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Compositing 101
In theory, compositing sounds like a very basic effect: put one layer of video on top of another to create a composite. In practice, though, compositing is one of the most powerful tools at your disposal. At the simplest level, you can use compositing tools to superimpose custom titles and graphics over video, or stack clips on top of each other to create a video collage. More sophisticated compositing tools allow you to do everything from mixing computergenerated elements with live video to placing your actors inside virtual sets. Although good compositing tools are the foundation of all special effects work, they can also be used for more everyday concerns, such as fixing bad pixels or letterboxing to mix SD and HD footage.
In most non-linear editing packages, you can create a simple composite by creating multiple video tracks, each containing a different clip or image (see Figure 17.12). For every-day compositing, your editing software will provide a set of tools that will suffice for all but the most specialized tasks.
Dedicated compositing programs such as Adobe After Effects provide more compositing power by letting you stack many layers of video and stills, and they provide more robust control over how those layers interact. After Effects also lets you animate the properties of layers to create sophisticated, animated effects.
No matter which program you use, the process of compositing is fairly simple. First, you import your video clips and stack them in the appropriate order. Obviously, if you put one piece of video on top of another, the upper clip will simply obscure the lower clip. Therefore, after creating your stack of video, you must define the method by which each layer will reveal the contents of the underlying layers. The method you choose will depend on the type of footage with which you are working.
Compositing methods fall into two categories: keys and mattes.
Figure 17.12
Composites are made by stacking tracks or layers on top of each other and then specifying how those layers will combine.
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Keys
If you’ve ever seen a local TV weatherman standing in front of a weather-filled map of the country, then you’ve seen an example of keying. The weatherman is not really standing in front of a map, of course. Rather, he’s standing in front of a blue or green screen that is electronically keyed out and replaced with the image of the map, as shown in Figure 17.13.
Figure 17.13
The upper image was shot in front of a blue screen, which can easily be “keyed out” and replaced with another image, such as this billowing smoke.
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Most editing apps provide keying functions that let you superimpose footage shot in front of a colored screen over another video clip, animation, or still image. In these programs, you import both your blue-screen footage and your underlying layers, stack them up, and then tell the program to apply a chroma key (sometimes called a color key) to the uppermost layer. The chroma key feature will provide controls to let you select which color you want to key out.
When the image is rendered, the computer will look for every occurrence of the specified key color and make it transparent, allowing the underlying layer to show through.
A luminance key functions the same way, but rather than keying out pixels of a certain color, a luma key keys out pixels of a certain brightness. Many programs offer variations of these keys, such as screen, multiply, or difference. Consult your manual for details.
Because you must shoot in front of a specially colored screen and perform very precise lighting, chroma key effects are not ideal for every compositing task. (The screens are blue or green because there is rarely any blue or green in the skin tone of a healthy human.) Chroma keys are usually used for situations where you want to superimpose a person in front of another video clip or still. Luminance keys can be used for situations where you can’t put a blue screen behind your foreground element. For example, you could use a luma key to key out a bright sky behind a building. Luminance keys can also be used to touch up parts of a chroma key composite that have not keyed out properly.
Shooting blueor green-screen footage requires a great deal of care and expertise.
Tutorial
Creating a Luminance Key
A luminance key lets you superimpose one layer over another by knocking out either the very bright or very dark parts of a layer to expose underlying layers. In this tutorial, we’ll use a luminance key to superimpose a flash of gunfire over another video clip (see Figure 17.14).
For this tutorial, you’ll need an editing or effects package that provides luminance keys (for example, Adobe Premiere, Apple’s Final Cut Pro, Adobe After Effects, Sony Vegas, Avid Media Composer). This tutorial assumes that you understand how to add clips to a project, search through them using a “source monitor,” add clips to a timeline, and add luminance keys to a clip. If you’re unclear on any of these, check your editing software’s manual.
STEP 1: SET UP YOUR PROJECT
In your editing package, create a project. Import the files hand-gun.mov and GS125.mov from the Chapter 17 page on the companion Web site at www.thedigitalfilmmakinghandbook. com/chapter17. Hand-gun.mov is a short clip showing a hand “firing” bullets. It was shot with a Canon GL-1. GS125.mov is from the ArtBeats Gun Stock Digital Film Library and is a short movie showing a number of muzzle flashes shot in near darkness.
Place the hand-gun.mov clip on the first video track in your timeline. Create a second video track above this one. Place the GS125.mov in the upper video track.
STEP 2: FIND THE FIRST MUZZLE FLASH
Load the GS125.mov clip in your timeline into your source monitor. Scrub through the clip to find a muzzle flash you like. Position the playback head on the first frame of one of the muzzle flashes and set an in-point. Scrub forward one or two frames to be sure the flash has ended and set an out-point.
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Figure 17.14
In this tutorial, we’re going to superimpose the footage of a muzzle flash over footage of a hand to create a final composite that will
show a hand firing bullets.
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Finding the Muzzle Flashes
Note that most of these flashes are only one frame. Depending on your computer’s performance, you might not see each frame during playback. Consequently, you’ll probably have to scrub to find the single frames. Look for places in the clip where the hand recoils. This is a good way to zero in on the flash.
STEP 3: POSITION THE FLASH IN THE TIMELINE
In the timeline, scrub through your footage and watch the program monitor to find the frame where the finger seems to fire. This is where you want the muzzle flash to appear (see Figure 17.15). Position the GS125.mov clip in a higher track so that it begins at the point in time where the finger fires. Note that the program monitor will now be filled with the muzzle flash on the black image of the GS125.mov clip. Your background plate that shows the finger will be completely obscured, but not for long.
Figure 17.15
Your muzzle flash footage should start right when the finger begins to recoil.
STEP 4: DEFINE YOUR LUMINANCE KEY
Add a luminance key to the GS125.mov clip. If you’re unsure of how to add a key, consult your manual for details. Some luminance keys allow you to choose between keying out light areas or dark areas. Others default to keying out darker areas. If your luma key provides an option, set it to key out darker areas.
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Most luminance keys provide at least two sliders for adjusting the key effect: one that specifies which values will be keyed out, and another that controls how transparent those areas will become. Adjust the sliders in your luminance key filter until the black areas are gone and only the flash remains. Your goal is to find a balance of settings that eliminates the background without reducing the luminance of the flash too much. Pay particular attention to the edges of the flash. Even though it’s only one frame, a hard edge or a black fringe will be noticeable to the viewer (see Figure 17.16).
Figure 17.16
Experiment to find luminance key settings that eliminate the background, without dulling the intensity of the flash and without creating a hard, distinct edge on the flash.
STEP 5: POSITION THE FLASH IN THE FRAME
With your key defined, you should be able to see the background beneath the muzzle flash. Unfortunately, the flash is probably over at the left side of the screen. Obviously, we want it to look like it’s coming out of the finger. Using your software’s motion controls, reposition the flash so that it sits just off the tip of the finger. You might also want to try scaling it down so that it doesn’t overwhelm the composition.
Finally, many of the flashes in the GS125.mov clip have a visible gun barrel on the left side of the frame. Your editing software should provide a motion control tool that allows you to crop the frame so that the barrel is removed from view without affecting the flash (see Figure 17.17). (See your user documentation if you don’t know how to do this.)
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Figure 17.17
Move the muzzle flash to the end of the “barrel” and scale it down to be more appropriate to the “caliber” of this finger.
STEP 6: NOW, DO THE REST
Render your footage and take a look. Pay close attention to the timing of the flash; it might need to be moved forward or backward a frame or two to match the recoil of the finger. You might also see that you need to adjust the settings of your luminance key. When you’re satisfied, choose another four muzzle flashes and edit them into the appropriate locations, as shown
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Filtered Footage
If you don’t have or don’t want to use clip footage of a muzzle flash, consider using a computergenerated one. DigiEffects’ Delirium collection of After Effects plug-ins includes an excellent Muzzle Flash filter that can generate muzzle flashes for you.
That’s it! With the simple application of some stock footage and a luminance key we’ve created . . . well . . . a kind of silly special effect. However, you can see that, in a more practical situation, such effects would save you the danger and expense of using a real gun. In the next chapter, we’ll increase the realism of the shot through the use of a little rotoscoping, but first, let’s see if we hit anything during our little target practice.
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Figure 17.18
When you’ve placed all of your muzzle flashes, your timeline should have five clips on it.
Tutorial
Using a Chroma Key
In the last tutorial, we used a luminance key to knock out dark areas of a layer. However, not all footage is suited to luminance keying. With chroma keying, we can knock out a specific color to reveal underlying layers.
A chroma key is usually used to knock out a background to superimpose an element over other footage. That’s what we’re going to do here to create footage of gunshots hitting a door. (The results of this footage can be edited onto the end of our previous tutorial to create a finished shot of a hand firing bullets that hit a door.)
For this tutorial, you’ll need an editing or compositing package that supports chroma keying.
STEP 1: SET UP YOUR PROJECT
Create a project and load the following media from the Chroma Key Tutorial folder: door still.psd, bullethole still.psd, bullethole.mov. Place door still.psd in your timeline. If your editing program does not support Photoshop files, you can try the TIFF version of the documents. (Note that, in the interest of saving space on the Web site, we have chosen to use a still of the door, rather than a movie. In practice, it’s a better idea to use a video of the door, even though there’s no movement in the image. A movie will show changes in noise and grain, while a still will have a slightly different quality that will be recognizable to your viewers.)
STEP 2: ADD A BULLET HOLE
Add the bullet hole still to a layer or track above the door’s layer. (This bullet hole is a still from the same ArtBeats Gun Stock collection that provided the muzzle flashes.) The door will be completely obscured by the blue surrounding the bullet hole.
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STEP 3: ADD A CHROMA KEY
Add a chroma key to the bullet hole still. Next, you need to specify the color you want to “key out” or eliminate. Many chroma key functions provide an Eyedropper tool that you can use to sample the key color directly from the image in your program monitor. Other programs require you to specify the color numerically or with a slider.
Don’t be surprised if you don’t see any changes after selecting the color. Most blue screens have a degree of variation in them, so selecting one single color isn’t going to eliminate an entire field of slightly varying hue. Fortunately, most chroma keys also have some type of tolerance control that lets you specify a degree of variation from your chosen color. Adjust the tolerance control until your background is removed.
Your chroma key function also probably provides controls for thinning and blurring (or feathering) the edges of the keyed areas. Adjust these to eliminate any remaining blue fringes around the bullet holes, and to soften the edges to better blend them into the door (see Figure 17.19).
Figure 17.19
By feathering the edge of our chroma key, we can blend the bullet hole into the underlying video.
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STEP 4: POSITION AND SCALE THE BULLET HOLE
At its normal size, this bullet hole is rather large (after all, it was plainly a small-caliber finger). Scale down the bullet hole to something more appropriate and then position the hole somewhere on the door. In After Effects, we can scale the bullet hole layer by dragging one of its corners to resize. We can then reposition by dragging the image to another location. Other programs might require the use of a dialog box or special motion command.
STEP 5: TINT THE HOLE
At this point, the bullet hole plainly looks like it’s floating over the door layer. But why? What’s not quite right? As we’ve discussed throughout this book, video is composed of luminance and chrominance information (lightness and color). It’s good to think in those terms when trying to match two pieces of video, either when editing shots into each other or compositing them on top of each other. Is there a chrominance difference between the bullet hole and the door? Unfortunately, because the door footage was underlit, it has a very slight greenish hue to it. This is in fairly sharp contrast to the strong reddish tones of the splinters in the bullet hole layer. Using the color correction facilities in your package, apply a slight green tint to the bullet hole.
Next, look at the luminance of the hole. It’s a little bright compared to the duller tones of the door. Again, using the color correction tools at your disposal, adjust the luminance to darken the bullet hole a little bit. Because the bullet hole has few highlights, and because its blacks are very black, you can probably concentrate your luminance adjustments to the mid-tones. See Color Plate 38 for before and after examples of this color correction.
STEP 6: STILL NOT QUITE RIGHT
It still looks like a decal, doesn’t it? Consider the overall image quality of the two layers. Is there any difference? The bullet hole looks sharper and more detailed than the door. This makes sense, since the bullet hole was shot using 35mm film and digitized at high-resolution. Apply a slight blur to the bullet hole layer (see Figure 17.20).
STEP 7: ADD THE REST
When you’re satisfied with your first bullet hole, make two more copies of it and stack them on separate layers. Drag each copy to a different location on the door. You should also apply different scale amounts and a little rotation to each bullet hole to hide the fact that they’re really all the same image. Separate them by a few frames or seconds in the timeline to correspond with the timing of the bullet hits. There were four shots fired from our “handgun.” For the last one, let’s see it actually impact the door.
STEP 8: ADD THE LAST HIT
Bullethole.mov (www.thedigitalfilmmakinghandbook.com/chapter17 ) actually shows a whole bullet hit. Drag it into yet another layer and apply the same chroma key, color balance, and blur filters that you applied to the still images. Position, scale, and rotate the layer
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That’s it! Now, you can render out a final shot and edit it into your footage of the handgun firing. If you cut to the door shot right before the impact of the last bullet, you’ll have a somewhat dynamic little scene.
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Figure 17.20
Our final composite looks good, but as you can see, it takes a good number of filters to get
it there.