A pulse of light travels from the left side of the screen toward two
observers. The lower observer is making measurements in an intertial
frame that is stationary relative to the frame of the movie. The upper
observer is making measurements using an intertial frame that is moving
to the left. Each
observer measures the speed of light by noting the time that the pulse
passes each of the two clocks (say times t1 and t2). The speed can then
be determined by taking the distance between clocks (say L) divided by the time difference t2-t1. That is v = L/(t2-t1). Note that
since the upper observer is moving toward the light pulse, this
observer measures a shorter time interval between clocks than the lower
obsever, and thus the larger speed of light.
This is because the upper observer's second clock is moving toward the light pulse, so the
pulse has a shorter distance to travel between clocks.