# In single image render trail of moving objects

I want to render a single frame of an animation and would like to have some of the moving objects to be rendered as a trail between either

• the beginning of the animation (frame 0),
• a particular number of n frames or
• the previous keyframe,

and the current frame.

The objects I want the trail to be rendered of are markers on a human body (like, little red dots, i.e. spheres with a 100% emissive red material, on the shoulder, elbow, hand of a humanoid figure)

The reason that I want the animation being rendered as a trail of the markers is that I want to use the image in a printed technical documentation that describes a particular motion. For obvious reasons an actual animation is not possible in this case.

I have already tried rendering the trail of the markers by using motion blur but the results were far from what I would like them to look. Furthermore the motion blur does for some reason not really follow the actual trail of the markers (I don't know why) and is also influenced by motion of the camera.

I would like to avoid post-composition of multiple frames by hand.

One idea would be to render only the markers for the n frames before the current frame with transparent background, fully render only the current frame and blend the frames together.

Is this possible in blender? Can it be automated? How?

## 1 Answer

It is possible by using Animation nodes.

Here is a setup for duplicating and distributing objects on every frame:

there are multiple markers that I would like to render the trails of. Furthermore it seems impossible to set a maximum lengh of how far the trail goes into the past.

For more control nodes are more complex. This node-setup gets objects from group and arranges markers between selected frames:

I provide blend-file (note, AN must be enabled):

• This is an interesting approach, but a little cumbersome since there are multiple markers that I would like to render the trails of. Furthermore it seems impossible to set a maximum lengh of how far the trail goes into the past. – Hendrik Wiese Jul 14 '17 at 9:06
• @HendrikWiese In AN nodes there is no such definition as "impossible", You get the blocks and build anything that you want. – Crantisz Jul 14 '17 at 11:00
• Ah, I see. Yeah, sure, there's no such thing as "impossible" at all. It all depends on the effort you want to put into something. I was hoping for something a little less complex. But it's okay, I guess I'll use this method. Thanks for your help! – Hendrik Wiese Jul 14 '17 at 11:30
• Okay, one last thing I'm struggling with: I'm trying to create the trail not from a static from to a static to but from current frame - n to current frame. So that the trail instances have kind of a lifetime. I've tried setting the from and to data inputs from time info frame and time info frame - some input age. That sadly does not work. How would I do that? – Hendrik Wiese Jul 14 '17 at 12:44