Expression to show next layer every X frames

Question

I'm making a video where I talk to the camera with constantly cycling different lengths of hair. I recorded each scene entirely with different lengths of hair and some scenes have more takes than others. I would like to cycle through each layer (length of hair) changing to the next layer every frame. So far my method has been to have a "layer selector" slider on a separate layer that I animate with a loop expression and then tell each video layer to have this expression so the layer only becomes visible when the slider calls it's number

if(index==Math.round(thisComp.layer("Adjustment Layer 6").effect("Layer Selector")("Slider"))){100}else{0};

But this gets messy when the "layer selector" slider animates from say 2-6 and doesn't align exactly with the frames of the video and we get layers which show slightly longer than others. The frame rate is also something I'd like to experiment with so my goal would really be to have the "layer selector" slider use an expression rather than keyframes. Something like this is what I'm thinking:

var startlayer = 2

var endlayer = 6
var framerate = 3

loop from start to end changing every [framerate] frames

Any help much appreciated!

Dan Ebberts · Accepted Answer

I think the expression for your layer selector slider would look like this:startlayer = 2;endlayer = 6;framerate = 3;numLayers = endlayer - startlayer + 1;currentLayer = Math.floor(timeToFrames(time)/framerate)%numLayers + startlayer;

Rick Gerard · Answer

How are you handling audio sync?

I would probably start in Premiere Pro, syncing the multiple takes close to where the audio overlaps. Premiere will do this automatically. Once you get the audio layers synced up for every few words, you can then stack the layers in After Effects and use a simple expression that changes opacity based on layer index. If you want to see a different layer for every frame, the expression would be:

t = time / thisComp.frameDuration;
if (t == index - 1)
	100
else
	0

You can adjust the math or fiddle with frame rates to keep each layer visible for more than one frame. If you want each layer to play for 10 frames, the expression would be:

t = time / thisComp.frameDuration;
n = Math.floor(t/10);
if (n == index - 1)
	100
else
	0

As I said, the hardest part is going to be making sure that the audio stays in sync. Make a comp for every one or two seconds of your video and then render or nest each of the comps to put the whole thing together. That is how I would handle the problem.

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