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Replicating "Marching Ants" Photoshop selection effect in After Effects

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Mikey Zoppo
Replicating "Marching Ants" Photoshop selection effect in After Effects
on Oct 22, 2018 at 9:07:54 pm

Marching ants, the dashed lines that slowly travel over any selection you make in Photoshop. I'm trying to recreate those in AE.

It's pretty simple right? Well, I'm a big dumb dumb.

I've got a square, it is stroked with dashes. If I animate the "offset" parameter, the dashed stroke will travel across my shape much like the Marching ants travel across a selection in photoshop. THE ONLY DIFFERENCE! The Marching ants move in small intervals, as opposed to one continuous smooth motion. Much like a clock hands ticks from one second to the next, it's not smooth.

I can easily get what I want by just animating the offset and applying posterize time with a super low number.

But this seems like a really useful thing to know how to do with an expression. I'm assuming it's kind of like a blink expression, except instead of oscillating between two values, we just want our value to continue growing/shrinking in those stuttering intervals.

Basically an expression that is a counter, that counts up/down at a rate you define, and will pause at an interval you define, how long the pause is would also be a value you define.


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Kalleheikki Kannisto
Re: Replicating "Marching Ants" Photoshop selection effect in After Effects
on Oct 23, 2018 at 8:16:35 am
Last Edited By Kalleheikki Kannisto on Oct 23, 2018 at 8:25:36 am

It would be a Math.round() expression on the time remapping, if you want to emulate posterize time.

Math.round(time) would give you a change once per second
Math.round(time*5)/5would give you a change 5 times per second
etc.

Or a similar expression on the offset, using time, but making the output 2-dimensional, for example

my_offset = 5*Math.round(time*5);
[my_offset, my_offset]


That would move 5 pixels over and down 5 times a second.

Adjust values to taste.

Kalleheikki Kannisto
Senior Graphic Designer


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Filip Vandueren
Re: Replicating "Marching Ants" Photoshop selection effect in After Effects
on Oct 23, 2018 at 1:24:42 pm

Hey Mikey,

If you look closely at Marching Ants, you'll notice that they're not actually doing the same as the offset on a dashed line.
A dashed line turns around corners, and continuous along the path until it's back where it started,
whereas the marching ants are a 45° slanted zebra pattern that is masked within a 1px stroke.
The easiest way to get the effect is to use an animated pattern, and use your shape as a track matte.

Alternatively, if you want to keep it in 1 layer I have a solution using Gradients and Colorama effect.
(see attached FFX)
12828_marchingants.ffx.zip



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