A molecular trajectory can be difficult to interpret when you only watch it in one direction. A change in conformation may happen gradually, or a small but important movement may be easy to miss in the middle of a long sequence. Playing the same path in reverse gives you another way to inspect the transition: start from the final frame and follow the motion back toward its beginning.
In SAMSON, the Play reverse path animation plays a selected path—or multiple selected paths—backward between two animation frames. This is useful when reviewing a trajectory, comparing conformations, or preparing an animation that makes the order of a structural change easier to follow.
Start with the path, not the animation
A path node stores a trajectory in SAMSON. Before adding the animation, select the path you want to review. Then, in the Animator, double-click Play reverse path in the Animation panel. The selected path will play in reverse between the animation’s two keyframes.
This distinction helps avoid a common source of confusion: the animation’s keyframes determine the interval over which it plays, while the selected path supplies the sequence of frames being shown. If the playback does not cover the part of the trajectory you want to inspect, move the animation keyframes as needed. They can be repositioned after the animation has been added.
When reverse playback helps
Watching a transition from its endpoint can make it easier to ask focused questions: Which movement happens just before the structure reaches this conformation? Does a change appear to begin in one region and then spread to another? Is the transition easier to explain when the viewer sees the final arrangement first?
Reverse playback is a viewing aid, not a new trajectory calculation. It changes the order in which the stored path is displayed; it does not, by itself, establish that the molecular process can physically occur in reverse. Keep that distinction in mind when interpreting or presenting a trajectory.
Reviewing several paths together
If you select multiple paths before adding the animation, SAMSON synchronizes them during playback. This can help when you want to view several stored paths over the same animation interval. Before drawing conclusions from the comparison, check that the paths represent the structures or conditions you intend to compare. Synchronization aligns their playback; it does not explain why the paths differ.
Check smoothing when the motion looks unexpected
The number of frames in the animation may not match the number of frames in the path. When they differ, the path is smoothed. Smoothing can make playback appear more continuous, but it may also affect how the transition looks between stored frames. If you need to inspect the path without smoothing, you can switch smoothing off in the Inspector.
It is also possible to change how parameters are interpolated between frames by adjusting the animation’s easing curve. If the timing or feel of the playback seems unsuitable for a review or presentation, check this setting along with the animation keyframes. These controls change the animation’s presentation, so use the stored path itself as the reference when assessing the underlying trajectory.
A simple review routine
- Select the path, or paths, you want to examine.
- Add Play reverse path from the Animator’s Animation panel.
- Adjust the animation keyframes to cover the interval you want to review.
- Watch the transition from its endpoint, and note which changes are easiest to identify in reverse.
- If the motion looks different from the stored frames, check whether smoothing is enabled and review the easing curve.
For the full instructions, see the Play reverse path documentation.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.