[mapguide-users] Resample or Clip expression for WMS connection

[Jackie Ng]

Actually you're not too far off.

You use the expression capabilities as a programmatic way to answer the
question of "What functions I can use for expressions against this
particular FDO provider?" by checking if your function of interest (CLIP)
is in the collection of supported functions.

If you know in your code that the feature source in question is always
pointing to a WMS connection, then you know (from the provider
documentation) that CLIP() is supported and you can bypass this capability
check. Otherwise, you would use the various capabilities from the root
IConnection to ask questions of what this connection can or cannot do.

As for what to do with the command. your CreateCommand call is close, but
you are actually supposed to cast that ICommand down to a derived command
interface. The ICommand interface itself is pretty barebones and not
something you'll ever use directly. The CommandType enum you're passing in
is the hint as to what command interface you're actually supposed to cast
to.

CommandType.CommandType_Select -> cast the result to ISelect
CommandType.CommandType_DescribeSchema -> cast the result to IDescribeSchema
CommandType.CommandType_SelectAggregates -> cast the result to
ISelectAggregates

etc, etc.

Here's a snippet adapted from the "FDO Cookbook - Select Data" example from
that devguide link that gives a rough idea of what you're supposed to do
once you've created a select command:

// The MgLayerBase has the name of the feature class
Identifier className = new Identifier(lyr.GetFeatureClassName());
// Make the select command
ISelect select = conn.CreateCommand(CommandType.CommandType_Select) as
ISelect;
// Set the name of the feature class this select command is querying from
select.FeatureClassName = className;
// The MgLayerBase also knows the name of the designated geometry/raster
property for the layer, which you need to know for the first argument of
CLIP()
// note the name "clipped_raster", which is our alias for this computed
property
ComputedIdentifier computedId =
Expression.Parse($"CLIP({lyr.GetFeatureGeometryName()}, {minX}, {minY},
{maxX}, {maxY}) AS clipped_raster") as ComputedIdentifier;
// Add this computed property to the select command
select.PropertyNames.Add(computedId);
// Execute the command
IFeatureReader reader = select.Execute();
// You need to advance the reader in order to get data out of it
try {
    while (reader.ReadNext()) {
        // Access the raster by the computed alias name
        var raster = reader.GetRaster("clipped_raster");
        // TODO: Do something with the raster data
    }
}
finally {
    // Always close the feature readers afterwards when done. Putting this
in the finally section of
    // a try/finally block ensures this always happens (regardless of
exceptions thrown in the try block)
    reader.Close();
}


You'll need to provide your own minX, minY, maxX, maxY values for the
CLIP() function call.

Hope that helps.

- Jackie

You wrote:

I am using Autodesk Civil 3D/Map 3D (contains a MapGuide component) and I
need to create a custom command to clip or resample raster image via WMS
connection. I am a C# developer.

I see the appropriate expression capabilities listed here, but I cannot
find any examples how to apply these expressions to the WMS connection.

FDO Developer's Guide: FDO Provider for WMS Capabilities (autodesk.com)
<http://docs.autodesk.com/MAP/2014/ENU/Developer_Guides/index.html>



Can anyone provide an example on how to use expressions? This is what I
have so far, but don’t feel like I’m close.



*using* Autodesk.Gis.Map.Platform;

*using* OSGeo.FDO.Commands;

*using* OSGeo.FDO.Connections;

*using* OSGeo.FDO.Connections.Capabilities;

*using* OSGeo.FDO.Expression;

*using* OSGeo.MapGuide;



*namespace* MapServices

{

    *public* *class* WmsLayer

    {

        *public* *static* *void* ClipMapLayer(*string* layerName)

        {

            AcMapFeatureService fs = AcMapServiceFactory.GetService(
MgServiceType.FeatureService) *as* AcMapFeatureService;



            //Get the current Map.

            AcMapMap currentMap = AcMapMap.GetCurrentMap();



            //Get the collecton of Map 3D layers

            MgLayerCollection layers = currentMap.GetLayers();

            *if* (layers.Contains(layerName)) //Make sure the specified Map
3D layer exists

            {

                //Get the specified Map 3D layer with the Raster image.

                MgLayerBase lyr = layers.GetItem(layerName);



                //Get the resource identifier

                MgResourceIdentifier identifier = *new* MgResourceIdentifier
(lyr.FeatureSourceId);



                //Get the FDO connection from the resource

                IConnection con = fs.GetFdoConnection(identifier);



                //I can get the Expression capabilities from the
connection, but what do I do with that??

                IExpressionCapabilities expCaps = con.ExpressionCapabilities
;

                ExpressionType[] types = expCaps.ExpressionTypes;

                FunctionDefinitionCollection funcs = expCaps.Functions;



                //Is this how to create the expression?

                //How do I pass the inputs into the expression?

                //How is a BLOB created from the raster?

                Expression wmsCmd = Expression.Parse("CLIP(raster, minX,
minY, maxX, maxY)");



                //Do I somehow pass the expression into an ICommand???  No
idea what command type is correct.

                ICommand cmd = con.CreateCommand(CommandType.
CommandType_Select);

            }

        }

    }

}



Thank you for your assistance!





*Keith Sowinski, P.E.*

Civil 3D Implementation Engineer

Methods Development Unit

WisDOT Bureau of Project Development

Office: (920) 492-4132

wisconsindot.gov




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