function tracks,img,n_tracks=n_tracks,thickness=thickness,seed

;to apply particle tracks to image.

new_img=img

if keyword_set(n_tracks) then n_tracks=n_tracks else n_tracks=255
if keyword_set(thickness) then thickness=thickness else thickness=30
coords=fix(randomu(seed,2,n_tracks)*511)
azimuth=randomu(seed,n_tracks)*360.
length=sin(randomu(seed,n_tracks)*!pi/2.)/sin(!pi/thickness)

for i=0,n_tracks-1 do begin
	new_img(coords(0,i),coords(1,i))=1023
	if (length(i) > 1.) then begin
		xtrack=coords(0,i)+findgen(1>length(i))*sin(azimuth(i)*!pi/180.)
		ytrack=coords(1,i)+findgen(1>length(i))*cos(azimuth(i)*!pi/180.)
		for j=0,length(i)-1 do begin
				new_img((xtrack(j)>0)<511,(ytrack(j)>0)<511)=512
		endfor
	endif
endfor


new_img=(new_img > 0.) < 1023.

return,new_img
end