fix: draw 8-bit RGB GeoTIFFs as-is instead of re-stretching them

server/mapcontrol_server/services/geotiff_service.py

@@ -335,9 +335,21 @@ def normalize_band(arr, m):
         normalized[m] = 0.0
         return np.clip(normalized, 0, 255).astype(np.uint8)
 
-    r_u8 = normalize_band(r, mask)
-    g_u8 = normalize_band(g, mask)
-    b_u8 = normalize_band(b_arr, mask)
+    if all(ds.dtypes[b - 1] == "uint8" for b in bands):
+        # 8-bit RGB is already display-ready (a true-colour product, or a
+        # writer that applied its own fixed stretch). Re-stretching it per
+        # band lets the scene set the colours: an ocean-dominated image has
+        # its noise amplified and its colour balance shifted.
+        def as_is(arr, m):
+            out = arr.copy()
+            out[m] = 0.0
+            return out.astype(np.uint8)
+
+        r_u8, g_u8, b_u8 = as_is(r, mask), as_is(g, mask), as_is(b_arr, mask)
+    else:
+        r_u8 = normalize_band(r, mask)
+        g_u8 = normalize_band(g, mask)
+        b_u8 = normalize_band(b_arr, mask)
 
     # Alpha channel: fully opaque for valid pixels, transparent for nodata/NaN
     # (actual translucency is handled by OpenLayers layer opacity, not baked into PNG)

server/tests/test_geotiff_masking.py

@@ -93,3 +93,22 @@ def test_singleband_honors_mask_and_nodata(tmp_path):
     assert (alpha[3, :] == 0).all()
     assert alpha[1, 1] == 0            # value 5 = nodata tag
     assert alpha[0, 0] == 255 and alpha[2, 3] == 255
+
+
+def test_uint8_rgb_is_drawn_as_is_without_restretching(tmp_path):
+    data = np.full((3, 4, 4), 12, dtype=np.uint8)       # dark ocean, display-scaled
+    data[:, 0, 0] = (180, 150, 90)                      # one bright land pixel
+    data[:, 3, 3] = 0                                   # no-data
+    rgba, ds = _process_rgb(_write(tmp_path / "a.tif", data, nodata=0), [1, 2, 3], 1.0, None)
+    ds.close()
+    assert tuple(rgba[1, 1, :3]) == (12, 12, 12)        # not stretched up to mid-grey
+    assert tuple(rgba[0, 0, :3]) == (180, 150, 90)
+    assert rgba[3, 3, 3] == 0 and rgba[1, 1, 3] == 255
+
+
+def test_non_uint8_rgb_is_still_stretched(tmp_path):
+    data = np.full((3, 4, 4), 1200, dtype=np.uint16)
+    data[:, 0, 0] = 3000
+    rgba, ds = _process_rgb(_write(tmp_path / "a.tif", data), [1, 2, 3], 1.0, None)
+    ds.close()
+    assert rgba[0, 0, 0] == 255 and rgba[1, 1, 0] == 0