"Fix" for [#25184] Forces for growing hair - update inconsistency - as discussed with Jahka on Saturday on IRC

* New option to "Regrow hair" for each frame.
* This was perhaps more a feature request, but there was a similar useful feature called "animated hair" in particles at some point.
* The previous behavior for hair growing was inconsistent to say the least, so this is a nice option to have.

6 files changed, 13 insertions, 4 deletions

diff --git a/release/scripts/ui/properties_particle.py b/release/scripts/ui/properties_particle.py
index 1565ece3b40..af60c5fd784 100644
--- a/release/scripts/ui/properties_particle.py
+++ b/release/scripts/ui/properties_particle.py
@@ -125,7 +125,9 @@ class PARTICLE_PT_context_particles(ParticleButtonsPanel, bpy.types.Panel):
                     if psys.is_edited:
                         split.operator("particle.edited_clear", text="Free Edit")
                     else:
-                        split.label(text="")
+                        row = split.row()
+                        row.enabled = particle_panel_enabled(context, psys)
+                        row.prop(part, "regrow_hair")
                     row = split.row()
                     row.enabled = particle_panel_enabled(context, psys)
                     row.prop(part, "hair_step")
diff --git a/source/blender/blenkernel/intern/particle.c b/source/blender/blenkernel/intern/particle.c
index a81406f0af1..2ec90eb29db 100644
--- a/source/blender/blenkernel/intern/particle.c
+++ b/source/blender/blenkernel/intern/particle.c
@@ -3473,7 +3473,7 @@ static void default_particle_settings(ParticleSettings *part)
 	part->bb_uv_split=1;
 	part->bb_align=PART_BB_VIEW;
 	part->bb_split_offset=PART_BB_OFF_LINEAR;
-	part->flag=PART_REACT_MULTIPLE|PART_HAIR_GEOMETRY|PART_EDISTR|PART_TRAND;
+	part->flag=PART_EDISTR|PART_TRAND;
 
 	part->sta= 1.0;
 	part->end= 200.0;
diff --git a/source/blender/blenkernel/intern/particle_system.c b/source/blender/blenkernel/intern/particle_system.c
index 60f54cfebf2..c7a65b8a5ee 100644
--- a/source/blender/blenkernel/intern/particle_system.c
+++ b/source/blender/blenkernel/intern/particle_system.c
@@ -4006,7 +4006,7 @@ static void psys_prepare_physics(ParticleSimulationData *sim)
 static int hair_needs_recalc(ParticleSystem *psys)
 {
 	if(!(psys->flag & PSYS_EDITED) && (!psys->edit || !psys->edit->edited) &&
-		((psys->flag & PSYS_HAIR_DONE)==0 || psys->recalc & PSYS_RECALC_RESET)) {
+		((psys->flag & PSYS_HAIR_DONE)==0 || psys->recalc & PSYS_RECALC_RESET || (psys->part->flag & PART_HAIR_REGROW && !psys->edit))) {
 		return 1;
 	}
 
diff --git a/source/blender/blenloader/intern/readfile.c b/source/blender/blenloader/intern/readfile.c
index 44fbf9ef317..2661f046178 100644
--- a/source/blender/blenloader/intern/readfile.c
+++ b/source/blender/blenloader/intern/readfile.c
@@ -11173,6 +11173,8 @@ static void do_versions(FileData *fd, Library *lib, Main *main)
 		for(part = main->particle.first; part; part = part->id.next) {
 			if(part->boids)
 				part->boids->pitch = 1.0f;
+
+			part->flag &= ~PART_HAIR_REGROW; /* this was a deprecated flag before */
 		}
 
 		for (sc= main->screen.first; sc; sc= sc->id.next) {
diff --git a/source/blender/makesdna/DNA_particle_types.h b/source/blender/makesdna/DNA_particle_types.h
index 105fa6d5289..e336d3056ea 100644
--- a/source/blender/makesdna/DNA_particle_types.h
+++ b/source/blender/makesdna/DNA_particle_types.h
@@ -273,7 +273,7 @@ typedef struct ParticleSystem{				/* note, make sure all (runtime) are NULL's in
 		/* for dopesheet */
 #define PART_DS_EXPAND		8
 
-#define PART_HAIR_GEOMETRY	16
+#define PART_HAIR_REGROW	16	/* regrow hair for each frame */
 
 #define PART_UNBORN			32	/*show unborn particles*/
 #define PART_DIED			64	/*show died particles*/
diff --git a/source/blender/makesrna/intern/rna_particle.c b/source/blender/makesrna/intern/rna_particle.c
index 649ba0f3a1d..90aeba65118 100644
--- a/source/blender/makesrna/intern/rna_particle.c
+++ b/source/blender/makesrna/intern/rna_particle.c
@@ -1183,6 +1183,11 @@ static void rna_def_particle_settings(BlenderRNA *brna)
 	RNA_def_property_ui_text(prop, "Multi React", "React multiple times");
 	RNA_def_property_update(prop, 0, "rna_Particle_reset");
 
+	prop= RNA_def_property(srna, "regrow_hair", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_sdna(prop, NULL, "flag", PART_HAIR_REGROW);
+	RNA_def_property_ui_text(prop, "Regrow", "Regrow hair for each frame");
+	RNA_def_property_update(prop, 0, "rna_Particle_redo");
+
 	prop= RNA_def_property(srna, "show_unborn", PROP_BOOLEAN, PROP_NONE);
 	RNA_def_property_boolean_sdna(prop, NULL, "flag", PART_UNBORN);
 	RNA_def_property_ui_text(prop, "Unborn", "Show particles before they are emitted");