How does a composition read motion-blur on other comps nested inside?

[Defineyourgoal]

Why can some comps have the motion blur + vectorize button on while others can't?

Most common for me is that I have comps, inside a comp that's inside a comp. Now - I want to have the vectorize button checked on everything as well as motion blur. What's the rules I should think of since this doesn't seem to work? I just can't seem to wrap my head around how the comps work/"see through each other".

/Pär

[Andrew Yoole · Accepted solution]

The short answer is:  You can turn motion blur on for vector based layers.  You cannot turn motion blur on for nested compositions, but the layers in that nested comp can have motion blur applied..

The long answer is:  The button you call the "vectorise button" has two functions.  Hopefully this explanation isn't too complicated.

On layers that contain vector-based content, like Illustrator graphics or text layers, the button acts as the Continuous Rasterisation button.  That means it will render the vectors of that layer at the native resolution of the current comp regardless of the transformation properties like scale.  Meaning your objects always look sharp and pristine.  If you don't turn on Continuous Rasterisation, objects scaled over 100% will look soft and possibly pixelated, just like pixel-based layers.  You can apply motion blur to vector layers just like any other standard layer.

On nested compositions the button takes on a different function:  Collapse Transformations.  This means that After Effects does some complicated mathematics and combines all the transform properties (like scale, rotation, position etc) in the Nested Precomp, with the transform properties of the nested layer in the Main Composition.

So, for example, if you have Illustrator Layer A in your Nested Precomp, scaled at 200% (with Continuous Rasterisation turned on so it looks nice and sharp) , and you have the nested layer in the Main Comp scaled at 200%, it will still look soft and pixelated, because you're scaling the precomp, not the original Illustrator Layer.  The Illustrator layer is being scaled cleanly to 200%, then scaled ANOTHER 200% without vectorisation.

When you Collapse Transformations of your nested layer in the Main Comp, you combine the transformation of the first 200% and the second 200% as if you were working directly on the precomposed layers.  The result is vectorisation that is sharp and pristine in the Main Comp, with the combined 200% + 200% scale (400%).

But it wouldn't be logical to process the motion blur in the precomp, and then apply another round of motion blur in the main comp, because the transformation properties are no longer seperate - they've been combined into one calculation.  And so Motion Blur is disabled on nested layers, and After Effects looks at the individual layer settings inside the Nested Comp to determine if Motion Blur is on or not.

So if you've nested vector-based content into a precomp, turning motion blur on inside the precomp for each layer will provide motion blur for those layers in the main comp, respecting the combined transformation of both stages that the layer has been animated.

Here's a decent tutorial if you want further info:

https://www.schoolofmotion.com/tutorials/collapse-transformations-continuously-rasterize-after-effec...

[Defineyourgoal]

Omg Andrew!!! Insane how well you describe everything. Can't thank you enough. So happy!

Some questions popped up:

1. How come, when having 3D layers in a comp that has motion blur on them+are parented to a "camera-mover-null", the main comps vectorize button needs to be off? Cause when I press the vectorize button in the main-comp the camera doesn't move at all. If I recall correctly I also need to press the blur button in the main comp for the blur to function (could also be that I had the wrong Render Setting for that comp). 

2. What makes a comp able to have both the rasterize button on AND motion blur? What's the rule?

3. I work mainly with 2D-vector layers. From your experience with nested comps, what is the worst workflow you have found brings the most damage later on?

All the best

/Pär