MS RFC 101: Add Support for Content Dependent Legend Responses

Author:Thomas Bonfort
Version:MapServer 6.4

1. Summary

The WMS GetLegendGraphic currently returns a legend image whose content matches the classes that are defined in the mapfile (which may also be overridden by an SLD), as specified by the WMS specification. This RFC proposes an extension to the GetLegendGraphic response by returning a legend that corresponds to the features that are actually present/rendered in the requested extent.

The use-case of this RFC is to be able to display a legend alongside a mapview, with the content of the legend that only matches features that are actually present in the mapview.

1.1 Example

Let us suppose we have a legend for a road network, comprising three classes for motorways, major roads and minor roads.

  • When fully zoomed out, a GetLegendGraphic call will return an image containing the 3 road legend icons, one for each class.
  • When zooming into a remote rural area (i.e. with only small roads), a GetLegendGraphic call will return an image with a single road icon, for minor roads.
  • When zooming into the ocean, a GetLegendGraphic will return an image with no icons for road features.

2. Implementation Details

2.1 Activating the feature

Support for content dependent legends will be supported for WMS GetLegendGraphic calls on POINT, LINE and POLYGON layers, when the client includes the (non-standard) BBOX, SRS/CRS and WIDTH/HEIGHT parameters (i.e. like for a GetMap request) in the request URL. If these parameters are not present, the classical legend rendering will occur.


2.2 Code Impacts

  • Inside msWMSDispatch(), if the BBOX parameter is detected during a GetLegendGraphic request, then the processing will skip the current GetLegendGraphic codepath, and instead pass through the initialization phases of GetMap requests (setting up layer groups, applying SLDs, etc...)
  • Once the GetMap initialization phases have occurred without error, go through msHitTestMap() instead of msDrawMap(). This new function mimics msDrawMap in the sense that it loops through active layers and calls msLayerWhichShapes/msLayerNextShape on them, but instead of rendering the features it marks their LAYER/CLASS/STYLE/LABEL as being activated or not by the map draw.
  • msDrawLegend/msDrawLegenIcon get passed a new parameter that corresponds to the hit-test result and will draw the map’s legend icons or not depending if they are activated in the hit-test or not.
  • mode=maplegend and mode=maplegendicon are added to the traditional mapserver cgi. The mapserverObj struct gets an added map_hittest member that is used by the legend templating code to produce content dependent html legends.

2.3 Quirks and Backwards Incompatibility Issues

  • Clients that send BBOX, SRS and WIDTH/HEIGHT parameters expecting to obtain a non content dependent legend will be returned erroneous results. It can be argued that these clients should not be sending out-of-spec parameters in the first place.
  • MapServer currently treats WIDTH and HEIGHT as the size to use for the legend icons. For content dependent legends these two parameters are used along with BBOX to compute the scale at which the current mapview is being rendered, and not for the size of the returned legend icons. While other parameter names might have been chosen to avoid this overlap, the implementation sticks with WIDTH and HEIGHT as on the one hand they permit an implementation that reuses the current GetMap codepaths, and on the second hand these are the parameters that are already being used by the (only?) other WMS implementation to support this extension, namely Cubewerx.
  • RULE is ignored for content-dependant legend requests
  • GetLegendGraphic uses a LAYER= parameter to specify which layer to use (MapServer has the extension to support multiple layers, by using LAYER=layer1,layer2), whereas GetMap requests use the plural LAYERS= . For content dependent GetLegendGraphic calls the code will duplicate the LAYER parameter into the LAYERS parameter in order to use the GetMap codepaths. The GetLegendGraphic response will be undefined if the client has included a LAYERS parameter in the URL that does not coincide with the LAYER parameter.

2.4 Performance impacts, mapscripts

  • No performance impacts (aside from the fact that content dependent legend rendering will be obviously slower than traditional legend rendering)
  • No user visible mapscript impacts (the mapscript code itself will need a few tweaks when calling the msDrawLegend() and msDrawLegenIcon() functions.
  • The result of a hittest will not be available to mapscripts. Work in this direction may be accomplished in the longer run once the APIs and hittest structs have stabilized.

3. Miscellaneous

3.2 Voting History

+1 from ThomasB, MikeS, TomK, StephanM, PerryN, SteveL, SteveW and JeffM