Move hairs that are inside the geometry back to the surface (VEX)

Example: After using noise functions on hair, some of the points might end up inside the geometry. This script will move the points that are inside a geometry to the surface.

Logic around the script:

• Check if the point is inside a geometry

  • find the nearest point on the skin geometry with xyzdist() vex function

  • store the nearest primitive number and barycentric UV coordinate to variables

  • find the nearest point position on the skin geo with primuv() vex function

  • calculate the direction vector to the surface

  • calculate the closest primitives normal at the nearest point position

  • if the direction vector and evaluated surface normal vectors dot product is greater than zero the vectors are pointing away from each other, therefore the point is inside the geometry

• for the points inside geometry, move along the surface direction vector multiplied by the distance to the surface.

• To add padding create a slider.

float dist;
int near_prim;
vector near_primuv;
vector near_pos;
vector dir_surface;
vector primN;
float padding = chf("padding");

dist = xyzdist(1,@P,near_prim,near_primuv);

near_pos = primuv(1, "P", near_prim, near_primuv);

dir_surface = normalize(near_pos - @P);

primN = prim_normal(1, near_prim, near_primuv);


float dot = dot(dir_surface, primN);

if ( dot > 0){  
    @P += (dist  + padding) * dir_surface;
}