I need to create some graphics to teach energy. One of the illustrations is about the sound energy that I represent using linear_shade function. In the following example the first illustration is represented correctly, but the second call of the function SoundEnergy implies the loss of gradient. What am I doing wrong?

Thanks in advance

Jorge

\startMPinclusions

def SoundEnergy =

 path pr ; pr := unitsquare xscaled .3cm yscaled 2cm ;

 currentpicture:=nullpicture;

 for i=0 step 2 until 10:

   linear_shade(pr shifted (i*.3cm,0),0,white,black);

   linear_shade(pr shifted ((i+1)*.3cm,0),0,black,white);

 endfor;  

 clip currentpicture to (0,0)--(0,2cm)--(2cm,2cm)--(3cm,1cm)--(2cm,0)--cycle;

 draw (0,0)--(0,2cm)--(2cm,2cm)--(3cm,1cm)--(2cm,0)--cycle withcolor .625red; 

 addto finalpicture also currentpicture shifted(avanco*3.5cm,0) ;

 currentpicture := nullpicture ; avanco := avanco+1 ;     

enddef;

\stopMPinclusions

\starttext

\startMPcode

picture finalpicture ; finalpicture := nullpicture ;

numeric avanco ; avanco := 0 ; bboxmargin := 0pt ;

%% first call is OK

SoundEnergy;

%% second call loss the gradient

SoundEnergy;

currentpicture := finalpicture ;

\stopMPcode

\stoptext