Foot Placement with Inverse Kinematics

[Josh]

I wanted to make a character's feet align to the ground when standing on a slope or on stairs. This was very easy to get a basic version set up.

The code here only handles the feet, so the legs currently will get stretched out. Each model may require a little bit different final rotation.

The next step is to adjust the knee bones. The hip, knee, and foot form a triangle. One of the constructors for the Plane class accepts three points in space, so those three bones can be used to find a plane. Then you transform their positions to a matrix facing in the direction of the plane normal, solve the triangle, transform back, and that's how IK works.

void MyComponent::UpdateFeet()
{
    auto entity = GetEntity();
    auto world = entity->GetWorld();
    if (not world) return;
    for (int n = 0; n < footbones.size(); ++n)
    {
        if (not footbones[n]) continue;
        auto pos = footbones[n]->GetPosition(true);
        pos.y = 0;
        pos = TransformPoint(pos, entity, NULL);
        auto pickmode = entity->GetPickMode();
        entity->SetPickMode(PICK_NONE);
        auto pickinfo = world->Pick(pos + Vec3(0, footradius + 0.75, 0), pos - Vec3(0, footreach, 0), footradius, true);
        entity->SetPickMode(pickmode);
        if (pickinfo.entity)
        {
            pos = TransformPoint(pickinfo.position, NULL, entity);
            footbones[n]->SetPosition(pos, true);
            Vec3 normal = TransformNormal(pickinfo.normal, NULL, entity);
            footbones[n]->AlignToVector(-normal, 1);
            footbones[n]->Turn(0, 180, 0);
        }
    }
}