This Help Article includes example code that can be used to subset an image using a vector file and to mask the areas outside of the vector boundaries. This is similar functionality to ENVI's 'Subset Data via ROI' found under the Basic Tools menu, but allows for the use of a vector file.
The following code provides methods to subset an image with an ENVI vector file and to mask the background values to only the area within the vector. The code example includes a few unexposed routines and keywords which make this a relatively simple process. The example uses ENVI's 'world_dem' image and an ENVI vector created from the 'states.shp' sample vector file.
'. Because these routines are not currently exposed to users, the syntax is subject to change in future versions of ENVI. Exelis VIS support of these routines is limited.
pro subset_by_evf
compile_opt idl2
;select input file to subset
fname = 'C:\Program Files\ITT\IDL\IDL80\products\envi48\data\world_dem'
;fname = files
envi_open_file, fname, r_fid=fid
if (fid eq -1) then return
envi_file_query, fid, bnames=bnames, sname=sname, nb=nb
pos=lindgen(nb)
;open vector file to use for subset and get id
evf_fname = 'C:\Program Files\ITT\IDL\IDL80\products\envi48\data\vector\states_.evf'
;make sure envi vector routine is available
envi_check_save, /vector
;open the vector and get its ID
evf_open, evf_fname, vec=vec, /no_warning
evf_id = vec.id
;compute the spatial boundary based on the evf using ROI routines (undocumented routine) - this will provide the DIMS to use to subset
roi_ids = envi_get_roi_ids(fid=fid, /include_evfs)
envi_roi_compute_spatial_boundry, roi_ids, dims, fid=fid
;resize the original image by the new dimensions
envi_doit, 'resize_doit', fid=fid, dims=dims, pos=pos, $
rfact=[1.,1.], /in_memory, r_fid=sub_fid
envi_file_query, sub_fid, ns=ns, nl=nl, dims=dims
;generate a mask from the evf (undocumented routine) - you need to mask areas outside of EVF boundaries if the EVF is not a rectangle
; this is the same as using Basic Tools ->Subset data via ROIs and masking the outside of the ROI/EVF boundary
envi_mask_doit, evf_id=evf_id, evf_fid=sub_fid, ns=ns, nl=nl, and_or=0, $
/inside, /in_memory, r_fid=m_fid
;apply the mask to the subset image
out_name=sname+'_subset'
envi_mask_apply_doit, fid=sub_fid, pos=pos, dims=dims, m_fid=m_fid, m_pos=0l, $
value=-9999, out_bname=bnames, out_name=out_name
envi_evf_close, evf_id
;clear out open files to avoid FID confusion when opening new files
fids=envi_get_file_ids()
for j=0, n_elements(fids)-1 do begin
envi_file_mng, id=fids[j], /remove
endfor
END