormap).

char *TIFFFieldInfo.field_name¶: A name for the tag. Normally mixed case (studly caps) like StripByteCounts, and relatively short.

Within tif_dirinfo.c file, the built-in TIFF tags are defined with TIFFField structure that has additional parameters defining the var_arg interface of TIFFSetField() and TIFFGetField().

Various functions exist for getting the internal TIFFFieldInfo definitions, including _TIFFFindFieldInfo(), and _TIFFFindFieldInfoByName(). See tif_dirinfo.c for details.

Default Tag Auto-registration¶

In libtiff 3.6.0 a new mechanism was introduced allowing libtiff to read unrecognised tags automatically. When an unknown tags is encountered, it is automatically internally defined with a default name and a type derived from the tag value in the file. Applications only need to predefine application specific tags if they need to be able to set them in a file, or if particular calling conventions are desired for TIFFSetField() and TIFFGetField().

When tags are autodefined like this the field_readcount and field_writecount values are always TIFF_VARIABLE2 (-3). The field_passcount is always TRUE, and the field_bit is FIELD_CUSTOM. The field name will be Tag %d where the %d is the tag number.

Thus, to read anonymous auto-registered tags use the following:

uint32_t count;
void* value;  //has to be a pointer to the expected value type.
TIFFGetField(tif, TIFFTAG_UNKNOWN_TO_LIBTIFF, &count, &value);

Defining Application Tags¶

For various reasons, it is common for applications to want to define their own tags to store information outside the core TIFF specification. This is done by calling TIFFMergeFieldInfo() with one or more TIFFFieldInfo.

The libgeotiff library provides geospatial information extensions within a TIFF file. First, an array of TIFFFieldInfo is prepared with information on the new tags:

static const TIFFFieldInfo xtiffFieldInfo[] = {

    /* XXX Insert Your tags here */
    { TIFFTAG_GEOPIXELSCALE,        -1,-1, TIFF_DOUBLE,     FIELD_CUSTOM,
      TRUE, TRUE,   "GeoPixelScale" },
    { 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" }
};

In order to define the tags, we call TIFFMergeFieldInfo() on the desired TIFF handle with the list of TIFFFieldInfo.

#define     N(a)    (sizeof (a) / sizeof (a[0]))

/* Install the extended Tag field info */
TIFFMergeFieldInfo(tif, xtiffFieldInfo, N(xtiffFieldInfo));

The tags need to be defined for each TIFF file opened - and when reading they should be defined before the tags of the file are read, yet a valid TIFF* is needed to merge the tags against. In order to get them registered at the appropriate part of the setup process, it is necessary to register our merge function as an extender callback with libtiff. This is done with TIFFSetTagExtender(). We also keep track of the previous tag extender (if any) so that we can call it from our extender allowing a chain of customizations to take effect.

static TIFFExtendProc _ParentExtender = NULL;

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);
}

The extender callback is looks like this. It merges in our new fields and then calls the next extender if there is one in effect.

static void
_XTIFFDefaultDirectory(TIFF *tif)
{
    /* Install the extended Tag field info */
    TIFFMergeFieldInfo(tif, xtiffFieldInfo, N(xtiffFieldInfo));

    /* 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);
}

The above approach ensures that our new definitions are used when reading or writing any TIFF file. However, since on reading we already have default definitions for tags, it is usually not critical to pre-define them. If tag definitions are only required for writing custom tags, you can just call TIFFMergeFieldInfo() before setting new tags. The whole extender architecture can then be avoided.

Adding New Builtin Tags¶

A similar approach is taken to the above. However, the _TIFFField information should be added to the tiffFields[] list in tif_dirinfo.c. This _TIFFField structure is like TIFFFieldInfo structure but has additional members:

The tags in the tiffFields[] list need not to be in sorted order by the tag number. Sorting is done when setting up the TIFFFieldArray at IFD initialization.

typedef struct _TIFFField {
  uint32_t             field_tag;        /* field's tag */
  short                field_readcount;  /* read count / TIFF_VARIABLE / TIFF_VARIABLE2 / TIFF_SPP */
  short                field_writecount; /* write count / TIFF_VARIABLE / TIFF_VARIABLE2 */
  TIFFDataType         field_type;       /* type of associated data */
  uint32_t             field_anonymous;  /* if true, this is a unknown / anonymous tag */
  TIFFSetGetFieldType  set_field_type;   /* type to be passed to TIFFSetField and TIFFGetField */
  TIFFSetGetFieldType  get_field_type;   /* not used */
  unsigned short       field_bit;        /* bit in fieldsset bit vector */
  unsigned char        field_oktochange; /* if true, can change while writing */
  unsigned char        field_passcount;  /* if true, pass dir count on set */
  char                *field_name;       /* ASCII name */
  TIFFFieldArray      *field_subfields;  /* if field points to child ifds, child ifd field definition array */
};

uint32_t _TIFFField.field_anonymous¶: Used internally to indicate auto-registered tags. See TIFFFieldIsAnonymous().

TIFFSetGetFieldType _TIFFField.set_field_type¶: The enummeration identifier gives a hint and defines the vararg parameters passed to TIFFSetField() and TIFFGetField().

For example TIFF_SETGET_UINT64 defines a single uint64_t parameter to be passed to TIFFSetField() and TIFFGetField(), respectively.

To distinguish the three different array types, there are three strings before the value type within the enummeration identifier (e.g. TIFF_SETGET_C0_UINT64):

"_C0_" for fixed arrays and thus no count parameter is required for TIFFSetField() and TIFFGetField(), respectively. The array length is constant and given by field_readcount or field_writecount, respectively.

"_C16_" and "_C32_" are for variable arrays with uint16_t or uint32_t count parameter required for TIFFSetField() and TIFFGetField(), respectively. But then, field_readcount and field_writecount has to be set to -1 for _C16_ and -3 for _C32_, respectively.

TIFFSetGetFieldType _TIFFField.get_field_type¶: Currently, this parameter is not used for TIFFGetField(). Nevertheless, it is set as a copy of set_field_type or set to TIFF_SETGET_UNDEFINED.

Normally new built-in tags should be defined with FIELD_CUSTOM and then only two points need to be done:

Define the tag in tiff.h.
Add an entry in the tiffFields[] array list defined at the top of tif_dirinfo.c.

However, if it is desirable for the tag value to have its own field in the TIFFDirectory structure, then you will also need to #define a new FIELD_ value for it, and add appropriate handling as follows:

Add a field to the TIFFDirectory structure in tif_dir.h and define a FIELD_* bit number (also update the definition of FIELD_CODEC to reflect your addition).
Add entries in _TIFFVSetField() and _TIFFVGetField() for the new tag.
(optional) If the value associated with the tag is not a scalar value (e.g. the array for TransferFunction) and requires special processing, then add the appropriate code to TIFFReadDirectory() and TIFFWriteDirectory(). You're best off finding a similar tag and cribbing code.
Add support to TIFFPrintDirectory() in tif_print.c to print the tag's value.

If you want to maintain portability, beware of making assumptions about data types. Use the typedefs (uint16_t, etc. when dealing with data on disk and t*_t when stuff is in memory) and be careful about passing items through printf or similar vararg interfaces.

Adding New Codec-private Tags¶

To add tags that are meaningful only when a particular compression algorithm is used follow these steps:

Define the tag in tiff.h.
Allocate storage for the tag values in the private state block of the codec.
Insure the state block is created when the codec is initialized.

At TIFFInitfoo() time override the method pointers in the TIFF structure for getting, setting and printing tag values. For example,

sp->vgetparent = tif->tif_vgetfield;
tif->tif_vgetfield = fooVGetField;        /* hook for codec tags */
sp->vsetparent = tif->tif_vsetfield;
tif->tif_vsetfield = fooVSetField;        /* hook for codec tags */
tif->tif_printdir = fooPrintDir;  /* hook for codec tags */

(Actually you may decide not to override the tif_printdir method, but rather just specify it).

Create a private TIFFFieldInfo array for your tags and merge them into the core tags at initialization time using _TIFFMergeFieldInfo(); e.g.
```
_TIFFMergeFieldInfo(tif, fooFieldInfo, N(fooFieldInfo));
```
(where N is a macro used liberaly throughout the distributed code).
Fill in the get and set routines. Be sure to call the parent method for tags that you are not handled directly. Also be sure to set the FIELD_* bits for tags that are to be written to the file. Note that you can create "pseudo-tags" by defining tags that are processed exclusively in the get/set routines and never written to file (see the handling of TIFFTAG_FAXMODE in tif_fax3.c for an example of this).
Fill in the print routine, if appropriate.

Note that space has been allocated in the FIELD_* bit space for codec-private tags. Define your bits as FIELD_CODEC+<offset> to keep them away from the core tags. If you need more tags than there is room for, just increase td_fieldsset at the top of tif_dir.h.

Defining New TIFF Tags¶

TIFFFieldInfo¶

Default Tag Auto-registration¶

Defining Application Tags¶

Adding New Builtin Tags¶

Adding New Codec-private Tags¶

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