(define (elastic)
    (define (update-mass n)
        (let ((v (vmul (vsub (pdata-get "pref" n) (pdata-get "p" n)) 0.01)))
        (pdata-set "vel" n (vmul (vadd (pdata-get "vel" n) v) 0.95)))
        (if (zero? n)
            0
            (update-mass (- n 1))))

    (update-mass (pdata-size))
    (pdata-op "+" "p" "vel"))

(define (perturb strength)
    (define (loop n)
        (pdata-set "vel" n (vadd (pdata-get "vel" n) 
            (vmul (vadd (vector (flxrnd) (flxrnd) (flxrnd)) 
            (vector -0.5 -0.5 -0.5)) (gh n))))
        (if (zero? n)
            0
            (loop (- n 1))))
    (loop (pdata-size)))

(clear)
(blur 0.1)
(backfacecull 0)
;(hint-none)
(point-width 5)
(wire-colour (vector 0 0 1))
(line-width 2)
;(hint-normal)
(hint-points)
;(hint-wire)
;(opacity 0.5)
(texture (load-texture "mud.png"))

(define o (build-nurbs-sphere 8 12))
(grab o)
(pdata-copy "p" "pref")
(pdata-add "vel" "v")
(perturb 1)
(ungrab)
(gain 1)

(define (render)
    (grab o)
    (perturb 1)
    (elastic)
    (recalc-normals 0)
    (ungrab))
    
(every-frame (render))