Properly center screen

src/app.rs

@@ -16,7 +16,7 @@ use crate::{
     game::{for_each_tile_on_screen, Biome, GameState},
     graphics::{self, ScreenPosScalar, NATIVE_RESOLUTION},
     input::Input,
-    math::{px_per_frame_to_km_h, WorldPos, M_PER_PX, TILE_SIZE},
+    math::{center_offset, px_per_frame_to_km_h, WorldPos, M_PER_PX, TILE_SIZE},
     res::Res,
     world::{Tile, TilePosScalar, CHUNK_EXTENT},
 };
@@ -243,6 +243,11 @@ impl App {
         self.rt.display();
         let mut spr = Sprite::with_texture(self.rt.texture());
         spr.set_scale((self.scale as f32, self.scale as f32));
+        let rw_size = self.rw.size();
+        let rt_size = self.rt.size() * self.scale as u32;
+        let x = center_offset(rt_size.x as i32, rw_size.x as i32);
+        let y = center_offset(rt_size.y as i32, rw_size.y as i32);
+        spr.set_position((x as f32, y as f32));
         self.rw.clear(Color::rgb(40, 10, 70));
         self.rw.draw(&spr);
         self.sf_egui

src/math.rs

@@ -1,3 +1,5 @@
+use num_traits::Signed;
+
 use crate::world::{TilePos, TilePosScalar};
 
 pub type WorldPosScalar = u32;
@@ -50,6 +52,23 @@ pub fn wp_to_tp(wp: WorldPosScalar) -> TilePosScalar {
     (wp / TILE_SIZE as WorldPosScalar) as TilePosScalar
 }
 
+// Get the offset required to center an object of `xw` width inside an object of `yw` width.
+//
+// For example, let's say `xw` (+) is 10 and we want to center it inside `yw` (-), which is 20
+//
+// ++++++++++           (x uncentered)
+// -------------------- (y)
+//      ++++++++++      (x centered)
+//
+// In this case, we needed to add 5 to x to achieve centering.
+// This is the offset that this function calculates.
+//
+// We can calulate it by subtracting `xw` from `yw` (10), and dividing it by 2.
+pub fn center_offset<N: From<u8> + Copy + Signed>(xw: N, yw: N) -> N {
+    let diff = yw - xw;
+    diff / N::from(2)
+}
+
 #[test]
 fn test_wp_to_tp() {
     assert_eq!(wp_to_tp(0), 0);