[gdal-dev] Proposed updates to the GDAL feature style spec

[Even Rouault]

Alan,

Thanks a lot for this refresh ! I've just a single remark

> 5. [...] change the definition of
> ogr-brush-1 to mean a solid fill in the selected background color;
> change the suggested default for BRUSH bc to transparent (#FFFFFF00).
> 
> Rationale: The spec is not explicit on how fc and bc are meant to
> work. A true null brush seems unnecessary, as the BRUSH() style tool
> can simply be omitted. If a null brush really is required, ogr-brush-1
> will continue to act as a null brush by default, assuming software
> uses a fully transparent color as a default for bc.

I'd suggest that we add a rule so as to keep the current semantics: if ogr-brush-1 is used, then 
bc must be omitted (or set to a color with alpha=0). It would be confusing to have the 
possibility to have 2 ways to express a solid fill with either ogr-brush-0 + fc or ogr-brush-1 + 
bc. So people wanting a solid fill should use ogr-brush-0 (or don't specify it)

For example MapServer (likely the most comprehensive user of OGR feature style) has a 
hard-coded behaviour when encountering ogr-brush-1 where it completely ignores bc:
https://github.com/mapserver/mapserver/blob/branch-7-0/mapogr.cpp#L5132

Even

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