The HighPass-LowPass Roofing Filter was created by John Ehlers (Cycle Analytics For Traders pg.78)

This is a variation of a roofing filter that will let you know which direction the stock is trending. If it falls below 0 then the stock is in a downtrend and if it rises above 0 then it is an uptrend. Buy when the indicator line is green and sell when it is red.