code: public void initialize(PyramidDiscrete<I> image ) {
  		if( previousDerivX == null || previousDerivX.length != image.getNumLayers()
  				|| previousImage.getInputWidth() != image.getInputWidth() || previousImage.getInputHeight() != image.getInputHeight() ) {
  			declareDataStructures(image);
  		}
  
  		for( int i = 0; i < image.getNumLayers(); i++ ) {
  			gradient.process(image.getLayer(i), previousDerivX[i], previousDerivY[i]);
  		}
  
  		previousImage.setTo(image);
  	}
docs: Call for the first image being tracked
  
  @param image Most recent video image.