Size by Life module + Ribbon = random Size?

Hi there,

I am not sure if it is a bug or my understanding of the Size By Life module in combination with ribbon particles is flawed, but it certainly feels like a bug. The issue is this:

When i create a ribbon particle system it all behaves as expected until I add a “Size by Life” module. The module itself works as expected in that it scales the ribbon’s width along it’s length according to my curve, but it also appears to add a random scale to the width depending on when the ribbon is spawned.
I am using constants in Initial Size, Initial Velocity and Lifetime just to exclude those as the possible source.

When i repeatedly restart the particle system in the level the width is always different. Most of the time the difference is barely visible, but sometimes it is quite obvious.

Was tested both in 4.13 and 4.12.5.

Hello divi,

Size by life shouldn’t be the issue here based off your screenshot. Could you provide screenshot of your initial size module? If you are using a Uniform Float distribution that would be your issue.


This issue has been reproduce and a bug report has been logged here: Unreal Engine Issues and Bug Tracker (UE-36802)


Hello Ed,

like I said, Lifetime, Initial Size and Initial Velocity all have constant values.
Just tested it on another machine and the issue is easily reproduced.


I hope the gif better explains the issue I’m having. All I do in the gif is to click the “Restart Level” button in cascade while recording 3 copies of the same particle system in the level.

Best regards

I was banging my head against a wall on this one. I found the answer! If you reorder “Initial Size” lower on the cascade list than “Size By Life” it fixes the issue. Good luck! :slight_smile:

(I should mention I was using 4.14. Almost certain this will fix it for any version though).

Just ran into this in 4.24.