Simple hair children: Make twist affected by texture

This completes twist feature, which is now possible to also control by
texture. Since textures can not easily contain negative values as well,
same trick with 0.5 neutral as vertex groups is used.

All in all, this twist features allows to do following things.

Original hair:

{F2287535}

Hair with scientifically calculated twist value of 0.5:

{F2287540}

And we can also twist braids in opposite directions dependent on left/right
side:

{F2287548}

7 files changed, 18 insertions, 2 deletions

diff --git a/release/scripts/startup/bl_ui/properties_texture.py b/release/scripts/startup/bl_ui/properties_texture.py
index 25e9ef3ee73..46b27c94419 100644
--- a/release/scripts/startup/bl_ui/properties_texture.py
+++ b/release/scripts/startup/bl_ui/properties_texture.py
@@ -1173,6 +1173,7 @@ class TEXTURE_PT_influence(TextureSlotPanel, Panel):
             col = split.column()
             factor_but(col, "use_map_length", "length_factor", "Length")
             factor_but(col, "use_map_clump", "clump_factor", "Clump")
+            factor_but(col, "use_map_twist", "twist_factor", "Twist")
 
             col = split.column()
             factor_but(col, "use_map_kink_amp", "kink_amp_factor", "Kink Amplitude")
diff --git a/source/blender/blenkernel/intern/particle.c b/source/blender/blenkernel/intern/particle.c
index 87322b5fddc..d7e08233601 100644
--- a/source/blender/blenkernel/intern/particle.c
+++ b/source/blender/blenkernel/intern/particle.c
@@ -3568,6 +3568,7 @@ static void get_cpa_texture(DerivedMesh *dm, ParticleSystem *psys, ParticleSetti
 			SET_PARTICLE_TEXTURE(PAMAP_KINK_AMP, ptex->kink_amp, mtex->kinkampfac);
 			SET_PARTICLE_TEXTURE(PAMAP_KINK_FREQ, ptex->kink_freq, mtex->kinkfac);
 			SET_PARTICLE_TEXTURE(PAMAP_DENS, ptex->exist, mtex->padensfac);
+			SET_PARTICLE_TEXTURE(PAMAP_TWIST, ptex->twist, mtex->twistfac);
 		}
 	}
 
@@ -3668,6 +3669,7 @@ void psys_get_texture(ParticleSimulationData *sim, ParticleData *pa, ParticleTex
 			SET_PARTICLE_TEXTURE(PAMAP_GRAVITY, ptex->gravity, mtex->gravityfac);
 			SET_PARTICLE_TEXTURE(PAMAP_DAMP, ptex->damp, mtex->dampfac);
 			SET_PARTICLE_TEXTURE(PAMAP_LENGTH, ptex->length, mtex->lengthfac);
+			SET_PARTICLE_TEXTURE(PAMAP_TWIST, ptex->twist, mtex->twistfac);
 		}
 	}
 
diff --git a/source/blender/blenkernel/intern/particle_child.c b/source/blender/blenkernel/intern/particle_child.c
index fae304d6580..48ecdb04348 100644
--- a/source/blender/blenkernel/intern/particle_child.c
+++ b/source/blender/blenkernel/intern/particle_child.c
@@ -358,6 +358,7 @@ void psys_apply_child_modifiers(ParticleThreadContext *ctx, struct ListBase *mod
 		totkeys = ctx->segments + 1;
 		max_length = ptex->length;
 
+		printf("%f\n", ptex->twist);
 		for (k = 0, key = keys; k < totkeys; k++, key++) {
 			ParticlePathIterator iter;
 			psys_path_iter_get(&iter, keys, totkeys, parent_keys, k);
diff --git a/source/blender/blenkernel/intern/texture.c b/source/blender/blenkernel/intern/texture.c
index fcedd880615..71b90641e02 100644
--- a/source/blender/blenkernel/intern/texture.c
+++ b/source/blender/blenkernel/intern/texture.c
@@ -412,6 +412,7 @@ void BKE_texture_mtex_default(MTex *mtex)
 	mtex->kinkfac = 1.0f;
 	mtex->kinkampfac = 1.0f;
 	mtex->roughfac = 1.0f;
+	mtex->twistfac = 1.0f;
 	mtex->padensfac = 1.0f;
 	mtex->lifefac = 1.0f;
 	mtex->sizefac = 1.0f;
diff --git a/source/blender/makesdna/DNA_particle_types.h b/source/blender/makesdna/DNA_particle_types.h
index 8dd4d46a596..9dbd1a18703 100644
--- a/source/blender/makesdna/DNA_particle_types.h
+++ b/source/blender/makesdna/DNA_particle_types.h
@@ -614,7 +614,8 @@ typedef enum eParticleTextureInfluence {
 	PAMAP_KINK_AMP	= (1<<12),
 	PAMAP_ROUGH		= (1<<9),
 	PAMAP_LENGTH	= (1<<4),
-	PAMAP_CHILD		= (PAMAP_CLUMP | PAMAP_KINK_FREQ | PAMAP_KINK_AMP | PAMAP_ROUGH | PAMAP_LENGTH),
+	PAMAP_TWIST	= (1<<13),
+	PAMAP_CHILD		= (PAMAP_CLUMP | PAMAP_KINK_FREQ | PAMAP_KINK_AMP | PAMAP_ROUGH | PAMAP_LENGTH | PAMAP_TWIST),
 } eParticleTextureInfluence;
 
 #endif
diff --git a/source/blender/makesdna/DNA_texture_types.h b/source/blender/makesdna/DNA_texture_types.h
index 995d7645dc0..0eb54a9b5b3 100644
--- a/source/blender/makesdna/DNA_texture_types.h
+++ b/source/blender/makesdna/DNA_texture_types.h
@@ -86,7 +86,7 @@ typedef struct MTex {
 	float timefac, lengthfac, clumpfac, dampfac;
 	float kinkfac, kinkampfac, roughfac, padensfac, gravityfac;
 	float lifefac, sizefac, ivelfac, fieldfac;
-	int pad2;
+	float twistfac;
 
 	/* lamp */
 	float shadowfac;
diff --git a/source/blender/makesrna/intern/rna_particle.c b/source/blender/makesrna/intern/rna_particle.c
index caf120e0ea1..2710a184367 100644
--- a/source/blender/makesrna/intern/rna_particle.c
+++ b/source/blender/makesrna/intern/rna_particle.c
@@ -1894,6 +1894,10 @@ static void rna_def_particle_settings_mtex(BlenderRNA *brna)
 	RNA_def_property_ui_text(prop, "Length", "Affect the child hair length");
 	RNA_def_property_update(prop, 0, "rna_Particle_redo_child");
 
+	prop = RNA_def_property(srna, "use_map_twist", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_sdna(prop, NULL, "mapto", PAMAP_TWIST);
+	RNA_def_property_ui_text(prop, "Twist", "Affect the child twist");
+	RNA_def_property_update(prop, 0, "rna_Particle_redo_child");
 
 	/* influence factors */
 	prop = RNA_def_property(srna, "time_factor", PROP_FLOAT, PROP_NONE);
@@ -1975,6 +1979,12 @@ static void rna_def_particle_settings_mtex(BlenderRNA *brna)
 	RNA_def_property_ui_range(prop, 0, 1, 10, 3);
 	RNA_def_property_ui_text(prop, "Rough Factor", "Amount texture affects child roughness");
 	RNA_def_property_update(prop, 0, "rna_Particle_redo_child");
+
+	prop = RNA_def_property(srna, "twist_factor", PROP_FLOAT, PROP_NONE);
+	RNA_def_property_float_sdna(prop, NULL, "twistfac");
+	RNA_def_property_ui_range(prop, 0, 1, 10, 3);
+	RNA_def_property_ui_text(prop, "Twist Factor", "Amount texture affects child twist");
+	RNA_def_property_update(prop, 0, "rna_Particle_redo_child");
 }
 
 static void rna_def_particle_settings(BlenderRNA *brna)