From 896a6ef5959043db5463637d84ed524ae7bade1e Mon Sep 17 00:00:00 2001
From: 3gg <3gg@shellblade.net>
Date: Sat, 11 Apr 2026 16:35:25 -0700
Subject: Render only if there was an update

---
 simloop/include/simloop.h   |  1 +
 simloop/src/simloop.c       | 17 +++++++++++++----
 simloop/test/simloop_test.c | 14 ++++++++------
 3 files changed, 22 insertions(+), 10 deletions(-)

(limited to 'simloop')

diff --git a/simloop/include/simloop.h b/simloop/include/simloop.h
index 6ee3b98..1e5b4e0 100644
--- a/simloop/include/simloop.h
+++ b/simloop/include/simloop.h
@@ -67,6 +67,7 @@ typedef struct Simloop {
   uint64_t        frame;  ///< Frame counter.
   Timer*          timer;
   bool            first_iter;
+  bool            updates_since_last_render;
 } Simloop;
 
 /// Create a simulation loop.
diff --git a/simloop/src/simloop.c b/simloop/src/simloop.c
index aa2b6b7..606f5ed 100644
--- a/simloop/src/simloop.c
+++ b/simloop/src/simloop.c
@@ -19,6 +19,7 @@ Simloop simloop_make(const SimloopArgs* args) {
                                       .last_step = args->timer->start_time},
       .timer      = args->timer,
       .first_iter = true,
+      .updates_since_last_render = false,
   };
 }
 
@@ -57,11 +58,19 @@ void simloop_update(Simloop* sim, SimloopOut* out) {
   assert(sim);
   assert(out);
 
-  out->should_update = step_update(sim, &sim->update);
-  out->should_render =
-      step_render(sim, &sim->render) ||
+  // Simulation update.
+  const bool updated             = step_update(sim, &sim->update);
+  out->should_update             = updated;
+  sim->updates_since_last_render = sim->updates_since_last_render || updated;
+  // Simulation render.
+  const bool rendered =
+      (sim->updates_since_last_render && step_render(sim, &sim->render)) ||
       (sim->first_iter); // Trigger an initial render on the first frame.
-  sim->frame += (out->should_update ? 1 : 0);
+  out->should_render = rendered;
+  sim->updates_since_last_render =
+      sim->updates_since_last_render && !out->should_render;
+  // Loop state update.
+  sim->frame += (updated ? 1 : 0);
   sim->first_iter     = false;
   out->frame          = sim->frame;
   out->render_elapsed = time_elapsed(sim, sim->render.last_step);
diff --git a/simloop/test/simloop_test.c b/simloop/test/simloop_test.c
index c79ee32..9f11e86 100644
--- a/simloop/test/simloop_test.c
+++ b/simloop/test/simloop_test.c
@@ -44,7 +44,7 @@ TEST_CASE(simloop_initial_render_not_retriggered) {
 
 /// A simulation loop with no render frame cap:
 ///   1. Updates based on the desired update frame rate.
-///   2. Renders at every loop.
+///   2. Renders at every loop (provided there are updates).
 TEST_CASE(simloop_no_render_frame_cap) {
   constexpr int    UPDATE_FPS    = 10;
   const time_delta EXPECT_UPDATE = sec_to_time_delta(1.0 / (double)UPDATE_FPS);
@@ -59,8 +59,10 @@ TEST_CASE(simloop_no_render_frame_cap) {
   for (time_delta t = 0; t < SIM_TIME_SEC; t += STEP) {
     timer_advance(&timer, t);
     simloop_update(&simloop, &simout);
-    TEST_TRUE(simout.should_render);
-    TEST_EQUAL((t > 0) && ((t % EXPECT_UPDATE) == 0), simout.should_update);
+    const bool expect_update = (t > 0) && ((t % EXPECT_UPDATE) == 0);
+    // A render is still expected at time 0.
+    TEST_EQUAL(simout.should_render, (t == 0) || expect_update);
+    TEST_EQUAL(simout.should_update, expect_update);
   }
 }
 
@@ -83,9 +85,9 @@ TEST_CASE(simloop_with_render_frame_cap) {
   for (time_delta t = 0; t < SIM_TIME_SEC; t += STEP) {
     timer_advance(&timer, t);
     simloop_update(&simloop, &simout);
-    // Also expecting initial render at t=0.
-    TEST_EQUAL((t % EXPECT_RENDER) == 0, simout.should_render);
-    TEST_EQUAL((t > 0) && ((t % EXPECT_UPDATE) == 0), simout.should_update);
+    // A render is still expected at time 0.
+    TEST_EQUAL(simout.should_render, (t % EXPECT_RENDER) == 0);
+    TEST_EQUAL(simout.should_update, (t > 0) && ((t % EXPECT_UPDATE) == 0));
   }
 }
 
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