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

[John Ellson]

Behdad Esfahbod wrote:
> Yeah, see:
>
> http://lists.cairographics.org/archives/cairo/2007-November/012026.html
>
> behdad
>   

Good.  Is there an implemenation developing?

I was hoping for something slightly more abstract, that could support 
anchors as enclosing blocks in
SVG and VML, not just as shapes as in PDF or imagemaps.

Also, I was thinking that the cairo user should not have to define a new 
path for the sensitive area; that instead an appropriate bounding
region could extracted from a pushed group, or a mini surface, or 
something like that.    No coordinates are required in the case of SVG.

A question of style:  cairo doesn't need to know anything about URLs, 
targets, tooltips, etc.   Is it better to introduce (a possibly evolving 
set of)
getters and setters for these properties, or just to have the user 
populate a structure that cairo can pass transparently through to the 
backends?

John
> On Wed, 2008-02-06 at 13:09 -0500, John Ellson wrote:
>   
>> 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
>>
>>
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