"Fix" for [#30098] Particle rotation wrong / explode modifier

- The main problem was that in order to be accurate all particle
  rotations have to be calculated incrementally so the only working
  solution is to store rotations to the point cache (previously
  this was only done for dynamic rotations). This can nearly double
  the point cache size so it's not ideal to have this as a default
  as in many cases you don't care about particle rotations.
- Particle rotation panel now has a new "enable" checkbox that
  enables rotation calculations and the storing of rotations to
  point cache.
- Old files will have rotations enabled via do_versions so that in
  the worst case old files will only get bigger point caches, but no
  sudden loss of particle rotations.

release/scripts/startup/bl_ui/properties_particle.py

@@ -380,6 +380,7 @@ class PARTICLE_PT_velocity(ParticleButtonsPanel, Panel):
 
 class PARTICLE_PT_rotation(ParticleButtonsPanel, Panel):
     bl_label = "Rotation"
+    bl_options = {'DEFAULT_CLOSED'}
     COMPAT_ENGINES = {'BLENDER_RENDER'}
 
     @classmethod
@@ -394,6 +395,15 @@ class PARTICLE_PT_rotation(ParticleButtonsPanel, Panel):
         else:
             return False
 
+    def draw_header(self, context):
+        psys = context.particle_system
+        if psys:
+            part = psys.settings
+        else:
+            part = context.space_data.pin_id
+
+        self.layout.prop(part, "use_rotations", text="")
+
     def draw(self, context):
         layout = self.layout
 
@@ -403,7 +413,7 @@ class PARTICLE_PT_rotation(ParticleButtonsPanel, Panel):
         else:
             part = context.space_data.pin_id
 
-        layout.enabled = particle_panel_enabled(context, psys)
+        layout.enabled = particle_panel_enabled(context, psys) and part.use_rotations
 
         layout.prop(part, "use_dynamic_rotation")
 

source/blender/blenkernel/intern/particle_system.c

@@ -2684,6 +2684,12 @@ static void basic_rotate(ParticleSettings *part, ParticleData *pa, float dfra, f
 {
 	float rotfac, rot1[4], rot2[4]={1.0,0.0,0.0,0.0}, dtime=dfra*timestep;
 
+	if((part->flag & PART_ROTATIONS)==0) {
+		pa->state.rot[0]=1.0f;
+		pa->state.rot[1]=pa->state.rot[2]=pa->state.rot[3]=0;
+		return;
+	}
+
 	if((part->flag & PART_ROT_DYN)==0) {
 		if(part->avemode==PART_AVE_SPIN) {
 			float angle;

source/blender/blenkernel/intern/pointcache.c

@@ -315,9 +315,10 @@ static void ptcache_particle_read(int index, void *psys_v, void **data, float cf
 		}
 	}
 
-	/* determine rotation from velocity */
+	/* default to no rotation */
 	if(data[BPHYS_DATA_LOCATION] && !data[BPHYS_DATA_ROTATION]) {
-		vec_to_quat( pa->state.rot,pa->state.vel, OB_NEGX, OB_POSZ);
+		pa->state.rot[0]=1.0f;
+		pa->state.rot[1]=pa->state.rot[2]=pa->state.rot[3]=0;
 	}
 }
 static void ptcache_particle_interpolate(int index, void *psys_v, void **data, float cfra, float cfra1, float cfra2, float *old_data)
@@ -815,13 +816,14 @@ void BKE_ptcache_id_from_particles(PTCacheID *pid, Object *ob, ParticleSystem *p
 		pid->read_extra_data = ptcache_particle_extra_read;
 	}
 
-	if(psys->part->rotmode!=PART_ROT_VEL
-		|| psys->part->avemode!=PART_AVE_SPIN || psys->part->avefac!=0.0f)
-		pid->data_types|= (1<<BPHYS_DATA_AVELOCITY) | (1<<BPHYS_DATA_ROTATION);
-
-	if(psys->part->flag & PART_ROT_DYN)
+	if(psys->part->flag & PART_ROTATIONS) {
 		pid->data_types|= (1<<BPHYS_DATA_ROTATION);
 
+		if(psys->part->rotmode!=PART_ROT_VEL
+			|| psys->part->avemode!=PART_AVE_SPIN || psys->part->avefac!=0.0f)
+			pid->data_types|= (1<<BPHYS_DATA_AVELOCITY);
+	}
+
 	pid->info_types= (1<<BPHYS_DATA_TIMES);
 
 	pid->default_step = 10;

source/blender/blenloader/intern/readfile.c

@@ -13292,6 +13292,13 @@ static void do_versions(FileData *fd, Library *lib, Main *main)
 		}
 	}
 
+	{
+		/* Default for old files is to save particle rotations to pointcache */
+		ParticleSettings *part;
+		for (part = main->particle.first; part; part = part->id.next)
+			part->flag |= PART_ROTATIONS;
+	}
+
 	/* put compatibility code here until next subversion bump */
 	{
 

source/blender/makesdna/DNA_particle_types.h

@@ -321,7 +321,7 @@ typedef struct ParticleSystem
 #define PART_TRAND			128	
 #define PART_EDISTR			256	/* particle/face from face areas */
 
-//#define PART_STICKY			512	/*collided particles can stick to collider*/
+#define PART_ROTATIONS		512	/* calculate particle rotations (and store them in pointcache) */
 #define PART_DIE_ON_COL		(1<<12)
 #define PART_SIZE_DEFL		(1<<13) /* swept sphere deflections */
 #define PART_ROT_DYN		(1<<14)	/* dynamic rotation */

source/blender/makesrna/intern/rna_particle.c

@@ -1661,6 +1661,12 @@ static void rna_def_particle_settings(BlenderRNA *brna)
 	RNA_def_property_ui_text(prop, "Size Deflect", "Use particle's size in deflection");
 	RNA_def_property_update(prop, 0, "rna_Particle_reset");
 
+	prop = RNA_def_property(srna, "use_rotations", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_sdna(prop, NULL, "flag", PART_ROTATIONS);
+	RNA_def_property_clear_flag(prop, PROP_ANIMATABLE);
+	RNA_def_property_ui_text(prop, "Rotations", "Calculate particle rotations");
+	RNA_def_property_update(prop, 0, "rna_Particle_reset");
+
 	prop = RNA_def_property(srna, "use_dynamic_rotation", PROP_BOOLEAN, PROP_NONE);
 	RNA_def_property_boolean_sdna(prop, NULL, "flag", PART_ROT_DYN);
 	RNA_def_property_clear_flag(prop, PROP_ANIMATABLE);