Use this procedure to perform supervised image classification using a support vector machine (SVM). The SVM is trained on a set of input ROIs for the image. Output class names, colors, and rule image band names are inherited from the input ROIs. You can use the optional keywords KERNEL_TYPE, KERNEL_DEGREE, KERNEL_GAMMA, KERNEL_BIAS, PENALTY, THRESH, PYRAMID_LEVELS, and PYRAMID_RECLASS_THRESH to control options specific to the SVM classifier. These options include kernel type and definition, penalty parameter, and can also be used to enable the use of hierarchical processing, which provides coarser overall results with reduced processing time. If not specified, the SVM classifier uses ENVI Classic’s default SVM options.
Syntax
ENVI_DOIT, 'ENVI_SVM_DOIT', DIMS=array, FID=file ID [, /IN_MEMORY] [, KERNEL_BIAS=value] [, KERNEL_DEGREE=value] [, KERNEL_GAMMA=value] [, KERNEL_TYPE=value] [, OUT_BNAME=string array], OUT_NAME=string [, PENALTY=value], POS=array, [, PYRAMID_LEVELS=value] [, PYRAMID_RECLASS_THRESH=value] [, R_FID=variable], ROI_IDS=array [, RULE_FID=variable] [, /RULE_IN_MEMORY] [, RULE_OUT_NAME=string] [, THRESH=value]
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).
IN_MEMORY (optional)
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).
KERNEL_BIAS (optional)
Use this keyword to specify the kernel bias value to use for the SVM classification. This keyword only applies if a sigmoid or polynomial kernel is selected. The default is 1.
KERNEL_DEGREE (optional)
Use this keyword to specify the degree of kernel to use for the SVM classification. This keyword only applies if a polynomial kernel is selected. The default is 2.
KERNEL_GAMMA (optional)
Use this keyword to specify the kernel gamma value to use for the SVM classification. This keyword is ignored if the kernel type is set to linear. The default is the inverse of the number of bands in the input image.
KERNEL_TYPE (optional)
Use this keyword to specify the type of kernel to use for the SVM classification. Allowable values are:
- 0: Linear
- 1: Polynomial
- 2: Radial Basis Function (RBF) (default)
- 3: Sigmoid
OUT_BNAME (optional)
Use this keyword to specify a string array of output band names.
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.
PENALTY (optional)
Use this keyword to specify the penalty parameter to use for the SVM classification. The default is 100. It is a floating point value greater than zero.
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'
PYRAMID_LEVELS (optional)
Use this keyword to specify the number of hierarchical resolution levels to process. This value is an integer value from 0 (no special processing), which implies the input image will only be processed at full resolution. If set to an integer larger than 0, this value will specify the number of pyramid levels that will be processed. The maximum value varies depending on the image size the user selects. It is determined by the criteria that the highest pyramid-level image is larger than 64x64. For example, for an image size of 24000 x 24000, the maximum level available would be 8. The default is 0.
PYRAMID_RECLASS_THRESH (optional)
Use this keyword to specify the probability threshold which a pixel must meet to avoid being reclassified at a finer resolution. This keyword is ignored if PYRAMID_LEVELS is not set to a value greater than 0. This is a floating point value ranging between 0.0 and 1.0 (all pixels classified). The default value is 0.9.
R_FID (optional)
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.
ROI_IDS
Use this keyword to specify an array of ROI IDs returned from a call to ENVI_GET_ROI_IDS. Each ID in the array will use the corresponding ROI to train the support vector machine.
RULE_FID (optional)
Use this keyword to specify a named variable that will contain the file ID for the processed rule image. This file ID can be used to access the processed data. If neither the RULE_OUT_NAME nor RULE_IN_MEMORY keywords are used, the return value of this keyword is undefined.
RULE_OUT_NAME (optional)
Use this keyword to specify an output filename for the rule image. If this keyword contains a valid filename, the rule image is saved.
RULE_IN_MEMORY (optional)
Set this keyword to store output rule images in memory. If this option is set, the rule image is saved.
THRESH (optional)
Use this keyword to specify the minimum probability a pixel must have in order to be classified. Pixels with all class probabilities less than this threshold will not be classified. This value is a floating point value ranging between 0.0 (all pixels classified) and 1.0 (no pixels classified). The default is 0.0.
API Version
4.2