[gdal-dev] OGR Feature Style Specification

[Ari Jolma]

Tamas Szekeres kirjoitti:
> Ari,
>
> This sounds interesting, however I cannot imagine the actual
> implementation of that. There's a couple of questions I have in my
> mind related to the feature styles, some of these may be due to the
> lack of my knowledge about the SVG feature styles.
>
> 1. Do we have an exact mapping between the OGR and the SVG style
> parameters. Is OGR currently have a subset of the SVG style set?
>   

That I leave for later research. I believe the current OGR is a subset 
of SVG styles.

> 2. The OGR style string is fairly compact and easy to parse. How about
> the SVG style representation from this aspect? Note that we might want
> to store the style strings in a separate field per feature even if the
> data source has no built in style support.
>   

SVG is XML but styles are encoded into elements, for example: 
style="font-size:12;fill:url(#linearGradient578);fill-opacity:1;fill-rule:evenodd;stroke:#000000;stroke-width:0.920943pt;"

i.e. a string of key:value;key:value;... but this is not the whole story 
since as in this example the fill is a reference to gradient described, 
which needs more explaining than a simple value, elsewhere in the document

> 3. Will this change affect the OGR Support Classes described in
> http://www.gdal.org/ogr/ogr_feature_style.html
>   

No. :)

> 4. Can someone consider which drivers have built in style support and
> what are their requirements to incorporate? The current approach seems
> to be a mapinfo-ish implementation but in the meantime there might be
> other drivers have been added to the pool with different requirements.
>   

Since SVG is a pretty general it should be able to handle all styles and 
mapping to it is then just a technical problem?

> 5. How about the compatibility issues with the existing applications?
> Would it be easy to migrate their existing scripts. For example if the
> application use the WHERE OGR_STYLE LIKE 'BRUSH' according to
> http://www.gdal.org/rfc6_sqlgeom.html ?
>   

That kind of statement assumes quite much and I'm not sure if such 
constraints should be encouraged. There is no "brush" in SVG styles 
AFAIK. Maybe SVG styles should be implemented as a "new styles" and 
deprecate the old styles eventually?

> Further issues:
>
> 6. Can we support the dataset/layer level default styles as described
> in the OGR Feature Style specification but haven't been implemented
> yet?
>   

Isn't that the matter of just adding a style member into the 
dataset/layer class? Do features have knowledge of the layer they belong 
to? Then they can return the default style if they do not have a style 
of their own.

Ari

>
> Best regards,
>
> Tamas
>
>
> 2008/4/17, Ari Jolma <ari.jolma at tkk.fi>:
>   
>> People,
>>
>>  As discussed in the OSGeo-Discuss list, there might be an interest, need,
>> etc. to rethink feature styles. My suggestion would be to look into the SVG
>> specification and refer to it as much as possible. Also something meta to
>> that would be useful, i.e., saying that this feature is to be rendered using
>> this symbol, and the symbol would be presented as a SVG snippet. I'd also
>> like to render raster cells using symbols, the best known example being
>> perhaps the flow direction arrow of raster DEMs.
>>
>>  The issue of rules - which symbols to use and tieing symbols and their
>> properties into feature attributes - is perhaps unavoidable. It could be
>> handled using the OGC symbology encoding spec (which also uses SVG styles),
>> but does the whole thing get too big or out of scope for GDAL? One easily
>> gets carried away. For example once we have SVG style and the geometry
>> available, we could spit out SVG paths... which brings up the idea of having
>> line simplification in OGR, etc...
>>
>>  If thought sensible in the first place I could try writing down this as a
>> RFC.
>>
>>  Regards,
>>
>>  Ari
>>
>>  --
>>  Prof. Ari Jolma
>>  Geoinformatiikka / Geoinformatics
>>  Teknillinen Korkeakoulu / Helsinki University of Technology
>>  tel: +358 9 451 3886 address: POBox 1200, 02015 TKK, Finland
>>  Email: ari.jolma at tkk.fi URL: http://www.tkk.fi/~jolma
>>
>>
>>  _______________________________________________
>>  gdal-dev mailing list
>>  gdal-dev at lists.osgeo.org
>>  http://lists.osgeo.org/mailman/listinfo/gdal-dev
>>
>>     


-- 
Prof. Ari Jolma
Geoinformatiikka / Geoinformatics
Teknillinen Korkeakoulu / Helsinki University of Technology
tel: +358 9 451 3886 address: POBox 1200, 02015 TKK, Finland
Email: ari.jolma at tkk.fi URL: http://www.tkk.fi/~jolma