code: public void setImage(ImagePyramid<InputImage> image,
  						 DerivativeImage[] derivX, DerivativeImage[] derivY) {
  		if( image.getNumLayers() != derivX.length || image.getNumLayers() != derivY.length )
  			throw new IllegalArgumentException("Number of layers does not match.");
  
  		this.image = image;
  		this.derivX = derivX;
  		this.derivY = derivY;
  	}
docs: Sets the current input images for the tracker to use.
  @param image Original image pyramid.
  @param derivX Derivative along x-axis.
  @param derivY Derivative along y-axis.