Merge pull request #143 from MIERUNE/fix/geographic-based-scale

scale correction for EPSG 3857

ui/main_dialog.py

@@ -22,7 +22,7 @@
 from translator.vector.label import generate_label_vector
 from ui.progress_dialog import ProgressDialog
 from translator.thread import ProcessingThread
-from utils import write_json, get_tempdir
+from utils import write_json, get_tempdir, get_scale
 
 
 class MainDialog(QDialog):
@@ -50,6 +50,11 @@ def init_ui(self):
             ]
         )
 
+        # calculate export scale and show to ui
+        self._update_ui_scale()
+        # update export scale shown in ui when change map extent
+        iface.mapCanvas().extentsChanged.connect(self._update_ui_scale)
+
         # close dialog when project cleared to avoid crash: Issue #55
         QgsProject.instance().cleared.connect(self.close)
 
@@ -138,7 +143,7 @@ def _run(self):
                 self.ui.mExtentGroupBox.outputExtent().xMaximum(),
                 self.ui.mExtentGroupBox.outputExtent().yMaximum(),
             ],
-            "scale": iface.mapCanvas().scale(),
+            "scale": get_scale(),
             "layers": layers_processed_successfully,  # layer_0,2,5..
             "assets_path": "assets",
         }
@@ -272,3 +277,6 @@ def _process_node_recursive(self, node, parent_node):
 
             if child_type == "group":
                 self._process_node_recursive(child, item)
+
+    def _update_ui_scale(self):
+        self.ui.label_scale_value.setText(str(get_scale()))

ui/main_dialog.ui

@@ -46,6 +46,31 @@
    <item>
     <widget class="QgsExtentGroupBox" name="mExtentGroupBox"/>
    </item>
+   <item>
+    <layout class="QHBoxLayout" name="export_scale">
+     <item>
+      <widget class="QLabel" name="label_scale">
+       <property name="text">
+        <string>出力縮尺</string>
+       </property>
+      </widget>
+     </item>
+     <item alignment="Qt::AlignRight">
+      <widget class="QLabel" name="label_scale_1">
+       <property name="text">
+        <string>1:</string>
+       </property>
+      </widget>
+     </item>
+     <item>
+      <widget class="QLabel" name="label_scale_value">
+       <property name="text">
+        <string>1</string>
+       </property>
+      </widget>
+     </item>
+    </layout>
+   </item>
    <item>
     <layout class="QHBoxLayout" name="horizontalLayout">
      <item>

utils.py

@@ -3,7 +3,16 @@
 from typing import Union
 
 
-from qgis.core import QgsRenderContext, QgsUnitTypes
+from qgis.core import (
+    QgsRenderContext,
+    QgsUnitTypes,
+    QgsProject,
+    QgsRectangle,
+    QgsPoint,
+    QgsScaleCalculator,
+    QgsCoordinateReferenceSystem,
+    QgsCoordinateTransform,
+)
 from qgis.utils import iface
 
 
@@ -52,3 +61,48 @@ def get_tempdir(output_dir: str) -> str:
         os.mkdir(os.path.join(output_dir, temp_dir_path))
 
     return os.path.join(output_dir, temp_dir_path)
+
+
+def get_scale() -> float:
+    """get scale from map canvas.
+    For web mercator projection (EPSG:3857) case,
+    calculate scale with map extent correction according to scale factor"""
+
+    if QgsProject.instance().crs().authid() == "EPSG:3857":
+        canvas = iface.mapCanvas()
+        # get map canvas center coordinates in geographic
+        transform = QgsCoordinateTransform(
+            canvas.mapSettings().destinationCrs(),
+            QgsCoordinateReferenceSystem("EPSG:4326"),
+            QgsProject.instance(),
+        )
+        center_geographic = transform.transform(canvas.center())
+        center_point = QgsPoint(center_geographic.x(), center_geographic.y())
+
+        # calculate scale_factor from center_point
+        # https://en.wikipedia.org/wiki/Mercator_projection#Scale_factor
+        scale_factor_x = (
+            QgsProject.instance().crs().factors(center_point).parallelScale()
+        )
+        scale_factor_y = (
+            QgsProject.instance().crs().factors(center_point).meridionalScale()
+        )
+
+        # determine extension corrected with scale factor
+        extent = canvas.extent()
+        delta_x = (extent.width() * scale_factor_x) - extent.width()
+        delta_y = (extent.height() * scale_factor_y) - extent.height()
+        corrected_extent = QgsRectangle(
+            extent.xMinimum() - delta_x / 2,
+            extent.yMinimum() - delta_y / 2,
+            extent.xMaximum() + delta_x / 2,
+            extent.yMaximum() + delta_y / 2,
+        )
+
+        # calculate scale based on corrected map extent
+        scale_calculator = QgsScaleCalculator(
+            canvas.mapSettings().outputDpi(), canvas.mapUnits()
+        )
+        return scale_calculator.calculate(corrected_extent, canvas.size().width())
+    else:
+        return iface.mapCanvas().scale()