Building a Ping-Pong Frame Sequence That Avoids Repeated Endpoints

A back-and-forth animation can feel smoother when its loop visits each endpoint only once, since naively appending a reversed frame list repeats the first and last frames and creates an unwanted pause. The article explains that for frames 0 to 3 the correct loop is 0, 1, 2, 3, 2, 1, whereas the duplicated version repeats frame 3 at the turn and frame 0 at the loop boundary. It provides a Python function, pingpong(frame_count), that generates this sequence, raises an error for invalid input, and produces a loop of 2N − 2 entries for N greater than one. The author notes that not every action looks natural in reverse, so separate return artwork or explicit endpoint holds may be needed, and that phase should be managed when switching animation states. The article also advises keeping per-frame timing data when intervals are not uniform, since the index sequence assumes equal display durations.