No objections, great to have a useful solution.  Is the opacity of the
background color also defined?

Out of curiosity why as part of a single UserStyle, since an SLD may have
several? Would it make sense to provide a background color as part of
StyledLayerDescriptor which describes an entire
map with several layers?

On Wed, Sep 11, 2019 at 10:23 AM Andrea Aime <andrea.a...@geo-solutions.it>
wrote:

> Hi,
> in Testbed 15 we are asked to implement a way to set the map background
> color in the
> style itself, at least for styles that are meant to be used as basemaps.
>
> The SLD specification does not allow for that, which makes sense in the
> context of "overlay" WMS,
> with a client deciding the set of layers to be displayed, and controlling
> the background color from the
> GetMap itself.
>
> However, the need to set the background color is real and we have seen in
> GeoServer since the
> beginning: anyone forgot about the "giant_polygon" layer used in the NYC
> demo layer group? :-D
> Looking at the MBStyles code I see a similar solution, the code is taking
> the bgcolor specification
> found in Mapbox GL and turning it into a inline user style with a GML
> giant polygon defined inside.
>
> The giant polygon approach has been labelled as clumsy and ineffective,
> and indeed, I have to agree:
>
>    - Why expose a dedicated, user visible layer with a massive polygon
>    just to set the color of the painting canvas
>    - Reprojection issues abund when going to local projections, the
>    advanced projection handling help here by cutting the geometry, but it's
>    not available for all projections
>
> The idea proposed in the testbed is to add a BGColor element inside
> UserStyle that would allow setting the background
> color for the entire style (imagine a multi-layer stylesheet, also known
> as style group, in GeoServer):
>
> <UserStyle>
>    <BGColor>FFAA00</BGColor>
>    ...
> </UserStyle>
>
> The idea seems useful and has no backwards compatibility issues, so I'd
> like to implement it, at least in the
> SLD parser, to give a simple solution to.. well... a simple problem.
> The renderer would then simply use the information to paint the canvas
> with the first bgcolor found in the layer stack.
> If the map is requested to be transparent (&transparent=true) then it
> would ignore the bgcolor, allowing the layers
> to be used both as suitable backgrounds but also as overlays
>
> Objections? :)
>
> Cheers
> Andrea
>
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-- 
--
Jody Garnett
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