Object Dissolve Motion Blur Problem

[evolross]

I have a scene where I'm dropping pieces of wooden lathe from a cracking, growing hole in a ceiling. As the lathe falls, I am swapping different models of the ceiling with a hole that's growing. Each model has a slightly bigger hole, which is how I get the effect I'm going for. I'm using multiple models because of the way my comp is laid out.

So I'm trying to cleanly swap my models as the timeline progresses. My problem is I HAVE to use motion blur for my falling lathe pieces, but the motion blur screws up my model swapping. I was using object dissolves to swap the models' visibility. When that didn't work, I went to animated clip masks (per Mr Rid's very good technique here http://newtek.com/forums/showthread.php?t=77832). All of my swaps are over exactly one key frame with linear keys.

With no motion blur it looks perfect. When I turn on motion blur, the one frame before each swap gets rendered incorrectly because the motion blur is blending the surrounding frames together.

I've ran into this problem before in other scenarios. Is there a way around it? I know there's something called Object List under the object properties which has to do with swapping, but I don't know how to use it.

[evolross]

I'm surprised no one has a solution to this. It seems like a common problem.

[walfridson]

You wont like this but one solution would be to do the blur in post. Render out blur buffer image, blur in post. Ye the problem might still be there but it will be a easy fix since you have image files for the blur area.
If you are using 9.5.1, fog it

[toonafish]

Create a object replacement list in the following format eg:

#LW Object Replacement List
0
T:\fluids\sim2\anim_obj_0000.lwo
1
T:\fluids\sim2\anim_obj_0001.lwo

[evolross]

After all this time, I finally figured out the solution to this problem that's cursed me so many times in the past. I was watching a Kurv tutorial today and the guy mentioned the exact same issue.

The trick is to used STEPPED keyframes, not linear, for your one-frame dissolve/model swaps. For example, I was swapping a solid model with a broken up model, over one keyframe. If the render has motion blur, you'll see a strange opacity error on the object on the first keyed framed of the dissolve. In your render, it'll look like a little pop.

If you change it to stepped, the render comes out as expected. You would think it would work on linear too, but for some reason it doesn't.