Render differes from viewport

I just got finished rendering a 104 frame sequence with each frame taking roughly 1.5 hours.

The scene has a bunch of gigantic flies flying past the camera away from it. There’s a lot of them, so they’re each just a particle with a group as the display type.

Well after spending 100+ hours on rendering this thing, I realize that the flies aren’t flying into the shot, no they’re randomly appearing in the middle of the shot, and I don’t know why. In the viewport they fly into the frame just fine, but in the render, they’re just not there for a few frames until suddenly, they are. It’s not all of them doing this, but it’s enough.

I went back and rendered 1 more frame just for the heck of it (one where the fly was in it in the viewport, but not in the original render), and it rendered just fine!!! What is going on???

I noticed that when I first open the file, I have to scrub through the sequence before the flies will all appear. Could this be the problem?


Did you bake your particle system before rendering?

I sure did.

make sure your bake starts well before you see them on camera

Ah that might be it. The bake starts right when we can start seeing them. (IIRC)