;*******************************************************************************
;*Usage: to calculate the substration between two multi-bands images.          *
;*******************************************************************************
;*Parameters:                                                                  *
;*BandSubstract,img1,img2,data_type,outimg                                     *
;* img1: the subtrahend image                                                  *
;* img2: the minuend image
;* data_type: a label is used to declare if the data_type of the images    *
;*            are byte format.                                                 *
;* out_img: the result for output.                                             *
;*******************************************************************************   

pro BandSubstract,img1,img2,data_type,outimg
;*to check if we can open the two images
 envi_open_file,img1,r_fid=fid1
 if fid1 eq -1 then begin
     message,'Cannot open file '+img1
 endif
 envi_open_file,img2,r_fid=fid2
 if fid2 eq -1 then begin
     message,'Cannot open file '+img2
 endif
 ;*to query the size parameters
 envi_file_query,fid1,ns=ns,nl=nl,nb=nb
 ;*to create an new images for output
 openw,lun,outimg,/get_lun
 dims=[-1,0,ns-1,0,nl-1]
 ;*make a substracting calculation band by band
 for i=0,nb-1 do begin
     data1=envi_get_data(fid=fid1,dims=dims,pos=i)
     data2=envi_get_data(fid=fid2,dims=dims,pos=i)
     if data_type eq 1 then begin
         out=int(data2)-int(data1)
     endif
     if data_type eq 1 then begin
          out=data2-data1
      endif
     writeu,lun,out
 endfor
 close,lun
 free_lun,lun
 envi_setup_head,fname=outimg,ns=ns,nl=nl,nb=nb,data_type=2,$
          interleave=0,offset=0, /write,/open
end