code: protected Double getCubicInterpolationValue(Double[] values, double offset) {
  		Double value = null;
  		if (values != null) {
  			value = getCubicInterpolationValue(values[0], values[1], values[2],
  					values[3], offset);
  		}
  		return value;
  	}
docs: Interpolate 4 values using the offset between value1 and value2
  
  @param values
  coverage data values
  @param offset
  offset between the middle two pixels
  @return value coverage data value