Can now set a lower bound on the number of subframes (i.e. an upper bound on the time step size) for fluid simulations. Previously, the "subframes" parameter was not available when the adaptive time step was enabled; now they can both be set. The two settings can be used together to greatly increase simulation stability.

1 files changed, 27 insertions, 7 deletions

diff --git a/source/blender/blenkernel/intern/particle_system.c b/source/blender/blenkernel/intern/particle_system.c
index 90889d7c09e..d5fabf60079 100644
--- a/source/blender/blenkernel/intern/particle_system.c
+++ b/source/blender/blenkernel/intern/particle_system.c
@@ -3781,11 +3781,12 @@ static void save_hair(ParticleSimulationData *sim, float UNUSED(cfra))
 
 /* Code for an adaptive time step based on the Courant-Friedrichs-Lewy
  * condition. */
-#define MIN_TIMESTEP 1.0f / 101.0f
+static const float MIN_TIMESTEP = 1.0f / 101.0f;
 /* Tolerance of 1.5 means the last subframe neither favors growing nor
  * shrinking (e.g if it were 1.3, the last subframe would tend to be too
  * small). */
-#define TIMESTEP_EXPANSION_TOLERANCE 1.5f
+static const float TIMESTEP_EXPANSION_FACTOR = 0.1f;
+static const float TIMESTEP_EXPANSION_TOLERANCE = 1.5f;
 
 /* Calculate the speed of the particle relative to the local scale of the
  * simulation. This should be called once per particle during a simulation
@@ -3802,14 +3803,31 @@ static void update_courant_num(ParticleSimulationData *sim, ParticleData *pa,
 	if (sim->courant_num < speed * dtime / sphdata->element_size)
 		sim->courant_num = speed * dtime / sphdata->element_size;
 }
+static float get_base_time_step(ParticleSettings *part) {
+	return 1.0f / (float) (part->subframes + 1);
+}
 /* Update time step size to suit current conditions. */
 static float update_timestep(ParticleSystem *psys, ParticleSimulationData *sim, float t_frac)
 {
+	float dt_target;
 	if (sim->courant_num == 0.0f)
-		psys->dt_frac = 1.0f;
+		dt_target = 1.0f;
+	else
+		dt_target = psys->dt_frac * (psys->part->courant_target / sim->courant_num);
+
+	/* Make sure the time step is reasonable.
+	 * For some reason, the CLAMP macro doesn't work here. The time step becomes
+	 * too large. - z0r */
+	if (dt_target < MIN_TIMESTEP)
+		dt_target = MIN_TIMESTEP;
+	else if (dt_target > get_base_time_step(psys->part))
+		dt_target = get_base_time_step(psys->part);
+
+	/* Decrease time step instantly, but expand slowly. */
+	if (dt_target > psys->dt_frac)
+		psys->dt_frac = interpf(dt_target, psys->dt_frac, TIMESTEP_EXPANSION_FACTOR);
 	else
-		psys->dt_frac *= (psys->part->courant_target / sim->courant_num);
-	CLAMP(psys->dt_frac, MIN_TIMESTEP, 1.0f);
+		psys->dt_frac = dt_target;
 
 	/* Sync with frame end if it's close. */
 	if (t_frac == 1.0f)
@@ -4307,14 +4325,16 @@ static void system_step(ParticleSimulationData *sim, float cfra)
 		if ((int)cfra == startframe && part->sta < startframe)
 			totframesback = (startframe - (int)part->sta);
 
+		/* Initialise time step. This may change if automatic subframes are
+		 * enabled. */
 		if (!(part->time_flag & PART_TIME_AUTOSF)) {
 			/* Constant time step */
-			psys->dt_frac = 1.0f / (float) (part->subframes + 1);
+			psys->dt_frac = get_base_time_step(part);
 		}
 		else if ((int)cfra == startframe) {
 			/* Variable time step; use a very conservative value at the start.
 			 * If it doesn't need to be so small, it will quickly grow. */
-			psys->dt_frac = 1.0;
+			psys->dt_frac = get_base_time_step(part);
 		}
 		else if (psys->dt_frac < MIN_TIMESTEP) {
 			psys->dt_frac = MIN_TIMESTEP;