Comp motion blur changes position of 3d layers

[J5DDE]

I've noticed that whenever I have motion blur enabled for a comp the masks do not align, even for the layers with motion blur switched off.

Here is an example:

I have a comp with a 3d camera that has some movement and a 3d layer that has some other movement. I draw some paths for rotoscoping purposes with the Pen tool and used the Stroke effect to create a white matte.

• So, if I have motion blur enabled for the comp (Shutter Angle 173º/Shutter Phase -86º) and for the layer, here's what happens:

• If I then disable the motion blur for the layer, it is still misalligned, but without the motion blur:

• However, if I disable motion blur for the whole comp, the path and the strokes allign, but in the position where the strokes were being presented in the last two scenarios. So the strokes never appear where I first drew them while I was rotoscoping. It's hard to tell from the screenshot, but the difference between picture 2 and picture 3 is that the paths of the mask have moved to the left towards the white strokes, not the other way around.

I have noticed that it's not just the mask paths, but the whole layer what moves, since the 3d anchor point of the layer moves too when I disable the motion blur for the comp. The position values are not changing, it's just the "apparent position" from the camera view.

I have been looking for a solution for this that keeps the comp motion blur... The only idea I can think of is drawing the paths with the motion blur disabled and then enabling it afterwards, but I'm sure there's some better way.

Does anyone know why this happens or how I can avoid it?

Thank you!

[Rick Gerard]

Adjusting the shutter phase determines when the motion blur occurs in real-time. Setting Shutter Phaze to zero will put the blur in front of the motion, and setting it to -180º will put the motion blur behind the action when using a 180º shutter. This happens because the CTI (Current Time Indicator) is always at the start of a frame.  

When you add a camera move to the equation, you multiply the timing by the camera motion. Pick a spot where the camera is at rest, and the path should align with the motion-blurred image. 

Try using Mocha AE instead of a Solid with a mask or Shape layer for this kind of manual Rotoscoping.  I find it easier because you can motion-track moving parts of the frame and tie the Roto shapes to the tracked layer. If you want to stick with animating paths, try using a shape layer instead of a mask path with a stroke. That will let you use an Alpha track matte instead of a Luma Matte, and it will render faster.

[J5DDE]

Hi Rick! Thank you for your answer.

The reason I use 3d layers is because I import the already tracked 3d camera and 3d layers from previous steps in the workflow (other software where I create some 3d elements) and that way I do one very good 3d tracking that I can use throughout. The mask path with a stroke was very specific for this rotoscoping situation, but I also use all types of shapes. I will try your tip on Shape Layers!

Regarding the motion blur, my confusion with the way After Effects works comes from the fact that when enabling motion blur, it seems to be moving the whole layer, instead of just affecting the way the layer looks in the viewer (as 2d motion blur does). So when I'm rotoscoping, matching some shape from the background image with a mask, if I 'm looking at it on the viewer with motion blur enabled for the comp (even if that layer has the motion blur disabled), the disallignment happens (see the second picture from the original post). And even if the path matches in those frames where the camera and layer are not moving, since they are almost always moving, this misallignment causes the rotoscoping to be off by a few pixels all the time.

From what you are saying, I guess what I'm trying to do is not really possible with After Effects at the moment, unless I disable motion blur for those comps.

[Rick Gerard]

You can do it using Mocha AE. It's faster, and the motion blur works perfectly. That's how the pros do detailed roto work. If you do a lot of it, it might pay to invest in Mocha Pro. 

Check out this training series. Boris Mocha AE Training.

My rotoscoped mattes are always all 2D, but Mocha can add the motion blur to the matte, and you can even manually adjust and animate the feathered edge. The motion blur will come through automatically if I need to add animated 3D layers or objects to the scene.

You can also always add CC Force Motion Blur or Pixel Motion Blur to a layer that needs to but is having problems matching the motion blur in the shot.

[J5DDE]

Very interesting! Thank you again for your answer, Rick!

I will check it out!