Bug or feature: opacity vs fillOpacity for polygons?

[rdewit]

The SLD spec supports fill-opacity for polygons, Geostyler has fillOpacity, but the openlayers parser assumes the field is called opacity and doesn't know about fillOpacity, therefore cannot render transparent polygons converted from SLDs.

Is there consensus about what the internal name should be for the fill opacity of a polygon?

[jansule]

Thanks for opening the discussion @rdewit, that is definitely a point we should get straight.

In SLD (SE), there is a clear distinction between fill-opacity (used in Fill element), stroke-opacity (used in Stroke element), and opacity(used in Graphic, RasterSymbolizer, ImageOutline). When styling a polygon, one has to use the fill-opacity parameter and the stroke-opacity parameter, to style the inside and the border of the polygon, respectively. AFAIK, there is no general opacity element that sets the value for both fill and stroke.

So I think, we should follow the same approach in GeoStyler and get rid of the opacity property in BaseSymbolizer (https://github.com/geostyler/geostyler-style/blob/master/style.ts#L126). The opacity value is a result of the early stages of geostyler-style, where an overall opacity property was defined for all symbolizers. Later, we added the more precise properties.

This change would probably affect multiple repositories, including https://github.com/geostyler/geostyler, where we currently provide 3 input fields for specifying the opacity of a fillSymbolizer Fill-Opacity, Opacity, Stroke-Opacity - with Opacity apparently having no impact on the style.

It would also be great to get the opinions of at least @dnlkoch @KaiVolland @chrismayer.

Some related issues:

• Still some problems with opacity handling in Symbolizer Editor #1359
• Parser does not recognize 'fill-opacity' parameter for PointSymbolizer #915

[KaiVolland]

The opacity is more or less the overall opacity of the Symbolizer. It can be an additonal opacity when used in combination with fill-opacity and stroke-opacity. Even if it is not the way SLD handles it, i can imagine styles (e.g. OL) where it make sense to combine all those three values.

[jansule]

Do you see the overall opacity as just an UI tweak for more convenience, or as an actual value for the style definition?

How would you handle setting values in parsers, where there is no overall opacity e.g. SLD?

[KaiVolland]

We may just add it to the unsupportedProperties.

[rdewit]

I can confirm that the fill-opacity vs opacity issue within the openlayers parser has been fixed by geostyler/geostyler-openlayers-parser#757. Thanks, @KaiVolland !