This routine has been replaced with the ForwardPCATransform task.

Use this procedure to calculate the principal components rotation transform.


This example calculates a forward PC rotation on a hyperspectral image. When processing is complete, open the Data Manager to verify that the result contains 50 bands.

; Start ENVI
e = ENVI()
; Open a data file
File = Filepath('', Root_Dir = e.Root_Dir, $
   Subdir = ['classic', 'data'])
Raster = e.OpenRaster(File)
; Determine an output file
OutFile = e.GetTemporaryFilename()
; Return a file ID
fid = ENVIRasterToFID(Raster)
; Query the file
ENVI_File_Query, fid, DIMS=dims, NB=nb
; Compute statistics
ENVI_Doit, 'ENVI_Stats_Doit', $
   FID = fid, $
   DIMS = dims, $
   POS = Lindgen(nb), $
   MEAN = Avg, $
   EVAL = Eval, $
   EVEC = Evec, $
   COMP_FLAG = 5
; Perform PC rotation
ENVI_Doit, 'PC_Rotate', $
   FID = fid, $
   DIMS = dims, $
   POS = Lindgen(nb), $
   MEAN = Avg, $
   EVAL = Eval, $
   EVEC = Evec, $
   OUT_DT = 4, $
   OUT_NB = nb, $
   /FORWARD, $
   OUT_NAME = OutFile


ENVI_DOIT, 'PC_ROTATE', DIMS=array, EVEC=value, EVAL=value, FID=file ID, /FORWARD, /IN_MEMORY, MEAN=value [, /NO_PLOT], OUT_BNAME=string array, OUT_DT={1 | 4 | 5} [, OUT_NB=integer], OUT_NAME=string, POS=array, R_FID=variable



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]: Unused for this routine; 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


Use this keyword to specify eigenvalues.


Use this keyword to specify eigenvectors.


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).


Set this keyword to specify that PC_ROTATE perform a forward principal components rotation. Otherwise, the inverse rotation is performed.


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).


Use this keyword to specify the mean of each band.

NO_PLOT (optional)

Set this keyword to disable the resulting principal components eigenvalue plot.


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


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)
  • 4: Floating-point (32 bits)
  • 5: Double-precision floating-point (64 bits)


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.

OUT_NB (optional)

Use this keyword for forward principal components rotation to specify the output number of bands for the principal components. The value must be less that than the number of elements of POS, and the default is the number of elements of POS. Only the first OUT_NB bands are stored in the resulting ENVI Classic file.

If you perform an inverse principal components rotation, the output number of bands will be the number of elements of the MEAN keyword.


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:

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]
envi_doit, 'cf_doit', dims=dims, fid=fid_array


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.