reworking the zoom behaviour

labelImg.py

@@ -181,6 +181,7 @@ class MainWindow(QMainWindow, WindowMixin):
             Qt.Vertical: scroll.verticalScrollBar(),
             Qt.Horizontal: scroll.horizontalScrollBar()
         }
+        self.scrollArea = scroll
         self.canvas.scrollRequest.connect(self.scrollRequest)
 
         self.canvas.newShape.connect(self.newShape)
@@ -805,10 +806,56 @@ class MainWindow(QMainWindow, WindowMixin):
         self.setZoom(self.zoomWidget.value() + increment)
 
     def zoomRequest(self, delta):
+        # get the current scrollbar positions
+        # calculate the percentages ~ coordinates
+        h_bar = self.scrollBars[Qt.Horizontal]
+        v_bar = self.scrollBars[Qt.Vertical]
+
+        # get the current maximum, to know the difference after zooming
+        h_bar_max = h_bar.maximum()
+        v_bar_max = v_bar.maximum()
+
+        # get the cursor position and canvas size
+        # calculate the desired movement from 0 to 1
+        # where 0 = move left
+        #       1 = move right
+        # up and down analogous
+        cursor = QCursor()
+        pos = cursor.pos()
+
+        cursor_x = pos.x()
+        cursor_y = pos.y()
+
+        w = self.canvas.width()
+        h = self.canvas.height()
+
+        w = self.scrollArea.width()
+        h = self.scrollArea.height()
+
+        print("current: ")
+        print(cursor_x, cursor_y)
+        print(w, h)
+        print()
+
+        margin = 0.1
+        move_x = (cursor_x - margin * w) / (w - 2 * margin * w)
+        move_y = (cursor_y - margin * h) / (h - 2 * margin * h)
+
+        # zoom in
         units = delta / (8 * 15)
         scale = 10
         self.addZoom(scale * units)
 
+        d_h_bar_max = h_bar.maximum() - h_bar_max
+        d_v_bar_max = v_bar.maximum() - v_bar_max
+
+        # get the new scrollbar values
+        new_h_bar_value = h_bar.value() + move_x * d_h_bar_max
+        new_v_bar_value = v_bar.value() + move_y * d_v_bar_max
+
+        h_bar.setValue(new_h_bar_value)
+        v_bar.setValue(new_v_bar_value)
+
     def setFitWindow(self, value=True):
         if value:
             self.actions.fitWidth.setChecked(False)