I've created a shield material with animating lasers or bars. One set is vertical and the second set is horizontal. Each moving the opposite direction they lay. My horizontal lines loop but with the distance from top and bottom, there's a delay unlike the vertical lines moving horizontally. There's a simple answer hidden in there, I just can't find it right now.
What I'm seeing as it animates are the horizontal lines moving downward past the Mask's cutoff, then continuing on to the "end" of the image file and coming back from the top. I've tried Sine and Cosine to try a different animation but wasn't liking it too much.