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}

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")

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);
 		}
 	}
 

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);

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;

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

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;

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)