How to make a raster layer transparent via MapServer WMS

Question

I loaded a PostGIS raster data through OpenLayers and MapServer.

The code inside the OpenLayers is as following :

localWMS = new OpenLayers.Layer.WMS( 
    "Local WMS layers",
    "http://127.0.0.1:8080/cgi-bin/mapserv.exe?MAP=C:/MapServer/raster.map",
    {
        layers: 'polygon',
        transparent: true,
        alpha:true
    },
    { 
        projection:new OpenLayers.Projection("EPSG:4326"),
        units:'m',
        isBaseLayer: false
    }
);

map.addLayer(localWMS);

And here is my map file, I already set opacity to each class of the layer but nothing happens:

LAYER
    DATA "PG:host=*** dbname=*** schema=public table=london where='rid=3' mode='1'"
    NAME "postgis"
    PROCESSING "SCALE=AUTO"
    PROCESSING "NODATA=-9999"
    STATUS DEFAULT
    TYPE RASTER
    UNITS METERS
    CLASS
      NAME "Minor Impact"
      EXPRESSION ([pixel] >= 0 AND [pixel] < 1.29525335532 )
      STYLE
        ANGLE 360
        OPACITY 50
        COLOR 122 182 245
        SYMBOL 0
        WIDTH 1
      END
    END
    CLASS
      NAME "Moderate Impact"
      EXPRESSION ([pixel] >= 1.29525335532 AND [pixel] < 2.59050671063 )
      STYLE
        ANGLE 360
        OPACITY 50
        COLOR 76 230 0
        SYMBOL 0
        WIDTH 1
      END
    END
    CLASS
      NAME "Severe Impact"
      EXPRESSION ([pixel] >= 2.59050671063 AND [pixel] < 5.69911476339 )
      STYLE
        ANGLE 360
        OPACITY 50
        COLOR 252 228 91
        SYMBOL 0
        WIDTH 1
      END
    END
    CLASS
      NAME "Catastrophic"
      EXPRESSION ([pixel] >=5.69911476339 )
      STYLE
        OPACITY 30
        COLOR 232 16 20
      END
    END
END

The map looks like following :

Then, I tried to set one of the opacity to less than 1, the red color just disappeared.

But if I set the Opacity of the Layer to 50

LAYER
    DATA "PG:host=*******************'"
    NAME "postgis"
    PROCESSING "SCALE=AUTO"
    PROCESSING "NODATA=-9999"
    STATUS DEFAULT
    TYPE RASTER
    OPACITY 50
    UNITS METERS

Even more strange map appeared.

Does anyone know what's the problem about that? Or Does anyone know the proper way to set a transparent raster layer via MapServer?

Answer 1

probably what you need is to set both OFFSITE and TRANSPARENCY in the layer section, something like this:

LAYER
NAME 'Raster'
TYPE RASTER
CONNECTIONTYPE postgis
CONNECTION "user=postgres password=... dbname=... host=..."
DATA "bytea FROM rasters"
STATUS OFF
OFFSITE 0 0 0
TRANSPARENCY 100
END

being OFFSITE the color you want to be transparent and TRANSPARENCY which controls the transparency level.

Hope this helps,

Answer 2

I set OPACITY in the mapfile for the entire layer (not for each style). Not sure what the OFFSITE does (I don't use that). Like this:

LAYER
DATA "PG:host=*** dbname=*** schema=public table=london where='rid=3' mode='1'"
NAME "postgis"
PROCESSING "SCALE=AUTO"
PROCESSING "NODATA=-9999"
STATUS DEFAULT
TYPE RASTER
OPACITY 50

UNITS METERS
CLASS
  NAME "Minor Impact"
  EXPRESSION ([pixel] >= 0 AND [pixel] < 1.29525335532 )
  STYLE
    ANGLE 360
    COLOR 122 182 245
    SYMBOL 0
    WIDTH 1
  END
END
...

Then in your OpenLayers (javascript) you set the layer to transparent:

 var mapParams = {
                  'layers': 'postgis',
                  'transparent': true
                   }
 var layer_wms = new OpenLayers.Layer.WMS(
                mapParams['layers'],
                serverURL,
                mapParams
            );
 map.addLayers([layer_wms]);