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Code Example: Support Vector Machine Classification Using API Objects

Code Example: Support Vector Machine Classification Using API Objects

This code example performs an end-to-end SVM classification using ENVI API objects. It performs the following steps:

  • Extracts an ENVIExamples object from an attribute image and training data ROIs.
  • Normalizes the examples to a common range of data values.
  • Shuffles the examples to create a random distribution of data.
  • Splits the examples into training and evaluation sets.
  • Uses an iterative trainer to train a SVM supervised classifier.
  • Plots a loss profile.
  • Prints a confusion matrix and accuracy metrics from the trained classifier.
  • Runs the classifier.
  • Displays the classification image.

Follow these steps to run the code example:

  1. Get the sample data files from our website or from the ENVI Resource DVD in the classification directory. Copy these files to your local drive.
  2. Copy and paste the code into the IDL Editor.
  3. Update the following lines to include the full path where you saved the sample data:
    • file = 'AttributeImage.dat'
    • ROIfile = 'TrainingDataROIs.xml'
  4. Update the following line to include the full path where you will save the trained classifier:
    • classifierURI = 'TrainedSVMClassifier.epo'
  5. Save the file as SVMClassificationUsingObjects.pro.
  6. Compile and run the code.
PRO SVMClassificationUsingObjects
COMPILE_OPT IDL2
 
; Start the application
e = ENVI()
 
; Open an attribute raster to classify
file = 'AttributeImage.dat'
raster = e.OpenRaster(file)
 
; Open training data ROIs
ROIfile = 'TrainingDataROIs.xml'
rois = e.OpenROI(ROIfile)
 
; Extract examples and class values
outExamples = ENVIExtractExamplesFromRaster(raster, rois)
 
; Normalize the data
normalizedExamples = ENVIApplyGainOffsetToExamples( $
  outExamples, $
  OUTPUT_OFFSET=offset, $
  OUTPUT_GAIN=gain)
Print, 'Gain: ',gain
Print, 'Offset: ',offset
 
; Shuffle the examples to create a random distribution
shuffledExamples = ENVIShuffleExamples(normalizedExamples)
 
; Split the examples into training and evaluation sets
splitExamples = ENVISplitExamples(shuffledExamples, $
  SPLIT_FRACTION=0.8)
 
; Define the SVM classifier inputs
classifier = ENVISVMClassifier( $
  NATTRIBUTES=outExamples.NATTRIBUTES, $
  NCLASSES=outExamples.NCLASSES, $
  CLASS_NAMES=outExamples.CLASS_NAMES)
 
; Define the trainer inputs
trainer = ENVIIterativeTrainer( $
  CONVERGENCE_CRITERION=0.0001, $
  MAXIMUM_ITERATIONS=1)
 
; Train the classifier
ENVITrainClassifier, trainer, classifier, splitExamples[0], $
  LOSS_PROFILE=lossProfile
 
; Save the trained classifier to disk
classifierURI = 'TrainedSVMClassifier.epo'
classifier.Save, URI=classifierURI
 
; Evaluate the result
confusionMatrix = ENVIEvaluateClassifier(splitExamples[1], classifier)
 
; Print the confusion matrix
Print, confusionMatrix.Confusion_Matrix
 
; Print the column totals
columnTotals = confusionMatrix.ColumnTotals()
FOR i=0, (outExamples.NCLASSES)-1 DO $
  Print, 'Ground truth total for ', $
  outExamples.CLASS_NAMES[i],': ', $
  columnTotals[i]
 
; Print the row totals
rowTotals = confusionMatrix.RowTotals()
FOR i=0, (outExamples.NCLASSES)-1 DO $
  Print, 'Predicted total for ', $
  outExamples.CLASS_NAMES[i],': ', $
  rowTotals[i]
 
; Print the accuracy metrics
accuracy = confusionMatrix.Accuracy()
Print, 'Overall accuracy: ', accuracy
kappa = confusionMatrix.KappaCoefficient()
Print, 'Kappa coefficient: ', kappa
commissionError = confusionMatrix.CommissionError()
Print, 'Error of commission: ', commissionError
omissionError = confusionMatrix.OmissionError()
Print, 'Error of omission: ', omissionError
F1 = confusionMatrix.F1()
Print, 'F1 value: ', F1
precision = confusionMatrix.Precision()
Print, 'Precision: ', precision
producerAccuracy = confusionMatrix.ProducerAccuracy()
Print, 'Producer accuracy: ', producerAccuracy
recall = confusionMatrix.Recall()
Print, 'Recall: ', recall
userAccuracy = confusionMatrix.UserAccuracy()
Print, 'User accuracy: ', userAccuracy
 
; Normalize the data
normalizedRaster = ENVIGainOffsetRaster(raster, gain, offset)
 
; Classify the normalized attribute image
classRaster = ENVIClassifyRaster(normalizedRaster, classifier)
 
; Display the result
view = e.GetView()
layer = view.CreateLayer(raster)
layer2 = view.CreateLayer(classRaster)
view.Zoom, /FULL_EXTENT
 
END

This code saves the trained classifier to a binary file named TrainedSVMClassifier.epo. You can apply this classifier to other similar datasets that have the same attributes and data representation. The following example shows how to do this.

  1. Get the sample data files (AttributeImage2.dat and TrainingDataROIs.xml) from our website or from the ENVI Resource DVD in the classification directory. Copy this file to your local drive.
  2. Copy and paste the code below into the IDL Editor.
  3. Update the following lines to include the full path where you saved the sample data:
    • file = 'AttributeImage2.dat'
    • ROIfile = 'TrainingDataROIs.xml'
  4. Update the following line to include the full path where you saved the trained classifier:
    • trainedClassifierURI = 'TrainedSVMClassifier.epo'
  5. Save the file as ApplySVMTrainedClassifier.pro.
  6. Compile and run the code.

Tip: The classification image in this example overlaps with the image in the first example. As long as the ENVI application is still open from the first example, you can compare the results where they overlap. The classification in the overlapping region will not be exactly the same because the attribute values are different in each image; however, the results should be similar.

PRO ApplySVMTrainedClassifier
COMPILE_OPT IDL2
 
; Start the application
e = ENVI()
 
; Open an attribute raster to classify
file = 'AttributeImage2.dat'
raster = e.OpenRaster(file)
 
; Normalize the attribute image using the
; previously recorded gains and offsets
gain = [0.68837909, 0.039032014, 1.3670539, $
  1.2792631, 1.2774655, 1.0320982]
offset = [-0.14015123, -0.13057724, -0.12848703, $
  -0.16770373, -0.18784684, -0.28932331]
normalizedRaster = ENVIGainOffsetRaster(raster, gain, offset)
 
; Get the trained classifier. This is a persistable object
; in the form of an IDL save file that needs to be restored
; from disk
trainedClassifierURI = 'TrainedSVMClassifier.epo'
trainedClassifier = ENVIRestoreObject(trainedClassifierURI)
 
; Classify a different attribute image
classRaster = ENVIClassifyRaster(normalizedRaster, trainedClassifier)
 
; Get the classification raster metadata
numClasses = classRaster.Metadata['Classes']
classNames = classRaster.Metadata['Class Names']
 
; Display the result
view = e.GetView()
layer = view.CreateLayer(raster)
layer2 = view.CreateLayer(classRaster)
view.Zoom, /FULL_EXTENT
 
END

You can optionally use the ENVICalculateConfusionMatrixFromRaster function to evaluate the classifier. This function takes in a classification raster and training ROIs, and it calculates a confusion matrix and accuracy metrics. Add the following lines of code to the previous code example, after classifying the attribute image and before displaying the results. Update the ROIfile variable to include the full path where you saved the sample data:

; Open an ROI file that will be used to 
; evaluate the classifier
ROIfile = 'TrainingDataROIs.xml'
rois = e.OpenROI(ROIfile)
 
; Evaluate the classifier
confusionMatrix = ENVICalculateConfusionMatrixFromRaster( $
  classRaster, rois)
 
; Print the confusion matrix
Print, confusionMatrix.Confusion_Matrix
 
; Print the column totals
columnTotals = confusionMatrix.ColumnTotals()
FOR i=0, (numClasses)-1 DO $
  Print, 'Ground truth total for ', $
  classNames[i],': ', $
  columnTotals[i]
 
; Print the row totals
rowTotals = confusionMatrix.RowTotals()
FOR i=0, (numClasses)-1 DO $
  Print, 'Predicted total for ', $
  classNames[i],': ', $
  rowTotals[i]
 
; Print the accuracy metrics
accuracy = confusionMatrix.Accuracy()
Print, 'Overall accuracy: ', accuracy
kappa = confusionMatrix.KappaCoefficient()
Print, 'Kappa coefficient: ', kappa
commissionError = confusionMatrix.CommissionError()
Print, 'Error of commission: ', commissionError
omissionError = confusionMatrix.OmissionError()
Print, 'Error of omission: ', omissionError
F1 = confusionMatrix.F1()
Print, 'F1 value: ', F1
precision = confusionMatrix.Precision()
Print, 'Precision: ', precision
producerAccuracy = confusionMatrix.ProducerAccuracy()
Print, 'Producer accuracy: ', producerAccuracy
recall = confusionMatrix.Recall()
Print, 'Recall: ', recall
userAccuracy = confusionMatrix.UserAccuracy()
Print, 'User accuracy: ', userAccuracy



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