Green Screening and Rotoscoping in After Effects

Green screening (or blue screening if you want to use a blue background – or any other colour, although bright blue or green come out best) links quite closely with rotoscoping. Rotoscoping however can cover a variety of things, although each involves editing a filmed clip, either by cutting out/removing areas (very much the same as green screening), or by drawing over filmed footage to animate a drawn character in a realistic way – Disney used this for the dancing scenes in Snow White in 1937, now it can be done easily on the computer.

Photoshop will be one of the best for hand drawn rotoscoping due to all the tools/paintbrushes, and you can also import movie files into Photoshop (I think this came in the CS5 package, so packages before then, this tool might not be available.) - to be able to edit a film clip, you will want to go to 'Window' and go down to 'Animation'. Click on it and you will see an 'Animation (Timeline)' bar appears at the bottom.

To open a movie clip, just go to 'File', 'Open' and select the movie file you want, just the same as when editing an illustration or photo. You will now see the first frame of the footage in the workspace, and it will show in the 'Layers' bar to the side that there is 'Layer 1' selected – it will also have a small image of film reel in the corner of the thumbnail, showing that it is a movie clip. You will also see in the timeline a bar appear, showing the length of your clip. It will also say how many frames per second it runs at, and whether it has sound. The bar is very similar to that in After Effects, and works in the same way. You can also edit the position, opacity and style in a drop down menu – if you click the little arrow next to 'Layer 1' in the timeline – just like After Effects.

In the same way as After Effects, you can change the frame rate by right clicking on the original file in the side menu (in this case 'Layer 1') and selecting 'Interpret Footage'. Here a window will appear and you can change the frame rate.

To draw over the footage, you generally go through frame by frame to draw the footage, but on some frames you may want the image to stay the same. If you go to the right end of the Timeline, there is a tiny downwards facing arrow next to four horizontal lines. Click on this and a drop down menu will appear. Here you can copy keyframes and it gives you other options to edit your footage. You can edit each frame just as you would for an illustration etc, copying and pasting layer styles etc.

On some footage, you may want to hand draw a character to interact with the footage you have filmed. You could easily hand draw it and film it with the rostrum, but you'd save a lot of time and effort (and pens and paper) by drawing straight onto the footage in Photoshop. When you drawn by hand on a lightbox, you put the two keyframes together and then create the inbetween shots. This can be done in Photoshop by using 'Onion Skinning'. To enable Onion Skinning, click the little onion image at the bottom of the timeline, which is placed next to the little dustbin. To edit the settings (such as how many frames before and after you see, frame spacing, opacity and the blend mode), click the drop down menu to the end of the timeline (the little arrow and horizontal lines menu) and click on 'Onion Skin Settings'. A window will pop up giving you the Onion Skinning options. You can now see the keyframes, and can inbetween the frames easily. I would recommend using a pen tablet if you are drawing into Photoshop – makes life way easier than using a mouse, unless you are some kind of mouse mistro... To save the finished footage, simply save as usual. Save it as a Photoshop (.psd) file, and import the file into After Effects, where you can add sound effects and render as usual – the footage will play through!

To remove green screening from your footage, you can either do this frame by frame in Photoshop (just magic wand out the colour ect and save your footage as explained above), or you can do the same in After Effects, but remove the green screen from the whole footage clip in one go, which is a tad quicker... Open up After Effects and create your composition as usual, then Import the footage file you wish to use, and drag the footage into the work area/timeline. Now go to 'Effects and Presets' down the right hand side, and click on the downwards arrow for 'Keying'. Find 'Colour Key' and double click it. You will see to the left that the usual menu changes and you have the Colour Key options. Click the blue box – a window will pop up, like in Photoshop when you change the paintbrush colour. Here you can select the colour you wish to remove. To do this click the little 'Eyedropper' button in the window (located just above 'Preview') – now hover over the footage and select the colour you wish to remove by clicking.

Sometimes, depending on lighting etc, not all of the green disappears – sometimes only a small area is removed. To extend the amount the colour key removes, click the yellow '0' next to 'Colour Tolerance', and drag it to the right. You will see it removes a large area of the green. It will also remove parts of your footage you want to keep though, so be careful not to remove to much. Just move the little arrow back to the left to remove less. You can also change the 'Edge Thin' and the 'Edge Feather'. Play about with the tools to see what effects you can get. There are also tools in 'Colour Correction' that may help to fix the colours before you key out the green. When the green is removed, there will be a black background – this is 'invisible' when you place another piece of footage underneath this footage – pretty useful...

Another way of green screening (or cutting a character/object out of a ready filmed piece of footage) is the rotoscoping tool in After Effects. Have your footage ready in the timeline, and click on the 'Roto Brush Tool' up on the top bar, next to the eraser tool. Click on the footage in the workspace. A window should appear telling you to use the tool in a Layer Panel. So double click on the layer to the left of the screen and a Layer Panel will open up in the workspace. When you hover over the footage, a little green circle will appear with a green plus in it. If you hold down 'Alt' on the keyboard, the circle will turn red and the plus will turn to a dash sign. When the circle is green, you will be selecting the area to stay in the footage. When it is red, you are removing an area.

Draw a line over part of the footage you want to keep, just a simple line, it doesn't have to follow the edges etc. You will see a pink outline which has selected an area of the footage – it will automatically find the closest edge and select that. To add more to the selected area, draw another line and the pink outline will extend to that area. Sometimes you will have to be more delicate with the brush strokes to make sure you get everything you need for the character/object. You can change the size of the brush by changing the 'Diameter' in the 'Brushes' section to the right side of the screen.

Now, it is likely that the Roto Brush Tool has selected an area you don't want. To remove it, hold the 'Alt' key and do the same as when selecting the area, but this time click on the area you wish to remove. You will see it removes the section.

In the Layer Panel, there will be a timeline, like the main bar at the bottom. Move the Layer Panel timeline bar along to the next frame. You will see that the same area is selected on the next frame, and most likely a few frames after that. There will be a small green bar underneath the timeline, showing how many frames are affected by the Roto Brush Tool. You can go through frame by frame and edit any areas which need selecting or removing, but the pink outline area will move with the character/object. Sometimes it will need adjusting however. If you click back to the 'Composition' Layer (the other tab from the Layer Panel), you will see the effects of the Roto Brush Tool. The area you removed is black, and is therefore invisible, so you can now place the background you wish into this space, just by layering as usual in After Effects.