Keying Tips

Most editing packages have the ability to chroma key. Some may have more features than others, but the techniques are generally the same.

The main issues you will face will be from badly lit screens, “green spill” on the actors, and problems introduced by the compression used by miniDV. The first two issues have been addressed when filming, but as pointed out, it is easier said than done! The miniDV standard just pours salt into the wound. Chroma keying relies on the colour information provided by the clip. The 5:1 compression technique achieved using DV, throws away “redundant” information in the frame. As stated before, our eyes can be very forgiving in certain situations. For every 4 samples of luminance, miniDV stores 1 sample of colour information. This results in there being less colour information for the computer to work with, causing blockiness around the edges of the foreground. You can counteract this by applying a 4 pixel horizontal blur to the colour components of your clip, but not to the luminance, as the luminance holds the detail information. Our eyes are more sensitive to luminance than colour. If your editing package doesn’t allow you to blue just the colour components, don’t apply a blur at all, you will make things worse!

You will also need to ensure when filming both your green screen and backgrounds (or plates as they are known!), that you pay attention to lighting in order that both the foreground and background will blend seamlessly together.

Using your editing package you could just load in your green screen clip and your background, apply your chroma key and you’re done right? Not so, for the problems stated above. Here are a couple of tips to get better results.

Some software packages allow you to work on your green screen footage in two parts. One will be the actual foreground, and one will be used to chroma key (they are essentially the same clip but treated differently for the purposes of chroma keying). As there are more software packages than we have red apples, I will explain a simple and generic technique.
  1. Load in your green screen clip to your editor.
  2. Now apply a saturation and contrast filter. Turn up the saturation to make that green go really green and use the contrast to iron out any other areas as below (doesn’t look nice, but compare the green portion to the earlier clip). Then filter out all but the green channel from the clip.
  1. Now apply your chroma key to key out the green and create a black and white “mask” to be used next. How you achieve this with your actual editing package varies and is out of the scope of this article, you will need to experiment with different filters and functions, but what you are looking for is a clip like this. You may have to save the mask clip as a separate file, again depending on your package.

What the above clip will now be used for is to tell the computer what it should treat as transparent and what it should leave alone. i.e. the black will become transparent and the white area will be left. Remember this is a mask not the final image.

 
  1. Load in your original green screen clip and apply a video overlay or mask to the clip. Use the source as the mask clip you created before. The editing software will then use the mask to remove the areas of green from your clip.

There are loads of resources and tips on the internet for getting good green screen scenes. The above technique I have found to be very good at cleaning up bad screens. As with most things, you need to experiment to get good results.

Declan Smith
mg35@btinternet.com