pro plot_pixels_color,data,lon,lat,pix_size=pix_size,levels=levels,color=color
;
;   plot pixels with a given size or determined by the pixel distances 
;
;
   num=n_elements(data)
   if (n_elements(data) ne n_elements(lat) and n_elements(data) ne n_elements(lon)) then begin
   print,'data must have same numbers as x and y'  
   return
   endif
   if(n_elements(pix_size) lt 1) then begin
    pix_size=fltarr(2)
    dis=(lon(0:n_elements(lon)-2)-lon(1:n_elements(lon)-1))^2+$
         (lat(0:n_elements(lat)-2)-lat(1:n_elements(lat)-1))^2
    pix_size(0)=min(sqrt(dis))*0.75
    pix_size(1)=min(sqrt(dis))*0.75
   print,'pixels size is:',pix_size
   print,'Note: the pixel size calculated may not be correct. Then use: ,pix_size=[xsize,ysize] in the option'
   endif
   if(n_elements(levels) lt 1 or n_elements(color) lt 1) then begin
      restore,'~/idllib/pub/c_liu.clr'
      ind=5+indgen(21)*12
     color=c_liu.colors(5,ind)
     color(0)=c_liu.white
     levels=min(data)+(max(data)-min(data))/22.0*(indgen(22))
   endif 
    step=levels(1)-levels(0)
   for k=0L,num-1 do begin
     ind=where(data(k) le levels[where(data(k) ge levels)+1])
     colorindex=ind[0]
     if(colorindex gt n_elements(color)-1) then colorindex=n_elements(color)-1
     if(colorindex gt n_elements(color)) then print,'impossible'
     if(colorindex ge 0) then ccolor=color(colorindex)
     if(colorindex lt 0) then ccolor=0
     polyfill,[lon(k)-pix_size(0),lon(k)+pix_size(0),lon(k)+pix_size(0),lon(k)-pix_size(0),lon(k)-pix_size(0)],$
              [lat(k)+pix_size(1),lat(k)+pix_size(1),lat(k)-pix_size(1),lat(k)-pix_size(1),lat(k)+pix_size(1)],$
              color=ccolor
   endfor
end