Refactor contrast stretching

terracotta/encode_decode.py

@@ -30,3 +30,26 @@ def array_to_img(arr, alpha_mask):
     img = Image.fromarray(alpha_mask, mode='LA')
 
     return img
+
+
+def contrast_stretch(tile, val_range):
+    """Scale an image to between 0 and 255.
+
+    Parameters
+    ----------
+    val_range: (int, int)
+        min and max value of input tile
+
+    Returns
+    -------
+    out: numpy array
+        input tile scaled to 0 - 255.
+    """
+
+    _, max_val = val_range
+    if max_val == 0:
+        tile[:] = 0
+    else:
+        tile *= 255 // max_val
+    tile = tile.astype(np.uint8)
+    return tile

terracotta/tile.py

@@ -148,7 +148,7 @@ def _load_file_meta(files):
         return meta
 
     def tile(self, tile_x, tile_y, tile_z, ds_name,
-             timestep=None, tilesize=256, scale_contrast=False):
+             timestep=None, tilesize=256):
         """Load a requested tile from source.
 
         Parameters
@@ -181,14 +181,13 @@ def tile(self, tile_x, tile_y, tile_z, ds_name,
             raise TileOutOfBoundsError('Tile {}/{}/{} is outside image bounds'
                                        .format(tile_z, tile_x, tile_y))
 
-        nodata = self.get_nodata(ds_name)
         mercator_tile = mercantile.Tile(x=tile_x, y=tile_y, z=tile_z)
         tile_bounds = mercantile.xy_bounds(mercator_tile)
         tile = self._load_tile(fname, tile_bounds, tilesize)
+
+        nodata = self.get_nodata(ds_name)
         alpha_mask = np.full((tilesize, tilesize), 255, np.uint8)
         alpha_mask[tile == nodata] = 0
-        if scale_contrast:
-            tile = contrast_stretch(tile, self._datasets[ds_name]['meta']['range'])
 
         return tile, alpha_mask
 
@@ -253,26 +252,3 @@ def tile_exists(bounds, tile_z, tile_x, tile_y):
         and (tile_x >= mintile.x) \
         and (tile_y <= maxtile.y + 1) \
         and (tile_y >= mintile.y)
-
-
-def contrast_stretch(tile, val_range):
-    """Scale the image to between 0 and 255.
-
-    Parameters
-    ----------
-    val_range: (int, int)
-        min and max value of input tile
-
-    Returns
-    -------
-    out: numpy array
-        input tile scaled to 0 - 255.
-    """
-
-    _, max_val = val_range
-    if max_val == 0:
-        tile[:] = 0
-    else:
-        tile *= 255 // max_val
-    tile = tile.astype(np.uint8)
-    return tile

terracotta/tile_api.py

@@ -6,7 +6,7 @@
 
 import terracotta.tile as tile
 from terracotta.tile import TileNotFoundError, TileOutOfBoundsError, DatasetNotFoundError
-from terracotta.encode_decode import array_to_img
+import terracotta.encode_decode as ed
 
 
 DEFAULT_CACHE_SIZE = 1000000000  # 1GB
@@ -32,8 +32,7 @@ def get_tile(dataset, tile_z, tile_x, tile_y, timestep=None):
     """Respond to tile requests"""
 
     try:
-        img, alpha_mask = tilestore.tile(tile_x, tile_y, tile_z, dataset, timestep,
-                                         scale_contrast=True)
+        img, alpha_mask = tilestore.tile(tile_x, tile_y, tile_z, dataset, timestep)
     except TileNotFoundError:
         if current_app.debug:
             raise
@@ -43,7 +42,9 @@ def get_tile(dataset, tile_z, tile_x, tile_y, timestep=None):
         img = np.full((256, 256), nodata, dtype=np.uint8)
         alpha_mask = np.zeros((256, 256), dtype=np.uint8)
 
-    img = array_to_img(img, alpha_mask)
+    range = tilestore.get_meta(dataset)['range']
+    img = ed.contrast_stretch(img, range)
+    img = ed.array_to_img(img, alpha_mask)
 
     sio = BytesIO()
     img.save(sio, 'png', compress_level=0)