WMS maps are not displayed

[gpsaliola]

Mapsui Version
5.0.2

Device
Android version 16

Describe the bug
WMS maps are not displayed.

This is the code I've used. What's wrong? Thank you.

                    WmsProvider? lProvider = null;
                    XmlDocument xml;
                    try
                    {
                        using HttpClient http = new HttpClient();

                        // Set provider properties
                        if (!string.IsNullOrEmpty(mReferer))
                            http.DefaultRequestHeaders.Referrer = new Uri(mReferer);
                        http.DefaultRequestHeaders.UserAgent.ParseAdd(AppGeneral.cHttpValueUserAgent);

                        // Load xml document
                        string wmsXmlString = await http.GetStringAsync(lUrlWMS);
                        xml = new XmlDocument();
                        xml.LoadXml(wmsXmlString);

                        // Create provider
                        lProvider = new WmsProvider(xml, async (uri) =>
                        {
                            using HttpRequestMessage? req = new HttpRequestMessage(HttpMethod.Get, uri);
                            if (!string.IsNullOrEmpty(mReferer))
                                req.Headers.Referrer = new Uri(mReferer);

                            using var resp = await http.SendAsync(req);
                            resp.EnsureSuccessStatusCode();
                            return await resp.Content.ReadAsByteArrayAsync();
                        });
                    }
                    catch (Exception ex)
                    {
                        throw new Exception(ex.Message + AppGeneral.cCarriageReturn + ( ex.InnerException == null ? "" : ex.InnerException.Message));
                    }

                    // Set provider properties
                    lProvider.ContinueOnError = true;
                    lProvider.TimeOut = cTimeOut;
                    lProvider.UserAgent = AppGeneral.cHttpValueUserAgent;

                    // Set CRS
                    string lCrs;
                    if (lProvider.IsCrsSupported("EPSG:3857") == true) lCrs = "EPSG:3857";
                    else if (lProvider.IsCrsSupported("EPSG:4326") == true) lCrs = "EPSG:4326";
                    else if (lProvider.IsCrsSupported("EPSG:6706") == true) lCrs = "EPSG:6706";
                    else throw new Exception(AppTexts.EPSGNotSupported);
                    lProvider.CRS = lCrs;

                    // Set layers
                    string[] lLayers = layer.Split(Convert.ToChar(","));
                    foreach (string lLayer in lLayers)
                    {
                        lProvider.AddLayer(lLayer);
                    }

                    // Set mime type
                    string lMimeType;
                    if (lProvider.OutputFormats.Contains("image/png")) lMimeType = "image/png";
                    else if (lProvider.OutputFormats.Contains("image/jpeg")) lMimeType = "image/jpeg";
                    else if (lProvider.OutputFormats.Contains("image/gif")) lMimeType = "image/gif";
                    else lMimeType = lProvider.OutputFormats[0];
                    lProvider.SetImageFormat(lMimeType);

                    // Return layer
                    ImageLayer imageLayer = new ImageLayer(AppGeneral.cLayerWebMap)
                    {
                        DataSource = lProvider,
                        Style = new RasterStyle(),
                    };
                    return imageLayer;

[gpsaliola]

It seems like an old issue with the Xamarin version.
With this map:
https://sgi2.isprambiente.it/arcgis/services/raster/igm25k_lazio_wgs/ImageServer/WMSServer?request=getcapabilities&service=wms
I think there's an Extent issue with the layer (they're reported in decimal degrees and not in projected x,y coordinates).
Could this be the cause?
In the Xamarin version, I adopted this workaround:

if (imageLayer.Extent == null)
{
    // Create the positions
    var minPosition = new Mapsui.UI.Maui.Position(AppGeneral.cWebMercatorMinLat, AppGeneral.cWebMercatorMinLon);
    var maxPosition = new Mapsui.UI.Maui.Position(AppGeneral.cWebMercatorMaxLat, AppGeneral.cWebMercatorMaxLon);
    
    // Convert positions to MPoints using ToMapsui()
    var minPoint = minPosition.ToMapsui();
    var maxPoint = maxPosition.ToMapsui();
    
    // Replace the actual WMS bounds
    imageLayer.SetExtent(new MRect(minPoint.X, minPoint.Y, maxPoint.X, maxPoint.Y));
}

and I created a custom class like this:

    public class CustomImageLayer : ImageLayer
    {
        // Classe ImageLayer custom per poter modificare la proprietà Extent
        public CustomImageLayer(string name) : base(name) { }

        public void SetExtent(MRect extent)
        {
            Extent = extent;
        }
    }

but now in Maui it doesn't seem to work.
Any help would be greatly appreciated.

[pauldendulk]

It is possible that something related has changed within the ImageLayer. The Extent is now set in the DataSource setters, so you could need to call SetExtent after you assign the DataSource.

To investigate this issue you could include the entire ImageLayer into your own code and step through it.

The WMS has projection specific bounding boxes. Perhaps the problem is cause by "EPSG:0" which should probably be "EPSG:3857"

Did you try setting EPSG:0 on the CRS of the WmsProvider? Here it tries to fetch the corresponding boundingbox from the capabilities.

    public string? CRS { get; set; }

    public string? UserAgent { get; set; }

    public MRect? GetExtent()
    {
        if (CRS != null && _wmsClient != null && _wmsClient.Layer.BoundingBoxes.TryGetValue(CRS, out var value))
        {
            if (AxisOrder.IsNaturalOrder())
            {
                return value;
            }

            // change x with y
            var temp = value;
            return new MRect(temp.MinY, temp.MinX, temp.MaxY, temp.MaxX);

        }

        return null;
    }

[gpsaliola]

Thanks for your reply.
If I use EPSG:0 in this map (https://sgi2.isprambiente.it/arcgis/services/raster/igm25k_lazio_wgs/ImageServer/WMSServer?request=getcapabilities&service=wms), the Extent property is correctly set and the limits are expressed in projected coordinates (x, y).
However, the map still isn't displayed.
In this case IsNaturalOrder is true.

There are other maps (for example, https://geoportale.regione.lazio.it/geoserver/ows?request=getcapabilities&service=wms) that display correctly only if I manually set the Extent at world bounds:

if (imageLayer.Extent == null)
{
// Create the positions
var minPosition = new Mapsui.UI.Maui.Position(AppGeneral.cWebMercatorMinLat, AppGeneral.cWebMercatorMinLon);
var maxPosition = new Mapsui.UI.Maui.Position(AppGeneral.cWebMercatorMaxLat, AppGeneral.cWebMercatorMaxLon);

// Convert the positions to MPoints using ToMapsui()
var minPoint = minPosition.ToMapsui();
var maxPoint = maxPosition.ToMapsui();

// Replace the actual WMS bounds 
imageLayer.SetExtent(new MRect(minPoint.X, minPoint.Y, maxPoint.X, maxPoint.Y)); 
}

[pauldendulk]

btw, you do not really need a extent if some other layer in your project has a bigger extent. What if you set it to null?

In this case IsNaturalOrder is true.

Is that different in the other cases?

[pauldendulk]

Another possible workaround is to download the capabilities, fix the change the EPSG:0 into EPSG:3857 and then create and XmlDocument from that local file and feed it to the WmsProvider.

[gpsaliola]

I ran some tests with 60 WMS maps.
The CRSs mentioned below are all supported by the tested maps.
EPSG:3857 => This doesn't always work. Also, sometimes Extent isn't set, and in that case the map isn't displayed. Setting it manually only fixes the map display in some cases.
EPSG:4326 => This is a geographic CRS (not projected, and therefore Extent is expressed in geographic coordinates). The map is displayed, but the coordinates and zoom factor are incorrect, and the map appears distorted.
CRS:84 => This is like CRS:4326, but the coordinate order is reversed (lon/lat). The map isn't displayed.
EPSG:0 => This isn't an official CRS, and even if selected, the map isn't displayed. Replacing it with EPSG:3857 doesn't work either.
EPSG:900913 => This is an old representation of EPSG:3857; the map isn't displayed. Even replacing it with EPSG:3857 doesn't work.
Important! Layers containing special characters (e.g., ":") aren't recognized.
It would still be helpful to have a List<string> SupportedCRS property containing all supported CRSs.
I hope I've answered your previous questions.
I'm attaching an Excel file with my tests.
The green color indicates that the map is displayed, unlike the red color.

Maps servers list.xlsx

I hope it helps.
Thank you.