Line + Round Caps + Trim Paths + Motion blur = Ghosting Artifact

[Dierenwinkel Doc]

Hi this is a bug that has been tormenting me since 2017 but as a young lad I didn't know these forums existed.

When you make a line with a stroke add round caps to that line. Add a trim paths & enable motion blur.

Than animate the start from 0 to 100%.

You get a nasty ghosting artifact when the line should completely dissapear.

You don't have this issue with the butt cap.

https://i.gyazo.com/c8b4fd218704ca1dc4b179399ffbc1f2.mp4 

Here is a gif of the issue. I also dropped a download link below.

With the project file. 

Hopefully you guys & gals can look into this 🙂

M1 Max, Ae 23.0.0

Much love,

Dierenwinkel

[JohnColombo]

Hi @Dierenwinkel Doc,

Thanks for making this report. To be honest, the gif and project file you've shared both appear to show Motion Blur behaving as it's expected to by blending previous and upcoming frames together. In the project file, I don't see quite the same issue as in the gif: once the time reaches the second keyframe on the Start property, the line is disappearing as I would expect.

Have you tried adjusting the Motion Blur settings for your composition to change the final render result?

Thanks again,

- John, After Effects Engineering Team 

[Dierenwinkel Doc]

Hi since the reply button is still broken I will do it this way. Sorry for giving u guys extra work but this is the only way to communicate for me atm.

@JohnColombo 

The bug is really really weird. So when I reopened the project I didn't get the bug but I got the correct behavior. With a round cap!

Now change the round cap to butt cap in your project and it will have the bug.

It's gone if you reopen the project.

https://i.gyazo.com/72fe249b7134b34b410481a7387e0279.mp4

Here is a gif of the issue in action.

Hopefully this also works on your end.

M1 Max, AE 23.0.0

Much love,

Dierenwinkel

[JohnColombo]

Thank you for that extra info, @Dierenwinkel Doc. We are able to see the issue; it appears that the trimmed Stroke renders differently on the last frame of the animation depending on where the CTI is placed. We've now got a ticket opened to investigate this behavior.

Thanks again for reporting this and for the additional hint as to how to reproduce the issue,

- John, After Effects Engineering Team