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Motion Blur Problem - layer disappears

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Steven KovarMotion Blur Problem - layer disappears
by on Mar 22, 2010 at 4:15:26 pm

I'm having a problem with Motion Blur in AE CS 4. System: Mac Pro, 10 gigs ram, Snow Leopard.

I've got a bunch of 3d layers, sort of like photos or postcards that are sitting flat on a 3d layer that is like a table or large work surface. I'm moving a camera around to different areas of this "table" with a camera parented to a 3d null.

The problem I am having is that when I tick motion blur on for a layer, it disappears underneath the 3d background surface. I have to set it's z-position to about -175 in order to raise the layer above my main background layer. When Motion Blur is off, I can see the layers just fine on top of the background layer. Z-position for the background layer is set to 0.

It seems strange, and it will definitely cause a problem when I start adding lights and shadows. Has anyone experienced this?

Thanks for any help.

- Steven K

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Dave LaRondeRe: Motion Blur Problem - layer disappears
by on Mar 22, 2010 at 4:56:21 pm

It sounds like you could precomp the work surface, then add the other elements in the precomp.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA

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Steven KovarRe: Motion Blur Problem - layer disappears
by on Mar 22, 2010 at 5:18:31 pm

Thank you for the suggestion. That seems like a reasonable solution, I'll try it out.

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Tristan NietoRe: Motion Blur Problem - layer disappears
by on Mar 24, 2010 at 2:34:45 pm

The way AE does motion blur is not totally realistic, in that it calculates each individual later separately then composites them together. EG - If you take a blue square, place it behind a red square of equal size and animate them flying across the screen, your red square will have a purple tint at the edges. This happens because AE calculates the blur on the blue square, then on the red square and then places them together, allowing you to see the blue motion blur through the red motion blur. Realistically you should never see the blue square because at every point in time it's being covered completely by the red square.

This can cause havoc in lots of situations, most noticably in Z-axis motion and X or Y axis rotation. It also makes things messy when:

Layers are closer together than the distance they cover within a single frame.
Layers with motion blur enabled interact with layers without motion blur, or with effects that don't support motion blur.
Layers use roving keyframes, or sub-frame animation.

It also can mess with your head if you aren't aware of the shutter phase of your composition. AE used to default to 0 degrees (putting the frame at the start of the motion blur), but now defaults to 90 degrees. This can be a problem if you shorten your shutter angle to less than 90 but don't adjust the phase - turning on motion blur will cause your object to lag behind its own keyframes.

And it gets better when you think that nested comps can preserve their shutter phase, some effects can override your native shutter and Masks can be set to have their own motion blur separate to the composition.

So basically, just try to avoid conflicts and keep everything simple, because there is no "fix" for it.

Tristan Nieto
Visual Effects & Motion Graphics

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Steven KovarRe: Motion Blur Problem - layer disappears
by on Mar 24, 2010 at 10:12:33 pm

Thanks so much for that information. I use motion blur a lot, so that will help me track down any problems with it more easily. I hadn't even considered shutter phase, so I'll read up on that too.

For the time being, I "lifted" the problem comps to a slightly higher z-level, since it didn't make any visual difference this time.

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