Keep integer dtype in mercator_transform

folium/utilities.py

@@ -273,6 +273,11 @@ def mercator(x):
         for j in range(nblayers):
             out[:, i, j] = np.interp(latslats, mercator(lats), array[:, i, j])
 
+    # Keep integer images (e.g. uint8 RGBA) in their original dtype, otherwise
+    # write_png treats them as float data and rescales each channel.
+    if np.issubdtype(array.dtype, np.integer):
+        out = np.round(out).astype(array.dtype)
+
     # Eventually flip the image.
     if origin == "upper":
         out = out[::-1, :, :]

tests/test_raster_layers.py

@@ -4,6 +4,7 @@
 
 """
 
+import numpy as np
 import pytest
 import xyzservices
 
@@ -113,6 +114,16 @@ def test_image_overlay():
     assert bounds == [[0, -180], [90, 180]], bounds
 
 
+def test_image_overlay_mercator_project_keeps_uint8_colors():
+    data = np.full((4, 3, 4), (100, 50, 200, 255), dtype="uint8")
+    bounds = [[40, 0], [50, 10]]
+
+    plain = folium.raster_layers.ImageOverlay(data, bounds)
+    projected = folium.raster_layers.ImageOverlay(data, bounds, mercator_project=True)
+
+    assert projected.url == plain.url
+
+
 @pytest.mark.parametrize(
     "tiles", ["CartoDB DarkMatter", xyzservices.providers.CartoDB.DarkMatter]
 )

tests/test_utilities.py

@@ -12,6 +12,7 @@
     get_obj_in_upper_tree,
     if_pandas_df_convert_to_numpy,
     javascript_identifier_path_to_array_notation,
+    mercator_transform,
     normalize_bounds_type,
     parse_font_size,
     parse_options,
@@ -171,6 +172,15 @@ def test_normalize_bounds_type(bounds, expected):
     assert normalize_bounds_type(bounds) == expected
 
 
+def test_mercator_transform_keeps_integer_dtype():
+    # A uint8 image must stay uint8, otherwise write_png treats the result as
+    # float data and stretches every channel to its maximum.
+    data = np.full((4, 3, 4), (100, 50, 200, 255), dtype="uint8")
+    out = mercator_transform(data, (40, 50))
+    assert out.dtype == np.uint8
+    np.testing.assert_array_equal(out, data)
+
+
 def test_camelize():
     assert camelize("variable_name") == "variableName"
     assert camelize("variableName") == "variableName"