Add an API call to export intersection geometry to geojson, for experimental streetmix3d integration.

To get an example:
> cargo run --bin headless -- --port=1234
> curl http://localhost:1234/map/get-intersection-geometry?id=291

Cargo.lock

@@ -1278,6 +1278,7 @@ name = "headless"
 version = "0.1.0"
 dependencies = [
  "abstutil",
+ "geojson",
  "geom",
  "hyper",
  "lazy_static",
@@ -1286,6 +1287,7 @@ dependencies = [
  "rand",
  "rand_xorshift",
  "serde",
+ "serde_json",
  "sim",
  "tokio",
  "url",

book/src/dev/api.md

@@ -90,6 +90,10 @@ are missing, etc. A summary of the commands available so far:
     `--edits=name_of_edits`.
   - **GET /map/get-edit-road-command?id=123**: Returns an object that can be
     modified and then added to map edits.
+  - **GET /map/get-intersection-geometry?id=123**: Returns a GeoJSON object with
+    one feature for the intersection and a feature for all connecting roads. The
+    polygon coordinates are measured in meters, with the origin centered at the
+    intersection's center.
 
 ## Working with the map model
 

geom/src/polyline.rs

@@ -75,6 +75,17 @@ impl PolyLine {
         PolyLine::new(pts)
     }
 
+    /// Like make_polygons, but make sure the points actually form a ring.
+    pub fn to_thick_ring(&self, width: Distance) -> Ring {
+        let mut side1 = self.shift_with_sharp_angles(width / 2.0, MITER_THRESHOLD);
+        let mut side2 = self.shift_with_sharp_angles(-width / 2.0, MITER_THRESHOLD);
+        side2.reverse();
+        side1.extend(side2);
+        side1.push(side1[0]);
+        side1.dedup();
+        Ring::must_new(side1)
+    }
+
     pub fn to_thick_boundary(
         &self,
         self_width: Distance,

headless/Cargo.toml

@@ -6,6 +6,7 @@ edition = "2018"
 
 [dependencies]
 abstutil = { path = "../abstutil" }
+geojson = "0.19.0"
 geom = { path = "../geom" }
 hyper = "0.13.8"
 lazy_static = "1.4.0"
@@ -14,6 +15,7 @@ map_model = { path = "../map_model" }
 rand = "0.7.0"
 rand_xorshift = "0.2.0"
 serde = "1.0.116"
+serde_json = "1.0.57"
 sim = { path = "../sim" }
 tokio = { version = "0.2.22", features = ["full"] }
 url = "2.1.1"

headless/src/main.rs

@@ -325,6 +325,10 @@ fn handle_command(
                 &map.edit_road_cmd(r, |_| {}).to_perma(map),
             ))
         }
+        "/map/get-intersection-geometry" => {
+            let i = IntersectionID(params["id"].parse::<usize>()?);
+            Ok(abstutil::to_json(&export_geometry(map, i)))
+        }
         _ => Err("Unknown command".into()),
     }
 }
@@ -415,3 +419,57 @@ impl LoadSim {
         (map, sim)
     }
 }
+
+fn export_geometry(map: &Map, i: IntersectionID) -> geojson::GeoJson {
+    use geojson::{Feature, FeatureCollection, GeoJson, Geometry, Value};
+
+    let i = map.get_i(i);
+    // Translate all geometry to center around the intersection, with distances in meters.
+    let center = i.polygon.center();
+
+    // The intersection itself
+    let mut props = serde_json::Map::new();
+    props.insert("type".to_string(), "intersection".into());
+    props.insert("id".to_string(), i.orig_id.to_string().into());
+    let mut features = vec![Feature {
+        bbox: None,
+        geometry: Some(Geometry::new(Value::Polygon(vec![i
+            .polygon
+            .translate(-center.x(), -center.y())
+            .points()
+            .iter()
+            .map(|pt| vec![pt.x(), pt.y()])
+            .collect()]))),
+        id: None,
+        properties: Some(props),
+        foreign_members: None,
+    }];
+
+    // Each connected road
+    for r in &i.roads {
+        let r = map.get_r(*r);
+        let mut props = serde_json::Map::new();
+        props.insert("type".to_string(), "road".into());
+        props.insert("id".to_string(), r.orig_id.osm_way_id.to_string().into());
+        features.push(Feature {
+            bbox: None,
+            geometry: Some(Geometry::new(Value::Polygon(vec![r
+                .center_pts
+                .to_thick_ring(2.0 * r.get_half_width(map))
+                .into_points()
+                .into_iter()
+                .map(|pt| pt.offset(-center.x(), -center.y()))
+                .map(|pt| vec![pt.x(), pt.y()])
+                .collect()]))),
+            id: None,
+            properties: Some(props),
+            foreign_members: None,
+        });
+    }
+
+    GeoJson::from(FeatureCollection {
+        bbox: None,
+        features,
+        foreign_members: None,
+    })
+}