Basic Compositing Techniques: Chroma Keying and Matting, Study notes of History of film

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Basic Compositing Techniques

The world of compositing has definitely changed

over the past half century. From traveling matte film systems to modern-

day green screen and roto masking done on a laptop, they all use similar

concepts to produce the desired result: joining foreground elements onto

the background plate. I’ll cover many different techniques of shooting and

post-production compositing in subsequent chapters. This chapter will

show you a few of the basic concepts in producing chroma keys, extracted

matte composites and roto masks with common desktop software.

Overview of Matting Compositing Techniques

The process of chroma keying, also called color difference matting, is a method of subject extraction done either with hardware, such as the Ultimatte system, or with a desktop soft- ware solution. The process involves first selecting a background color range in the spectrum (such as green from a green screen) from a foreground plate, then extracting the subject from this background color, and finally creating an alpha matte that allows only the subject image data to be visible. This process lets you composite the foreground plate and background plate together, creating the illusion that the two plates were created as one through the lens of the camera (see Figure 4.1). If you’re using a hardware compositor, such as the Ultimatte HD, then the compositing is done by combining the live foreground and the background plate and piping the compos- ited imagery live to broadcast (such as a news or weather program) or to a storage device. This is how most live news programs, televised sporting events, and virtual-set entertainment

■ Overview of Matting Compositing Techniques 59

Basic Chroma Keying with Final Cut Pro

The first example (see Figure 4.2) uses Final Cut Pro to create a composite from a piece of footage shot against a green screen (with uneven lighting) and a still image for the back- ground plate. The foreground footage has some motion blur in the first few seconds, which can complicate the composite if you attempt to create a hard-edged chroma key from the footage.

Figure 4.2 Simple green screen footage in Final Cut Pro

By applying the built-in Blue and Green Screen effect (Effects → Video Filters → Key → Blue And Green Screen), you can get a basic chroma key out of the green color channel from the foreground footage (see Figure 4.3).

Figure 4.3 Applying the built-in Blue and Green Screen effect in Final Cut Pro

Adjusting the Color Level, Color Tolerance, Edge Thin, and Edge Feather sliders, you can get a fairly good, hard-edged result. Further applying a spill-suppressor effect (Effects → Video Filters → Key → Spill Suppressor – Green) helps knock out any remaining green in the

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foreground footage (see Figure 4.4). This would probably be acceptable for a simple educa- tional classroom project or internal corporate communications video; but if the subject in the footage isn’t clean without motion blur against a correctly lit green screen, your results won’t look professional.

Figure 4.4 Adjusting the sliders in the chroma keyer effect and applying spill suppression clean up the composite.

Using a Third-Party Plug-in with Final Cut Pro

To take this composite a step further into a more professional look, I applied a green screen matte extraction feature from a third-party plug-in called Boris Continuum Complete 6 (www.borisfx.com/). After applying the effect to your foreground footage, you can make many fine adjustments to the matte to get a good, clean result right away. Previewing the matte by itself, you can see how the edges look and whether you have any holes in your matte (see Figure 4.5). You can further refine the composited foreground subject by using the Matte Choker to pull in the stray edges of the green screen. But the feature that makes Boris Continuum Complete 6 stand out is Light Wrap. It doesn’t just add a glow around your subject but uses the background plate as its reference point to create a light spill around the edges of your subject (see Figure 4.6). This gives a much more believable and professional composited result.

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Matting and Compositing Layers in After Effects

Adobe After Effects is the ultimate affordable desktop compositing tool. It gives you a lot of flexibility and control when you’re working on several layers at a time. After Effects CS or newer includes a powerful matte extraction tool called Keylight. This plug-in handles motion blur and deals with spill suppression quite well, although it’s not as refined as, say, dvGarage’s Conduit or Ultimatte’s matting software plug-ins. The process is straightforward. Create a composition, and import the foreground and background files, stacking the foreground layer on top of the background layer in the Timeline panel (see Figure 4.7).

Figure 4.7 Stacking the foreground and background layers in After Effects in preparation for compositing

Select the foreground green screen layer in the Timeline, and apply the Keylight plug-in (Effect → Keying → Keylight). Keylight’s controls open in the Effects panel on the left. Use the eyedropper to select an average area of the green screen background around the subject. Change the View to Screen Matte so you can make adjustments with the Clip Black and Clip White sliders to create a clean alpha matte (see Figure 4.8). Keylight includes a lot of controls you can adjust for the edges, foreground color cor- rection, and more. It works well for a built-in matte extraction tool. However, it may have issues with reflectivity or color collisions that would be problematic for any chroma keyer or matte tool and that require hand-roto masking, as I’ll discuss later in this chapter. Notice in Figure 4.9 the bad reflection on the briefcase, which causes it to be partially invisible for a few frames. I’ll cover ways to avoid this during a shoot in Chapter 9.

■ Overview of Matting Compositing Techniques 63

Figure 4.8 Applying Keylight to the green screen foreground and cleaning up the Screen Matte

Figure 4.9 Reflectivity on the briefcase creates problems in a few frames of the foreground footage.

To sell a composite, you need to create a sense of interaction with the foreground and background plates. This often requires shadows, reflections, light casting, and so on. One quick method is to add a drop shadow of your subject cast on the background. In this case, I used a Drop Shadow Layer Style on the foreground plate (Layer → Layer Styles → Drop Shadow) and altered it to be faint and distant, giving it a subtle effect on the background plate (see Figure 4.10).

■ Overview of Layer Masks, Mattes and Roto-Masking Techniques 65

Figure 4.11 A green screen subject is first extracted with Keylight in After Effects.

Then, import a background animation video on a layer beneath the foreground plate layer. Temporarily hide the foreground plate, and create a new solid white layer between the foreground and background layers (Layer → New → Solid). With the new white solid layer selected in the Timeline panel, apply an Alpha Track Matte (Layer → Track Matte → Alpha Matte), as in Figure 4.12. This process creates a solid colored silhouette of the extracted foreground subject (see Figure 4.13). The effect can use several matted layers and elements to create an interesting effect with animated characters in a two-dimensional composite. Another variation is to use the foreground subject layer as an alpha matte for the ani- mation, revealed against a solid background or other video footage (see Figure 4.14). The process is the same: whatever layer you place beneath the matted subject layer can have an alpha matte applied to it.

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Figure 4.12 Layering the background animation with the keyed foreground plate and applying an Alpha Track Matte to a new solid white layer

Figure 4.13 The matted character is silhouetted against the background animation.

Roto-Spline Masking Roto-spline masking has many uses. For instance, you can use it to garbage-matte the green/blue screen, clean up holes in an extracted matte, or manually mask out a character or object in motion in a piece of footage that wasn’t intentionally shot with extraction in mind. You can apply it to completely extract your subject or merely mask it from an applied effect to the rest of the image. You do this by creating a roto-bezier spline mask on a footage layer and tracking it over time.

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Starting at Frame 0, I opened the Mask options on the Timeline in After Effects and selected the Stopwatch to set the first keyframe (see Figure 4.17). Doing so established the base position of all the points on the mask and let me modify it over time to match the position of the tire/wheel in the video. Advancing down the timeline, I moved the Current Time Indicator to a position and started moving and adjusting the points on the mask edge to match the tire’s edge. After Effects automatically tweened the positions of these points between keyframes.

Figure 4.16 The original video layer is placed beneath the masked layer and adjusted so it’s easier to see where the track points need to be moved over time.

Figure 4.17 Clicking the stopwatch on the Mask properties in After Effects automatically generates keyframes to track the mask’s point positions along the timeline.

In this example, I applied a zooming effect to the original video footage following the road as the car travels down the mountain. The masked layer is on top and remains sharp and in focus throughout the entire clip (see Figure 4.18). Note the various keyframes in the Timeline that were generated by tracking this process over the 10-second clip.

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Figure 4.18 An effect is applied to the bottom video footage layer, and the masked footage on top remains sharp and clear.

Where to Learn More?

Products Mentioned in This Chapter

• Apple Final Cut Pro : www.apple.com
• Adobe After Effects : www.adobe.com
• Boris Continuum Complete 6 : www.borisfx.com/bcc/
• dvGarage Conduit : www.dvgarage.com/ Image and Footage Credits:
• iStockphoto : www.istockphoto.com You can find a complete list of references and suggested continued reading/learning from this chapter in Appendix A.