So, you have your green screen footage captured…now what?
For starters, you need an editing program in order to key, though that should be obvious. I use the Adobe CC suite, and key in either Premiere or After Effects. It tend to use the Ultra Key in Premiere most often, though Keylight in After Effects is no slouch. The entire Adobe CC suite is available with a monthly paid subscription and can also be used for free as a 30 day trial. If you’re a student or teacher, you can get it as cheap as $19.99/mo which is a bargain.
If you are truly on a shoe-string budget, there are freeware editing programs out there as well as some that are very inexpensive. Though I have not tried them, both Wax and Jahshaka look fairly robust, especially considering the price (free!) and are capable of keying. Whatever your software used for keying, the basic premise remains the same. Using a color selector, you will sample a 1-5 pixel section of the video and all areas containing the same color (or similar – this is controllable) will be rendered transparent and anything placed beneath the video on the timeline will become visible through the transparent areas. Now, if only it were that simple.
This is where Green Screen Tips & Tricks Parts 1 & 2 really come into play. If you had a good color and material for your background and lit it evenly, then you will have very little additional work to do after selecting the color to key out. However, you will inevitably need to tweak the key to get it perfect.
I will cover some of the main points based off of keying in Premiere (using Ultra Key), as that is what I am most familiar with, though Keylight in After Effects works very similarly, though a few of the settings carry different names.
An Opacity mask was applied to this clip with key frames to help track movement prior to applying the Ultra Key.
First, apply the Ultra Key effect by dragging it from the Effects panel over to the clip in the timeline or onto the Effect Controls if you have the clip already selected. Then, using the eyedropper, select an area of the clip which is the closest to the midtone of the the color you are keying out. I typically find that slightly towards the shadowed/darker side of the keying area is better than a brighter area and will result in less issues as you adjust the key.
Once you have done this, clip on the drop down menu above the eyedropper that says Composite. Move down and select the option Alpha Channel. This will allow you to see the clip as a negative and make cleaning up the key much easier. With the clip in Alpha Channel, areas that are black will be transparent and areas that are white will be opaque. You’ll want to make settings adjustments to the Ultra Key in order to get it perfect, and you will need to scrub the timeline in order to be certain that it is clean throughout the entire clip. If any white shows in areas that should be black (transparent) or any black shows in areas that should be white (opaque) then begin to make adjustments to the settings.
The first place I go to make adjustments is the pedestal. I will usually bring this up to anywhere between 65-100. This is the quickest way to reduce excess image noise created by opaque (white) areas that need to be transparent (black). The effects of this change are always immediate, and you will notice a significant improvement in the key, assuming you color selected from a darker mid-tone of the green screen. If more correction is needed, I tend to adjust the Contrast followed by the Mid Point. Usually this gets even a difficult clip to the point that minor adjustments to the Highlight and Shadow will fix any remaining issues. Occasionally, very minor adjustments to the Transparency setting will yield good results, but it is usually best not to go much below 38.
Once satisfied, go back up and change from Alpha Channel back to Composite and see the results.
Lastly, a quick word about color correction. Ultra Key, more so than Keylight, tends to desaturate the color. Adjustments can be made within Ultra Key to the saturation, as well as hue and luminance. This can be particularly useful for compositing, however color correction effects placed beneath the Ultra Key effect on the Effect Control panel, such as RGB Curves, will allow you to make corrections to the color without effecting the key. Contrary to this, placing color correction effects ABOVE the Ultra Key in the Effect Control panel will directly affect the key. This can be used to your benefit, often on particularly difficult clips, allowing you to make changes, with RGB Curves for example, and see an immediate impact on the key. Increasing or decreasing the greens and reds with curves often yields good results.
Don’t be afraid to make adjustments to the settings. If things go horribly wrong, click the reset arrow on the far right of the setting or effect and start over. The more you play with the settings, the better you will become at making a really clean key – fast!