How To combine multiple shots into a seamless whole
Step 4: Mask Traces of Compositing
(CLICK FOR FULL-RES)
If any of the objects in your composite float oddly above the background instead of blending in with it (as Acosta’s motorcycle did at first), it may help to “ground” the object by creating a shadow beneath it. Duplicate the object’s layer, flip the image vertically, rotate it if necessary, and reduce the layer’s opacity. Again, use the Move tool (V) to place the object’s shadow correctly below it.