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.