move clip_altitudes() to base RenderEngine

src/c3nav/mapdata/render/engines/base.py

@@ -2,6 +2,8 @@ import math
 from abc import ABC, abstractmethod
 from typing import Optional
 
+from shapely.ops import unary_union
+
 
 class FillAttribs:
     __slots__ = ('color', 'opacity')
@@ -42,11 +44,26 @@ class RenderEngine(ABC):
 
         self.background_rgb = tuple(int(background[i:i + 2], 16) for i in range(1, 6, 2))
 
+        # keep track which area of the image has which altitude currently
+        self.altitudes = {}
+        self.last_altitude = None
+
     @abstractmethod
     def get_png(self) -> bytes:
         # render the image to png.
         pass
 
+    def clip_altitudes(self, new_geometry, new_altitude=None):
+        # register new geometry with an altitude
+        # a geometry with no altitude will reset the altitude information of its area as if nothing was ever there
+        if self.last_altitude is not None and self.last_altitude > new_altitude:
+            raise ValueError('Altitudes have to be ascending.')
+
+        if new_altitude in self.altitudes:
+            self.altitudes[new_altitude] = unary_union([self.altitudes[new_altitude], new_geometry])
+        else:
+            self.altitudes[new_altitude] = new_geometry
+
     def add_geometry(self, geometry, fill: Optional[FillAttribs] = None, stroke: Optional[StrokeAttribs] = None,
                      altitude=None, height=None, shape_cache_key=None):
         # draw a shapely geometry with a given style