Simple hair children: Make twist affected by vertex group

The idea is to give a control over direction of twist, and maybe amount of
twist as well. More concrete example: make braids on left and right side of
character head to be twisting opposite directions.

Now, tricky part: we need some negative values to flip direction, but weights
can not be negative. So we use same trick as displacement map and tangent normal
maps, where 0.5 is neutral, values below 0.5 are considered negative and values
above 0.5 are considered positive.

4 files changed, 13 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_particle.h b/source/blender/blenkernel/BKE_particle.h
index 747a6b65bb0..0cda7dceb33 100644
--- a/source/blender/blenkernel/BKE_particle.h
+++ b/source/blender/blenkernel/BKE_particle.h
@@ -112,6 +112,7 @@ typedef struct ParticleTexture {
 	float damp, gravity, field;           /* used in physics */
 	float length, clump, kink_freq, kink_amp, effector;  /* used in path caching */
 	float rough1, rough2, roughe;         /* used in path caching */
+	float twist;  /* used in path caching */
 } ParticleTexture;
 
 typedef struct ParticleSeam {
@@ -159,6 +160,7 @@ typedef struct ParticleThreadContext {
 	float *vg_length, *vg_clump, *vg_kink;
 	float *vg_rough1, *vg_rough2, *vg_roughe;
 	float *vg_effector;
+	float *vg_twist;
 
 	struct CurveMapping *clumpcurve;
 	struct CurveMapping *roughcurve;
diff --git a/source/blender/blenkernel/intern/particle.c b/source/blender/blenkernel/intern/particle.c
index ea27e23ed2f..723bb86cb47 100644
--- a/source/blender/blenkernel/intern/particle.c
+++ b/source/blender/blenkernel/intern/particle.c
@@ -2136,6 +2136,7 @@ static bool psys_thread_context_init_path(
 	ctx->vg_rough1 = psys_cache_vgroup(ctx->dm, psys, PSYS_VG_ROUGH1);
 	ctx->vg_rough2 = psys_cache_vgroup(ctx->dm, psys, PSYS_VG_ROUGH2);
 	ctx->vg_roughe = psys_cache_vgroup(ctx->dm, psys, PSYS_VG_ROUGHE);
+	ctx->vg_twist = psys_cache_vgroup(ctx->dm, psys, PSYS_VG_TWIST);
 	if (psys->part->flag & PART_CHILD_EFFECT)
 		ctx->vg_effector = psys_cache_vgroup(ctx->dm, psys, PSYS_VG_EFFECTOR);
 
@@ -3502,6 +3503,7 @@ static void get_cpa_texture(DerivedMesh *dm, ParticleSystem *psys, ParticleSetti
 	ptex->ivel = ptex->life = ptex->exist = ptex->size = ptex->damp =
 	ptex->gravity = ptex->field = ptex->time = ptex->clump = ptex->kink_freq = ptex->kink_amp =
 	ptex->effector = ptex->rough1 = ptex->rough2 = ptex->roughe = 1.0f;
+	ptex->twist = 1.0f;
 
 	ptex->length = 1.0f - part->randlength * psys_frand(psys, child_index + 26);
 	ptex->length *= part->clength_thres < psys_frand(psys, child_index + 27) ? part->clength : 1.0f;
@@ -3579,6 +3581,7 @@ void psys_get_texture(ParticleSimulationData *sim, ParticleData *pa, ParticleTex
 	ptex->ivel = ptex->life = ptex->exist = ptex->size = ptex->damp =
 	ptex->gravity = ptex->field = ptex->length = ptex->clump = ptex->kink_freq = ptex->kink_amp =
 	ptex->effector = ptex->rough1 = ptex->rough2 = ptex->roughe = 1.0f;
+	ptex->twist = 1.0f;
 
 	ptex->time = (float)(pa - sim->psys->particles) / (float)sim->psys->totpart;
 
@@ -3744,6 +3747,8 @@ static void get_child_modifier_parameters(ParticleSettings *part, ParticleThread
 		ptex->roughe *= psys_interpolate_value_from_verts(ctx->dm, cpa_from, cpa_num, cpa_fuv, ctx->vg_roughe);
 	if (ctx->vg_effector)
 		ptex->effector *= psys_interpolate_value_from_verts(ctx->dm, cpa_from, cpa_num, cpa_fuv, ctx->vg_effector);
+	if (ctx->vg_twist)
+		ptex->twist *= psys_interpolate_value_from_verts(ctx->dm, cpa_from, cpa_num, cpa_fuv, ctx->vg_twist);
 }
 /* get's hair (or keyed) particles state at the "path time" specified in state->time */
 void psys_get_particle_on_path(ParticleSimulationData *sim, int p, ParticleKey *state, const bool vel)
diff --git a/source/blender/blenkernel/intern/particle_child.c b/source/blender/blenkernel/intern/particle_child.c
index 53bf577d4f4..fae304d6580 100644
--- a/source/blender/blenkernel/intern/particle_child.c
+++ b/source/blender/blenkernel/intern/particle_child.c
@@ -734,6 +734,7 @@ static void do_twist(const ParticleChildModifierContext *modifier_ctx,
 {
 	ParticleThreadContext *thread_ctx = modifier_ctx->thread_ctx;
 	ParticleSimulationData *sim = modifier_ctx->sim;
+	ParticleTexture *ptex = modifier_ctx->ptex;
 	ParticleSettings *part = sim->psys->part;
 	/* Early output checks. */
 	if (part->childtype != PART_CHILD_PARTICLES) {
@@ -757,6 +758,9 @@ static void do_twist(const ParticleChildModifierContext *modifier_ctx,
 	twist_get_axis(modifier_ctx, time, axis);
 	/* Angle of rotation. */
 	float angle = part->twist;
+	if (ptex != NULL) {
+		angle *= (ptex->twist - 0.5f) * 2.0f;
+	}
 	if (twist_curve != NULL) {
 		const int num_segments = twist_num_segments(modifier_ctx);
 		angle *= curvemapping_integrate_clamped(twist_curve,
diff --git a/source/blender/blenkernel/intern/particle_system.c b/source/blender/blenkernel/intern/particle_system.c
index bae76354705..7851cd5e801 100644
--- a/source/blender/blenkernel/intern/particle_system.c
+++ b/source/blender/blenkernel/intern/particle_system.c
@@ -497,6 +497,8 @@ void psys_thread_context_free(ParticleThreadContext *ctx)
 		MEM_freeN(ctx->vg_rough2);
 	if (ctx->vg_roughe)
 		MEM_freeN(ctx->vg_roughe);
+	if (ctx->vg_twist)
+		MEM_freeN(ctx->vg_twist);
 
 	if (ctx->sim.psys->lattice_deform_data) {
 		end_latt_deform(ctx->sim.psys->lattice_deform_data);