From 7a85d9d38499ad59f32b4ce0245711546ca655dc Mon Sep 17 00:00:00 2001
From: "Daniel J. Hofmann" <daniel@trvx.org>
Date: Fri, 20 Jul 2018 12:16:15 +0200
Subject: [PATCH] Generalizes post-processing handlers across zoom levels,
 closes #51

---
 robosat/features/parking.py | 14 ++++++++++----
 robosat/tiles.py            | 24 ++++++++++++++++++++++++
 robosat/tools/features.py   |  3 +++
 3 files changed, 37 insertions(+), 4 deletions(-)

diff --git a/robosat/features/parking.py b/robosat/features/parking.py
index ec01f014..cdce4086 100644
--- a/robosat/features/parking.py
+++ b/robosat/features/parking.py
@@ -5,20 +5,26 @@
 
 import shapely.geometry
 
+from robosat.tiles import meters_per_pixel
 from robosat.features.core import denoise, grow, contours, simplify, featurize, parents_in_hierarchy
 
 
 class ParkingHandler:
-    kernel_size_denoise = 20
-    kernel_size_grow = 20
+    kernel_size_denoise_m = 10
+    kernel_size_grow_m = 10
     simplify_threshold = 0.01
 
     def __init__(self):
         self.features = []
 
     def apply(self, tile, mask):
-        if tile.z != 18:
-            raise NotImplementedError("Parking lot post-processing thresholds are tuned for z18")
+        resolution = meters_per_pixel(tile, mask.shape[0])
+
+        kernel_size_denoise_px = round(self.kernel_size_denoise_m / resolution)
+        kernel_size_grow_px = round(self.kernel_size_grow_m / resolution)
+
+        assert kernel_size_denoise_px >= 1, "denoising kernel is at least a single pixel in size"
+        assert kernel_size_grow_px >= 1, "growing kernel is at least a single pixel in size"
 
         # The post-processing pipeline removes noise and fills in smaller holes. We then
         # extract contours, simplify them and transform tile pixels into coordinates.
diff --git a/robosat/tiles.py b/robosat/tiles.py
index 24a09a1e..56b11d6b 100644
--- a/robosat/tiles.py
+++ b/robosat/tiles.py
@@ -8,6 +8,7 @@
 See: https://wiki.openstreetmap.org/wiki/Slippy_map_tilenames
 """
 
+import math
 import csv
 import io
 import os
@@ -16,6 +17,29 @@
 import mercantile
 
 
+def meters_per_pixel(tile, size):
+    """Returns the resolution for a tile.
+
+    Args:
+      tile: the mercantile.tile to calculate the resolution for.
+      size: the size in pixels for this tile (e.g. 256).
+
+    Returns:
+      The resolution in meters per pixel.
+    """
+
+    # https://wiki.openstreetmap.org/wiki/Slippy_map_tilenames#Resolution_and_Scale
+
+    z = tile.z
+    lat = mercantile.ul(tile).lat
+
+    radius = 6378137
+    mpp = radius * 2 * math.pi / size
+    resolution = mpp * math.cos(math.radians(lat)) / (2 ** z)
+
+    return resolution
+
+
 def pixel_to_location(tile, dx, dy):
     """Converts a pixel in a tile to a coordinate.
 
diff --git a/robosat/tools/features.py b/robosat/tools/features.py
index 062441ac..17750d1a 100644
--- a/robosat/tools/features.py
+++ b/robosat/tools/features.py
@@ -46,6 +46,9 @@ def main(args):
         image = np.array(Image.open(path).convert("P"), dtype=np.uint8)
         mask = (image == index).astype(np.uint8)
 
+        h, w = mask.shape[:2]
+        assert h == w
+
         handler.apply(tile, mask)
 
     handler.save(args.out)