In this lesson we will look at how to pull keys (often called green/blue screening, chroma keying or simply keying). Firstly, please download some footage to practice with from the following link: http://www.hollywoodcamerawork.us/greenscreenplates.html
Hollywood Camerawork is a great resource for training material. I chose the “hair detail” clip because you’ll often have to key hair and it can be tricky. Note that the files download in .zip format so you’ll need to extract it before you can import it into after effects. They come as an image sequence so when you go to import them inside after effects, choose any image and make sure you click the Sequence button.
Because image sequences are multiple images, they have no inherent duration. By default after effects plays image sequences at 30 frames per second. That may be fine for you, though HDTV is 25 frames per second so I have set my default to 25 in the preferences (edit > preferences > import).
You can also change the footage from the default import by right clicking on footage in the project panel and clicking interpret footage > main.
Drag the footage into the new composition icon. If your footage looks stretched or squashed, you can change the composition settings with control + K. I used the HDTV 720 25 preset.
Head over to the effects panel and expand the keying section, we have a lot of different options.
Drag the colour key effect onto the footage. Click on the eye dropper next to the key colour and select the green that we want to remove in the composition window.
Nothing appears to happen because although we have chosen which colour to key, we also need to choose a tolerance. Increasing the tolerance increases the range of colours (next to our chosen colour) that will be removed from the image. Have a play with the settings and see what works best.
It’ll be impossible to get a good looking image just by playing with the tolerance. The feather and thin settings can help us refine our key. I’m sure you all know what the feather setting does, and the thin setting erodes pixels adjacent to already keyed pixels.
To help you tweak settings you can choose to view the alpha to make sure that we don’t have holes in our matte (alt+4). You can also turn on transparency grid by clicking this button or create a coloured solid behind the footage layer.
It’s difficult to pull a good key with the colour key, so remove it from our footage by selecting it in the effects panel and pressing delete. Let’s use the Keylight effect which is a much more powerful keyer.
You can choose which colour you’d like to key using the screen colour. I simply picked the green and the result looks good already. This is a combination of keylight being a really great keyer and the footage being evenly lit with a smooth greenscreen. Let’s tweak some of the settings:
Screen pre blur is a setting that blurs the original image, pulls the key from that blurred image but uses the unblurred image for the RGB channels. This creates softer edges.
Additional settings can be found in the dropdown menus. Screen matte is particularly important, go ahead and expand it.
When tweaking the settings it’s often much easier to be looking at the alpha channel rather than the red green blue channels (Alt+4 is the shortcut).
- Clip Black: this setting will remove transparent parts of the image that have alphas less than this value. For example if I had a green tracking marker in the background that I wanted to remove and it was 19% opaque, I could set the clip black to 20 (this setting is measured as a %) and the tracking marker would be completely removed. Note that when working in 8 bits per channel (as we are), the alpha in our info panel is not measured as a %, it is measured as a value from 0 to 255).
- Clip White works the same as clip black but instead of being a threshold that lowers transparency, it is a threshold that increases transparency.
- Clip rollback is a value that removes any changes to the two above settings. This is useful for tweaking the intensity of both settings at once.
- Screen shrink is similar to edge thin in the colour key, it erodes or expands the matte. If you have a green fringe around your image, screen shrink can come in handy.
- Screen softness controls the softness of your edge. Increasing this in subtle amounts is a good way to improve jagged edges.
- Replace method is a setting for spill suppression. Often when a character is in a green screen studio, the green may spill onto them (especially if they are wearing white). Spill suppression attempts to remove the spill. The default setting of soft colour does a good job but sometimes I find it can be quite grainy, which forces me to use the source setting. In our specific example there is no spill on the character so these settings won’t make much of a difference.
Some general tips for keying:
Often the softness required to key hair will not be suitable for other parts of the image such as clothes or skin. In this case you will need to pull separate keys, one for the hair and one for the other parts. Each key will have to be on a separate layer. Separate each area with masks and use feathering to blend them together.
If you have holes in your character but don’t want to crank the clip white to fill the holes (this might decrease the fidelity of your edges) you can always duplicate the layer, remove the keylight and create masks to fill the gaps.
In the next lesson, we will look at general workflow practices and rendering our files.