tl;dr: Become Marty McFly within Nuke.

Time travel is cool, Nuke is cool, therefore time traveling on Nuke is…you name it.
Anyway, time-traveling sort of means that you can experience time as a solid state dimension which you can transverse back and forth, forth and back, so on a so forth. Would not it be cool to have a slider to go back (or forwards) in time sometimes? I´m pretty sure it would, although it might be a little weird at first.
Now, the little Python-powered Gizmo I got this time to introduce you guys attempts to let you do so, on Nuke. Maybe you are pretty happy with how linear time works in general and will never have to use it at all, but in case you happened to, it may come in handy.
1 – What´s this (really) about.
As you well know, time managing in Nuke is quite flexible, and there is already a lot of tools dealing with time (Retime, TimeWarp, TimeEcho…). In a way, my so-called “B_traail” tool is an “on steroids” version of the TimeEcho Nuke Node, in that it allows you to see in a still image the difference in movement occurred during a given lapse of time. Here´s an example.

On steroids, because that´s pretty much all the TimeEcho Node allows you to do, whereas my little Gizmo creature adds a fair amount of extended functionality. Other than just to “plotting” time, you can also retract both ends of the animation, apply different temporal reduction factors, select multiple merging operations, apply and invert different gradients (linear, quadratic & double-quadratic), time-recolor, and more. 

To better illustrate this, I´ve prepared a simple radial animation with a Roto node, a bunch of Transform nodes and a Merge. Without applying any additional operation, the animation looks like this.

Ok, let´s now bring in a B_traail gizmo and see what we can do with this amazing animation. And just to give you an idea before we get a closer look at the tool, here we have a few examples of what kind of results we could end up having out of the animation by playing a little with it.

2 –  Understanding it.

If you look carefully at these images, you will notice that they represent the animation plotted in one image: 75 frames in only one. Additionally, you may also perceive that there are some alterations in the way those moving dots are shown in the present moment, with respect to how they were: different gradients and merging operations have been applied, both ends of the animation can be retracted to limit the range that we see in one frame, color-over-time has also been applied, and even the temporal density has been dealt with according to different reducing factors. Last but not least, all those parameters can, of course, be animated.

At this point, I´m hoping that the idea has become clear. Probably you have already figured that we could apply this tool to any moving, masked out character to create a trail that we can play with later on, or to a bunch of moving cars to create a long exposure kind of effect. The applications are endless and I can´t wait to see what you guys come up with.
3 – User guide

Let´s now take an in-depth look at the properties bin of the B_traail tool to examine its knobs and see what they do. It´s only a 2 minutes video, don´t be lazy and check it out

4 – Installation guide

Like I said in the beginning, my B_traail Gizmo tool has a Pythonic soul. Behind the scenes, pretty much all the Nuke knobs available on the node´s properties panel trigger a piece of Python code that enables its functionality. More specifically, they call to a number of 34 functions allocated within the context of the module, along 414 lines of code.

This obviously means that you have to save the Python module within the Nuke Plugin Path context for the tool to work. Just download it using the link below and save it where Nuke can find it. NOTE: it´s crucially important to name it “b_traail” exactly. Do not rename the file!.

Great, with that being done, now download the Gizmo file itself and save it so that you can call it from Nuke (ideally in the “Gizmos” folder within the Nuke Plugin Path context).

B_traail.gizmo download link

Now, if you are of a curious type, you´ll probably want to convert the Gizmo into a Group and take a look inside. If you do so before running the tool, what you´ll find will look somehow like this:

No worries, it´s totally normal! Just introduce a frame range, run the tool and its Python code will automatically build-up the setup on the fly. If you take a look inside again, you should be able to see something like this:

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