From dadaf61c45d675f0e8b88fbc231748ad8247a736 Mon Sep 17 00:00:00 2001
From: 3gg <3gg@shellblade.net>
Date: Mon, 20 Apr 2026 18:03:09 -0700
Subject: Percent frame interpolation factor for smooth animation

---
 simloop/include/simloop.h | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

(limited to 'simloop/include/simloop.h')

diff --git a/simloop/include/simloop.h b/simloop/include/simloop.h
index c5a0372..7774c35 100644
--- a/simloop/include/simloop.h
+++ b/simloop/include/simloop.h
@@ -5,14 +5,17 @@
  * simulation must be updated and/or the result rendered.
  *
  * The simulation is updated at a fixed time step given a desired frame rate.
- * Rendering frame rate can likewise be capped or be unlimited. In any case, the
- * loop guarantees that the same frame is not rendered twice.
+ * Rendering frame rate can likewise be capped or be unlimited.
+ * In any case, an interpolation factor is computed for smooth animation between
+ * updates.
+ * The implementation also guarantees that the same frame is not rendered twice
+ * if time does not advance.
  *
  * Generally, the simulation's update logic should be able to keep up with the
  * requested frame rate; it is the application's responsibility to ensure this.
  * Should the update logic not be able to keep up, then the loop requests a
  * single update per iteration, effectively "degrading" to match the update
- * logic frame rate, and giving the update logic a chance to catch up with
+ * logic frame rate, giving the update logic a chance to catch up with
  * subsequent loop iterations.
  *
  * Under a variable time delta, the loop could simply update the simulation
@@ -57,8 +60,10 @@ typedef struct SimloopOut {
   simloop_time_t render_elapsed; ///< Amount of time elapsed in the rendering.
   simloop_time_t update_elapsed; ///< Amount of time elapsed in the simulation.
   simloop_time_t update_dt;      ///< Delta time for simulation updates.
-  bool           should_update;  ///< Whether the simulation should update.
-  bool           should_render;  ///< Whether the simulation should be rendered.
+  double         percent_frame;  ///< Percent progress between this frame and
+                                 ///< the next. Used for smooth animation.
+  bool should_update;            ///< Whether the simulation should update.
+  bool should_render;            ///< Whether the simulation should be rendered.
 } SimloopOut;
 
 typedef struct SimloopTimeline {
@@ -71,8 +76,8 @@ typedef struct Simloop {
   uint64_t        frame;  ///< Frame counter, number of updates done.
   SimloopTimeline update; ///< Update timeline.
   SimloopTimeline render; ///< Render timeline.
+  double          percent_frame;
   bool            first_iter;
-  bool            updates_since_last_render;
 } Simloop;
 
 /// Create a simulation loop.
-- 
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