<refentry id="iptc-overview" revision="23 Feb 2005">
<refmeta>
	<refentrytitle>libiptcdata Overview</refentrytitle>
	<manvolnum>3</manvolnum>
	<refmiscinfo>libiptcdata Library</refmiscinfo>
</refmeta>

<refnamediv>
	<refname>libiptcdata Overview</refname>
	<refpurpose>how to use libiptcdata in an application</refpurpose>
</refnamediv>

<refsect1 id="jpeg-loading">
	<title>Extracting IPTC data from a JPEG file</title>

	<para>
	 When using libiptcdata, the first task you are probably interested in
	 is reading the IPTC metadata from an existing file.  At present, JPEG is
	 the only format supported by libiptcdata.  The API is designed in
	 a multi-level fashion so that if your application already does some form
	 of JPEG decoding, you can integrate libiptcdata without having to parse
	 the file twice.
	</para>

	<para>
	 The <ulink url="http://www.iptc.org/IIM/">IPTC IIM standard</ulink>
	 was originally conceived so that the JPEG file itself
	 would be encapsulated inside an IPTC wrapper.  Since this is not backward
	 compatable with regular JPEGs, Adobe devised its own standard where an IPTC
	 data block is placed inside the Adobe Photoshop application-specific header.
	 This Photoshop
	 metadata is stored in the APP13 header section of a JPEG file.  So reading
	 the IPTC data block is a two-step process:  First, extract Photoshop metadata
	 from the APP13 header, and second, extract the IPTC data block from the
	 Photoshop metadata.
	</para>

	<para>
	 <ulink url="http://controlledvocabulary.com/imagedatabases/iptc_naa.html">More
	 information about the history of IPTC metadata</ulink> is available at
	 controlledvocabulary.com.
	</para>

	<para>
	 For the impatient, the IPTC data can be extracted with a single
	 line of code using the <function><link linkend="iptc-data-new-from-jpeg">iptc_data_new_from_jpeg</link>()</function>
	 function if you have no need for fine-grained error checking or you don't
	 want to share code with a pre-existing JPEG decoding routine in your
	 application.  Here's an example:
	</para>

	<programlisting>
IptcData * d;
char * filename = "foo.jpg";

d = iptc_data_new_from_jpeg (filename);
if (!d) {
	fprintf (stderr, "Error reading IPTC data\n");
	return 1;
}
	</programlisting>

	<para>
	 However, for all but the most simplistic of applications, you will probably
	 want finer control of the process.  For that, you use three functions.
	 The first, <function><link linkend="iptc-jpeg-read-ps3">iptc_jpeg_read_ps3</link>()</function>
	 extracts the
	 Photoshop metadata from the APP13 header.  If your application already
	 decodes the JPEG file, you don't need this function:  Simply store the
	 contents of the APP13 header as you decode the JPEG.  See
	 <link linkend="iptc-libjpeg">libjpeg Interoperability</link> for
	 an example.
	</para>

	<para>
	 The second function,
	 <function><link linkend="iptc-jpeg-ps3-find-iptc">iptc_jpeg_ps3_find_iptc</link>()</function>
	 extracts the IPTC data block from the Photoshop metadata.
	</para>

	<para>
	 The third function, <function><link linkend="iptc-data-new-from-data">iptc_data_new_from_data</link>()</function>
	 takes the IPTC data block and decodes it into a data structure of type
	 <structname><link linkend="IptcData">IptcData</link></structname> which you can use with the rest of the
	 libiptcdata API.  Here's an example of all three functions in use:
	</para>

	<programlisting>
FILE * infile;
int ps3_len, iptc_off, iptc_len;
IptcData * d;
char * filename = "foo.jpg";
unsigned char buf[256*256];

infile = fopen (filename, "r");
if (!infile) {
	fprintf (stderr, "Error opening %s\n", filename);
	return 1;
}

ps3_len = iptc_jpeg_read_ps3 (infile, buf, sizeof(buf));
fclose (infile);
if (ps3_len &lt; 0) {
	fprintf (stderr, "Error parsing JPEG file\n");
	return 1;
}

if (ps3_len == 0) {
	fprintf (stderr, "File contains no PS3 header\n");
	return 1;
}

iptc_off = iptc_jpeg_ps3_find_iptc (buf, ps3_len, &amp;iptc_len);
if (iptc_off &lt; 0) {
	fprintf (stderr, "Error parsing PS3 header\n");
	return 1;
}

if (iptc_off == 0) {
	fprintf (stderr, "PS3 header contains no IPTC data\n");
	return 1;
}

d = iptc_data_new_from_data (buf + iptc_off, iptc_len);
if (!d) {
	fprintf (stderr, "Error parsing IPTC data\n");
	return 1;
}
	</programlisting>

</refsect1>

<refsect1 id="creating-iptc">
	<title>Creating IPTC data from scratch</title>

	<para>
	 If you want to create an empty <structname><link linkend="IptcData">IptcData</link></structname> structure from scratch without
	 reading it from a preexisting file, use the <function><link linkend="iptc-data-new">iptc_data_new</link>()</function> function:
	</para>

	<programlisting>
IptcData * d;

d = iptc_data_new ();
	</programlisting>
</refsect1>

<refsect1 id="viewing-iptc">
	<title>Viewing IPTC data</title>

	<para>
	 IPTC metadata is simply a list of "datasets."  Each dataset contains a
	 record number, dataset number, and the data itself.  The record number
	 and dataset number, taken together, determine the purpose of the dataset,
	 according to the IPTC IIM specification.  For example, dataset 2:120 (record 2,
	 dataset 120) contains an image caption.  Some datasets can be repeated, such
	 as dataset 2:25, which contains a single image keyword.  Each repeated dataset
	 would contain a separate keyword.  The IPTC IIM specification determines
	 the content of each dataset, the type (string, byte, short, etc.), and any
	 restrictions such as minimum/maximum length, repeatability, etc.
	</para>

	<para>
	 The following code snippet shows how to iterate through all the datasets in
	 an <structname><link linkend="IptcData">IptcData</link></structname> object
	 and print their contents.  The <function><link linkend="iptc-tag-get-title">iptc_tag_get_title</link>()</function>
	 function gets the string name of a dataset from its record and dataset numbers.
	 <function><link linkend="iptc-dataset-get-format">iptc_dataset_get_format</link>()</function>
	 retrieves the type of the data contained in the dataset.  For datasets with
	 integer types, <function><link linkend="iptc-dataset-get-value">iptc_dataset_get_value</link>()</function>
	 gets the contents of the tag, and for datasets with string types,
	 <function><link linkend="iptc-dataset-get-data">iptc_dataset_get_data</link>()</function>
	 will do the same.
	 <function><link linkend="iptc-dataset-get-as-str">iptc_dataset_get_as_str</link>()</function>
	 formats the data as a string regardless of its type.
	</para>

	<programlisting>
int i;

printf("%6.6s %-20.20s %-9.9s %6s  %s\n", "Tag", "Name", "Type",
		"Size", "Value");
printf(" ----- -------------------- --------- ------  -----\n");

for (i=0; i &lt; d-&gt;count; i++) {
	IptcDataSet * e = d-&gt;datasets[i];
	char buf[256];

	printf("%2d:%03d %-20.20s %-9.9s %6d  ",
			e-&gt;record, e-&gt;tag,
			iptc_tag_get_title (e-&gt;record, e-&gt;tag),
			iptc_format_get_name (iptc_dataset_get_format (e)),
			e-&gt;size);
	switch (iptc_dataset_get_format (e)) {
		case IPTC_FORMAT_BYTE:
		case IPTC_FORMAT_SHORT:
		case IPTC_FORMAT_LONG:
			printf("%d\n", iptc_dataset_get_value (e));
			break;
		case IPTC_FORMAT_BINARY:
			iptc_dataset_get_as_str (e, buf, sizeof(buf));
			printf("%s\n", buf);
			break;
		default:
			iptc_dataset_get_data (e, buf, sizeof(buf));
			printf("%s\n", buf);
			break;
	}
}
	</programlisting>

	<para>
	 This next code snippet prints out the list of keywords stored in the
	 IPTC metadata.  First, in case we don't know the record and dataset
	 numbers for the "Keywords" tag, the
	 <function><link linkend="iptc-tag-find-by-name">iptc_tag_find_by_name</link>()</function>
	 function retrieves them from the IPTC specification.  Next, the
	 <function><link linkend="iptc-data-get-next-dataset">iptc_data_get_next_dataset</link>()</function>
	 function iterates through each Keyword tag, printing out the contents.
	</para>

	<para>
	 Another important concept presented in this example is the use of reference
	 counts.  Whenever a function, such as <function><link linkend="iptc-data-get-next-dataset">iptc_data_get_next_dataset</link>()</function>
	 returns a pointer, the reference count for the associated object is incremented by one.
	 It is the application's responsibility to later unreference that pointer
	 so that the count does not keep growing.  When the count reaches zero, the
	 object is automatically freed.  This must be done for both
	 <structname><link linkend="IptcData">IptcData</link></structname>
	 (with <function><link linkend="iptc-data-unref">iptc_data_unref</link></function>) and
	 <structname><link linkend="IptcDataSet">IptcDataSet</link></structname> (with
	 <function><link linkend="iptc-dataset-unref">iptc_dataset_unref</link></function>) objects.
	</para>

	<programlisting>
IptcDataSet * ds = NULL;
IptcRecord record;
IptcTag tag;
char buf[256];

if (iptc_tag_find_by_name ("Keywords", &amp;record, &amp;tag) &lt; 0) {
	fprintf (stderr, "Invalid tag name\n");
	return 1;
}

while (1) {
	ds = iptc_data_get_next_dataset (d, ds, record, tag);
	if (!ds)
		break;
	iptc_dataset_get_data (ds, buf, sizeof(buf));
	printf("%s\n", buf);
	iptc_dataset_unref(ds);
}
	</programlisting>
</refsect1>

<refsect1 id="adding-tag">
	<title>Adding a new tag</title>

	<para>
	 New tags can be added to an
	 <structname><link linkend="IptcData">IptcData</link></structname>
	 object with the <function><link linkend="iptc-data-add-dataset">iptc_data_add_dataset</link>()</function> function.
	 This new dataset can be obtained from another
	 <structname><link linkend="IptcData">IptcData</link></structname> object,
	 or it can be created from scratch with the
	 <function><link linkend="iptc-dataset-new">iptc_dataset_new</link>()</function> function.
	 If created from scratch,
	 <function><link linkend="iptc-dataset-set-tag">iptc_dataset_set_tag</link>()</function>
	 is first used to set the record and dataset number for the tag.  Next,
	 <function><link linkend="iptc-dataset-set-tag">iptc_dataset_set_data</link>()</function>
	 is used to set the actual contents of the tag.  The following code snippet
	 demonstrates adding a new keyword, "travel" to an existing set of IPTC
	 data:
	</para>

	<programlisting>
IptcDataSet * ds;
IptcRecord record;
IptcTag tag;

if (iptc_tag_find_by_name ("Keywords", &amp;record, &amp;tag) &lt; 0) {
	fprintf (stderr, "Invalid tag name\n");
	return 1;
}

ds = iptc_dataset_new ();
iptc_dataset_set_tag (ds, record, tag);
iptc_dataset_set_data (ds, "travel", 6, IPTC_DONT_VALIDATE);
iptc_data_add_dataset (d, ds);
iptc_dataset_unref (ds);
	</programlisting>
</refsect1>

<refsect1 id="modifying-tag">
	<title>Modifying an existing tag</title>

	<para>
	 Combining the functions we have seen above, the following code
	 snippet modifies an existing caption in the IPTC data.  It first
	 finds the Caption tag with the
	 <function><link linkend="iptc-data-get-dataset">iptc_data_get_dataset</link>()</function>
	 function and then changes the data with
	 <function><link linkend="iptc-dataset-set-data">iptc_dataset_set_data</link>()</function>:
	</para>

	<programlisting>
IptcDataSet * ds;
IptcRecord record;
IptcTag tag;
char * str = "This is a caption";

if (iptc_tag_find_by_name ("Caption", &amp;record, &amp;tag) &lt; 0) {
	fprintf (stderr, "Invalid tag name\n");
	return 1;
}

ds = iptc_data_get_dataset (d, record, tag);
if (!ds) {
	fprintf (stderr, "Tag not found\n");
	return 1;
}
iptc_dataset_set_data (ds, str, strlen (str), IPTC_DONT_VALIDATE);
iptc_dataset_unref (ds);
	</programlisting>
</refsect1>

<refsect1 id="deleting-tag">
	<title>Deleting a tag</title>

	<para>
	 A dataset can be deleted using the
	 <function><link linkend="iptc-data-remove-dataset">iptc_data_remove_dataset</link>()</function> function, as seen in this example, which deletes the Caption from the
	 IPTC data:
	</para>

	<programlisting>
IptcDataSet * ds;
IptcRecord record;
IptcTag tag;

if (iptc_tag_find_by_name ("Caption", &amp;record, &amp;tag) &lt; 0) {
	fprintf (stderr, "Invalid tag name\n");
	return 1;
}

ds = iptc_data_get_dataset (d, record, tag);
if (!ds) {
	fprintf (stderr, "Tag not found\n");
	return 1;
}
iptc_data_remove_dataset (d, ds);
iptc_dataset_unref (ds);
	</programlisting>
</refsect1>

<refsect1 id="jpeg-saving">
	<title>Saving IPTC data to a JPEG file</title>

	<para>
	 Saving an <structname><link linkend="IptcData">IptcData</link></structname>
	 object back to a JPEG file is a multi-step process requiring several
	 different function calls.  Although this may seem cumbersome at first,
	 it allows you to integrate the process into any existing JPEG-saving
	 code already written in your application.
	</para>

	<para>
	 First, a call to
	 <function><link linkend="iptc-data-sort">iptc_data_sort</link>()</function>
	 sorts the datasets by tag number prior to saving.  This step
     is optional.
	</para>

	<para>
	 Next, by calling
	 <function><link linkend="iptc-data-save">iptc_data_save</link>()</function>
	 we convert the
	 <structname><link linkend="IptcData">IptcData</link></structname>
	 object into a bytestream.  This IPTC data block is then encapsulated
	 inside a Photoshop header using
	 <function><link linkend="iptc-jpeg-ps3-save-iptc">iptc_jpeg_ps3_save_iptc</link>()</function>.
	 This function will use an existing Photoshop header, if available, so that
	 all unrelated portions of the Photoshop header are preserved.  If no
	 pre-existing Photoshop header is provided, a new one will be created from
	 scratch.
	</para>

	<para>
	 The new Photoshop header can be saved inside the APP13 header of the JPEG
	 file using your application code, or you can use
	 <function><link linkend="iptc-jpeg-save-with-ps3">iptc_jpeg_save_with_ps3</link>()</function> to do the same thing.
	</para>

	<para>
	 The following example code demonstrates the entire process:
	</para>

	<programlisting>
unsigned char * iptc_buf = NULL;
unsigned char outbuf[256*256];
char tmpfile[strlen(filename)+8];
int v;

iptc_data_sort (d);

if (iptc_data_save (d, &amp;iptc_buf, &amp;iptc_len) &lt; 0) {
	fprintf (stderr, "Failed to generate IPTC bytestream\n");
	return 1;
}
ps3_len = iptc_jpeg_ps3_save_iptc (buf, ps3_len,
		iptc_buf, iptc_len, outbuf, sizeof(outbuf));
iptc_data_free_buf (d, iptc_buf);
if (ps3_len &lt; 0) {
	fprintf (stderr, "Failed to generate PS3 header\n");
	return 1;
}

infile = fopen (filename, "r");
if (!infile) {
	fprintf (stderr, "Can't reopen input file\n");
	return 1;
}
sprintf (tmpfile, "%s.%d", filename, getpid());
outfile = fopen (tmpfile, "w");
if (!outfile) {
	fprintf (stderr, "Can't open temporary file for writing\n");
	return 1;
}

v = iptc_jpeg_save_with_ps3 (infile, outfile, outbuf, ps3_len);
fclose (infile);
fclose (outfile);
if (v &lt; 0) {
	unlink (tmpfile);
	fprintf (stderr, "Failed to save image\n");
	return 1;
}

if (rename (tmpfile, filename) &lt; 0) {
	fprintf (stderr, "Failed to save image\n");
	unlink (tmpfile);
	return 1;
}
fprintf (stderr, "Image saved\n");
	</programlisting>

</refsect1>

</refentry>