question: private void cleanDirtyRegion ( float eventX , float eventY ) { dirtyRect . left = Math . min ( lastTouchX , eventX ) ; dirtyRect . right = Math . max ( lastTouchX , eventX ) ; dirtyRect . top = Math . min ( lastTouchY , eventY ) ; dirtyRect . bottom = Math . max ( lastTouchY , eventY ) ; }
target:  Retrieve the region needing to be redrawn