Chromanator Tutorials: Compositing
Once you have prepared your footage, it is time to use Chromanator. Greenscreening is the easiest technique for compositing – presuming the footage and screen itself is of good quality – and requires only a couple of tools:
- Colour Difference Key – removes the majority of the green.
- Spill Suppression – fixes the green 'fringing' around the edges of the key.
Higher layers obscure lower layers in Chromanator, so always make sure to put your greenscreen clip above the background clip on the timeline.
Colour Difference Key
This is a simple tool to use. It is designed specifically for green/bluescreening, with various optimisations that result in excellent keying results. It can be found in the Key -> Key toolset in the Object View.
For this example I'll use a simple shot of an actor against a greenscreen:
The first thing to do is choose the colour. This is done using the tool's drop-down menu. As mentioned in the previous section, greenscreens tend to be the most effective when working with miniDV:
The white point determines the amount of keying. At 255 there will be only a small change in the footage. As you lower the value, the chosen colour will be gradually removed. The required value will vary depending on the clip being used:
Keep lowering the white point until the green has disappeared. Any problematic areas – perhaps where a dark shadow or a bright light spoil the greenscreen – may have to be dealt with using the garbage matte system, which is covered in other tutorials.
The black point adjusts the sharpness of the edge of the key. The higher the value, the sharper the edge. The amount required is mostly down to trial and error and your own aesthetic tastes. Some shots may require a very sharp edge, whilst others might benefit from a softer one. Here you can see the difference between a soft and hard edge:
- If the black point value is greater than the white point, the edge may begin to break up. It is generally wise to keep the black point lower than the white point value.
Even the best greenscreen is likely to leave slight green fringing around the edges of the keyed subject. The spill suppression tool can be used to eradicate this fringing. It can be found in the Grade toolset.
This tool is extremely simple to use and just requires activation.
- The screen colour is selected using the drop-down menu. If a greenscreen was used, be sure to set the spill suppression to green.
- After the green fringing is removed the image itself may become discoloured. Turning on the extended function will automatically re-grade the image back to its normal colours.
Here are the three stages of spill suppression, and how they affect the edges:
That is essentially all that you need to do when using good quality greenscreen footage.
Of course, filming does not always go according to plan. If you find problems keying your greenscreen footage, you may need to use a garbage matte, which we will examine in the next section.