Why warp the second image back to the first one instead of opposite?

[ProNoobLi]

Based on my understanding, the flow filed always indicates the flow of each pixel from the previous frame to the current one. However, why and how do you do the warping from the second back to the first one?

[deqings]

Please check this example for the difference between forward and backward warping https://www.researchgate.net/figure/Forward-and-backward-image-warping-In-the-case-of-foward-warping-A-holes-can-occur_fig2_267946997