Add --center-lat, --center-lon, --zoom CLI arguments

Allow overriding the map centre and OSM zoom level from the command
line, keeping the existing values as defaults.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main.rs

@@ -34,6 +34,18 @@ struct Args {
     /// Disable OpenStreetMap base layer (use flat green background)
     #[arg(long)]
     no_basemap: bool,
+
+    /// Map centre latitude in degrees (default: 52.56)
+    #[arg(long, value_name = "LAT", default_value_t = render::DEFAULT_CENTER_LAT)]
+    center_lat: f64,
+
+    /// Map centre longitude in degrees (default: 13.08)
+    #[arg(long, value_name = "LON", default_value_t = render::DEFAULT_CENTER_LON)]
+    center_lon: f64,
+
+    /// OSM tile zoom level (default: 10)
+    #[arg(long, value_name = "ZOOM", default_value_t = render::DEFAULT_OSM_ZOOM)]
+    zoom: u8,
 }
 
 fn main() -> Result<()> {
@@ -134,12 +146,12 @@ fn main() -> Result<()> {
     let basemap = if args.no_basemap {
         None
     } else {
-        let proj = render::make_projection();
+        let proj = render::make_projection(args.center_lat, args.center_lon, args.zoom);
         let tile_cache = tiles::fetch_tiles(
             &proj,
             render::MAP_W,
             render::MAP_H,
-            render::OSM_ZOOM,
+            args.zoom,
             &PathBuf::from(CACHE_DIR),
         )?;
         Some(tiles::rasterize_basemap(
@@ -153,12 +165,12 @@ fn main() -> Result<()> {
     let labels = if args.no_basemap {
         None
     } else {
-        let proj = render::make_projection();
+        let proj = render::make_projection(args.center_lat, args.center_lon, args.zoom);
         let label_cache = tiles::fetch_label_tiles(
             &proj,
             render::MAP_W,
             render::MAP_H,
-            render::OSM_ZOOM,
+            args.zoom,
             &PathBuf::from(CACHE_DIR),
         )?;
         Some(tiles::rasterize_labels(
@@ -200,7 +212,7 @@ fn main() -> Result<()> {
         let out_name = format!("clct_{:06}.png", step + 1);
         let out_path = cache_dir.join(&out_name);
 
-        render::render_frame(&out_path, &lats, &lons, &cloud, &title, basemap.as_deref(), labels.as_deref())
+        render::render_frame(&out_path, &lats, &lons, &cloud, &title, basemap.as_deref(), labels.as_deref(), args.center_lat, args.center_lon, args.zoom)
             .with_context(|| format!("Rendering frame {}", step))?;
 
         eprintln!("  [{:02}/{:02}] {}", step + 1, args.prediction_hours, out_name);

src/render.rs

@@ -15,27 +15,27 @@ pub const MARGIN: u32 = 8;
 pub const MAP_W: u32 = IMG_WIDTH - LEGEND_W - MARGIN * 2;
 pub const MAP_H: u32 = IMG_HEIGHT - TITLE_H - MARGIN;
 
-/// Map image centre and OSM zoom level.
-pub const CENTER_LAT: f64 = 52.56;
-pub const CENTER_LON: f64 = 13.08;
-/// OSM tile zoom level. The viewport half-extents are derived from this so that
+/// Default map image centre and OSM zoom level.
+pub const DEFAULT_CENTER_LAT: f64 = 52.56;
+pub const DEFAULT_CENTER_LON: f64 = 13.08;
+/// Default OSM tile zoom level. The viewport half-extents are derived from this so that
 /// one tile pixel maps to exactly one output pixel, eliminating interpolation
 /// artefacts on the OSM base layer.
-pub const OSM_ZOOM: u8 = 10;
+pub const DEFAULT_OSM_ZOOM: u8 = 10;
 
 /// Build the orthographic projection whose viewport is sized so that the
-/// output resolution matches OSM tiles at `OSM_ZOOM` exactly.
+/// output resolution matches OSM tiles at `zoom` exactly.
 ///
 /// At zoom `z`, Web-Mercator tiles cover `360 / 2^z` degrees of longitude per
 /// tile, each 256 px wide.  Near the centre latitude `φ`, 1° of longitude ≈
 /// `cos φ` arc-degrees, so the arc-degree scale is:
 ///   `pixels_per_arc_deg = 256 · 2^z / (360 · cos φ)`
 /// Half-extents follow directly from the map pixel dimensions.
-pub fn make_projection() -> OrthoProjection {
-    let scale = f64::from(256u32 << OSM_ZOOM as u32) / (360.0 * CENTER_LAT.to_radians().cos());
+pub fn make_projection(center_lat: f64, center_lon: f64, zoom: u8) -> OrthoProjection {
+    let scale = f64::from(256u32 << zoom as u32) / (360.0 * center_lat.to_radians().cos());
     let half_w = MAP_W as f64 / (2.0 * scale);
     let half_h = MAP_H as f64 / (2.0 * scale);
-    OrthoProjection::new(CENTER_LAT, CENTER_LON, half_w, half_h)
+    OrthoProjection::new(center_lat, center_lon, half_w, half_h)
 }
 
 const BACKGROUND: RGBColor = RGBColor(240, 248, 255); // pale sky-blue
@@ -179,8 +179,11 @@ pub fn render_frame(
     title: &str,
     basemap: Option<&[[u8; 3]]>,
     labels: Option<&[[u8; 4]]>,
+    center_lat: f64,
+    center_lon: f64,
+    zoom: u8,
 ) -> Result<()> {
-    let proj = make_projection();
+    let proj = make_projection(center_lat, center_lon, zoom);
 
     let root = BitMapBackend::new(output_path, (IMG_WIDTH, IMG_HEIGHT)).into_drawing_area();
     root.fill(&BACKGROUND).context("fill background")?;