user3592: I'm after a very specific effect and I haven't found an effective way to accomplish it.  Basically, I want dots to appear along a path and disappear over time.  The dots should not move after they're born, only fade out via alpha channel.  It's important that this is an actual alpha transformation, not just a color blend, because I plan to overlay this graphic on different source videos.  The paths also need to be easily editable and there will be hundreds of dots, so individual layers for each dot is out of the question.  
The best way I can describe the effect I'm after is: Imagine a person walking through the snow, the footprints appear instantly, and fade out over time.  I've seen plenty of tutorials on creating a series of elements which follow each other, as if you're dragging a string of pearls, but this is not what I'm after.
I've tried After Effects, and it's easy enough to make a dotted line with a shape layer, but a gradient stroke doesn't follow the path, or give me control over start position, end position, or offset.  (here's an illustration of the problem)

I've also tried using Cinema 4D and mograph, but keep running into similar problems.  I basically can't get a graduated shader effector to follow a spline.  
Any solutions which use C4D or AE are preferred.  All are welcome.  Thanks.

user6172: I tried to accomplish a similar effect and used the 'Vegas' and 'Echo' (or a similar) effect in AE to create an alpha matte.
The problem with the 'Vegas' effect is that when you set your line to be right before the beginning of the path, it appears at the end of it (try it out, you'll quickly notice what I mean).
My solution was to only use a little dot in the 'Vegas' effect but duplicate and fade it over time using an 'Echo' effect (or another of that sort).
The 'Echo' effect controls the length and falloff of the stroke, the color and width are controlled by 'Vegas'.
It's not very pleasingly clean, but it might get the job done.

user6685: A simple way in blender or any other 3d package would be to use a boolean modifier, if you want to subtract real geometry:


use a array modifier to duplicate your shape
create a path and aligning the objects on it
animate the object count of the array modifier
create a simple geometry for subtracting the path objects at the beginning
add a boolean modifier to your base object
animate the position of the boolean object to subtract the shape objects
make the boolean object invisible to the camera (e.g. with a transparent material)


.blend file

Edit: To get a soft transition you can render the bool object seperatly, blur the alpha channel and subract it from the another alpha. Or simply use a roto mask. 


.blend

user1950: here's a solution that I've used. It can be used to reveal a shape path, or as a way to add a smooth fade-out on the stroke effect as-is:

Create your dotted path using the pen tool (or whatever you like),
then use the path to create a mask on a new adjustment layer. I use
an expression so that I can dynamically change the path - just
alt-click the stopwatch on the Mask Path property and drag the
pick-whip to the shape path:

Apply the stroke effect to the mask path on the adjustment layer.
We'll be using the adjustment layer as an alpha matte for your
dotted line layer, but best to do that later so you can see what
you're doing. Set the Brush Size property so that the brush
covers the dotted line layer. Set the Paint Style property to
On Transparent.
Animate the Start and / or End property for the stroke
effect to animate the reveal / disappearance of your stroke. Now
this creates a hard start and end, so what you have to do is apply
the Echo effect — Effects>Time>Echo. set the decay to
something less than 1. You may want to increase the Number of
Echos and decrease the Echo Time to make the fade
in/out/smooth.

Now set your alpha matte on the shape layer et Voila!


If you want the stroke to fade in as well as out, you'll need to add another echo, but with the echo time set to a positive value (or is it negative) so that you're echoing from the future. You thought it was slow with just one echo effect applied? You're in for a world of waiting now! The reason it's so slow is that it has to compute the value for the previous frames for each of the echos, so for every frame you're rendering ten, twenty frames. That's why you might want to only ramp up the number of echos when it's time to send it off to the render.
Caveat: if the stroke crosses itself the alpha matte will reveal areas out of sequence. You can manually mask the offending areas out, or split the stroke path up into separate layers, this works, but it's a world of tediousness, I know from experience - I used it on an animation that featured hand-written text slowly fading in. Also Echo is dog slow, so you may want to ramp the settings up to final quality only once you done everything else you want to in the comp.

out-of sequence image appearing.
You can mitigate this by making sure that your brush size is as small as possible:

But if that's still a problem you can spit the stroke: add an extra point somewhere between the overlapping parts of the stroke

Now duplicate the shape layer and the adjustment layer (note that I've coloured the duplicated shape layer yellow)

Then you delete the points at either side of the point you created on each layer

one side

and the other. 
Make sure you adjust the expression in your duplicated adjustment layer to reflect the name of the new stroke layer - if it was thisComp.layer("Shape Layer **1**").content("Shape 1").content("Path 1").path you'll have to use the pick-whip or manually correct it to thisComp.layer("Shape Layer **2**").content("Shape 1").content("Path 1").path.
Now you just have to offset the keyframes in your adjustment layer so that one starts when the last one stops. You might need to juggle the position of the middle keyframes to deal with the echoes.

and you have a correctly revealing crossing:


3592: <p>I'm after a very specific effect and I haven't found an effective way to accomplish it.  Basically, I want dots to appear along a path and disappear over time.  The dots should not move after they're born, only fade out via alpha channel.  It's important that this is an actual alpha transformation, not just a color blend, because I plan to overlay this graphic on different source videos.  The paths also need to be easily editable and there will be hundreds of dots, so individual layers for each dot is out of the question.  </p>

<p>The best way I can describe the effect I'm after is: Imagine a person walking through the snow, the footprints appear instantly, and fade out over time.  I've seen plenty of tutorials on creating a series of elements which follow each other, as if you're dragging a string of pearls, but this is <strong>not</strong> what I'm after.</p>

<p>I've tried After Effects, and it's easy enough to make a dotted line with a shape layer, but a gradient stroke doesn't follow the path, or give me control over start position, end position, or offset.  (here's an illustration of the problem)</p>

<p><img src="https://i.stack.imgur.com/7Tjtt.png" alt="enter image description here"></p>

<p>I've also tried using Cinema 4D and mograph, but keep running into similar problems.  I basically can't get a graduated shader effector to follow a spline.  </p>

<p>Any solutions which use C4D or AE are preferred.  All are welcome.  Thanks.</p>
 6172: <p>I tried to accomplish a similar effect and used the 'Vegas' and 'Echo' (or a similar) effect in AE to create an alpha matte.</p>

<p>The problem with the 'Vegas' effect is that when you set your line to be right before the beginning of the path, it appears at the end of it (try it out, you'll quickly notice what I mean).<br>
My solution was to only use a little dot in the 'Vegas' effect but duplicate and fade it over time using an 'Echo' effect (or another of that sort).<br>
The 'Echo' effect controls the length and falloff of the stroke, the color and width are controlled by 'Vegas'.</p>

<p>It's not very pleasingly clean, but it might get the job done.</p>
 6685: <p>A simple way in blender or any other 3d package would be to use a boolean modifier, if you want to <strong>subtract real geometry</strong>:</p>

<p><img src="https://i.stack.imgur.com/sZLmG.png" alt="enter image description here"></p>

<ol>
<li>use a <strong>array modifier</strong> to duplicate your shape</li>
<li>create a path and <strong>aligning the objects</strong> on it</li>
<li><strong>animate the object count</strong> of the array modifier</li>
<li>create a <strong>simple geometry for subtracting the path objects</strong> at the beginning</li>
<li><strong>add a boolean modifier</strong> to your base object</li>
<li>animate the <strong>position of the boolean object</strong> to subtract the shape objects</li>
<li>make the <strong>boolean object invisible</strong> to the camera (e.g. with a transparent material)</li>
</ol>

<p><img src="https://i.stack.imgur.com/wlDKH.gif" alt="enter image description here"></p>

<p><a href="http://www.pasteall.org/blend/33840" rel="nofollow noreferrer"><strong>.blend file</strong></a></p>

<hr>

<p>Edit: To get a <strong>soft transition</strong> you can render the bool object seperatly, blur the <strong>alpha channel</strong> and subract it from the another alpha. Or simply use a <strong>roto mask</strong>. </p>

<p><img src="https://i.stack.imgur.com/alShv.png" alt="enter image description here"></p>

<p><img src="https://i.stack.imgur.com/ZHmif.png" alt="enter image description here"></p>

<p><a href="http://www.pasteall.org/blend/33845" rel="nofollow noreferrer"><strong>.blend</strong></a></p>
 1950: <p>here's a solution that I've used. It can be used to reveal a shape path, or as a way to add a smooth fade-out on the stroke effect as-is:</p>

<ol>
<li><p>Create your dotted path using the pen tool (or whatever you like),
then use the path to create a mask on a new adjustment layer. I use
an expression so that I can dynamically change the path - just
alt-click the stopwatch on the <strong>Mask Path</strong> property and drag the
pick-whip to the shape path:
<img src="https://i.stack.imgur.com/RGZOJ.png" alt="using pick-whip to synchronise paths"></p></li>
<li><p>Apply the stroke effect to the mask path on the adjustment layer.
We'll be using the adjustment layer as an alpha matte for your
dotted line layer, but best to do that later so you can see what
you're doing. Set the <strong>Brush Size</strong> property so that the brush
covers the dotted line layer. Set the <strong>Paint Style</strong> property to
<em>On Transparent</em>.</p></li>
<li><p>Animate the <strong>Start</strong> and / or <strong>End</strong> property for the stroke
effect to animate the reveal / disappearance of your stroke. Now
this creates a hard start and end, so what you have to do is apply
the <strong>Echo</strong> effect — <em>Effects>Time>Echo</em>. set the decay to
something less than 1. You may want to increase the <strong>Number of
Echos</strong> and decrease the <strong>Echo Time</strong> to make the fade
in/out/smooth.</p>

<p><img src="https://i.stack.imgur.com/iPCQp.png" alt="stroke effect fading smoothly"></p></li>
<li><p>Now set your alpha matte on the shape layer <em>et Voila!</em>
<img src="https://i.stack.imgur.com/BIEdC.png" alt="final result"></p></li>
</ol>

<p>If you want the stroke to fade in as well as out, you'll need to add another echo, but with the echo time set to a positive value (or is it negative) so that you're echoing from the future. You thought it was slow with just one echo effect applied? You're in for a world of waiting now! The reason it's so slow is that it has to compute the value for the previous frames for each of the echos, so for every frame you're rendering ten, twenty frames. That's why you might want to only ramp up the number of echos when it's time to send it off to the render.</p>

<p>Caveat: if the stroke crosses itself the alpha matte will reveal areas out of sequence. You can manually mask the offending areas out, or split the stroke path up into separate layers, this works, but it's a world of tediousness, I know from experience - I used it on an animation that featured hand-written text slowly fading in. Also <strong>Echo</strong> is dog slow, so you may want to ramp the settings up to final quality only once you done everything else you want to in the comp.</p>

<p><img src="https://i.stack.imgur.com/vKoRW.png" alt="uh oh">
out-of sequence image appearing.</p>

<p>You can mitigate this by making sure that your brush size is as small as possible:</p>

<p><img src="https://i.stack.imgur.com/muUG2.png" alt="enter image description here"></p>

<p>But if that's still a problem you can spit the stroke: add an extra point somewhere between the overlapping parts of the stroke
<img src="https://i.stack.imgur.com/woyVZ.png" alt="enter image description here">
Now duplicate the shape layer and the adjustment layer (note that I've coloured the duplicated shape layer yellow)
<img src="https://i.stack.imgur.com/gySub.png" alt="enter image description here"></p>

<p>Then you delete the points at either side of the point you created on each layer</p>

<p><img src="https://i.stack.imgur.com/4ytoF.png" alt="enter image description here"></p>

<p>one side</p>

<p><img src="https://i.stack.imgur.com/bIFOy.png" alt="enter image description here"></p>

<p>and the other. </p>

<p>Make sure you adjust the expression in your duplicated adjustment layer to reflect the name of the new stroke layer - if it was <code>thisComp.layer("Shape Layer **1**").content("Shape 1").content("Path 1").path</code> you'll have to use the pick-whip or manually correct it to <code>thisComp.layer("Shape Layer **2**").content("Shape 1").content("Path 1").path</code>.</p>

<p>Now you just have to offset the keyframes in your adjustment layer so that one starts when the last one stops. You might need to juggle the position of the middle keyframes to deal with the echoes.
<img src="https://i.stack.imgur.com/opFCA.png" alt="enter image description here"></p>

<p>and you have a correctly revealing crossing:</p>

<p><img src="https://i.stack.imgur.com/YTCJU.png" alt="enter image description here"></p>
