Link Tags

A hyperlink is specified by enclosing the hyperlink text with the CAIRO_TAG_LINK tag.

For example:

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cairo_tag_begin (cr, CAIRO_TAG_LINK, "uri='https://cairographics.org'");
cairo_move_to (cr, 50, 50);
cairo_show_text (cr, "This is a link to the cairo website.");
cairo_tag_end (cr, CAIRO_TAG_LINK);

The PDF backend uses one or more rectangles to define the clickable area of the link. By default cairo will use the extents of the drawing operations enclosed by the begin/end link tags to define the clickable area. In some cases, such as a link split across two lines, the default rectangle is undesirable.

rect : [optional] The "rect" attribute allows the application to specify one or more rectangles that form the clickable region. The value of this attribute is an array of floats. Each rectangle is specified by four elements in the array: x, y, width, height. The array size must be a multiple of four.

An example of creating a link with user specified clickable region:

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struct text {
    const char *s;
    double x, y;
};
const struct text text1 = { "This link is split", 450, 70 };
const struct text text2 = { "across two lines", 50, 70 };
cairo_text_extents_t text1_extents;
cairo_text_extents_t text2_extents;
char attribs[100];

cairo_text_extents (cr, text1.s, &text1_extents);
cairo_text_extents (cr, text2.s, &text2_extents);
sprintf (attribs,
         "rect=[%f %f %f %f %f %f %f %f] uri='https://cairographics.org'",
         text1_extents.x_bearing + text1.x,
         text1_extents.y_bearing + text1.y,
         text1_extents.width,
         text1_extents.height,
         text2_extents.x_bearing + text2.x,
         text2_extents.y_bearing + text2.y,
         text2_extents.width,
         text2_extents.height);

cairo_tag_begin (cr, CAIRO_TAG_LINK, attribs);
cairo_move_to (cr, text1.x, text1.y);
cairo_show_text (cr, text1.s);
cairo_move_to (cr, text2.x, text2.y);
cairo_show_text (cr, text2.s);
cairo_tag_end (cr, CAIRO_TAG_LINK);

There are three types of links. Each type has its own attributes as detailed below.

"page=3 pos=[3.1 6.2]"

"uri='https://cairographics.org'"

"file='document.pdf' page=16 pos=[25 40]"

Destination Tags

A destination is specified by enclosing the destination drawing operations with the CAIRO_TAG_DEST tag.

name : [required] A UTF-8 string specifying the name of this destination.

x : [optional] A float specifying the x coordinate of destination position on this page. If not specified the default x coordinate is the left side of the extents of the operations enclosed by the CAIRO_TAG_DEST begin/end tags. If no operations are enclosed, the x coordidate is 0.

y : [optional] A float specifying the y coordinate of destination position on this page. If not specified the default y coordinate is the top of the extents of the operations enclosed by the CAIRO_TAG_DEST begin/end tags. If no operations are enclosed, the y coordidate is 0.

internal : A boolean that if true, the destination name may be omitted from PDF where possible. In this case, links refer directly to the page and position instead of via the named destination table. Note that if this destination is referenced by another PDF (see File Links), this attribute must be false. Default is false.

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/* Create a hyperlink */
cairo_tag_begin (cr, CAIRO_TAG_LINK, "dest='mydest' internal");
cairo_move_to (cr, 50, 50);
cairo_show_text (cr, "This is a hyperlink.");
cairo_tag_end (cr, CAIRO_TAG_LINK);

/* Create a destination */
cairo_tag_begin (cr, CAIRO_TAG_DEST, "name='mydest'");
cairo_move_to (cr, 50, 250);
cairo_show_text (cr, "This paragraph is the destination of the above link.");
cairo_tag_end (cr, CAIRO_TAG_DEST);

Document Structure (PDF)

The document structure tags provide a means of specifying structural information such as headers, paragraphs, tables, and figures. The inclusion of structural information facilitates:

The list of structure types is specified in section 14.8.4 of the PDF Reference.

Note the PDF "Link" structure tag is the same as the cairo CAIRO_TAG_LINK tag.

The following example creates a document structure for a document containing two section, each with a header and a paragraph.

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cairo_tag_begin (cr, "Document", NULL);

cairo_tag_begin (cr, "Sect", NULL);
cairo_tag_begin (cr, "H1", NULL);
cairo_show_text (cr, "Heading 1");
cairo_tag_end (cr, "H1");

cairo_tag_begin (cr, "P", NULL);
cairo_show_text (cr, "Paragraph 1");
cairo_tag_end (cr, "P");
cairo_tag_end (cr, "Sect");

cairo_tag_begin (cr, "Sect", NULL);
cairo_tag_begin (cr, "H1", NULL);
cairo_show_text (cr, "Heading 2");
cairo_tag_end (cr, "H1");

cairo_tag_begin (cr, "P", NULL);
cairo_show_text (cr, "Paragraph 2");
cairo_tag_end (cr, "P");
cairo_tag_end (cr, "Sect");

cairo_tag_end (cr, "Document");

cairo_tag_begin ()

void
cairo_tag_begin (cairo_t *cr,
                 const char *tag_name,
                 const char *attributes);

Marks the beginning of the tag_name structure. Call cairo_tag_end() with the same tag_name to mark the end of the structure.

The attributes string is of the form "key1=value2 key2=value2 ...". Values may be boolean (true/false or 1/0), integer, float, string, or an array.

String values are enclosed in single quotes ('). Single quotes and backslashes inside the string should be escaped with a backslash.

Boolean values may be set to true by only specifying the key. eg the attribute string "key" is the equivalent to "key=true".

Arrays are enclosed in '[]'. eg "rect=[1.2 4.3 2.0 3.0]".

If no attributes are required, attributes can be an empty string or NULL.

See Tags and Links Description for the list of tags and attributes.

Invalid nesting of tags or invalid attributes will cause cr to shutdown with a status of CAIRO_STATUS_TAG_ERROR.

See cairo_tag_end().

Since: 1.16

cairo_tag_end ()

void
cairo_tag_end (cairo_t *cr,
               const char *tag_name);

Marks the end of the tag_name structure.

Invalid nesting of tags will cause cr to shutdown with a status of CAIRO_STATUS_TAG_ERROR.

See cairo_tag_begin().

Since: 1.16

CAIRO_TAG_DEST

#define CAIRO_TAG_DEST "cairo.dest"

Create a destination for a hyperlink. Destination tag attributes are detailed at Destinations.

Since: 1.16

CAIRO_TAG_LINK

#define CAIRO_TAG_LINK "Link"

Create hyperlink. Link tag attributes are detailed at Links.

Since: 1.16

CAIRO_TAG_CONTENT

#define CAIRO_TAG_CONTENT "cairo.content"

Create a content tag.

Since: 1.18

CAIRO_TAG_CONTENT_REF

#define CAIRO_TAG_CONTENT_REF "cairo.content_ref"

Create a content reference tag.

Since: 1.18