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Making corrective strokes

From: After Effects Apprentice 13: Paint, Roto, and Puppet

Video: Making corrective strokes

In the previous movie we tweaked Roto Brush's propagation parameters to help it find the changes in the segmentation boundary from frame to frame. This will greatly improve Roto Brush's performances and indeed is an essential step that many people miss. But, it does not make Roto Brush infallible; you will still need to teach it when it makes mistakes. To do this, you need to first start from your base frame and then step, frame by frame, later and earlier in time and look for mistakes.

Making corrective strokes

In the previous movie we tweaked Roto Brush's propagation parameters to help it find the changes in the segmentation boundary from frame to frame. This will greatly improve Roto Brush's performances and indeed is an essential step that many people miss. But, it does not make Roto Brush infallible; you will still need to teach it when it makes mistakes. To do this, you need to first start from your base frame and then step, frame by frame, later and earlier in time and look for mistakes.

When you make a corrective stroke, Roto Brush will propagate the new information you've taught it from that frame in the direction that the arrows in the Roto Brush Span are pointing. Don't jump many frames down the road, make a correction then go back closer to the base frame and go, "Oh, I'll look it missed something, let's make a correction here." Roto Brush will not propagate your changes against the direction of the arrows. It's very important to remember, these arrows make a big difference in how the information you teach Roto Brush is used on subsequent frames.

So, I'm back on my nice base frame. Let's start by trying to propagate all the way back to the beginning of this clip. I'm going to press Page Up or the number 1 to step one frame earlier in time, look for mistakes, and there's one right here. I'm going to zoom in, press Option or Alt, and correct Roto Brush's error right in that gap. I'll zoom in a little bit more and try to correct that error. Now as we discussed earlier, this pink segmentation boundary is just the centerline of the alpha channel edge Roto Brush is going to create.

It does create some anti-aliasing and semi-transparent areas depending on the softness of the edge, motion blur, etc. However, there may be times when you become a bit unnerved that this propagation boundary is not following sharp angles. If that's the case, you need to turn your attention to the Matte, Smooth parameter. That's basically an algorithm for how much Roto Brush is smoothing out this profile's drawing around the edge between your foreground and background. For example, I'll just take this down to zero initially and you'll see Roto Brush follows a lot more detail along that edge maybe even too much detail creating something very jagged.

Eventually, you're going to tweak the Smoothness parameter, they come up with a good balance between tightly following your edge but not creating little jaggies. For now I'm going to try to find a value that gives me confidence that Roto Brush is looking at my edge correctly and not over smoothing the edge and causing me to create incorrect foreground and background strokes. So I'd say right around here may be a good compromise value. If I go too far it's a little bit of a jaggy edge like through here and stir steps that are a little bit nicer.

I'll use this as my temporary value while creating the rest of my corrective strokes. Again, I can always tweak this later. Go back down to 100% and look for other potential mistakes in Roto Brush's propagation. I've got a little bit down here at the keyboard. So let's press Option or Alt and fix that edge, that's better. Now zoom in a little bit more here, hold Command or Ctrl, resize my brush to make a smaller brush, and make sure I'm getting this gap right in there. It's important to go all the way around your frame and make sure you're happy that Roto Brush is consistently finding that boundary between foreground and background that you defined back in your base frame.

We might be missing a little bit of finger through here so I'm just going to scooch that edge out a little bit right into there. You can go crazy trying to make these edges too perfect. When you have a lot of motion from frame to frame the user is not going to see a lot of these small errors and the inconsistencies. Perfection is something to strive for, but you'd drive yourself crazy in trying to achieve it.

But this looks like a pretty good balance for this frame. All the information that I have entered on that particular frame is actually being stored for the Roto Brush effect on this layer. I'll type PP and reveal all the individual strokes I have made to define the base frame and now to update the current frame. Quite often you don't need to look at these and do anything with them but they are there if you want to turn them on and off, delete them, etcetera. But I just want to show you that Roto Brush is indeed recording these strokes and using that information as it propagates earlier or later in time.

And I'll twirl that up for now. I'll press Page Up or the number 1 again and see how it's doing there. And let's zoom in and again do some corrective actions for Roto Brush by holding the Spacebar to get the Hand tool temporarily, panning around my image, and just looking for corrections I need to make to create a nice consistent Roto Brush outline from frame to frame.

Again, remembering that this pink segmentation boundary is just the centerline for partial transparency, it's not my final matte. Now I'm going through this fairly quickly because I don't want to drive you completely crazy with boredom. Normally, I would spend a lot more time making sure that I am defining all these little corrective strokes exactly the way that I want to.

Don't be afraid of giving Roto Brush too much information as long as it's correct information. You notice I was making multiple little strokes into this gap to make sure we got as much information there as possible. Even though the segmentation boundary didn't change, I didn't mind so much because I was teaching Roto Brush correct information. If I was teaching Roto Brush bad information, I would need to undo to unteach it and then make better corrective strokes.

If you get to another frame and there's no need to make a corrective stroke, that's fine. Don't make another corrective stroke. I need to keep tweaking this little gap between the fingers here. Put a smaller brush into the gap, that's a bit better. But if you were so fortunate that you don't need to make any corrections, there's one more pitfall you might run into with Roto Brush -- and excuse me while I talk and correct at the same time.

Oh, big mistake through here. This is why you have to look all the way around the outline on every frame to make sure you're teaching Roto Brush correctly. I moved one frame earlier and things are looking pretty good there. I think that gap is as good as I'm going to get it and anyway I'm getting beyond the edge of my screen here. Yup, that's looking pretty good. Page Up. If I did not have to make any correction on this frame, and for that matter if I did not have to make any corrections for the next 20 frames, I might find myself in a situation where I'm at the end of my Roto Brush Span.

If you go one more frame past the edge of your span, you'll see the pink segmentation boundary is no longer following the foreground and background, it's jumped to follow the entire frame. That means Roto Brush has not received any new information for 20 frames and does not feel comfortable propagating for 21 frames or more; it wishes it had more information. There's a couple of things you can do at this point. Go ahead and make any corrective stroke somewhere along the span and Roto Brush will extend the span another 20 frames from that stroke.

Or, if you really feel you don't need to make any more corrective strokes you can hover your cursor over the edges of the Roto Brush Span and drag it earlier or later in time to cover as many frames as you feel comfortable having Roto Brush automatically propagate your information. Now in my case, I am not comfortable. I'm seeing Roto Brush continue to make little errors from frame to frame. For example, there's a gap opening up between fingers here and I want to make sure Roto Brush gets that gap.

Those who have spent any time on the underground system in England feel free to start chanting Mind the Gap. Those who have been to England on the underground know what I'm talking about. The rest of you can look it up so you've got party trivia. And I want to make sure that I'm keeping a consistent edge between foreground and background as I pan around this image. Good. Oops! See, you have to look.

And if Roto Brush grabs too much information as long as your stroke was good you don't need to undo. I'll just add another corrective stroke to help teach Roto Brush what the difference is between foreground and background. It's a little bit of a mistake in through there. By the way, Roto Brush does like to start chewing away this bracelet I have found out because I've done a shot a bunch of times. That's another area for you to watch out for as you go around the shot.

I could grab a little bit more in that area. Again, I'm looking for the central line of my eventual transparency. Shadow areas get tricky. Here's definitely a case for that bracelet disappeared in the shadow and Roto Brush can't tell the difference between shadow and bracelet, so I'll help it define those areas until it does a better job at determining that edge. And that looks good.

One more frame earlier, you notice that Roto Brush did indeed realize that that gap was still there but I do need to teach it a little bit more to improve that gap. That's better. Let's make sure I'm not eroding too much of the fingers away. That's not bad. Good. Good. I could arguably go after a little gap there. Definitely, I have some issues through here.

Oops, it's missing a bit of bracelet there. See what I just did there? I held the wrong key down, and I made a background stroke through the foreground. Don't make another foreground stroke; two wrongs do not make a right. Undo and then make the correct stroke through that area. Some little touch ups. Big problems with the shadows here, so let's go ahead and correct that.

A little bit in that area and that area, major problem right in through there. But now I've got a reasonable centerline. Make sure you get a little bit more of this fingertip here but otherwise Roto Brush has been doing a pretty good job with the motion blurred fingertips and we'll show you how Roto Brush handles Motion Blur a little bit later on. Okay, that's good. Rather than keeping you here for another 20 minutes, half hour, however long it's going to take to set through all these frames, I am going to leave you the rest of these frames back to time zero as an exercise for you to go ahead and clean up.

I think by now you have an idea of what I'm doing and you might need to go back and forth through your frames multiple times to get this the way you want it to be. But now you know what you need to do to teach Roto Brush, the differences between foreground and background, and you might say to yourself "this seems like a lot of work," and you'd be right. Oops, wrong stroke. It is a lot of work, but it is far less work, and if I had to repaint the alpha channel all the way around the shape on every single frame.

I'm just making a couple of small corrective strokes here and there not having to repaint every single pixel of the alpha. So no matter how tedious this is, it could be and would be a lot worse. Okay, you can take it from here. Keep working until you have propagated your segmentation boundary all the way back to time zero. And when you're done, move on to the next movie.

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