[cairo] Howto: insert anchors into cairo-generated PDF or SVG graphics ?

[John Ellson]

Carl Worth wrote:
> On Tue, 05 Feb 2008 12:49:38 -0500, John Ellson wrote:
>   
>> Is there a cairo-api that can be used to place anchor annotations around
>> blocks of cairo graphic objects in output formats that support them,
>> such as SVG or PDF ?
>>     
>
> There's not any API for this yet. But we've had an item on our TODO
> list for just about forever saying that it would definitely be a nice
> thing to have.
>
> I think we're just waiting for someone to take it from the "it would
> be nice" stage to something more concrete by starting an
> implementation.
>
>   
>> Or can anyone suggest any hacks or kludges that would let me do
>> this?
>>     
>
> I hope you'll get some feedback to this question, since hacks and
> kludges might actually help direct the way toward a real solution.
>
> -Carl
>   

Carl,


Here are some thoughts to try get a development discussion started:

Goal:

   Support for "anchors" in cairo's various output formats.  i.e. 
regions that are sensitive
   to the mouse and which can raise a tooltip or map to a URL.

   This proposal is based on experience with generating anchors and maps 
in graphviz
   (and is pragmatically designed to minimally support the needs of 
graphviz).


Examples:

   SVG:

     [<!ATTLIST svg xmlns:xlink CDATA #FIXED 
"http://www.w3.org/1999/xlink">]
     ...
     <a xlink:href="some_url" xlink:title="some_tooltip" 
target="some_target">
        ... graphics ...
     </a>

   VML:

      <a href="some_url" title="some_tooltip" target="some_target">
        ... graphics ...
      </a>

   PDF:

      [ /Rect [ x1 y1 x2 y2 ]
        /Border [ 0 0 0 ]
        /Action << /Subtype /URI /URI some_url
        /Subtype /Link
      /ANN pdfmark

   Imagemaps

      <area shape="poly" href="some_url" title="some_tooltip" alt="" 
coords="x1 y1 ..... " />


Observations:

   SVG and VML anchors enclose the graphic primitives, PDF and Imagemaps
   describe a separate shape for the sensitive area.

   Imagemaps are just a surface with anchors, but no other graphics.  
Once anchors
   are supported it makes sense for cairo to support imagemaps as an 
output surface type.



Proposal for cairo api:

        typedef struct {
                char *url;        /* null permitted for case of tooltip 
only */
                char *tooltip;    /* null permitted */
                char *target;     /* null permitted */
        } cairo_anchor_data_t;

        cairo_status_t cairo_begin_anchor(cairo_t, cairo_anchor_data_t);
        cairo_status_t cairo_end_anchor(cairo_t);


    cairo_begin_anchor() ... cairo_end_anchor() would enclose the set of 
graphics
    that are to be mouse sensitive in the output format.

    Each backend would emit output using the fields in the 
cairo_anchor_data_t
    in the appropriate syntax for that format.   Null property values 
will cause
    the entire property=value to be omitted from the output.

    Backends have access to the contained graphics and so can calculate 
bounding shapes
    as needed.


John