Initially, I wanted to make a picture of snow and wind, but the difficulty I encountered was that it was difficult to make the mountain look like it was covered with snow. After adding the ramp and roto nodes, the mountain is more like frozen into ice, so it gave me inspiration. I simply let the hot air balloon be frozen into ice and fall to achieve a more interesting and creative effect.
The technical problem I encountered is that the order of a and b in merge3, if the hot air balloon is on top, it will be blocked by the ramp before 1040 frames, and vice versa, it will be blocked by mountains after 1040 frames. My solution was to duplicate a new hot air balloon and make it disappear after 1040 frames, and the two hot air balloons appeared at different time periods.
This poster taught me the tracking with perspective relationship (rather than the tracking of 2d plane), using cornerpin2d5 to control. I learned to add more pipes in the middle of roto when doing roto, because this is the interference content, which can make nuke calculations more accurate.