Adobe After Effects CS5 Visual Effects and Compositing STUDIO TECHNIQUES phần 4 pptx

ptg 143 II: E ects Compositing Essentials midtones and highlights pink (light red). If, instead, you adjust Input Black or Output White inward, the tinting moves in the opposite direction—toward cyan—in the corresponding shadows and highlights. As you probably know, each primary on the digital wheel of color (red, green, or blue) has an opposite (cyan, magenta, or yellow, respectively). As your color skills progress you will notice when your method of, say, reducing green spill has made fl esh tones too magenta, but when you’re start- ing out it’s enough simply to be aware that adjustments to each color channel proportionally affect its opposite (Figure 5.9). See the fi le Motionworks_ levels_and_curves. pdf, in the additional resources folder on the book’s disc for a reference on color adjustments to channels. Figure 5.8 Proper shooting with a low-dynamic-range digital video camera such as a DSLR requires that you shoot a flat- looking image with low contrast and then bracket the histogram’s white and black points, as it’s always possible to add contrast to optimize an image but not possible to remove it without losing detail. The only difference between the left and right sides of the image is a Levels adjustment transforming the flat source, left, into the richer image on the right. Figure 5.9 These charts were devised by John Dickinson at Motionworks (www.motionworks.com.au) after he read an earlier edition of this book; it shows the relationship of each color to its opposite when adjusting the Levels Effect. Download from WoweBook.com Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com ptg 144 Chapter 5 Color Correction Gradients are one thing, but the best way to make sense of this with a real image is to develop the habit of studying footage on individual color channels as you work. This is the key to effective color matching. Along the bottom of the Composition panel, all of the icons are monochrome by default save one: the Show Channel menu. It contains fi ve selections: the three color channels as well as two alpha modes. Each one has a short- cut that, unfortunately, is not shown in the menu: Alt+1 through Alt+4 (Opt+1 through Opt+4) toggle each color channel. A colored outline around the edge of the com- position palette reminds you which channel is displayed (Figure 5.10); toggling the active channel returns the image to RGB. Try adjusting a single channel of the gradient in Levels while displaying only that channel. The effect of brightness and contrast adjustment on a grayscale image is readily apparent. This is the way to work with individual channel adjustments, especially when you’re just beginning or if you have diffi culty distinguishing colors. As you work with actual images instead of gradients, the histogram can offer valuable information about the image. Figure 5.10 Four Views mode is generally intended for 3D use, but it can also be used to show RGB and individual red, green, and blue channels. This becomes extremely useful for color matching. Note differences in the three channels and the colored outline showing which is which. Same Difference: Levels (Individual Controls) The Levels effect and Levels (Individual Controls) contain identical controls. The sole difference is that Levels lumps all adjustments into a single keyframe property, which expressions cannot use. Levels (Individual Controls) is particularly useful to . animate and time Levels settings individually . link an expression to a Levels setting . reset a single Levels property (instead of the entire effect) Levels is more commonly used, but Levels (Indi- vidual Controls) is sometimes essential. Hold down Shift with the Alt+1–3 (Opt+1–3) shortcut for color channels, and each will display in its color. Shift with Alt+1–4 (Opt+1–4) displays the image with a straight alpha channel, as After Effects uses it internally. Download from WoweBook.com Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com ptg 145 II: E ects Compositing Essentials Levels: Histograms and Channels You might have noticed the odd appearance of the histo- gram for an unadjusted gradient. If you were to try this setup on your own, depending on the size of the layer to which you applied Ramp, you might see a histogram that is fl at along the top with spikes protruding at regular inter- vals (Figure 5.11). The histogram is exactly 256 pixels wide; you can think of it as a bar chart made up of 256 bars, each one pixel in width and corresponding to one of the 256 possible levels of luminance in an 8-bpc image. These levels are displayed below the histogram, above the Output controls. In the case of a pure gradient, the histogram is fl at because of the even distribution of luminance from black to white. If the image height in pixels is not an exact multiple of 256, certain pixels double up and spike. In any case, it’s more useful to look at real-world examples, because the histogram is useful for mapping image data Figure 5.11 Strange-looking histo- grams: A colored solid (top) shows three spikes, one each for the red, green, and blue values, and nothing else. With Ramp (bottom) the distribu- tion is even, but the spikes at the top are the result of the ramp not being an exact multiple of 255 pixels, caus- ing certain pixels to recur more often than others. Download from WoweBook.com Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com ptg 146 Chapter 5 Color Correction that isn’t plainly evident on its own. The point is to help you assess whether any color changes are liable to improve or harm the image. There is in fact no single typical or ideal histogram—they can vary as much as the images themselves, as seen back in Figure 5.8. Despite that fact, you can try a simple rule of thumb for a basic contrast adjustment. Find the top and bottom end of the RGB histogram—the highest and lowest points where there is any data whatsoever—and bracket them with the Input Black and Input White carets. To “bracket” them means to adjust these controls inward so each sits just outside its corresponding end of the histogram. The result stretches values closer to the top or bottom of the dynamic range, as you can easily see by applying a second Levels effect and studying its histogram. Try applying Levels to any image or footage from the disc and see for yourself how this works in practice. First densify the blacks (by moving Input Black well above the lowest black level in the histogram) and then pop the whites (moving Input White below the highest white value). Don’t go too far, or subsequent adjustments will not bring back that detail—unless you work in 32-bpc HDR mode (Chapter 11). Occasionally a stylized look calls for crushed contrast, but generally speaking, this is bad form. Black and white are not at all equivalent in terms of how your eye sees them. Blown-out whites are ugly and can be a dead giveaway of an overexposed digital scene, but your eye is much more sensitive to subtle gradations of low black levels. These low, rich blacks account for much of what makes fi lm look like fi lm, and they can contain a surprising amount of detail, none of which, unfortunately, shows up on the printed page. Look for it in the images themselves. The occasions on which you would optimize an image by raising Output Black or lowering Output White controls are rare, as this lowers dynamic range and the overall contrast. However, there are many uses in compositing for lowered contrast, to soften overlay effects (say, fog and clouds), high-contrast mattes, and so on. Examples follow in this chapter and throughout the rest of the book. Auto Levels serves up a result similar to bracketing Input White and Input Black to the edges of the histogram. If that by itself isn’t enough to convince you to avoid using Auto Levels, or really any “Auto” correction, consider also that they are processor intensive (slow) and resample on every frame. The result is not consistent from frame to frame, like with auto-exposure on a video camera—reality televi- sion amateurism. Footage is by its very nature dynamic, so it is essential to leave headroom for the whites and foot room for the blacks until you start working in 32 bits per channel. You can add contrast, but once the image blows out, that detail is gone. LCD displays, as a whole, lack the black detail that can be captured on film. The next time you see a movie in a cinema, notice how much detail you can see in the shadows and compare. Download from WoweBook.com Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com ptg 147 II: E ects Compositing Essentials Problem Solving Using the Histogram You may have noticed that the Levels histogram does not update as you make adjustments. After Effects lacks a panel equivalent to Photoshop’s Histogram palette, but you can, of course, apply a Levels effect just to view the histogram (as in Figure 5.11). The histogram reveals a couple of new wrinkles in the backlit shot from Figure 5.5, now adjusted with Levels to bring out foreground highlights (Figure 5.12). Spikes at the end of the second histogram (which is there just to evaluate the adjustment of the fi rst) indicate clipping at the ends of the spectrum, which seems necessary for the associated result. Clipping, then, is part of life. Note also the gaps that appear in the second histogram. Again, the net effect is a loss of detail, although in this case, the gaps are not a worry because they occur among a healthy amount of surrounding data. In more extreme Figure 5.12 Adjusted to empha- size the foreground as in Figure 5.5 (top), the values below midgray are stretched, resulting in clear gaps in a second histogram that indicate loss of detail. Those same gaps appear, to a lesser extent, with the more modest adjustment to emphasize the back- ground (bottom). Download from WoweBook.com Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com ptg 148 Chapter 5 Color Correction cases, in which there is no data in between the spikes what- soever, you may see a prime symptom of overadjustment, banding (Figure 5.13). Banding is typically the result of limitations of 8-bpc color. 16-bpc color mode was added to After Effects 5.0 specifi - cally to address this problem. You can switch to 16 bpc by Alt-clicking (Opt-clicking) on the bit-depth identifi er along the bottom of the Project panel (Figure 5.14) or by chang- ing it in File > Project Settings. Chapter 11 explains this in more detail. Figure 5.14 An entire project can be toggled from the default 8-bpc color mode to 16-bpc mode by Alt-clicking (Opt-clicking) the project color depth toggle in the Project panel; this prevents the banding seen in Figure 5.13. Curves: Gamma and Contrast Curves rocks. I heart Curves. The Curves control is particu- larly useful for gamma correction. . Curves lets you fully (and visually) control how adjust- ments are weighted and roll off. . You can introduce multiple gamma adjustments to a single image or restrict the gamma adjustment to just one part of the image’s dynamic range. . Some adjustments can be nailed with a single well- placed point in Curves, in cases where the equivalent adjustment with Levels might require coordination of three separate controls. It’s also worth understanding Curves controls because they are a common shorthand for how digital color adjustments are depicted; the Curves interface recurs in most color cor- rection toolsets. Figure 5.13 Push an adjustment far enough and you may see quantiza- tion, which appears as banding in the image. Those big gaps in the histogram are expressed as visible bands on a gradient. Switching to 16 bpc from 8 bpc is an instant fix for this problem in most cases. Download from WoweBook.com Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com ptg 149 II: E ects Compositing Essentials Curves does, however, have drawbacks, compared with Levels: . It’s not immediately intuitive and can easily yield hid- eous results if you don’t know what you’re doing. There are plenty of artists who aren’t comfortable with it. . Unlike Photoshop, After Effects doesn’t offer numeri- cal values corresponding to curve points, making it a purely visual control that can be hard to standardize. . In the absence of a histogram, you may miss obvious clues about the image (making Levels more suitable for learners). The most daunting thing about Curves may be its inter- face, a simple grid with a diagonal line extending from lower left to upper right. There is a Channel selector at the top, set by default to RGB as in Levels, and there are some optional extra controls on the right to help you draw, save, and retrieve custom curves. To the novice, the arbitrary map is an unintuitive abstraction that you can easily use to make a complete mess of your image. Once you under- stand it, however, you can see it as an elegantly simple description of how image adjustment works. You’ll fi nd a project containing the equivalent Curves graph to the previous Levels corrections on the book’s disc. Figure 5.15 shows the more fully featured Photoshop Curves, which better illustrates how the controls work. Figure 5.15 Photoshop’s more deluxe Curves includes a histogram, built-in presets, displays of all channels together, and fields for input and output values for a given point on the curve. Download from WoweBook.com Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com ptg 150 Chapter 5 Color Correction Figures 5.16 shows some basic Curves adjustments and their effect on an image. Figure 5.17 uses linear gradients to illustrate what some common Curves settings do. I encourage you to try these on your own. A C Figure 5.16 What you see in an image can be heavily influenced by gamma and contrast. A. The source image. B. An increase in gamma above the shadows. C. A decrease in gamma. D. Both corrections combined. D B Download from WoweBook.com Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com ptg 151 II: E ects Compositing Essentials A C E B D F Figure 5.17 This array of Curves adjustments applied to a gradient shows the results of some typical settings. A. The default gradient and Curves setting. B. An increase in gamma. C. A decrease in gamma. D. An increase in brightness and contrast. E. Raised gamma in the highlights only. F. Raised gamma with clamped black values. Download from WoweBook.com Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com ptg 152 Chapter 5 Color Correction Most interesting are the types of adjustments that only Curves allows you to do—or at least do easily. I came to realize that most of the adjustments I make with Curves fall into a few distinct types that I use over and over. The most common adjustment is to simply raise or lower the gamma with Curves, by adding a point at or near the middle of the RGB curve and then moving it upward or downward. Figure 5.18 shows the result of each. This pro- duces a subtly different result from raising or lowering the Gamma control in Levels because of how you control the roll-off (Figure 5.19). Figure 5.18 Two equally valid gamma adjustments via a single-point adjustment in the Curves control. Fine tuning follows in Figure 5.21. Download from WoweBook.com Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com [...]... point, then a second one to restrict its range, and a third as needed to bring the shape of one section back where I want it 1 54 Download from WoweBook.com II: Effects Compositing Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com Essentials Hue/Saturation: Color and Intensity The third of three essential color correction tools in After Effects is Hue/Saturation You can use this one... color pots and accompanying wheeled surface controllers This is also known as a three-way color corrector, and it has been the major missing color tool in the shipping version of After Effects until now Synthetic Aperture’s Color Finesse version 3, now included with After Effects, fills this gap Although Color Finesse is a full color correction application that has been included with After Effects for... Direction and Position An element generated in 3D software ideally contains multiple passes for more control Even with that, if the lighting direction and perspective of an element are wrong, there’s no practical way to make it match (Figure 5. 34) 170 Download from WoweBook.com II: Effects Compositing Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com Essentials On the other hand, compositing. .. all costs—Luma Key and Color Key The latter two are limited to a bitmap (black and white) selection; only by choking and blurring the result with Edge Thin and Edge Feather controls can you add threshold adjustment It’s a little unfortunate that Adobe hasn’t let these ancient effects go into the bin marked “obsolete” since the other, less intuitively named effects supersede them (and yet those not in... overall compared with the car Bring Blue Input White down to at least 0.95 while viewing the blue channel (Alt+3/Opt+3) and see if it doesn’t appear to be a better match Switch the view and Levels control to Red, and 1 64 Download from WoweBook.com II: Effects Compositing Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com Essentials Figure 5.29 Compare this integration to that of Figure... Download from WoweBook.com II: Effects Compositing Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com Essentials 3 Begin as if you are looking at a black -and- white pho- tograph, and match the element to this dark contrasty scene using Levels in the RGB channel If the element needs more contrast in the shadows and highlights, as this one does, raise Input Black and lower Input White; if... full Curves and Levels controls (with histogram), a Levels alternate called Luma Range, and a Secondary control for particular colors you might want to isolate and change 158 Download from WoweBook.com II: Effects Compositing Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com Essentials The top half of Color Finesse contains most of the professional modes of viewing and analyzing... light, and shadow There are even cases in which you can, to some degree, relight a shot in After Effects, introducing light direction, exchanging day for night, and so on You’ll discover more in Chapter 12, and on your own 172 Download from WoweBook.com CHAPTER Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 6 Color Keying Download from WoweBook.com Simpo PDF Merge and Split... Effects or used in most of the world’s leading compositing and color correction applications, including those you see on the list: Autodesk Lustre and Smoke, LUTher, Scratch, and Truelight Cube, among others Color Matching Now, having laid the groundwork with the toolset, it’s time for the bread -and- butter work of compositing: to match separate foreground and background elements so that the scene appears... Figure 5. 24 Color Finesse brings scopes into—or at least makes them available to After Effects CS5 You could perform all of your color corrections here, without opening the full Color Finesse interface, but when you do open it, you’ll find more ways to take complete control of the color look (Figure 5. 24) In the lower left are slider controls for all four color modes: HSL, RGB, its opposites CMY and the . Range, and a Secondary control for particular colors you might want to isolate and change. Figure 5. 24 Color Finesse brings scopes into—or at least makes them available to After Effects CS5. Download. Export and t format, and the application will create a fi le containing a 3D color lookup table that can be saved for use in After Effects or used in most of the world’s leading compositing and. corrector, and it has been the major missing color tool in the shipping version of After Effects until now. Synthetic Aperture’s Color Finesse version 3, now included with After Effects, fi