It ain't fugly if it works, right?

src/main.rs

@@ -21,6 +21,8 @@ use rocket::tokio::time::Instant;
 use rocket::tokio::{self, select};
 use rocket::{get, routes, State};
 
+use std::f64::consts::PI;
+
 pub(crate) use self::forecast::Metric;
 use self::forecast::{forecast, Forecast};
 pub(crate) use self::maps::{Maps, MapsHandle};
@@ -58,6 +60,17 @@ async fn forecast_geo(
     Json(forecast)
 }
 
+fn deg2rad(x: f64)->f64 {
+    let y: f64 = x * PI / 180.0;
+    return y;
+}
+
+/// Mercator projection from https://stackoverflow.com/questions/18838915/convert-lat-lon-to-pixel-coordinate
+
+fn mercator_y(lat: f64)->f64 {
+    return f64::ln(f64::tan(lat / 2.0 + PI / 4.0));
+}
+
 /// Handler for showing the current map with the geocoded position for a specific metric.
 ///
 /// Note: This handler is mosly used for debugging purposes!
@@ -83,16 +96,80 @@ async fn show_map(
         _ => return None, // Unsupported metric
     };
 
-    // Paint the provided position on the map using a 11×11 pixel square.
-    // FIXME: Use an actual correct calculation, instead of this very rough estimation!
-    let rel_x = (lon - 3.3) / 4.0; // 3.3..7.3, 4.0
-    let rel_y = 1.0 - ((lat - 50.7) / 3.0); // 50.7..53.7, 3.0
-    let x = (rel_x * image.height() as f64) as u32;
-    let y = (rel_y * image.width() as f64) as u32;
-    for px in x - 5..=x + 5 {
-        for py in y - 5..=y + 5 {
-            image.put_pixel(px, py, Rgba::from([0xff, 0x0, 0x0, 0x70]));
-        }
+    // GPS coordinates from Google Maps
+    let vlissingen_lat: f64 = deg2rad(51.44);
+    let vlissingen_lon: f64 = deg2rad(3.57);
+    let vlissingen_x: u32 = 84;
+    let vlissingen_y: u32 = 745;
+
+    let winschoten_lat: f64 = deg2rad(53.14);
+    let winschoten_lon: f64 = deg2rad(7.04);
+    let winschoten_x: u32 = 729;
+    let winschoten_y: u32 = 185;
+
+    let lauwersoog_lat: f64 = deg2rad(53.40);
+    let lauwersoog_lon: f64 = deg2rad(6.22);
+    let lauwersoog_x: u32 = 566;
+    let lauwersoog_y: u32 = 111;
+
+    let enschede_lat: f64 = deg2rad(52.22);
+    let enschede_lon: f64 = deg2rad(6.90);
+    let enschede_x: u32 = 694;
+    let enschede_y: u32 = 494;
+
+    let ref_x1 = vlissingen_x;
+    let ref_lon1 = vlissingen_lon;
+    let ref_y1 = vlissingen_y;
+    let ref_lat1 = vlissingen_lat;
+
+    let ref_x2 = enschede_x;
+    let ref_lon2 = enschede_lon;
+    let ref_y2 = lauwersoog_y;
+    let ref_lat2 = lauwersoog_lat;
+
+    let y_min = mercator_y(ref_lat1);
+    let y_max = mercator_y(ref_lat2);
+    let x_factor = ((ref_x2 - ref_x1) as f64) / (ref_lon2 - ref_lon1);
+    let y_factor = ((ref_y1 - ref_y2) as f64) / (y_max - y_min);
+    println!("x_factor: {}, y_factor: {}", x_factor, y_factor);
+
+    println!("y_min: {}, y_max: {}, lat: {}", y_min * y_factor, y_max * y_factor, mercator_y(deg2rad(lat)) * y_factor);
+    let mut x_f64 = (deg2rad(lon) - ref_lon1) * x_factor + ref_x1 as f64;
+    let mut y_f64 = (y_max - mercator_y(deg2rad(lat))) * y_factor + ref_y2 as f64;
+
+    println!("x: {}, y: {}", x_f64, y_f64);
+    if x_f64 < 0.0 {
+      x_f64 = 0.0;
+    }
+    if x_f64 > (image.width() - 1) as f64 {
+      x_f64 = (image.width() - 1) as f64;
+    }
+    if y_f64 < 0.0 {
+      y_f64 = 0.0;
+    }
+    if y_f64 > (image.height() - 1) as f64 {
+      y_f64 = (image.height() - 1) as f64;
+    }
+    println!("bounded to x: {}, y: {}", x_f64, y_f64);
+
+    for px in ref_x1 - 5..=ref_x1 + 5 {
+      for py in ref_y1 - 5..=ref_y1 + 5 {
+            image.put_pixel(px, py, Rgba::from([0x00, 0xff, 0xff, 0x70]));
+      }
+    }
+
+    for px in ref_x2 - 5..=ref_x2 + 5 {
+      for py in ref_y2 - 5..=ref_y2 + 5 {
+            image.put_pixel(px, py, Rgba::from([0xff, 0x00, 0xff, 0x70]));
+      }
+    }
+
+    for py in 0..(image.height() - 1) {
+      image.put_pixel(x_f64 as u32, py, Rgba::from([0x00, 0x00, 0x00, 0x70]));
+    }
+
+    for px in 0..(image.width() - 1) {
+      image.put_pixel(px, y_f64 as u32, Rgba::from([0x00, 0x00, 0x00, 0x70]));
     }
 
     // Encode the image as PNG image data.