Mapserver ANCHORPOINTS with SVG symbols, SVG output - What's going on?

map.svg

square.svg

svg.map

MAP
  NAME SVG_ICON_ANCHORPOINT_TEST
  STATUS ON
  SIZE 100 100
  EXTENT 0 0 0.001 0.001
  IMAGECOLOR 255 255 255

  IMAGETYPE svg

  CONFIG "MS_ERRORFILE" "stderr"

  SYMBOL
    NAME "square_0_0"
    TYPE svg
    IMAGE "./square.svg"
    ANCHORPOINT 0 0
  END
  SYMBOL
    NAME "square_05_05"
    TYPE svg
    IMAGE "./square.svg"
    ANCHORPOINT 0.5 0.5
  END
  SYMBOL
    NAME "square_1_1"
    TYPE svg
    IMAGE "./square.svg"
    ANCHORPOINT 1 1
  END
  SYMBOL
    NAME "DEBUG"
    TYPE ellipse
    FILLED true
    POINTS
      1 1
    END
  END

  LAYER
    TYPE POINT
    STATUS ON
    FEATURE
      POINTS
        0.0004 0.0005
      END
      TEXT "P"
    END
    CLASS
       STYLE
         SYMBOL "square_0_0"
       END
       STYLE
         COLOR 255 0 0
         SYMBOL "DEBUG"
       END
    END
  END
  LAYER
    TYPE POINT
    STATUS ON
    FEATURE
      POINTS
        0.0005 0.0005
      END
      TEXT "P"
    END
    CLASS
       STYLE
         SYMBOL "square_05_05"
       END
       STYLE
         COLOR 255 0 0
         SYMBOL "DEBUG"
       END
    END
  END
  LAYER
    TYPE POINT
    STATUS ON
    FEATURE
      POINTS
        0.0006 0.0005
      END
      TEXT "P"
    END
    CLASS
       STYLE
         SYMBOL "square_1_1"
       END
       STYLE
         COLOR 255 0 0
         SYMBOL "DEBUG"
       END
    END
  END

END

SVG symbols, SVG output, and Anchorpoints

Note the strange output produced by the above map file. It has:

• three layers, each containing a single inline feature
• uses svg output
• each layer has an svg symbol (square) with one of three anchorpoints
• additionally a circle symbol is used to mark the feature center

Note how the anchor points are not where they should be (see map.svg above). You have to zoom in to see properly.

I use this command to render the map:
shp2img -o map.svg -m svg.map

If I change the output type to png, I get the expected result, with the symbols anchored at the correct point:

It is possible that this is related to these issues, but I'm not sure:

MapServer/MapServer#5593
https://trac.osgeo.org/mapserver/ticket/4184

MapServer version 7.4.3 OUTPUT=PNG OUTPUT=JPEG OUTPUT=KML SUPPORTS=PROJ SUPPORTS=AGG SUPPORTS=FREETYPE SUPPORTS=CAIRO SUPPORTS=SVG_SYMBOLS SUPPORTS=RSVG SUPPORTS=ICONV SUPPORTS=FRIBIDI SUPPORTS=WMS_SERVER SUPPORTS=WMS_CLIENT SUPPORTS=WFS_SERVER SUPPORTS=WFS_CLIENT SUPPORTS=WCS_SERVER SUPPORTS=SOS_SERVER SUPPORTS=FASTCGI SUPPORTS=THREADS SUPPORTS=GEOS SUPPORTS=PBF INPUT=JPEG INPUT=POSTGIS INPUT=OGR INPUT=GDAL INPUT=SHAPEFILE

Also doesn't work with 7.6.2.

MapServer version 7.6.2 OUTPUT=PNG OUTPUT=JPEG SUPPORTS=PROJ SUPPORTS=AGG SUPPORTS=FREETYPE SUPPORTS=CAIRO SUPPORTS=SVG_SYMBOLS SUPPORTS=RSVG SUPPORTS=ICONV SUPPORTS=FRIBIDI SUPPORTS=WMS_SERVER SUPPORTS=WFS_SERVER SUPPORTS=WCS_SERVER SUPPORTS=FASTCGI SUPPORTS=GEOS SUPPORTS=POINT_Z_M INPUT=JPEG INPUT=POSTGIS INPUT=OGR INPUT=GDAL INPUT=SHAPEFILE

I found the issue! The SVG symbol (square.svg) does not have a width/height set. If the width/height is set, the symbol is correctly anchored at the set anchorpoints.

The reason the results are so strange seems to be related to the fractional maxx/maxy set in the viewbox: they are apparently being cast to ints! I verified this by adding a print statement in mapcairo.c::msPreloadSVGSymbol() [0]:

    rsvg_handle_get_dimensions_sub (cache->svgc, &dim, NULL);
    symbol->sizex = dim.width;
    symbol->sizey = dim.height;
    fprintf( stderr, "width: %d height: %d\n", dim.width, dim.height);

The dimensions returned by rsvg_handle_get_dimensions_sub(...) are always ints [1].

[0] https://github.com/MapServer/MapServer/blob/main/mapcairo.c#L1000
[1] https://github.com/ImageMagick/librsvg/blob/main/rsvg-base.c#L1458

Thanks for all of this research. Added to docs as a note (MapServer/MapServer-documentation#446)