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QGIS Enhancement: Opaque Layergroups in QGIS-Server

Date 2025/08/28

Author Oliver Jeker (@ojeker) / Dave Signer (@signedav)

Contact oliver dot jeker at bd dot so dot ch / david at opengis dot ch

Version QGIS 4.0 or 4.2

Summary

We would like to extend QGIS with the capability to serve opaque layergroups through OGC WMS. Opaque layergroups hide all their children, grandchildren, ... (named opaque childlayers in this QEP) in the layertree. In GeoServer it's called an opaque container, while in QGIS we would use "opaque" as the descriptive word as well and keep layer groups to follow QGIS-style wording.

Our motivation is to hide implementation details from the user. One usage example for an opaque layergroup is a detailed road_polygon dataset and an overview road_line dataset. The point and polygon layer of these two datasets are contained in the opaque layergroup "Roads".

roads

Intended behaviour

Most behaviour of an opaque layergroup is the same as in the "normal" already existing layergroup. This chapter explains the differences in behaviour between opaque and normal layergroups for all WMS operations.

Differing behaviour for

  • GetCapabilities
    Opaque layergroups hide all their opaque childlayers in the GetCapabilities response. Only the opaque layergroup itself is returned and looks like a "singlesource" layer (QgsVectorLayer, QgsRasterLayer) in the layer tree.

    <Layer queryable="1">
<Name>Roads</Name>
<Abstract>This is an opaque layergroup</Abstract>
<CRS>CRS:84</CRS>
<CRS>EPSG:4326</CRS>
<CRS>EPSG:3857</CRS>
<EX_GeographicBoundingBox>[...]</EX_GeographicBoundingBox>
<BoundingBox CRS="EPSG:3857" maxx="930992.313" maxy="5947431" minx="930871" miny="5947339"/>
<BoundingBox CRS="EPSG:4326" maxx="47.03281" maxy="8.363247" minx="47.032245" miny="8.362156"/>
<Style>[...]</Style>
</Layer>

    Without any <Layer> elements in this object.

  • GetContext
    Opaque childlayers are not listed.

  • GetProjectSettings
    Expose the new property in the group setting information.

    <Layer expanded="1" mutuallyExclusive="0" queryable="1" visibilityChecked="1" visible="1">
    <Name>Roads</Name>
    [...]
    <IsOpaque>1</IsOpaque>
    [...]

  • Requests on children (like GetMap, GetFeatureInfo, GetStyle(s), GetLegendGraphic(s), DescribeLayer)
    Requests containing a opaque childlayers in the "layers" parameter get the same error response as when requesting a layer that does not exist in the layer tree.

    <ServiceExceptionReport xmlns="http://www.opengis.net/ogc" version="1.3.0">
    <ServiceException code="LayerNotDefined">The layer 'RoadsInOpaqueGroup' does not exist.</ServiceException>
</ServiceExceptionReport>

    What is OGC_LayerNotDefined in source code.

Is "Layer does not exist" always the preferred feedback?

There is a different behavior on requesting a non-existent layer or an excluded layer.

Request Non-Existent Excluded
GetMap OGC_LayerNotDefined Not displayed (white canvas)
GetFeatureInfo OGC_LayerNotDefined OGC_LayerNotDefined
GetStyle / GetStyles OGC_LayerNotDefined Empty reply
GetLegendGraphic OGC_LayerNotDefined OGC_LayerNotDefined

We propose to handele the opaque childlayers same like the non-existent layer and return OGC_LayerNotDefined.

Same behaviour for

Normal layergroups and opaque layergroups on:

  • GetMap request on the opaque layergroup
    Requests containing opaque layergroups in the "layers" parameter are rendered the same way as normal layergroups.
  • GetFeatureInfo
  • GetLegendGraphic
  • etc.

Proposed Solution

Add a checkbox or dropdown to the form "Set Group WMS Data" to choose if the group is opaque or not.

group_wms_data.png

(As well we propose to change the dialog title to "Set Group WMS Properties")

A new member isOpaque will be added to the QgsMapLayerServerProperties of QgsLayerTreeGroup store this configuration.

In the server classes it can be handled similar to the wmsRestrictedLayers (the excluded layers), but with the difference, that they are not hidden on everything (e.g. rendered on GetMap requests of the group).

On QgsWmsRenderContext::initOpaqueGroupLayers() it checks the isOpaque on the groups' server properties and fills up the QStringList mOpaqueChildLayers. They should not be considered in removeUnwantedLayers, because it still should be rendered.

On a request on one of the opaque childlayers it should check QgsWmsParameter::LAYER against mOpaqueChildLayers and throw the OGC_LayerNotDefined error "The layer '%1' does not exist."

On GetCapabilties it should return in functions like appendLayersFromTreeGroup and be detected for removing in appendDrawingOrder etc.

Preferred behavior on duplicate layer names / short name in the layer tree

QGIS Server currently merges layers with identical names (or short names). The opaque layergroup proposed in this QEP allows for more control over grouping and thus will have higher priority. If doable without adding too much complexity, our targetted behavior will be:

  • When requesting the layer name (or short name), only the layer in the non-opaque part of the layer tree must be rendered.
  • When requesting the opaque layergroup, only the child of the opaque layergroup must be rendered, and not merged with another same named layer outside the opaque layergroup. But when requesting a opaque layergroup having the same name as another layer outside an opaque layergroup, the opaque layergroup should be merged with the same named layer.

But e.g. when having a opaque group "roads":

- roads
  |- roadline
  |- roadpoly
- roadline

When requesting "roads" only the first roadline is rendered. And when requesting roadline only the second roadline is rendered.

A layername will not be added to the mOpaqueGroupLayers, when there is a identical named layer outside an opaque layergroup. On requesting the layer outside the opaque layergroup it should care in QgsWmsRenderContext::searchLayersToRender() that it should not be "merged" with the identical named layer inside an opaque layergroup.