Use this procedure to apply an FFT filter image and perform the inverse FFT.

Note: If you select a user-defined filter, then it must exist as an ENVI annotation file prior to starting this function.

Example

; Start ENVI
e = ENVI()
 
; Open a data file
File = Filepath('qb_boulder_msi', Root_Dir = e.Root_Dir, $
   Subdir = ['data'])
Raster = e.OpenRaster(File)
 
; Determine an output file
OutFile = e.GetTemporaryFilename()
 
; Return a file ID
fid = ENVIRasterToFID(Raster)
 
; Perform forward FFT
ENVI_File_Query, fid, dims=dims, nb=nb
ENVI_Doit, 'FFT_Doit', $
   FID = fid, $
   DIMS = dims, $
   POS = Lindgen(nb), $
   OUT_NAME = OutFile, $
   R_FID = FilterFID
 
; Perform inverse FFT
ENVI_Doit, 'FFT_Inv_Doit', $
   FID = fid, $
   DIMS = dims, $
   POS = Lindgen(nb), $
   FILTER_FID = FilterFID, $
   FILTER_POS = [0], $
   OUT_DT = 4, $
   OUT_NAME = 'InverseFFTImage'

Syntax

ENVI_DOIT, 'FFT_INV_DOIT', DIMS=array, FID=file ID, FILTER_FID=file ID, FILTER_POS=integer, /IN_MEMORY, OUT_BNAME=string array, OUT_DT={1 | 2 | 3 | 4 | 5 | 6 | 9 | 12 | 13 | 14 | 15}, OUT_NAME=string, POS=array, R_FID=variable

Keywords

DIMS

The “dimensions” keyword is a five-element array of long integers that defines the spatial subset (of a file or array) to use for processing. Nearly every time you specify the keyword FID, you must also specify the spatial subset of the corresponding file (even if the entire file, with no spatial subsetting, is to be processed).

  • DIMS[0]: A pointer to an open ROI; use only in cases where ROIs define the spatial subset. Otherwise, set to -1L.
  • DIMS[1]: The starting sample number. The first x pixel is 0.
  • DIMS[2]: The ending sample number
  • DIMS[3]: The starting line number. The first y pixel is 0.
  • DIMS[4]: The ending line number

To process an entire file (with no spatial subsetting), define DIMS as shown in the following code example. This example assumes you have already opened a file using ENVI_SELECT or ENVI_PICKFILE:

  envi_file_query, fid, dims=dims

FID

The file ID (FID) is a long-integer scalar with a value greater than 0. An invalid FID has a value of -1. The FID is provided as a named variable by any routine used to open or select a file. Often, the FID is returned from the keyword R_FID in the ENVIRasterToFID routine. Files are processed by referring to their FIDs. If you work directly with the file in IDL, the FID is not equivalent to a logical unit number (LUN).

FILTER_FID

Use this keyword to specify the file ID of a filter image generated by ENVI_FILTER_DOIT.

FILTER_POS

Use this keyword to specify the band number for the filter image.

IN_MEMORY

Set this keyword to specify that output should be stored in memory. If you do not set IN_MEMORY, output will be stored on disk and you must specify OUT_NAME (see below).

OUT_BNAME

Use this keyword to specify a string array of output band names.

OUT_DT

This keyword indicates the IDL data type of the output data. Set the keyword to one of the following integer values:

  • 1: Byte (8 bits)
  • 2: Integer (16 bits)
  • 3: Long integer (32 bits)
  • 4: Floating-point (32 bits)
  • 5: Double-precision floating-point (64 bits)
  • 6: Complex (2x32 bits)
  • 9: Double-precision complex (2x64 bits)
  • 12: Unsigned integer (16 bits)
  • 13: Unsigned long integer (32 bits)
  • 14: Long 64-bit integer
  • 15: Unsigned long 64-bit integer

OUT_NAME

Use this keyword to specify a string with the output filename for the resulting data. If you set the keyword IN_MEMORY, you do not need to specify OUT_NAME.

POS

Use this keyword to specify an array of band positions, indicating the band numbers on which to perform the operation. This keyword indicates the spectral subset of bands to use in processing. POS is an array of long integers, ranging from 0 to the number of bands minus 1. Specify bands starting with zero (Band 1=0, Band 2=1, etc.) For example, to process only Bands 3 and 4 of a multi-band file, POS=[2, 3].

POS is typically used with individual files. The example code below illustrates the use of POS for a single file with four bands of data:

  pos=[0,1,2,3]
                  
envi_doit, 'envi_stats_doit', dims=dims, fid=fid, pos=pos, $
                  
comp_flag=3, dmin=dmin, dmax=dmax, mean=mean, stdv=stdv, hist=hist

But what if you need to create an output file consisting of data from different bands, each from different files? Library routines such as CF_DOIT and ENVI_LAYER_STACKING_DOIT can accomplish this, but they use the POS keyword differently. Suppose you have four files, test1, test2, test3, and test4, with corresponding FIDs of fid1, fid2, fid3, and fid4, respectively. In the following example, you want Band 3 from test1 in the first position, Band 2 from test2 in the second position, Band 6 from test3 in the third position, and Band 4 from test4 in the fourth position. The code should be as follows:

  fid_array = [fid1,fid2,fid3,fid4]
                  
pos=[2,1,5,3]
                  
envi_doit, 'cf_doit', dims=dims, fid=fid_array
                  
out_name='test_composite_file'

R_FID

ENVI Classic library routines that result in new images also have an R_FID, or “returned FID.” This is simply a named variable containing the file ID to access the processed data. Specifying this keyword saves you the step of opening the new file from disk.

API Version

4.2