Pair upper-left with lower-right and vice versa. Compare crossed straight references, their conditional-mean ODE trajectories, and trajectories after reflow with generated endpoint pairs. Improvement here is a toy observation, not universal one-round straightness.

Crossed reference paths 　 rectified ODE　 ODE after reflow
p₀ samples　 p₁ samples

These straight references cross at the center. They are training reference paths, not the generated ODE trajectories.