Smoothing out fps limit

Author: Interrupt

src/examples/forest.zig

@@ -146,6 +146,9 @@ fn on_init() void {
     // capture and hide the mouse!
     papp.captureMouse(true);
 
+    // set a FPS limit, to test that as well
+    papp.setTargetFPS(60);
+
     sprite_batch = batcher.SpriteBatcher.init(.{}) catch {
         debug.showErrorScreen("Fatal error during batch init!");
         return;

src/framework/platform/app.zig

@@ -10,6 +10,7 @@ const AppBackend = sokol_app_backend.App;
 
 const NS_PER_SECOND: i64 = 1_000_000_000;
 const NS_PER_SECOND_F: f32 = 1_000_000_000.0;
+const NS_FPS_LIMIT_OVERHEAD = 1_250_000; // tuned to ensure consistent frame pacing
 
 const state = struct {
     // FPS cap vars, if set
@@ -148,23 +149,44 @@ fn on_frame() void {
 
     // tell modules this frame is done
     modules.postDrawModules();
-}
 
-/// Get time elapsed since last tick. Also calculate the FPS!
-fn calcDeltaTime() f32 {
-    if (state.reset_delta) {
-        state.reset_delta = false;
-        state.game_loop_timer.reset();
-        return 1.0 / 60.0;
-    }
+    // keep under our FPS limit, if needed
+    limitFps();
+}
 
+fn limitFps() void {
     if (state.target_fps != null) {
         // Try to hit our target FPS!
-        const frame_len_ns = state.game_loop_timer.read();
 
+        // Easy case, just stop here if we are under the target frame length
+        const initial_frame_ns = state.game_loop_timer.read();
+        if (initial_frame_ns >= state.target_fps_ns)
+            return;
+
+        // Harder case, we are faster than the target frame length.
+        // Note: time.sleep does not ensure consistent timing.
+        // Due to this we need to sleep most of the time, but busy loop the rest.
+
+        const frame_len_ns = initial_frame_ns + NS_FPS_LIMIT_OVERHEAD;
         if (frame_len_ns < state.target_fps_ns) {
             time.sleep(state.target_fps_ns - frame_len_ns);
         }
+
+        // Eat up the rest of the time in a busy loop to ensure consistent frame pacing
+        while (true) {
+            const cur_frame_len_ns = state.game_loop_timer.read();
+            if (cur_frame_len_ns >= state.target_fps_ns)
+                break;
+        }
+    }
+}
+
+/// Get time elapsed since last tick. Also calculate the FPS!
+fn calcDeltaTime() f32 {
+    if (state.reset_delta) {
+        state.reset_delta = false;
+        state.game_loop_timer.reset();
+        return 1.0 / 60.0;
     }
 
     // calculate the fps by counting frames each second