Thank you Jackie! This is very helpful.
Now I’m getting and Exception when executing the ISelect. I think it is
because lyr.GetFeatureGeometryName() returns an empty string.
[cid:image002.png@01D99F56.364B5210]
From: mapguide-users <mapguide-users-boun...@lists.osgeo.org> On Behalf Of
Jackie Ng
Sent: Wednesday, June 14, 2023 6:45 AM
To: mapguide-users@lists.osgeo.org
Subject: Re: [mapguide-users] Resample or Clip expression for WMS connection
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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<http://wisconsindot.gov/>
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