Compositing a Green Screen Clip with a New Background
ACA Objective 2.3
ACA Objective 4.6
Now you’re ready to remove the green screen, revealing the weather map underneath.
Drawing a garbage matte
The first phase of green screen compositing is to draw an opacity mask, which is traditionally called a garbage matte. Although it is possible to simply have Premiere Pro remove the background based on the green color, using an opacity mask quickly masks off the areas that never need to be shown at any point in the clip, reducing the amount of potential variation in the green screen color and making background removal easier and faster.
In the Timeline panel, drag the green screen clip, weatherReport.mp4, to a higher track. For example, put a green screen clip on V2 so that you can put the new background under it on track V1.
Drag the weather map graphic, Washington_ref_2001/weatherMap.psd, to track V1 at the beginning of the clip.
With the Rate Stretch tool, stretch Washington_ ref_2001/weatherMap.psd to match the duration of weatherReport.mp4 (Figure 4.4). You need to use the Rate Stretch tool because Premiere Pro sees the PSD file as more of a video clip than a still image.
Figure 4.4 The weatherMap graphic set up with the correct position and duration in the Timeline panel
Make sure the weatherReport.mp4 clip is selected in the Timeline panel.
Scrub through or play back the sequence and note how far out weather reporter Joe’s hands extend during the presentation.
In the Effect Controls panel, expand the Opacity setting.
Select the Free Draw Bezier tool () (Figure 4.5).
Figure 4.5 The Free Draw Bezier tool selected in the Effect Controls panel and positioned over the Program Monitor
In the Program Monitor, click the Free Draw Bezier tool around Joe to draw a rough mask that stays outside the furthest reach of Joe’s hands during the presentation (Figure 4.6).
Figure 4.6 Drawing the path of an opacity mask
The mask doesn’t have to follow Joe’s outline tightly or precisely; leave a bit of margin between Joe and the mask path. Click only in the green areas; don’t click any points inside Joe, and don’t let any path segments cross over Joe.
When you’re ready to close the path, click the tip of the Free Draw Bezier tool on the first point you drew.
The path automatically closes. The area outside the mask becomes transparent (Figure 4.7).
Figure 4.7 A completed opacity mask path
Play back the sequence and see if any part of Joe’s body crosses over the mask at any time.
If you need to move a path point or make other adjustments to the path, use the Selection tool to reposition any points on the mask path. If you need to move a point outside the frame, zooming out will let you see outside the frame.
If you want to convert any straight segments to a curved segment, Alt-drag (Windows) or Option-drag (macOS) a point to extend Bezier handles. These handles curve the segments extending from a point (Figure 4.8).
Figure 4.8 Creating curved segments by extending Bezier handles
Keying out the green background
With the garbage matte in place, now you can have Premiere Pro concentrate on isolating and removing the green screen color that remains.
In the Effect Controls panel, find the Ultra Key video effect (remember to use the search feature in the Effects panel). Drag Ultra Key from the Effect Controls panel and drop it on the weatherReport.mp4 clip in the Timeline panel.
In the Effect Controls panel, scroll down to the Ultra Key settings and select the Key Color eyedropper (Figure 4.9).
Figure 4.9 The Key Color eyedropper selected in the Ultra Key settings; the pointer is now an eyedropper.
Click the Key Color eyedropper on the green screen in the Program Monitor.
This samples the green color that Ultra Key should remove, and what was the green screen color should now be transparent (Figure 4.10).
Figure 4.10 Before and after clicking the green screen color with the Key Color eyedropper
In the Effect Controls panel, go to the Ultra Key settings, click the Output menu, and choose Alpha Channel. This displays the mask created by Ultra Key so you can see whether it is clean enough (Figure 4.11).
Figure 4.11 After choosing the Alpha Channel setting
There is a saying that can help you remember how to read an alpha channel: “White reveals, black conceals.” White mask areas allow the clip to display because they represent opaque areas, whereas black areas are part of the mask that makes those areas of the clip transparent. Gray areas are partially transparent, so dark gray areas are mostly transparent but still let through some of those clip areas.
Scrub through the sequence to see if the mask is clean for the entire sequence.
If the mask is not clean (not fully black) in some areas, in the Effect Controls panel go to the Ultra Key settings, click the Settings menu, and choose a different option to see which one works best. Each option is a preset for the advanced settings below the Key Color option (Figure 4.12). If the advanced settings are expanded, you can see how they change when you choose a Settings preset.
A common reason for mask variations is uneven lighting on the green screen.
Figure 4.12 Settings presets change the values in the advanced settings.
If you’re feeling adventurous or already have a technical familiarity with keying, you can expand the Matte Generation, Matte Cleanup, Spill Suppression, and Color Correction settings and try adjusting them. If it feels like you have to work too hard to produce a clean mask, the fastest fix may be to sample a different Key Color by repeating steps 3–5.
Scrub through the sequence to see if the mask is now clean for the entire sequence. If it isn’t, try step 6 again.
In the Effect Controls panel, go to the Ultra Key settings, click the Output menu, and choose Composite.
This displays the composite result of the two tracks plus the mask applied to the upper track.
Play back the sequence to make sure it looks right. Watch out for irregularities in the keyed-out area, and keep an eye out for green spill on the subject.