<p>The thing about masks is that you <em>don't</em> have to do them on every frame. To quickly do rotoscoping you let the computer do as much of the work as it can, by interpolating wherever possible.</p>

<p>To mask out the hands with the pen tool, draw the mask for the first frame and then move as far along the timeline as you can until the point where the motion dramatically changes, e.g. if the hand is moving up, continue until it stops or starts moving down. Adjust the mask points, trying to keep the points close to the part of the hand they were near when you first drew them. It's good to give yourself enough points to work with when you first draw the mask. </p>

<p>Now go back between the two keyframes and find the point at which the maksk is the <em>furthest</em> from matching. Re-adjust, thus making a third keyframe. Now 
rinse, repeat: between each of your keyframes, pick the point where the mask is the worst and refine.</p>

<p>As you work you'll find that the mask needs less and less tweaking and eventually starts following the hand well. If your motion is really crazy this might not be until you've done every frame, but the adjustments you'll need to make will be smaller and smaller each time.</p>

<p>You'll find this will be tedious, but way less tedious than rendering as an image sequence and photoshoping each frame, or just opening the movie in photoshop (which is the answer to your question BTW). The difficulty in getting smooth motion that way would be immense. Think about the way hand drawn animation looks - the lines constantly 'boil' because each frame is painted from scratch. Painting the matte frame by frame will give you a result like that, which might be nice, but it doesn't seem like the look you're going for.</p>

<p>Put on a podcast or some music, make a coffee, get comfortable and get busy with the béziers.</p>

CC BY-SA 3.0
2016-11-25T02:50:34.173Z
Thanks for this. I ended up using the rotobrush and just trying to teach it best I can which eventually worked. If I need to do it again, I'd just open it in photoshop as you suggested.  

For other footage in the video, I've been using masks with the pen tool as they are more non-organic shapes in the foreground and it makes sense to use the pen tool for those. For things like this hand which changes its shape too much with blurring and softer edges, the pen tool isn't as useful.
Galaxy
CC BY-SA 3.0
2016-11-25T04:01:12.273Z
If you turn motion blurring on for the layers with the masks you'll find that the masks will be motion blurred as they move. There's also the feather tool (tap the `g` key twice) which allows you to variably feather a mask
stib
CC BY-SA 3.0
2016-11-24T13:16:57.747Z
2016-11-24T13:16:57.747Z
stib
<p>I'm trying to rotobrush/rotoscope a bunch of footage, see image below for what I mean.</p>

<p>Currently I'm using the rotobrush, but it is too time-consuming and slow - it keeps trying to guess the paths, which aren't accurate enough. I also don't want to draw pen masks, as it is too slow to do frame by frame and the hands are too complex.</p>

<p>Ideally, I just was to brush paint each frame and have that as a alpha mask. I'm sure this can be done, I just don't know the way to do it!</p>

<p>Any help would be awesome, thanks!</p>

<p><a href="https://i.sstatic.net/E9t76.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/E9t76.png" alt="enter image description here"></a></p>

<video><after-effects><alpha>
Using brush tool for alpha channels/rotoscoping in after-effects
Galaxy
# Using brush tool for alpha channels/rotoscoping in after-effects

(Asked by: username_0 on 2016-11-24)

I'm trying to rotobrush/rotoscope a bunch of footage, see image below for what I mean.

Currently I'm using the rotobrush, but it is too time-consuming and slow - it keeps trying to guess the paths, which aren't accurate enough. I also don't want to draw pen masks, as it is too slow to do frame by frame and the hands are too complex.

Ideally, I just was to brush paint each frame and have that as a alpha mask. I'm sure this can be done, I just don't know the way to do it!

Any help would be awesome, thanks!

[![enter image description here](https://i.sstatic.net/E9t76.png)](https://i.sstatic.net/E9t76.png)

**Tags:** video, after-effects, alpha


---

## 1 answers

### Answer by username_1 on 2016-11-24. Score: 1 | Accepted answer

The thing about masks is that you *don't* have to do them on every frame. To quickly do rotoscoping you let the computer do as much of the work as it can, by interpolating wherever possible.

To mask out the hands with the pen tool, draw the mask for the first frame and then move as far along the timeline as you can until the point where the motion dramatically changes, e.g. if the hand is moving up, continue until it stops or starts moving down. Adjust the mask points, trying to keep the points close to the part of the hand they were near when you first drew them. It's good to give yourself enough points to work with when you first draw the mask.

Now go back between the two keyframes and find the point at which the maksk is the *furthest* from matching. Re-adjust, thus making a third keyframe. Now
rinse, repeat: between each of your keyframes, pick the point where the mask is the worst and refine.

As you work you'll find that the mask needs less and less tweaking and eventually starts following the hand well. If your motion is really crazy this might not be until you've done every frame, but the adjustments you'll need to make will be smaller and smaller each time.

You'll find this will be tedious, but way less tedious than rendering as an image sequence and photoshoping each frame, or just opening the movie in photoshop (which is the answer to your question BTW). The difficulty in getting smooth motion that way would be immense. Think about the way hand drawn animation looks - the lines constantly 'boil' because each frame is painted from scratch. Painting the matte frame by frame will give you a result like that, which might be nice, but it doesn't seem like the look you're going for.

Put on a podcast or some music, make a coffee, get comfortable and get busy with the béziers.

> **Comments:**
> **username_0**: Thanks for this. I ended up using the rotobrush and just trying to teach it best I can which eventually worked. If I need to do it again, I'd just open it in photoshop as you suggested.    For other footage in the video, I've been using masks with the pen tool as they are more non-organic shapes in the foreground and it makes sense to use the pen tool for those. For things like this hand which changes its shape too much with blurring and softer edges, the pen tool isn't as useful.  
> **username_1**: If you turn motion blurring on for the layers with the masks you'll find that the masks will be motion blurred as they move. There's also the feather tool (tap the `g` key twice) which allows you to variably feather a mask  

---

