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Erasing different channels

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

Video: Erasing different channels

You can use the Eraser tool to remove parts of an image or just part of an existing paint stroke. You can also use it just in the Alpha channel. To get started, add any image to a composition, preferably one that has an interesting Alpha channel. I'll continue with the same composition I was using in the previous lesson. The first thing to do is to select the Eraser tool. Remember that the options in the Paint and Brushes panels are different depending of which tool is selected. So be sure to select the Eraser tool first.

Erasing different channels

You can use the Eraser tool to remove parts of an image or just part of an existing paint stroke. You can also use it just in the Alpha channel. To get started, add any image to a composition, preferably one that has an interesting Alpha channel. I'll continue with the same composition I was using in the previous lesson. The first thing to do is to select the Eraser tool. Remember that the options in the Paint and Brushes panels are different depending of which tool is selected. So be sure to select the Eraser tool first.

I will start with the default settings which is to set the Channels pop-up to RGB and Alpha, you have the same options as you had with the Brush tool; the Duration to Constant and again, these are same options we had for brushes. I won't be using all the different options for the Eraser; I am just going to set it to the Constant mode. The Erase options are only available when the Eraser tool is active. The default is to erase the Layer Source, the image itself, as well as any paint strokes that are applied to the image.

The other options are to only erase the paint strokes or to only erase the Last Stroke you painted. We'll cover these options in the next movie. For now, let's leave it set to layer Source & Paint. I will find an area of the image I would like erase. I can only zoom in so you can see things in the little more detail. Remember, the shortcut for setting the size of the brush; this also works for the eraser. I will press the Command key, size the brush and I will pick a size I think is pretty generous.

Let go of the Command key and then I can set the size to the hardest. I am just going to start erasing this corner of the image. Let's just say, I don't want that to be there. If you find your brushes too small or too large, it might be better to undo the stroke and set it to a different size and notice I'm trying to erase this area using one Brush stroke. I'm not dabbing at it. When I release the mouse, you can see in the timeline that the stroke I painted is called Eraser 1, and you can animate this just like you could with the paint brush.

And that's the reason I wasn't dabbing at it the way I might if I was erasing pixels because every time I paint with the eraser, I am creating a vector stroke just like with the Paint tool. So I don't want to have multiple strokes to manage. So that's the result when you paint with the Channel set to RGB and Alpha. You are painting to transparency. Anytime you want to check that, turn on the Transparency Grid and you can see that area is now transparent. I will undo to get back the original image and I will change the Channels pop-up to RGB.

Now when I erase, I will be replacing the image with the background color and you can see that as I start painting. Now you might be saying to yourself that looks just like the previous example. But when I toggle on the Transparency Grid, you can see I have replaced it with black and it's important to realize whenever you are using the eraser, you are always erasing to the background color. Remember, if you set the Comps background color to the same color as the background color in paint and you set Channels to RGB where you are replacing the image with the background color, you won't be able to tell the difference until you toggle on the Transparency Grid which is something to keep in mind.

Unfortunately, unlike the Paint Brush, the Eraser 1 stroke in the timeline does not have a Channels pop-up. So you are not able to change the mode for Channels after the fact. So I will just select the Eraser 1 and delete it or you can just press Undo. The last option on the Channels is to only use the Eraser tool in the Alpha channel. As soon as I set it to Alpha, you will notice that the colors change through black and white, even if you click on the foreground color or the background color and try to change it to a color, once you have the Channels set to Alpha, you will only be painting or erasing in shades of gray.

And by the way, in the next chapter, we will be painting in the Alpha channel. Remember, when you are using the Eraser tool, you are always erasing to the background color. So let's say I want to paint in the Alpha channel with black. Make sure black appears as the background color. You can just click this little switch here to quickly set it to black and white and then toggle the foreground and background colors to place Black in the background color. Now when I am painting, I am actually painting black in the Alpha.

And in this case, there is no difference between painting black in the Alpha and simply erasing the RGB and Alpha image. Of course, if you are painting in the Alpha channel, you may want to set the Channels pop-up to Alpha and then you can see the Black and White alpha as you are erasing. If you want to replace some pixels with white, you can switch the colors so that white is the background color. Remember, there is no reason why you couldn't click on the Paint Brush, make sure the Paint Brush is also set to Alpha and then you can paint in Black and White using the Paint tool.

There is really no difference, although I see here I have the Duration set to Single Frame. I will just switch that back to Constant. Of course, when I switch to using the Brush tool, the size of my brush was different than the size I had set for the Eraser. So keep an eye on that. You might want them to match. The big difference with using the Brush tool is that I will always be painting with the foreground color, in this case, that's black. If I want to paint with white, I need to make sure the white is the foreground color.

So again, that's the opposite of the Eraser tool. I am going to return to the Eraser tool and let me just explain one last thing that might trip you up. I will select all those brushes that I created by accident, quite a few of them here, and we will get back to the original image. Notice I am still painting in the Alpha channel and I am going to be erasing to the white color. Notice if I actually extend the Alpha channel and make it larger, you might see some artifacts in the Color channels.

I will switch back to viewing RGB and you can see that the edge of the Color channels looks really bad. I will undo and I will turn on the Transparency Grid. Normally, the edge of your image looks really smooth and that's because you are viewing it with a nice anti-aliased Alpha channel. But underneath the hood, After Effects is compositing this image with the Color channels in straight RGB mode and what that means is that the color pixels actually extend passed the edge of the Alpha channel.

You can see this very clearly if you set the Show channels to RGB Straight. Now you can see what the RGB channels actually look like before the Alpha channel is composited. So when you extend the white area in the Alpha channel, you are simply revealing these pixels which you probably don't want to reveal. So keep that in mind that this is actually how it's supposed to work if you happen to be painting the Alpha channel and you happen to extend the white area of the Alpha outside of the original Alpha channel. This is not a bug.

So let's undo again. So I think that covers the various options under Channels and return to erasing RGB and Alpha, and in the next movie, I will cover the various options in the Erase pop-up.

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