Use more descriptive names for variables and configuration parameters.

queries.yaml

@@ -613,7 +613,10 @@ post_process:
       source_layer: boundaries
       start_zoom: 8
       end_zoom: 11
-      factor: 11
+      # pixels (256px nominal) to require per letter in the name. if lines are shorter than this,
+      # the name gets dropped. name pairs (e.g: :left and :right) get dropped if either are longer
+      # than pixels_per_letter. larger values mean fewer lines are eligible for labelling.
+      pixels_per_letter: 11
 
   # drop names on boundary lines (country, region, macroregion) except zoom 7 from Natural Earth
   - fn: vectordatasource.transform.drop_properties

vectordatasource/transform.py

@@ -1967,10 +1967,10 @@ def _unicode_len(s):
     return None
 
 
-def _delete_labels_longer_than(max_label, props):
+def _delete_labels_longer_than(max_label_chars, props):
     """
     Delete entries in the props dict where the key starts with 'name' and the
-    unicode length of the value is greater than max_length.
+    unicode length of the value is greater than max_label_chars.
 
     If one half of a left/right pair is too long, then the opposite in the pair
     is also deleted.
@@ -1982,12 +1982,12 @@ def _delete_labels_longer_than(max_label, props):
         if not k.startswith('name'):
             continue
 
-        length = _unicode_len(v)
-        if length is None:
+        length_chars = _unicode_len(v)
+        if length_chars is None:
             # huh? name isn't a string?
             continue
 
-        if length <= max_label:
+        if length_chars <= max_label_chars:
             continue
 
         to_delete.add(k)
@@ -2013,7 +2013,8 @@ def drop_names_on_short_boundaries(ctx):
     layer_name = params.required('source_layer')
     start_zoom = params.optional('start_zoom', typ=int, default=0)
     end_zoom = params.optional('end_zoom', typ=int)
-    factor = params.optional('factor', typ=(int, float), default=10.0)
+    pixels_per_letter = params.optional('pixels_per_letter', typ=(int, float),
+                                        default=10.0)
 
     layer = _find_layer(ctx.feature_layers, layer_name)
     zoom = ctx.nominal_zoom
@@ -2022,26 +2023,33 @@ def drop_names_on_short_boundaries(ctx):
        (end_zoom is not None and zoom >= end_zoom):
         return None
 
-    tolerance = factor * tolerance_for_zoom(zoom)
+    # tolerance for zoom gives us a value in meters for a pixel, so it's
+    # meters per pixel
+    meters_per_letter = pixels_per_letter * tolerance_for_zoom(zoom)
 
     for shape, props, fid in layer['features']:
         geom_type = shape.geom_type
 
         if geom_type in ('LineString', 'MultiLineString'):
-            label_shape = shape.simplify(tolerance)
+            # simplify to one letter size. this gets close to what might
+            # practically be renderable, and means we're not counting any
+            # sub-letter scale fractal crinklyness towards the length of
+            # the line.
+            label_shape = shape.simplify(meters_per_letter)
+
             if geom_type == 'LineString':
-                shape_length = label_shape.length
+                shape_length_meters = label_shape.length
             else:
                 # get the longest section to see if that's labellable - if
                 # not, then none of the sections could have a label and we
                 # can drop the names.
-                shape_length = max(part.length for part in label_shape)
+                shape_length_meters = max(part.length for part in label_shape)
 
             # maximum number of characters we'll be able to print at this
             # zoom.
-            max_label = int(shape_length / tolerance)
+            max_label_chars = int(shape_length_meters / meters_per_letter)
 
-            _delete_labels_longer_than(max_label, props)
+            _delete_labels_longer_than(max_label_chars, props)
 
     return None