Avoid sign flip computation

app/api/hillshade.py

@@ -71,12 +71,6 @@ class LightSource:
             A 2d array of illumination values between 0-1, where 0 is
             completely in shadow and 1 is completely illuminated.
         """
-
-        # Because most image and raster GIS data has the first row in the array
-        # as the "top" of the image, dy is implicitly negative.  This is
-        # consistent to what `imshow` assumes, as well.
-        dy = -dy
-
         # compute the normal vectors from the partial derivatives
         e_dy, e_dx = np.gradient(vert_exag * elevation, dy, dx)
         

app/api/tiler.py

@@ -465,7 +465,7 @@ class Tiles(TaskNestedView):
                         _("Cannot compute hillshade of non-elevation raster (multiple bands found)"))
                 delta_scale = (maxzoom + ZOOM_EXTRA_LEVELS + 1 - z) ** 2
                 dx = src.dataset.meta["transform"][0] * delta_scale
-                dy = -src.dataset.meta["transform"][4] * delta_scale
+                dy = src.dataset.meta["transform"][4] * delta_scale
                 ls = LightSource(azdeg=315, altdeg=45)
                 
                 # Remove elevation data from edge buffer tiles

app/raster_utils.py

@@ -235,7 +235,7 @@ def export_raster(input, output, **opts):
                 if hillshade is not None and hillshade > 0:
                     delta_scale = ZOOM_EXTRA_LEVELS ** 2
                     dx = src.meta["transform"][0] * delta_scale
-                    dy = -src.meta["transform"][4] * delta_scale
+                    dy = src.meta["transform"][4] * delta_scale
                     ls = LightSource(azdeg=315, altdeg=45)
                     intensity = ls.hillshade(arr[0], dx=dx, dy=dy, vert_exag=hillshade)
                     intensity = intensity * 255.0