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Clip instead of squeeze out-of-range latitudes in mercator_transform #2305

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Clip instead of squeeze out-of-range latitudes in mercator_transform #2305

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7 changes: 4 additions & 3 deletions folium/utilities.py
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Original file line number Diff line number Diff line change
Expand Up @@ -232,7 +232,7 @@ def mercator_transform(
lat_bounds : length 2 tuple
Minimal and maximal value of the latitude of the image.
Bounds must be between -85.051128779806589 and 85.051128779806589
otherwise they will be clipped to that values.
otherwise the image will be clipped to that range.

origin : ['upper' | 'lower'], optional, default 'upper'
Place the [0,0] index of the array in the upper left or lower left
Expand Down Expand Up @@ -261,8 +261,9 @@ def mercator(x):
if origin == "upper":
array = array[::-1, :, :]

lats = lat_min + np.linspace(0.5 / height, 1.0 - 0.5 / height, height) * (
lat_max - lat_min
# The input rows span the full lat_bounds; only the output is clipped.
lats = lat_bounds[0] + np.linspace(0.5 / height, 1.0 - 0.5 / height, height) * (
lat_bounds[1] - lat_bounds[0]
)
latslats = mercator(lat_min) + np.linspace(
0.5 / height_out, 1.0 - 0.5 / height_out, height_out
Expand Down
2 changes: 1 addition & 1 deletion tests/test_raster_layers.py
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Expand Up @@ -94,7 +94,7 @@ def test_image_overlay():
url = (
"data:image/png;base64,"
"iVBORw0KGgoAAAANSUhEUgAAAAMAAAACCAYAAACddGYaAAA"
"AF0lEQVR42mP4z8AARFDw/z/DeiA5H4QBV60H6ABl9ZIAAAAASUVORK5CYII="
"AF0lEQVR42mP4z8AARFDw/z/DciC5DoQBV78H/mpWZYkAAAAASUVORK5CYII="
)
assert io.url == url

Expand Down
19 changes: 19 additions & 0 deletions tests/test_utilities.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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,
Expand Down Expand Up @@ -292,3 +293,21 @@ def test_parse_font_size_valid(value, expected):
def test_parse_font_size_invalid(value, error_message):
with pytest.raises(ValueError, match=error_message):
parse_font_size(value)


def test_mercator_transform_clips_bounds_beyond_mercator_limit():
# Each row holds the latitude of its centre, from -89.5 (bottom) to 89.5.
height = 180
lats = np.linspace(-90, 90, height + 1)[:-1] + 0.5
data = np.repeat(lats[:, None], 2, axis=1)

out = mercator_transform(data, (-90, 90), origin="lower")

# The output only covers the Web Mercator range, so every output row must
# hold (up to interpolation, well within one 1 degree input pixel) the
# latitude of its own centre: rows beyond +-85.05 are dropped, not
# squeezed into the output.
# Output row centres in Web Mercator y (degrees); +-85.05 maps to +-180.
y = np.linspace(-180, 180, height + 1)[:-1] + 180 / height
expected = np.degrees(np.arctan(np.sinh(np.radians(y))))
np.testing.assert_allclose(out[:, 0, 0], expected, atol=0.05)