<p>One way to do this would be to construct a matte layer out of white circles expanding as needed, then key one video layer over the other using the matte to reveal. You can animate graphic circles for the matte, as many as you like.</p>

CC BY-SA 3.0
2015-03-13T01:01:31.500Z
Thanks Jim, this is similar to what I was expecting but can you point me in the direction of how to create the matte? In my tests so far, I have created each circle on a seperate layer however this will be ridiculous to have to produce 100 or so layers
Ecks
CC BY-SA 3.0
2015-03-13T09:17:24.533Z
I'm not an AE maven so I can't give you specific directions, but there's likely a way to combine layers (mix down or render) each time you add, say, four layers. Then when you're done you have a single rendered layer you can import into your main project. Honestly, if I were doing this I'd write code (a script or similar) to create the circles but I'm not sure if AE can be scripted like that. Maybe someone else here knows.
Jim Mack
CC BY-SA 3.0
2015-03-13T13:58:24.293Z
There is. It's called pre-comping. It's been a while since I've done it, but if you check the docs for "pre comp" or "precompose" you should find more info.
user1118321
CC BY-SA 3.0
2015-03-13T00:52:39.600Z
2015-03-13T00:52:39.600Z
Jim Mack
<p>You could use the particle engine CC Particle World.</p>

<p>Set the particles to be shaded spheres, adjust the X&amp;Y radius of the producer to fill the screen, adjust the birth rate to produce particles at the rate you need, turn off gravity and velocity.  set the end size to be so big that they fill the screen.  Set the Particle Transfer Mode to "black matte", and you'll get something like this:</p>

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

<p>then precomp the particle layer, moving everything inside the comp</p>

<p>Now add your two layers (the files for before &amp; after the transition). Put the "before" layer at the bottom, then the "after" layer, then the "particle" comp layer.
Turn of the particle layer by clicking the eyeball.
In the example below, I'm using a pink and blue layer for the before and after.</p>

<p>Add a "<a href="https://helpx.adobe.com/after-effects/using/channel-effects.html#set_matte_effect" rel="nofollow noreferrer">Set Matte</a>" to the "after" layer, and set the "Take matte from layer" setting to your particle layer.</p>

<p>When you drag through the comp, your "before" layer should reveal the "after" layer via your circles.</p>

<p><img src="https://i.sstatic.net/sr3mk.png" alt="enter image description here"></p>

CC BY-SA 3.0
2015-03-14T00:58:19.180Z
That sound perfect i will try this asap. Thanks tomh. This takes me back to my days of playing with particles in Lightwave 3d.
Ecks
CC BY-SA 3.0
2015-03-13T14:38:53.087Z
2015-03-13T17:20:11.453Z
2015-03-13T17:20:11.453Z
tomh
<p>I need to do a transition that puts random circles that expand over a video but each circle needs to start at random times. </p>

<p>Does anyone know the best way to achieve this?</p>

CC BY-SA 3.0
2015-03-13T00:48:18.407Z
Do these circles act as a wipe between two full screen images, or do they contain individual images, or...? Does each circle cover only a portion of the screen, or would each eventually expand to cover the whole screen? Details, please -- maybe a drawing?
Jim Mack
CC BY-SA 3.0
2015-03-13T00:49:45.667Z
Two full screen images
Ecks
CC BY-SA 3.0
2015-03-13T00:58:25.240Z
I apologize Jim, I was checking on my phone and only saw half of your comment. Yes, it is two full screen images. Each circle only needs to grow to a certain size say 75 pixels but there needs to be enough to fill the screen at the end of the transition.
Ecks
<after-effects><transitions>
After affects transition help
Ecks
# After affects transition help

(Asked by: username_0 on 2015-03-12)

I need to do a transition that puts random circles that expand over a video but each circle needs to start at random times.

Does anyone know the best way to achieve this?

**Tags:** after-effects, transitions

> **Comments:**
> **username_1**: Do these circles act as a wipe between two full screen images, or do they contain individual images, or...? Does each circle cover only a portion of the screen, or would each eventually expand to cover the whole screen? Details, please -- maybe a drawing?  
> **username_0**: Two full screen images  
> **username_0**: I apologize Jim, I was checking on my phone and only saw half of your comment. Yes, it is two full screen images. Each circle only needs to grow to a certain size say 75 pixels but there needs to be enough to fill the screen at the end of the transition.  

---

## 2 answers

### Answer by username_3 on 2015-03-13. Score: 2 | Accepted answer

You could use the particle engine CC Particle World.

Set the particles to be shaded spheres, adjust the X&Y radius of the producer to fill the screen, adjust the birth rate to produce particles at the rate you need, turn off gravity and velocity. set the end size to be so big that they fill the screen. Set the Particle Transfer Mode to "black matte", and you'll get something like this:

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

then precomp the particle layer, moving everything inside the comp

Now add your two layers (the files for before & after the transition). Put the "before" layer at the bottom, then the "after" layer, then the "particle" comp layer.
Turn of the particle layer by clicking the eyeball.
In the example below, I'm using a pink and blue layer for the before and after.

Add a "[Set Matte](https://helpx.adobe.com/after-effects/using/channel-effects.html#set_matte_effect)" to the "after" layer, and set the "Take matte from layer" setting to your particle layer.

When you drag through the comp, your "before" layer should reveal the "after" layer via your circles.

![enter image description here](https://i.sstatic.net/sr3mk.png)

> **Comments:**
> **username_0**: That sound perfect i will try this asap. Thanks username_3. This takes me back to my days of playing with particles in Lightwave 3d.  

---

### Answer by username_1 on 2015-03-13. Score: 1

One way to do this would be to construct a matte layer out of white circles expanding as needed, then key one video layer over the other using the matte to reveal. You can animate graphic circles for the matte, as many as you like.

> **Comments:**
> **username_0**: Thanks Jim, this is similar to what I was expecting but can you point me in the direction of how to create the matte? In my tests so far, I have created each circle on a seperate layer however this will be ridiculous to have to produce 100 or so layers  
> **username_1**: I'm not an AE maven so I can't give you specific directions, but there's likely a way to combine layers (mix down or render) each time you add, say, four layers. Then when you're done you have a single rendered layer you can import into your main project. Honestly, if I were doing this I'd write code (a script or similar) to create the circles but I'm not sure if AE can be scripted like that. Maybe someone else here knows.  
> **username_2**: There is. It's called pre-comping. It's been a while since I've done it, but if you check the docs for "pre comp" or "precompose" you should find more info.  

---

