Fixed Timestep Interpolation (2D)

Adds fixed timestep interpolation to the rendering server (2D only).
Switchable on and off with a project setting (default is off).

Co-authored-by: lawnjelly <lawnjelly@gmail.com>

2 files changed, 16 insertions, 0 deletions

diff --git a/main/main.cpp b/main/main.cpp
index 9215f2e848..f1ba25bf02 100644
--- a/main/main.cpp
+++ b/main/main.cpp
@@ -3967,6 +3967,11 @@ bool Main::iteration() {
 		PhysicsServer3D::get_singleton()->flush_queries();
 #endif // _3D_DISABLED
 
+		// Prepare the fixed timestep interpolated nodes BEFORE they are updated
+		// by the physics server, otherwise the current and previous transforms
+		// may be the same, and no interpolation takes place.
+		OS::get_singleton()->get_main_loop()->iteration_prepare();
+
 		PhysicsServer2D::get_singleton()->sync();
 		PhysicsServer2D::get_singleton()->flush_queries();
 
diff --git a/main/main_timer_sync.cpp b/main/main_timer_sync.cpp
index 569930d427..d358d9fa93 100644
--- a/main/main_timer_sync.cpp
+++ b/main/main_timer_sync.cpp
@@ -299,6 +299,17 @@ int64_t MainTimerSync::DeltaSmoother::smooth_delta(int64_t p_delta) {
 // before advance_core considers changing the physics_steps return from
 // the typical values as defined by typical_physics_steps
 double MainTimerSync::get_physics_jitter_fix() {
+	// Turn off jitter fix when using fixed timestep interpolation.
+	// Note this shouldn't be on UNTIL 3d interpolation is implemented,
+	// otherwise we will get people making 3d games with the physics_interpolation
+	// set to on getting jitter fix disabled unexpectedly.
+#if 0
+	if (Engine::get_singleton()->is_physics_interpolation_enabled()) {
+		// Would be better to write a simple bypass for jitter fix but this will do to get started.
+		return 0.0;
+	}
+#endif
+
 	return Engine::get_singleton()->get_physics_jitter_fix();
 }