[mapserver-dev] Canvas support for MapServer

[Jan Hartmann]

Hi Brent,

First let me say that Canvas support is just an idea; I think it can be 
done but whether it is advisable to do so is up to you to decide. I 
liked the idea because the Canvas element is supported nowadays by most 
browsers, unlike SVG which needs a plugin. It is also much simpler, so 
it's easier to use for simple maps, and especially for simple 
interactive operations like digitizing, or combining server-side with 
client-side graphics. That's why I would prefer Canvas above SVG, let 
alone the fact that I much prefer to program in native Javascript above 
parsing XML. Most of my maps are simple (I would guess that goes for 
many people here), so SVG is decidedly overkill. The Canvas HTML element 
also fits nicely in the page layout and can be processed easily by 
web-editors.

Your suggestion that Canvas could be implemented at the client side by 
catching SVG output and transforming it into Canvas directives is 
certainly viable. That is also Paul Spencer's view in his reply to my 
email.  Yet I would prefer a server-side approach: it's more efficient 
(no second parsing of SVG code that had to be generated in the first 
place), and I think the server-side code can be easily adapted from the 
SVG driver. Canvas is a very simple format, just a few graphic 
primitives, more like GD than SVG actually.

Again, I don't know how difficult it would be to add a Canvas 
outputformat, and whether it fits at all into the MapServer design 
philosophy. However, when it could be done, it would give me an a far 
more easier tool than SVG  to combine server-side and client-side mapping.

Jan

Brent Fraser wrote:
> Jan,
>
>  My interest is in building a (or re-purposing an existing) tool for 
> Cartographic Layout, for printing or rendering a graphics format for 
> easy viewing (e.g. PDF).  Steve's comments were about using existing 
> Web formats and syntax to describe the layout, and existing(?) web 
> editors to do the placement/moving of the layout components.
>
>  My (very limited) understanding of the <canvas> HTML tag is that it's 
> a "container" recognized by the browser, and vector (and raster) 
> graphics are rendered into it using client-side JavaScript.  Sort of 
> the JavaScript equivalent of the SVG format.
>
>  It would be possible to add another output format to Mapserver to 
> generate JavaScript code of canvas rendering methods, but a more 
> elegant approach might be to write a small(?) JavaScript module to 
> ingest Mapserver's SVG and render the objects to the canvas.
>
>  Are there some specific features of the direct-canvas-output approach 
> not available in SVG that you need?
>
> Brent Fraser
>
>
> Jan Hartmann wrote:
>> Hi folks,
>>
>> A few weeks ago I suggested in a thread about PDF support that 
>> Mapserver could perhaps be made to support the new "Canvas" tag. 
>> There were no reactions to this, so I don't know if this suggestion 
>> is viable/advisable/practicable, or just downright stupid. Can anyone 
>> comment? 
>> (from 
>> http://lists.osgeo.org/pipermail/mapserver-dev/2009-January/008055.html)
>>
>>> I am wondering if MapServer support for the "Canvas" tag could do 
>>> what Steve suggests in a more simple way. Originally, only Safari, 
>>> Opera and Firefox supported this tag to allow (simple) 2D drawing, 
>>> but recently a surprisingly simple Javascript interface for IE has 
>>> become available, translating Canvas command to native IE VRML 
>>> commands. It requires just one single script tag in the web page.
>>>
>>> See the tutorial at the Mozilla site at: 
>>> https://developer.mozilla.org/en/Canvas_tutorial
>>> and the IE interface (from the Google stables) at: 
>>> http://excanvas.sourceforge.net/
>>>
>>> This is something I have long been looking for. The graphics are 
>>> very simple, so the functionality is nothing like PDF or SVG, but I 
>>> could imagine that a driver for MapServer could fulfill many needs. 
>>> I can use it already by letting Mapserver generate raw coordinates 
>>> and catching them with some sort of Ajax, but a separate driver 
>>> would be very neat of course.
>>>
>>> How about it?
>>>
>>> Jan
>>>
>>>
>>
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