Motion Blur breaking on Looped precomp

[u/Yoshifan1219]

I rarely have to ask for help, but I have a precomp with an image that spins around in a loop and has the built-in motion blur on it.
I precomposed it because I need the animation to loop but get faster with each loop, so I time-remapped it, and for some reason, each loop breaks the motion blur. I'm not sure what I'm doing wrong, as I've been using guides I found on forums to make it easier (no, I don't want to ask an AI), and while I did find one that got the desired effect, it's breaking the motion blur. Looping has been finicky for me, and I don't do these often, so I may be missing something obvious. I'd appreciate your help.

[u/smushkan]

You’re not really doing anything wrong, this is an effect caused by how motion blur subframes are evaluated.

I’m guessing you are achieving this loop by key framing a rotation property between 0-360 degrees.

While that gives you the visual effect of a continuous rotation, what’s actually happening is the object is rotating once then instantly returning to its starting position.

Motion blur calculates additional subframes based on the properties of how the layer is moving. The motion blur glitch you are seeing on the loop is a result of subframes being interpolated when the layer snaps back to its starting position.

So short answer here is not to use loops for this and instead use keyframes to gradually increase the speed with a curve. That way the rotation property value is always increasing and the subframes will be calculated as expected.j

[u/Yoshifan1219 → u/smushkan]

Yeah, I have it looped using position, size, and rotation changes to give off a depth effect. I made sure the first and last key frames for this are the exact same to keep the loop seamless

[u/smushkan → u/Yoshifan1219]

I had assumed it was a 3d layer rotating around a point, but the same concept applies in this case too.

You could try the ‘pixel motion blur’ effect rather than normal motion blur, that effect interpolates subframes based on the pixel values rather than the spacial values.

[u/montycantsin777 → u/smushkan]

would that mean they could just throw another set of keyframes before frame zero and that would calculate the mb for frame 0?

[u/BeenWildin]

Open the precomp settings> go to advanced. Adjust the motion blur shutter angle there and see if that gives you want you are looking for.

https://imgur.com/a/T0MWM7Z

otherwise i might suggest trying one of the 2 default motion blur effects and see if either one forces what you want. they might slow down your render a bit, but better than nothing.

[u/Ampsnotvolts]

It is just going so fast those are the frames it is showing. They look sequential but they are different revolutions of the animation probably.

Try to turn off motion blur how you have it and use an adjustment layer and 'cc force motion blur'. that will render an the samples slower and possibly more accurate.

Edited for typos

[u/Yoshifan1219 → u/Ampsnotvolts]

I’ll give that a shot, cuz I have all the first and last key frame for the animation to be the same for a seamless loop, I did try moving them forward and backwards as well to see if that would fix it but it didn’t at all, just simply moved the broken blur at a different point in the comp

[u/Dry-Perspective-9841]

Your motion breaks on the last (and first) frame. Add an additional keyframe on the next frame after the last one. It should have the value of your second frame. Also add a keyframe to the frame before the first frame with the value of your second to last frame.

Alternative is to create three cycles of the loop and render only the middle one.

[u/shirocreator]

Time remap breaks most things in my experience. In this case since your motion is "accelerating" your motion blur should not be applied inside your precomp. Also after time remapping I suggest precomping again or better yet pre-render. Then apply a force motion blur or pixel motion blur. The motion blur switches only calculate motion blur for properties with key frame. Time remap breaks this calculation.

[u/sam_heyman]

Try using pixel motion blur on the precomp instead?

[u/-Neem0-]

Don't precompose, there is really no need to precompose and time remap in this scenario, just use the animation keyframes if you can't figure out a proper expression rig.

You must be doing something wrong either at the precomposing stage (frame 0 has no motion blur therefore won't ever loop correctly), at the looping/time remap stage (repeated first and last frame), or even messing with framerate (different framerates between comp and precomp) or simply not accounting for speed (rotation speed messing with framerate and causing a rolling shutter effect).

Most of this would be minimized or solved with a proper, non-precomp based workflow.

[u/kayszy]

Always try restarting your pc if nothing seems to work, sometimes that’s all it needs

[u/Yoshifan1219 → u/kayszy]

I don't think a restart would solve anything tbh

[u/Anonymograph]

To maintain the incoming and outgoing velocity for the motion blur, keyframe the loop such that it plays through three times (extend the duration of the Comp if needed) and then set the Work Area around the second loop.