The JPEG XL bitstream format contains a "spline" primitive. Unfortunately, these don't get encoded particularly efficiently, and there aren't any encoders that

The JPEG XL bitstream format contains a "spline" primitive. Unfortunately, these don't get encoded particularly efficiently, and there aren't any encoders that can take advantage of it yet. JPEG XL can do animations, but it's not supposed to replace video codecs. It would be interesting to see similar features put into an actual video codec.