VRTTI: also support setting the metadata as a XML attached to the xml…

…:VRTTI metadata domain of a vector layer

autotest/gdrivers/vrttileindex.py

@@ -2553,3 +2553,46 @@ def test_vrttileindex_open_options(tmp_vsimem):
     assert vrt_ds.GetGeoTransform() == pytest.approx(
         (440720.0, 30.0, 0.0, 3751320.0, 0.0, -30.0)
     )
+
+
+def test_vrttileindex_xml_vrtti_embedded(tmp_vsimem):
+
+    index_filename = str(tmp_vsimem / "index.vrt.gpkg")
+
+    src_ds = gdal.Open(os.path.join(os.getcwd(), "data", "byte.tif"))
+    index_ds, lyr = create_basic_tileindex(index_filename, src_ds)
+
+    xml_content = """<VRTTileIndexDataset>
+  <ResX>60</ResX>
+  <ResY>60</ResY>
+  <SortField>location</SortField>
+  <SortFieldAsc>true</SortFieldAsc>
+  <Band band="1" dataType="UInt16">
+      <Description>my band</Description>
+      <Offset>2</Offset>
+      <Scale>3</Scale>
+      <NoDataValue>4</NoDataValue>
+      <UnitType>dn</UnitType>
+      <ColorInterp>Gray</ColorInterp>
+      <ColorTable/>
+      <CategoryNames><Category>cat</Category></CategoryNames>
+      <GDALRasterAttributeTable/>
+  </Band>
+</VRTTileIndexDataset>"""
+
+    lyr.SetMetadata([xml_content], "xml:VRTTI")
+    del index_ds
+
+    vrt_ds = gdal.Open(index_filename)
+    band = vrt_ds.GetRasterBand(1)
+    assert band.GetDescription() == "my band"
+    assert band.DataType == gdal.GDT_UInt16
+    assert band.GetOffset() == 2
+    assert band.GetScale() == 3
+    assert band.GetNoDataValue() == 4
+    assert band.GetUnitType() == "dn"
+    assert band.GetColorInterpretation() == gdal.GCI_GrayIndex
+    assert band.GetColorTable() is not None
+    assert band.GetCategoryNames() == ["cat"]
+    assert band.GetDefaultRAT() is not None
+    del vrt_ds

data/gdalvrtti.xsd

@@ -34,7 +34,9 @@
         <xs:complexType>
             <xs:sequence>
                 <xs:choice minOccurs="0" maxOccurs="unbounded">
-                    <xs:element name="IndexDataset" type="xs:string" minOccurs="1"/>
+                    <!-- IndexDataset is required for a standalone XML VRTTI -->
+                    <!-- It is ignored if the XML document is embedded in the xml:VRTTI metadata domain of a vector layer -->
+                    <xs:element name="IndexDataset" type="xs:string"/>
 
                     <xs:element name="IndexLayer" type="xs:string"/>
                     <xs:element name="Filter" type="xs:string"/>

doc/source/drivers/raster/vrtti.rst

@@ -223,6 +223,11 @@ In addition to those layer metadata items, the dataset-level metadata item
 ``TILE_INDEX_LAYER`` may be set to indicate, for dataset with multiple layers,
 which one should be used as the tile index layer.
 
+Alternatively to setting those metadata items individually, the corresponding
+information can be grouped together in a VRTTI XML document, attached in the
+``xml:VRTTI`` metadata domain of the layer (for drivers that support alternate
+metadata domains such as GeoPackage)
+
 VRTTI XML format
 ----------------
 
@@ -236,7 +241,7 @@ mentioned in the previous section.
 .. code-block:: xml
 
     <VRTTileIndexDataset>
-        <IndexDataset>PG:dbname=my_db</IndexDataset>   <!-- required -->
+        <IndexDataset>PG:dbname=my_db</IndexDataset>   <!-- required for stanalone XML VRTTI files. Ignored if embedded in the xml:VRTTI metadata domain of the layer  -->
         <IndexLayer>my_layer</IndexLayer>              <!-- optional, but required if there are multiple layers in IndexDataset -->
         <Filter>pub_date >= '2023/12/01'</Filter>      <!-- optional -->
         <SortField>pub_date</SortField>                <!-- optional -->

frmts/vrt/vrttileindexdataset.cpp

@@ -572,6 +572,26 @@ bool VRTTileIndexDataset::Open(GDALOpenInfo *poOpenInfo)
         return false;
     }
 
+    // Try to get the metadata from an embedded xml:VRTTI domain
+    if (!m_psXMLTree)
+    {
+        char **papszMD = m_poLayer->GetMetadata("xml:VRTTI");
+        if (papszMD && papszMD[0])
+        {
+            m_psXMLTree.reset(CPLParseXMLString(papszMD[0]));
+            if (m_psXMLTree == nullptr)
+                return false;
+
+            psRoot = CPLGetXMLNode(m_psXMLTree.get(), "=VRTTileIndexDataset");
+            if (psRoot == nullptr)
+            {
+                CPLError(CE_Failure, CPLE_AppDefined,
+                         "Missing VRTTileIndexDataset root element.");
+                return false;
+            }
+        }
+    }
+
     const auto GetOption = [poOpenInfo, psRoot, this](const char *pszItem)
     {
         if (psRoot)