<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.sstatic.net/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.sstatic.net/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.sstatic.net/alShv.png" alt="enter image description here"></p>

<p><img src="https://i.sstatic.net/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>

CC BY-SA 3.0
2015-01-16T14:06:01.307Z
Very close, but it's important that the tail fades out.  C4D has booleans too, and I could do something very similar.  It's just the "8. OR render the bool object as mask for creating a softer edge" that's tricky for me, and actually the crux of this whole question.  The most frustrating part about it is that I don't actually need the effect to be 3D.  I'd just settle for a 3D app solution if that's the only way I can get the control I need.  All I really want is an alpha gradient that can traverse a path.  But thank you for your answer, I'll upvote it.
Jason Conrad
CC BY-SA 3.0
2015-01-16T15:25:18.150Z
@JasonConrad To get a nice alpha gradient, you can render the bool object seperatly like this: https://i.sstatic.net/qm5ts.png, blur the alpha pass and combine both alphas.
p2or
CC BY-SA 3.0
2015-01-22T22:08:49.820Z
I would also suggest to do the alpha fade in post.
timonsku
CC BY-SA 3.0
2015-01-16T13:21:36.143Z
2015-01-18T10:38:57.950Z
2015-01-18T10:38:57.950Z
p2or
<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.sstatic.net/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.sstatic.net/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.sstatic.net/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.sstatic.net/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.sstatic.net/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.sstatic.net/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.sstatic.net/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.sstatic.net/4ytoF.png" alt="enter image description here"></p>

<p>one side</p>

<p><img src="https://i.sstatic.net/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.sstatic.net/opFCA.png" alt="enter image description here"></p>

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

<p><img src="https://i.sstatic.net/YTCJU.png" alt="enter image description here"></p>

CC BY-SA 3.0
2015-02-10T13:20:43.647Z
Thanks for the really spectacular answer.  Just wanted to say I'm sorry for not awarding you the full 300 in time, I totally meant to, but I got pretty sick for a week or so.  Anyway, much appreciation :)
Jason Conrad
CC BY-SA 3.0
2015-01-20T02:37:29.523Z
2015-01-21T04:26:39.907Z
2015-01-21T04:26:39.907Z
stib
<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>

CC BY-SA 3.0
2015-01-16T21:23:20.247Z
2015-01-16T21:23:20.247Z
j-maas
<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.sstatic.net/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>

CC BY-SA 3.0
2015-01-15T22:53:15.347Z
[Semi-related](http://video.stackexchange.com/questions/10391/how-to-achieve-this-kind-of-masks-and-transitions/10394#10394), though I'm sure there are better options.
AJ Henderson
CC BY-SA 3.0
2015-01-19T11:35:05.310Z
Feel free to comment if my answer don't solve your issue...
p2or
CC BY-SA 3.0
2015-01-19T17:04:52.207Z
It looks good.  The edits you added are helpful.  I just can't help but feel that there's an easier way.  I'm going to give it some more time.  Actually, maybe add a bounty.
Jason Conrad
CC BY-SA 3.0
2015-01-19T19:47:37.663Z
Ok, nice :) What exactly bothers you? Do you prefer After Effects? 2d is ok?
p2or
CC BY-SA 3.0
2015-01-19T22:10:00.133Z
I'm not too familiar with blender, and am not quite sure how to translate into c4d.  I guess it's the soft transition part that I don't quite get, or maybe it sounds a little cumbersome.  Honestly I haven't sat down and tried it yet, it might make more sense when I do.  And really, this is a compositing problem, so I'm going to need to see the background as I draw over it and decide where to place the lines.  I know how to do that, it just seems like a job for a compositing app.  Guess I'm disappointed it's not as simple as it seems like it should be.
Jason Conrad
<after-effects><animation><cinema4d>
Footprint or breadcrumb animation
Jason Conrad
# Footprint or breadcrumb animation

(Asked by: username_0 on 2015-01-15)

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)

![enter image description here](https://i.sstatic.net/7Tjtt.png)

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.

**Tags:** after-effects, animation, cinema4d

> **Comments:**
> **username_1**: [Semi-related](http://video.stackexchange.com/questions/10391/how-to-achieve-this-kind-of-masks-and-transitions/10394#10394), though I'm sure there are better options.  
> **username_2**: Feel free to comment if my answer don't solve your issue...  
> **username_0**: It looks good.  The edits you added are helpful.  I just can't help but feel that there's an easier way.  I'm going to give it some more time.  Actually, maybe add a bounty.  
> **username_2**: Ok, nice :) What exactly bothers you? Do you prefer After Effects? 2d is ok?  
> **username_0**: I'm not too familiar with blender, and am not quite sure how to translate into c4d.  I guess it's the soft transition part that I don't quite get, or maybe it sounds a little cumbersome.  Honestly I haven't sat down and tried it yet, it might make more sense when I do.  And really, this is a compositing problem, so I'm going to need to see the background as I draw over it and decide where to place the lines.  I know how to do that, it just seems like a job for a compositing app.  Guess I'm disappointed it's not as simple as it seems like it should be.  

---

## 3 answers

### Answer by username_2 on 2015-01-16. Score: 5

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

![enter image description here](https://i.sstatic.net/sZLmG.png)

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

![enter image description here](https://i.sstatic.net/wlDKH.gif)

[**.blend file**](http://www.pasteall.org/blend/33840)

---

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**.

![enter image description here](https://i.sstatic.net/alShv.png)

![enter image description here](https://i.sstatic.net/ZHmif.png)

[**.blend**](http://www.pasteall.org/blend/33845)

> **Comments:**
> **username_0**: Very close, but it's important that the tail fades out.  C4D has booleans too, and I could do something very similar.  It's just the "8. OR render the bool object as mask for creating a softer edge" that's tricky for me, and actually the crux of this whole question.  The most frustrating part about it is that I don't actually need the effect to be 3D.  I'd just settle for a 3D app solution if that's the only way I can get the control I need.  All I really want is an alpha gradient that can traverse a path.  But thank you for your answer, I'll upvote it.  
> **username_2**: @JasonConrad To get a nice alpha gradient, you can render the bool object seperatly like this: https://i.sstatic.net/qm5ts.png, blur the alpha pass and combine both alphas.  
> **username_3**: I would also suggest to do the alpha fade in post.  

---

### Answer by username_4 on 2015-01-20. Score: 5

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:

1. 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:
   ![using pick-whip to synchronise paths](https://i.sstatic.net/RGZOJ.png)
2. 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*.
3. 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.

   ![stroke effect fading smoothly](https://i.sstatic.net/iPCQp.png)
4. Now set your alpha matte on the shape layer *et Voila!*
   ![final result](https://i.sstatic.net/BIEdC.png)

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.

![uh oh](https://i.sstatic.net/vKoRW.png)
out-of sequence image appearing.

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

![enter image description here](https://i.sstatic.net/muUG2.png)

But if that's still a problem you can spit the stroke: add an extra point somewhere between the overlapping parts of the stroke
![enter image description here](https://i.sstatic.net/woyVZ.png)
Now duplicate the shape layer and the adjustment layer (note that I've coloured the duplicated shape layer yellow)
![enter image description here](https://i.sstatic.net/gySub.png)

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

![enter image description here](https://i.sstatic.net/4ytoF.png)

one side

![enter image description here](https://i.sstatic.net/bIFOy.png)

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.
![enter image description here](https://i.sstatic.net/opFCA.png)

and you have a correctly revealing crossing:

![enter image description here](https://i.sstatic.net/YTCJU.png)

> **Comments:**
> **username_0**: Thanks for the really spectacular answer.  Just wanted to say I'm sorry for not awarding you the full 300 in time, I totally meant to, but I got pretty sick for a week or so.  Anyway, much appreciation :)  

---

### Answer by username_5 on 2015-01-16. Score: 1

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.


---

