ves handbook of visual effects ch 3

116 Chapter 3 ACqUISITION/SHOOTING

Figure 3.11 Bruce Almighty (2003) water tank composite.(Image courtesy © 2003 Universal Studios Licensing, LLLP. All rights reserved.)

Controlling Spill LightAttentive use of flags and teasers on set lighting and black cloth on light-reflecting surfaces outside the frame will eliminate most spill light on the backing. (Even concrete stage floors reflect a sur-prising amount of light. To see spill light when the backing is lit, look through a red filter.) A small amount of white spill light from the set inevitably hits the backing. It often comes from the large, almost unflaggable soft sources that simulate skylight. Since the skylight is typically two or three stops down from the key light, the spill has little effect on the backing. Realistic lighting should be the paramount concern.

If white light is contaminating an area of the backing, a higher level of the alpha channel can be applied in post to darken it. Since there is no difference in color between, say, transparent white smoke or mist and white light of the same brightness falling on the backing, it’s clear that the less white light contamination there is to be cleaned up, the better. Otherwise, as the contami-nation disappears, so do all the transparent foreground pixels of the same color. Screen correction is invaluable in extracting the maximum detail from smoke and spray shot against white-lit backings.

If the foreground must be flat lit to simulate overcast con-ditions, a good approach is to bring most of the light in from overhead through a large, translucent silk. On stage, much of the overhead soft light may be kept off the backing with a series of horizontal black teasers hung directly beneath the silk, running its entire width parallel to the backing. The teasers are progressively longer top to bottom as they get near the backing, preventing the backing from “seeing” the silk (see Figure 3.11 above).

Chapter 3 ACqUISITION/SHOOTING 117

Lighting Virtual SetsInescapably, if one is lighting an actor and the surrounding floor with white light, there is no way to control the floor brightness independently of the actor, other than changing the floor paint or floor fabric. The only control available is the balance between the actor’s shadow and the rest of the floor and backing.

Lighting Procedure for Holding the Shadow (Petro Vlahos Technique)1. Turn on the key light to cast the desired shadow.2. Measure the brightness on the floor just outside the shadow

(use a spot brightness meter and green filter, assuming that it’s a green floor).

3. Light all the rest of the green floor to this measured brightness while adding as little light as possible to the shadow area.

4. Light the green walls to achieve the same brightness as the floor.5. Shadow density may be increased by blocking fill light from

the shadow area or lightened by adding fill light to the shadow area.Shadow density is controlled by adjusting the fill light, not

by adjusting the key light. Outside the shadow, the entire green set should appear to have equal and uniform intensity as seen from the camera position. Strive to stay within plus-or-minus 1/3 f-stop; screen correction can deal with brightness variations as great as plus or minus one f-stop.

The human eye quickly compensates for small light changes; it is not a good absolute measuring device. (It is, however, superb at comparisons.) It is necessary to use a spot brightness meter and green filter to check for uniform brightness. A digital camera with a computer display is also useful for making a quick check of lighting uniformity in the three-color channels.

In backlight, because of the shallow angle between the camera and floor, the floor will not appear as green as the back wall. A dif-fused, polarized white-light glare component is reflected by the floor because of the shallow angle. For holding good shadows in backlight, it is essential to use a polarizing filter over the camera lens. The HN38 is recommended. Rotate the filter until the floor glare is canceled. Ideally, the backlights should be polarized too, but it is rarely done. Large sheets of polarizing plastic are avail-able up to about 19 feet wide; they can be protected against heat with heat shield reflecting filter material. Of course, HMIs emit less heat than tungsten lamps to begin with.

Lighting to Eliminate the Shadow (Vlahos Technique)1. Light the entire green set uniformly with large-area diffused

light sources.


2. Check uniformity as noted above.3. Place the actor in position. If he casts a shadow, add additional

low-level lighting to return the light level in the shadow to its original level.

4. Add a modest key light to create the desired modeling, and ignore the shadow it casts. The added key light will cause a shadow to be visible to the eye, but because the key light did not affect the green intensity of the floor in the shadow it has created, the shadow can be made to drop out in compositing.

Tracking Marks on the ScreenWhen the foreground camera moves, the background must move appropriately. Unless the foreground and/or background can be photographed with a motion control camera, tracking data must be extracted from the foreground image and applied to the background during compositing. This process is called matchmoving.

Tracking marks applied to the otherwise featureless screen give the matchmovers fixed points to track. These marks must obviously show in the photographed scene, but ideally they should clear the foreground actors, or at least avoid their heads, since they must be removed in the composite. Marks are typically laid out in a rectangular pattern, with about 3 to 5 feet between them, depending on the lens used, the action, and the distance to the backing. Black or white tape pieces or crosses will usually suf-fice, though uniquely identifiable markers are very helpful if there is much tracking to do.

If camera shake or other sudden motion is required in the foreground photography, motion blur can obliterate the track-ing marks. The Aerocrane Strobe Tracking System created by Alvah Miller provides target arrays of LED lamps that strobe in sync with the camera shutter, giving well-defined marks on every frame even if they are not in focus. Cylindrical LEDs have uniform brightness even when viewed off-axis.

Sometimes it is desirable to light the tracking LEDs con-tinuously, allowing them to blur in motion. Valuable tracking information can be derived from the length of the blur. Consult the tracking team for their preference.

On-Set PreviewOn-set preview composites made with a still camera and cali-brated monitor, like the Kodak/Panavision Preview System, or a live composite made with a hardware Ultimatte device will alert the crew to problems before they are committed to film. A few video assist companies provide this specialized service.


Using the digital Ultimatte previewer (hardware device or soft-ware on a computer) on the motion picture set eliminates much guesswork and uncertainty. It provides great assistance when photographing actors who must be realistically integrated with people and objects in the background scene. Previewing with Ultimatte also immediately identifies the acceptable limits in lighting irregularities and wardrobe color.

If it’s a digital shoot, an output video stream must be available that’s compatible with the Ultimatte. An outboard processor may be needed. This yields the best preview available with all the fore-ground–background relationships visible at full quality.

For film shoots, a small, outboard color camera feeds the pre-viewer. (Film camera color taps, even when they can be switched to 100% video, are so starved for light that they usually cannot make good composites, although if their geometry is properly adjusted, they are fine for alignment purposes.) Playback from disk or tape provides the background scene.

Camera for Bluescreen or Greenscreen PhotographyFilm Photography: Choosing a Camera NegativeSome camera negatives are better suited to composite work than others. Ideally, one would choose the finest grained, sharpest film available. It is also important to have low cross-sensitivity between the color layers. Foreground and background film stocks do not have to match, but of course it’s helpful if they have simi-lar grain and color characteristics.

Kodak Vision 2, 100T and 200T (tungsten balance), films are ideal for green and blue backing work. The dye clouds are very tight and well defined. Vision 3, 500T, the latest in a series of remarkably fine-grain high-speed films, as one would expect, is still grainier than the lower speed films. Although the 500T film is not ideal, a well-exposed 500T negative is much better than a marginally exposed 200T negative!

An interlayer effect in these films produces a dark line around bright foreground objects (such as white shirts) when they are photographed against a green screen. Software can deal with this effect.

Kodak Vision 2, 50-speed daylight film and Fuji 64 daylight film produce superb results in sunlight, with very low shadow noise, but require high light levels on stage.

If these 100T and 200T films cannot be used for aesthetic rea-sons, one should still pick the finest grain emulsion compatible with lighting requirements. Be aware that additional image pro-cessing (and cost) may be required. A few negative emulsions have so much cross-sensitivity between the color layers that they should not be used.


Film emulsions are constantly evolving. As an example, recent improvements in red sensitivity in some emulsions have been accompanied by more sensitivity to infrared reflected from cos-tumes, altering their color noticeably. This effect is easily dealt with by filtration—if you know it’s there! A quick test of actors and costumes is always worthwhile.

Choosing a Digital CameraSince all three-color channels are used in creating the composite, an ideal camera would have high resolution and uncompressed color (bandwidth).

Three major factors affect color recording:1. spatial resolution,2. captured bit depth, and3. recorded bit depth and compression.

Spatial ResolutionSpatial resolution is broadly related to the number of photo-sites (light- sensitive elements) available for each color. In the commonly used Bayer array there are half as many blue pho-tosites as there are green photosites. Likewise, there are half as many red photosites as green photosites. The missing val-ues are derived through interpolation from adjacent pixels in the de-Bayering operation. Because human visual acuity is greatest in the green wavelengths, Bayer’s array gives excellent visual results from an optimally small number of photosites. Although they are not ideal for the purpose, Bayer array cam-eras can yield good composites with care in original photogra-phy and in post.

However, the blue and red image is still half the resolution of the green image, which limits the resolution and fine detail of the mask image.12 To address this and other image quality issues, a few high-end cameras like Panavision’s Genesis and Sony’s F35 (same sensor as the Genesis) have full resolution in all three col-ors. These cameras are ideal for composite work.

Color Bandwidth and CompressionAssuming your camera can produce a full-bandwidth, uncom-pressed RGB signal, much information can be lost when that sig-nal is compressed and recorded. Many HD VCRs are limited to 4:2:2 recording, which includes rolling off the green channel’s high

12 It should be noted that film builders use a roughly equivalent compromise: Green- and red-sensitive negative layers have more grain and less resolution than the green layer.


frequencies and applying half-bandwidth MPEG compression to blue and red.

The designation 4:2:2 does not refer directly to RGB band-width but rather to YUV. The Y channel carries the luma or bright-ness information, while U and V are the channels from which the color information is derived (similar to LAB color space in Photoshop). In a 4:4:4 recording, every channel is recorded at the full color depth. (The designation 4:4:4 is actually a misnomer, carried over from standard definition D1 digital video. Because it’s well understood to mean full bandwidth in all three channels, its use has continued into the high-definition and higher digital cinema world.)

Just as the classic Bayer array has a negligible effect on images intended for viewing but adversely affects composite quality, well-compressed images designed to look good on screen can have serious limitations when composited. Good software engi-neering can recover some of the lost bandwidth, but edge detail (fine hair and so forth) and shadow noise still suffer from com-pression artifacts. A laundry list of compression artifacts includes dark or light lines trailing or leading moving objects, banding in dark areas, and so forth. These problems are even more pro-nounced in “DV” and “SD” format cameras. With new cameras coming on line every day, testing on the actual subject matter is always worthwhile.

Edge Enhancement/Sharpening/Detail SettingsCamera edge enhancement/sharpening should be turned off! The artificial edges that sharpening produces will otherwise carry into the composite. If sharpening is needed, it can be done dur-ing compositing.

RecordingRecording in data mode gives maximum flexibility and best qual-ity in post. Data mode records the uncompressed data (as directly off the camera sensor as the camera’s design allows) to a hard disk. This is often called raw mode, but beware: At least one cam-era’s (Red) raw mode is in fact compressed. Since raw mode data cannot be viewed directly, a separate viewing conversion path is required to feed on-set monitors.

If recording in data mode is not possible, shoot material intended for post-compositing as uncompressed 4:4:4 full- bandwidth HD (or better) video onto a hard drive or use a full-bandwidth VCR, such as Sony’s 4:4:4 SR format machines.

To sum up, resolution numbers are not the whole story, since some cameras trade off resolution for color depth. Test your avail-able camera and recorder choices.


Because this is an imperfect world, you may have no choice but to shoot or record with 4:2:2 equipment. Although 4:2:2 is not ideal, don’t forget that the last two Star Wars films, which included thou-sands of greenscreen, were shot with 2/3-inch 4:2:2 cameras. Test the camera on the subject matter. Note that 4:2:2 can produce a satisfactory result in green screen (since the green channel has the highest resolution in these cameras), but one should not expect the ultimate in fine edge detail. (Consumer cameras typically record 4:1:1 and are not recommended for pro visual effects use.)

It bears repeating: Whatever the camera, any edge enhance-ment/sharpening should be turned off!

FiltrationIn general, no color or diffusion filters other than color-temperature correction should be used on the camera when shooting green-screen or bluescreen work. Compositing can be called “the struggle to hold edge detail”; obviously low-contrast, soft effects or diffusion filtering that affects the edge or allows screen illumination to leak into the foreground will have an adverse effect.

To ensure that the filter effect you desire will be duplicated in the composite, shoot a short burst of the subject with the chosen filter, making sure it is slated as filter effect reference.

Negative Scanning and Digital ConversionThe film frames, data recording, or video recording must be con-verted into frame-based digital files the software can use. It’s important not to lose information at this step.

The three layers of the color negative are sensitive exclusively to the red, green, and blue portions of the color spectrum. When the negative is scanned, the RGB densities of each pixel in the image are translated into red, green, and blue numerical levels in a digital memory. The three color records of each frame are referred to as the red, green, and blue channels. They are usually recorded as Cineon or DPX frames, which are uncompressed formats.

Video and data must be similarly converted into frames. This step is sometimes called digitization, which is really a misno-mer since the source is already digital. These frames are usually recorded in the DPX format.

Color CorrectionColor correction at the scanning/conversion stage can be a major source of data loss. It should not be built in to image files intended for compositing. On the other hand, a few frames recorded with the desired color and filtration will be an invaluable reference during the composite step.

ves handbook of visual effects ch 3

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