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.

1 files changed, 5 insertions, 0 deletions

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)