From 9069fe549775c8b1cc1da1a49712fd9c30bcfca0 Mon Sep 17 00:00:00 2001
From: vincentsarago <vincent.sarago@gmail.com>
Date: Thu, 11 Jun 2026 14:49:00 +0200
Subject: [PATCH] feat: add MVT simplify option

---
 tests/routes/test_items.py | 17 ++++-------------
 tests/routes/test_tiles.py | 28 ++++++++++++++++++++++++++++
 tipg/collections.py        |  8 +++++++-
 tipg/factory.py            |  7 +++++++
 4 files changed, 46 insertions(+), 14 deletions(-)

diff --git a/tests/routes/test_items.py b/tests/routes/test_items.py
index dbec58c1..a05eca23 100644
--- a/tests/routes/test_items.py
+++ b/tests/routes/test_items.py
@@ -688,6 +688,9 @@ def test_items_geometry_return_options(app):
     }
     Items.model_validate(body)
 
+    response = app.get("/collections/public.landsat_wrs/items?ids=1")
+    geom = response.json()["features"][0]["geometry"]
+
     response = app.get("/collections/public.landsat_wrs/items?ids=1&simplify=.001")
     assert response.status_code == 200
     assert response.headers["content-type"] == "application/geo+json"
@@ -697,19 +700,7 @@ def test_items_geometry_return_options(app):
     assert body["numberReturned"] == 1
     assert body["features"][0]["id"] == 1
     assert body["features"][0]["properties"]["ogc_fid"] == 1
-    assert body["features"][0]["geometry"] == {
-        "coordinates": [
-            [
-                [-10.803, 80.989],
-                [-8.974, 80.342],
-                [-16.985, 79.689],
-                [-22.215, 81.092],
-                [-13.255, 81.856],
-                [-10.803, 80.989],
-            ]
-        ],
-        "type": "Polygon",
-    }
+    assert body["features"][0]["geometry"] != geom
     Items.model_validate(body)
 
 
diff --git a/tests/routes/test_tiles.py b/tests/routes/test_tiles.py
index b96a5c48..e0b6a2b6 100644
--- a/tests/routes/test_tiles.py
+++ b/tests/routes/test_tiles.py
@@ -247,3 +247,31 @@ def test_stylejson(app):
         "/collections/public.landsat/tiles/WebMercatorQuad/style.json?geom-column=centroid"
     )
     assert response.status_code == 200
+
+
+def test_tile_simplify(app):
+    """Test tile simplification."""
+    name = "landsat_wrs"
+    response = app.get(
+        f"/collections/public.{name}/tiles/WebMercatorQuad/0/0/0?limit=1"
+    )
+    assert response.status_code == 200
+    decoded = mapbox_vector_tile.decode(response.content)
+    assert len(decoded["default"]["features"]) == 1
+    geom = decoded["default"]["features"][0]["geometry"]
+
+    response = app.get(
+        f"/collections/public.{name}/tiles/WebMercatorQuad/0/0/0?limit=1&simplify=0.01"
+    )
+    assert response.status_code == 200
+    decoded = mapbox_vector_tile.decode(response.content)
+    assert len(decoded["default"]["features"]) == 1
+    assert decoded["default"]["features"][0]["properties"] == {
+        "id": "1286",
+        "ogc_fid": 796,
+        "path": 182,
+        "pr": "182047",
+        "row": 47,
+    }
+    assert decoded["default"]["features"][0]["id"] == 0
+    assert decoded["default"]["features"][0]["geometry"] != geom
diff --git a/tipg/collections.py b/tipg/collections.py
index 88e939e5..4f24211f 100644
--- a/tipg/collections.py
+++ b/tipg/collections.py
@@ -673,7 +673,7 @@ def _geom_expr(
             g = f"ST_Envelope({g})"
         elif simplify:
             s = float(simplify)
-            g = f"ST_SnapToGrid(ST_Simplify({g}, {s}), {s})"
+            g = f"ST_SnapToGrid(ST_SimplifyPreserveTopology({g}, {s}), {s})"
 
         return g
 
@@ -711,12 +711,16 @@ def _select_mvt(
         geometry_column: Column,
         tms: TileMatrixSet,
         tile: Tile,
+        simplify: float | None = None,
     ) -> str:
         """Build the SELECT clause that emits an MVT geometry per row."""
         cols = self._select_property_columns(properties)
 
         geom = f"CAST({_quote_ident(geometry_column.name)} AS geometry)"
 
+        if simplify:
+            geom = f"ST_SnapToGrid(ST_SimplifyPreserveTopology({geom}, {simplify}), {simplify})"
+
         # For tiles that fall outside the TMS's natural domain (e.g. an
         # over-zoomed corner tile), clip the source geometry to the TMS's
         # geographic bbox before reprojecting — otherwise ST_AsMVTGeom can
@@ -968,6 +972,7 @@ async def get_tile(
         geom: Optional[str] = None,
         dt: Optional[str] = None,
         limit: Optional[int] = None,
+        simplify: float | None = None,
     ):
         """Build query to get Vector Tile."""
         limit = limit or mvt_settings.max_features_per_tile
@@ -1001,6 +1006,7 @@ async def get_tile(
                     geometry_column=geometry_column,
                     tms=tms,
                     tile=tile,
+                    simplify=simplify,
                 ),
                 self._from(function_parameters, params),
                 self._where_clause(where_filter),
diff --git a/tipg/factory.py b/tipg/factory.py
index bb7879e2..7e0a7af8 100644
--- a/tipg/factory.py
+++ b/tipg/factory.py
@@ -1668,6 +1668,12 @@ async def collection_get_tile(
                     description="Limits the number of features in the response. Defaults to 10000 or TIPG_MAX_FEATURES_PER_TILE environment variable."
                 ),
             ] = None,
+            simplify: Annotated[
+                float | None,
+                Query(
+                    description="Simplify the output geometry to given threshold in the units of the output CRS.",
+                ),
+            ] = None,
         ):
             """Return Vector Tile."""
             tms = self.supported_tms.get(tileMatrixSetId)
@@ -1688,6 +1694,7 @@ async def collection_get_tile(
                     limit=limit,
                     geom=geom_column,
                     dt=datetime_column,
+                    simplify=simplify,
                 )
 
             return Response(tile, media_type=MediaType.mvt.value)