Smooth camera and unit motion: display positions, camera target smoothing, pan speed setting

src/game.rs

@@ -288,12 +288,20 @@ pub struct Ent {
     pub target: Option<u8>, // attackers: side whose core (or gate) they march on
     #[serde(default)]
     pub chained: bool, // a lone creature chained in a sealed cave: joins when the cave is claimed
+    #[serde(skip, default = "zero_f32")]
+    pub disp_x: f32, // smoothed screen position (not part of the saved or synced world state)
+    #[serde(skip, default = "zero_f32")]
+    pub disp_y: f32,
 }
 
 fn yes() -> bool {
     true
 }
 
+fn zero_f32() -> f32 {
+    0.0
+}
+
 impl Ent {
     fn dummy() -> Ent {
         Ent {
@@ -350,6 +358,8 @@ impl Ent {
             side: 0,
             target: None,
             chained: false,
+            disp_x: 0.0,
+            disp_y: 0.0,
         }
     }
     pub fn tile(&self) -> usize {
@@ -1129,6 +1139,8 @@ impl Game {
         e.hero = hero;
         e.x = x;
         e.y = y;
+        e.disp_x = x;
+        e.disp_y = y;
         e.level = level;
         e.max_hp = s.hp * (1.0 + 0.15 * (level - 1) as f32);
         e.hp = e.max_hp;
@@ -1178,6 +1190,30 @@ impl Game {
 
     // ---------------------------------------------------------------- update
 
+    // smoothed positions for the screen only: the simulation stays fixed-step and deterministic
+    pub fn smooth_display(&mut self, dt: f32) {
+        if dt <= 0.0 {
+            return;
+        }
+        let k = (1.0 - (-22.0 * dt).exp()).clamp(0.0, 1.0);
+        for e in self.ents.iter_mut() {
+            if e.disp_x == 0.0 && e.disp_y == 0.0 {
+                e.disp_x = e.x;
+                e.disp_y = e.y;
+                continue;
+            }
+            let dx = e.x - e.disp_x;
+            let dy = e.y - e.disp_y;
+            if dx * dx + dy * dy > 4.0 {
+                e.disp_x = e.x;
+                e.disp_y = e.y;
+            } else {
+                e.disp_x += dx * k;
+                e.disp_y += dy * k;
+            }
+        }
+    }
+
     pub fn update(&mut self, dt: f32) {
         let mut steps = 0usize;
         let cmds = if self.netplay.is_some() {