How are the details holding up? And how does the foreground look in the actual composite? It’s easy to get so wrapped up in creating the perfect matte that you forget that some problems are harder to spot against some backgrounds. If she’s headed for a bluish environ- ment, why trouble yourself too much with spill suppres- sion? Would more blending of the background into the foreground (later in this chapter in the “Edge Selection”
section) or even light wrap help sell this composite
Figure 6.16 This isn’t how the actual comp looks;
it’s showing the core matte as the white center with the plate translucently revealing that nicely isolated edge, where all efforts are to be focused.
Figure 6.17 Blue matte line around the edges.
ptg 6. Isolate and refi ne further.
Do you need to isolate the hair for its own keying pass?
How about those motion-blurred hands? Are there holes in the matte, or problems with the core pass?
Does the talent, heaven forbid, make direct contact with the background, for example lying down on the colored fl oor?
For these types of issues, create holdout mattes.
Keylight for Color Keying
The core of Keylight is screen matte generation, and as mentioned, the most essential step is choosing the exact color to key. From that, Keylight makes weighted compari- sons between its saturation and hue and that of each pixel, as detailed in Table 6.1. From this, you see that the ideal background is distinct and saturated.
Figure 6.18 At this stage, prior to any color matching, the detail is well pre- served but the matte lines along the torso and arms, in particular, remain.
Holdout Matte
A holdout matte isolates an area of an image for separate treatment. I recommend that you think of a color key as an isolated edge matte surrounded by two holdout mattes: one for the core, one for the background. Details on creating these can be found ahead in the “Fine Tuning and Problem Solving” section.
TABLE 6.1 How Keylight Makes Its Key Decisions COMPAREDTOSCREENCOLOR, PIXELIS KEYLIGHTWILL
of a different hue consider it foreground, making it opaque of a similar hue and more saturated key it out completely, making it transparent
of a similar hue but less saturated subtract a mathematically weighted amount of the screen color and make it semitransparent
ptg My current advice is to leave all other top controls in Key-
light alone, but in case you’re curious about them, follow- ing is a bit of extra information on how each works.
Screen Gain
The ideal Screen Gain setting is 100, no change from the default. This adjustment is compensation for a poorly lit matte or a foreground contaminated with background color. While raising it may make the matte channel look better, you are also likely to see increased color grain and lost edge detail with values above the default. The alterna- tive with fewer side effects is to raise Clip Black.
Screen Gain boosts (or reduces) the saturation of each pixel before comparing it to the screen color. This effec- tively adds more desaturated background pixels into the keyed color range.
Screen Balance
Keylight relies on one of the three RGB color values being the dominant background color. It is even more effective when it knows whether one of the two remaining colors is more prevalent in the background, and which one. Screen Balance makes use of a dominant secondary background color.
The software automatically sets a balance of 95% with blue- screens (which typically contain a good deal of green) and leaves it at 50% for greenscreens (which tend to be more monochromatic). If you want to try adjusting it yourself, try alternate settings of either 5.0 or 95.0 to take it close to one or the other secondary color.
Figure 6.19 The Rosco colors: Ultimatte Blue, Ultimatte Green, and Ultimatte Super Blue. Blue is not pure blue but double the amount of green, which in turn is double the amount of red. Ultimatte Green is more pure, with only a quarter the amount of red and no blue whatsoever. Lighting can change their hue (as does converting them for print in this book).
The controls atop Keylight (from Screen Colour down to Screen Pre- blur) alter the actual generation of the matte. Everything from Screen Matte down adjusts the result of that first step.
A Rosco Ultimatte Blue screen con- tains quite a bit of green—much more than red, unless improperly lit. Ultimatte Green screens, meanwhile, are nearly pure green (Figure 6.19).
ptg Bias
The Bias settings color-correct the image by scaling the primary color component up or down (enhancing or reducing its difference from the other two components).
The Foundry recommends that in most cases you leave Alpha Bias at the default and that you click the Despill Bias eyedropper on a well-lit skin tone to be preserved; despill pivots around this value.
Bias has the unpleasant side effect of signifi cantly increas- ing graininess. If this happens to your footage, try other despill methods discussed later in this chapter.
Refinement
When you spot an area that looks like a candidate for refi nement, save (to hold an undo point should you need to use File > Revert), zoom in, and create a region of inter- est around the area in question.
Now take a look at the tools provided by Keylight to address some common problems.
Clip White, Black, Rollback
The double-matte method (core and edge) eliminates a lot of the tug of war that otherwise exists between a solid foreground and subtle edges. Even with this advantage, both mattes may require adjustments to the Clip White or Clip Black controls.
Keep the largest possible difference (or delta, if you prefer) between these two settings, as this is where all of the gray, semitransparent alpha pixels live. The closer the two num- bers get, the closer you are to a bitmap alpha channel, in which each pixel is pure black or white—a very bad thing indeed (Figure 6.20).
If you push too far, restore back toward the initial matte with Clip Rollback. Its value is the number of pixels from the edge that are rolled back relative to the original, unclipped screen matte. So if your edges were subtle but sizzling on the fi rst pass, and removing noise from the matte hardened them, then this tool may restore subtlety.
Hold down Alt (Opt) to center a zoom around your cursor.
Figure 6.20 Here’s how the hair looks without a separated edge matte. Not nice.
ptg Figure 6.21 The source (top left) can
be converted to YUV with Channel Combiner and the UV (chroma) or Y (luminance) blurred individually, then round-tripped back to RGB with Channel Combiner again. With heavy blur to the color data (top right) the image is still clear, albeit stylized, if the luminance is untouched, but blur the luminance and leave the color and the result is far less recognizable (lower left).
Figure 6.22 4:4:4 is just pixels, no chroma subsampling, where 4:2:2 and 4:1:1 group the nearest neighboring pixels, giving them identical luminance accord- ing to the patterns shown here.
Figure 6.23 Key a 4:1:1 image (left) and Keylight’s Status view (right) clearly shows the horizontal blocks associated with that type of chroma subsampling (images are shown at 400%).
Chroma Subsampling:
The 411 on 4:1:1, 4:2:2, and 4:2:0 Video images are RGB on your computer, but video devices themselves use Y’CrCb, the digital equivalent of YUV. Y’ is the luminance or brightness signal (or “luma”); Cr and Cb are color-difference signals (roughly corresponding to red-cyan and blue-yellow)—you could call them chrominance or “ chroma.”
It turns out that the human eye is much more particular about gradations in luma than chroma, as is amply demonstrated in Figure 6.21.
The standard types of digital video compression take advantage of this fact. Figure 6.22 shows the difference between straight RGB and 4:2:2 compression, which is common to popular formats including DVCPRO HD and DVCPRO50, ProRes 422 and cameras such as the Sony F900, as well as 4:1:1, which is used by DVCPRO and NTSC DV.
Almost as bad for keying purposes is 4:2:0, the MPEG-2 (DVD), HDV, and PAL DV format.
As you might imagine, chromatic compression is far less than ideal for color keying ( Figure 6.23), hence the workarounds in this section.
ptg Noise Suppression
For seriously sizzling mattes, Keylight includes a Screen Pre-blur option that I would reserve for footage with a clearly evident noise problem, such as heavy compression.
Blurring source footage before keying adds inaccuracy and is something of a desperation move. The footage itself does not appear blurred, but the matte does.
A better alternative for a fundamentally sound matte is Screen Softness, under the Screen Matte controls. This control blurs the screen matte itself, so it has a much better chance of retaining detail than a pre-blur approach. As shown in Chapter 3, edges in nature are slightly soft, and a modest amount of softness is appropriate even with a perfectly healthy matte.
The Despot cleanup tools are meant to fi ll matte holes, but at high levels they add blobbiness, so they are rarely use- ful. An alternative approach, particularly with DV formats (which, by the way, are guaranteed to add compression noise and are not recommended for bluescreen and green- screen work), is to do as follows:
1. Convert the footage to YUV using Channel Combiner (the From pop-up menu). This will make the clip look very strange, because your monitor displays images as RGB. Do not be alarmed (Figure 6.24).
Figure 6.24 This is how an image converted to YUV should look on an RGB monitor—weird. The point is not how YUV looks, but what you can do to adjust it before using Channel Combiner to round-trip it back to RGB.
ptg 2. Apply Channel Blur to the green and blue channels
only, at modest amounts. To gauge this, examine each channel as you work—press Alt+2 (Opt+2) or Alt+3 (Opt+3) while zoomed in on a noisy area. Make sure Repeat Edge Pixels is checked.
3. Round-trip back from YUV to RGB, using a second instance of Channel Combiner.
4. Apply Keylight.
Matte Choke
Besides mismatched lighting, fringing (excess edge opac- ity) and choking (lost edge detail) are the most common tells of a greenscreen comp. Screen Grow/Shrink deals with this issue directly. Don’t be afraid to use it, gently (a setting of around 1.0, or one pixel, won’t do your matte much harm, especially if combined with a bit of matte softness).
This is not the last resort for choking and spreading a matte; alternatives follow in “Fine Tuning and Problem Solving.”
Spill Suppression
Keylight suppresses color spill (foreground pixels contami- nated by refl ected color from the background) as part of the keying operation when displaying the fi nal result. Thus spill-kill can be practically automatic if you pull a good initial key.
There are a surprising number of cases in which Keylight’s spill suppression is not what you want, for the following reasons:
. Dramatic hue shifts occur to items whose colors are anywhere near green (for example, cyan) or opposite green (for example, magenta). It’s challenging enough to keep green off of a green set, let alone its neighbor- ing and opposite hues.
. These hue shifts can also add graininess, even to foot- age that was shot uncompressed and has little or no source grain.
YUV is the digital version of the broadcast video color space. It is used in component PAL television and is functionally similar to YIQ, the NTSC variant. In After Effects YUV, the red channel displays the luminance value (Y) of the shot, while the green and blue channels display blue and red weighted against green (U and V).
ptg InFigure 6.25, notice how the whole shape of the girl’s
face seems to change due to the removal of highlights via spill suppression.
Should Keylight’s spill suppression become unwieldy or otherwise useless for the preceding reasons, there is an easy out: Ordinarily, the View is set to Final Result, but set it to Intermediate Result for the matte applied to the alpha without any change to RGB. The CC Composite effect does the same thing, eliminating all RGB changes from preced- ing effects but keeping the alpha.
Keylight itself also includes spill suppression tools, under Edge Colour Correction, that infl uence only the edge pixels. Enable its check box and adjust the controls below, softening or growing the edge as needed to increase the area of infl uence. Sometimes adjusting Luminance or Satu- ration of edges is a quick fi x.
The next section, which goes beyond this tool, describes better ways to kill spill.
Fine Tuning and Problem Solving
The key here is to break it down. The above steps apply to most ordinary keying situations, but extraordinary ones happen all the time. The trick is to fi nd the areas that are closer to ordinary, deal with those, and isolate the extra- ordinary stuff separately. In this section we focus on how to break apart a key with various types of holdout mattes and keep procedural keys effective.
Although each shot is different, there are really only a few challenges that consistently come up with a color matte:
. Lighting: If the shot was not lit properly, everything that follows will be much more diffi cult.
Figure 6.25 Her face doesn’t even look the same without the highlights reflected with the green. Even worse, at this magnification, it’s easy to see that the amount of grain noise has increased significantly (right). It’s a definite case for pulling the matte on one pass and applying spill suppres- sion separately.
ptg . Image quality: Bluescreen and greenscreen keys put
footage quality to the test, and the worst cameras are those that lose the most data right at the time of cap- ture. Mini DV cameras used to be the main culprits, but nowadays it is sadly the mighty DSLR that is most often inappropriately used to shoot effects plates.
These two points are determined at the shoot itself; for more about that, see the next section, “Shoot for the Perfect Matte.”
. Fine detail such as hair, motion, or lens blur (Figure 6.26, top row).
. Costume contamination: shiny, refl ective, or transpar- ent subjects, or those simply containing colors close to that of the background, can present a fun keying chal- lenge but can also turn out to be more of a nightmare (Figure 6.26, bottom left).
. Set contact is always a huge challenge, whether simply a full-body shot including feet or talent interacting with objects painted green, sitting on a green stool, or lying on the green fl oor.
Figures 6.26 Fun challenges you may encounter when pulling a color key include wispy hair (top left), motion blur (top right), contamination of fore- ground elements by the background color (bottom left), and shadows (bottom right).