Tags and Links: Cairo: A Vector Graphics Library

## Tags and Links Tags and Links — Hyperlinks and document structure |

## Functions

void | cairo_tag_begin ()
void | cairo_tag_end ()

## Types and Values

#define | CAIRO_TAG_DEST
#define | CAIRO_TAG_LINK

## Description

The tag functions provide the ability to specify hyperlinks and document logical structure on supported backends. The following tags are supported:

- Link - Create a hyperlink

- Destinations - Create a hyperlink destination

- Document Structure Tags - Create PDF Document Structure

### 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.

- Internal Links - A link to a location in the same document

- URI Links - A link to a Uniform resource identifier

- File Links - A link to a location in another document

#### Internal Links

An internal link is a link to a location in the same document. The destination is specified with either:

`dest` : a UTF-8 string specifying the destination in the PDF file to link to. Destinations are created with the `CAIRO_TAG_DEST` tag.

or the two attributes:

`page` : An integer specifying the page number in the PDF file to link to.

`pos` : [optional] An array of two floats specifying the x,y position on the page.

An example of the link attributes to link to a page and x,y position:

    "page=3 pos=[3.1 6.2]"

#### URI Links

A URI link is a link to a Uniform Resource Identifier (RFC 2396).

A URI is specified with the following attribute:

`uri` : An ASCII string specifying the URI.

An example of the link attributes to the cairo website:

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

#### File Links

A file link is a link a location in another PDF file.

The file attribute (required) specifies the name of the PDF file:

`file` : File name of PDF file to link to.

The position is specified by either:

`dest` : a UTF-8 string specifying the named destination in the PDF file.

or

`page` : An integer specifying the page number in the PDF file.

`pos` : [optional] An array of two floats specifying the x,y position on the page. Position coordinates in external files are in PDF coordinates (0,0 at bottom left).

An example of the link attributes to PDF file:

    "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:

- Extraction of text and graphics for copy and paste

- Reflow of text and graphics in the viewer

- Processing text eg searching and indexing

- Conversion to other formats

- Accessability support

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");

## Functions

### 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()`.

#### Parameters

cr | a cairo context |
tag_name | tag name |
attributes | tag attributes |

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()`.

#### Parameters

cr | a cairo context |
tag_name | tag name |

Since: 1.16

## Types and Values

### 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

## See Also

cairo_pdf_surface_t