I happen to be working on a video tutorial for Motion 3 and 4 that addresses this problem. The bump map filter only effects the area where the water paint stroke is. So the work around is to place the water effect in a group so it feels the whole screen. Be sure to check the fixed resolution check box in the Group tab (F4). Then apply the group to the bump map source well and it will do the trick. Best of luck.
Stephen Smith
Utah Video Productions
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Graphics Demo Reel I did.
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