evaluate_capsule Function

    pure elemental function evaluate_capsule(this, pos) result(res)
        !! Evaluation function for Capsule SDF.

        use utils, only : clamp

        class(capsule), intent(in) :: this
        !> vector position to evaluate SDF at
        type(vector),   intent(in) :: pos
        real(kind=wp) :: res

        type(vector) :: pa, ba, p
        real(kind=wp) :: h

        p = pos .dot. this%transform

        pa = p - this%a
        ba = this%b - this%a
        h = clamp((pa .dot. ba) / (ba .dot. ba), 0._wp, 1._wp)
        res = length(pa - ba*h) - this%r

    end function evaluate_capsule