[processing] improve rasterize dialog UI, add help (#5008)

python/plugins/processing/algs/help/qgis.yaml

@@ -473,6 +473,17 @@ qgis:rasterlayerhistogram: >
 qgis:rasterlayerstatistics: >
   This algorithm computes basic statistics from the values in a given band of the raster layer.
 
+qgis:rasterize: >
+  This algorithm rasterizes map canvas content.
+
+  A map theme can be selected to render a predetermined set of layers with a defined style for each layer.
+
+  Alternatively, a single layer can be selected if not map theme is set.
+
+  If neither map theme nor the layer is set, the current map content will be rendered.
+
+  The minimum extent entered will internally be extended to be a multiple of the tile size.
+
 qgis:refactorfields: >
   This algorithm allows editing the structure of the attributes table of a vector layer. Fields can be modified in their type and name, using a fields mapping.
 

python/plugins/processing/algs/qgis/Rasterize.py

@@ -84,31 +84,10 @@ class RasterizeAlgorithm(QgisAlgorithm):
         with some other properties.
         """
         # The parameters
-        map_theme_param = QgsProcessingParameterString(
-            self.MAP_THEME,
-            description=self.tr(
-                'Map theme to render.'),
-            defaultValue=None, optional=True)
-
-        map_theme_param.setMetadata(
-            {'widget_wrapper': {
-                'class':
-                    'processing.gui.wrappers_map_theme.MapThemeWrapper'}})
-        self.addParameter(map_theme_param)
-
-        self.addParameter(
-            QgsProcessingParameterMapLayer(
-                self.LAYER,
-                description=self.tr(
-                    'Layer to render. Will only be used if the map theme '
-                    'is not set. '
-                    'If both, map theme and layer are not '
-                    'set, the current map content will be rendered.'),
-                optional=True))
         self.addParameter(
             QgsProcessingParameterExtent(self.EXTENT, description=self.tr(
-                'The minimum extent to render. Will internally be extended to '
+                'Minimum extent to render')))
-                'be a multiple of the tile sizes.')))
+
         self.addParameter(
             QgsProcessingParameterNumber(
                 self.TILE_SIZE,
@@ -125,6 +104,25 @@ class RasterizeAlgorithm(QgisAlgorithm):
             type=QgsProcessingParameterNumber.Double
         ))
 
+        map_theme_param = QgsProcessingParameterString(
+            self.MAP_THEME,
+            description=self.tr(
+                'Map theme to render'),
+            defaultValue=None, optional=True)
+
+        map_theme_param.setMetadata(
+            {'widget_wrapper': {
+                'class':
+                    'processing.gui.wrappers_map_theme.MapThemeWrapper'}})
+        self.addParameter(map_theme_param)
+
+        self.addParameter(
+            QgsProcessingParameterMapLayer(
+                self.LAYER,
+                description=self.tr(
+                    'Single layer to render'),
+                optional=True))
+
         # We add a raster layer as output
         self.addParameter(QgsProcessingParameterRasterDestination(
             self.OUTPUT,
@@ -133,7 +131,7 @@ class RasterizeAlgorithm(QgisAlgorithm):
 
     def name(self):
         # Unique (non-user visible) name of algorithm
-        return 'Rasterize'
+        return 'rasterize'
 
     def displayName(self):
         # The name that the user will see in the toolbox