Build difference matte with expression

[Kevin Camp]

i think you can do most of this with an animation preset…

create a new solid (solid should be the same size as the bg layer), add the compound arithmetic effect and set the second source layer to your bg layer (that should put a copy of the bg layer onto the solid)

to create the freeze frame, add the posterize time effect after the compound arithmetic effect and then add this expression to hold on the first frame:

thisComp.frameDuration/thisComp.duration

now add the difference matte effect, leaving the difference layer as the solid (which is now a freeze frame of the bg).

last, duplicate the compound arithmetic effect and move it to last in the effect stack (this will put the keyed objects back onto the solid).

now you can tweak the difference matte settings to refine the key.

if that works for you, you can save it as a preset to apply when needed. to do that, select the solid and type ‘uu’ to reveal modified properties. select the effects and also the frame rate property of the posterize time effect (with the expression) and maybe some of the difference matte settings that you want to keep, and then choose animation>save animation preset and save it in the after effects presets folder.

when you need it again, make sure now layers are selected in a comp, then find the preset in the effects & presets panel and apply it (double-click) and it will create a new solid and add those effects. all you’ll need to do is select the bg layer in the 2 compound arithmetic effects.

Kevin Camp
Art Director
KCPQ, KZJO & KRCW

[Trevor Asquerthian]

That is most excellent, and does exactly what I was asking for, thank you.

A couple of questions arising, if I may:

Is there an advantage to using the expression in the posterise time effect, rather than just setting the frame rate to 0?

[Kevin Camp] “last, duplicate the compound arithmetic effect and move it to last in the effect stack (this will put the keyed objects back onto the solid).”

I’m trying to understand the composting at work here. I would have thought that this extra copy of the source layer would be fully opaque, negating the difference composite, rather than having the (desired) blending effect back on top?

thanks again

Trevor

[Kevin Camp]

[Trevor Asquerthian] “Is there an advantage to using the expression in the posterise time effect, rather than just setting the frame rate to 0?”

doh! zero would probably work just fine… i was over thinking and trying to get too fancy.

[Trevor Asquerthian] “I would have thought that this extra copy of the source layer would be fully opaque, negating the difference composite, rather than having the (desired) blending effect back on top?”

by default the effect is set to composite only the rgb channels onto the layer and not effect the current alpha, however, you can set the ‘operate on channel’ setting to rgb & alpha (argb) or even just the alpha, both of which can be very useful when needed.

Kevin Camp
Art Director
KCPQ, KZJO & KRCW