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Adobe After Effects CS5 : Dynamic Range: Bit Depth and Film (part 3) - Video Gamma Space, Linearized Working Space

11/12/2013 2:45:13 AM
5. Video Gamma Space Because log spaces certainly don’t look natural, it probably comes as no surprise that they are bad color spaces to work in. But there is another encoding color space with which you definitely have a working familiarity: the video space of your monitor. You may have always assumed that 8-bit monitor code value 128, halfway between black and white, makes a gray that is half as bright as white. If so, you may be shocked to hear that this is not the case. In fact, 128 is much darker—not even a quarter of white’s brightness on most monitors. A system where half the input gives you half the output is described as linear, but monitors (like many things in the real world) are nonlinear. When a system is nonlinear, you can sometimes describe its behavior using the gamma function, shown in Figure 9 and in the equation Figure 9. Graph of monitor gamma (2.2) with file gamma (0.4545) and linear (1.0). These are the color curves in question, with 0.4545 and 2.2 each acting as the direct inverse of the other. `Output = inputgamma 0 <= input <= 1` In this function, the darkest and brightest values (0.0 and 1.0) are always fixed, and the gamma value determines how the transition between them behaves. Successive applications of gamma can be concatenated by multiplying them together. Applying gamma and then 1/gamma has the net result of doing nothing. A curve with a gamma of 1.0 is linear. Mac OS previously used a gamma of 1.8, while Windows used a gamma value of 2.2. Because the electronics in your screen are slow to react from lower levels of input voltage, a 1.0 gamma is simply too dark in either case; boosting this value compensates correctly. The reason digital images do not appear dark, however, is that they have all been created with the inverse gamma function baked in to prebrighten pixels before they are displayed (Figure 10). Yes, all of them. Figure 10. The gamma settings in the file and monitor complement one another to result in faithful image reproduction. Because encoding spaces are not compositing spaces, working directly with images that appear on your monitor can pose problems. Similar to log encoding, video gamma encoding allocates more values to dark pixels, so they weigh more than they should. Video color space is not much more valid than Cineon color space for re-creating the way light behaves in the world at large. Close-up: Gamma-ramaIn case all this gamma talk hasn’t already blown your mind, allow me to clarify how monitor gamma and human vision work together. The question often comes up—why is middle gray 18% and not 50%? And why does 50% gray look like middle gray on my monitor, but not on a linear color chart?It turns out that your eyes also have a nonlinear response to color—your vision brightens low light, which helps you to see where it’s dim, a survival advantage. The human eye is very sensitive to small amounts of light, and it gets less sensitive as brightness increases. Your eye effectively brightens the levels, and objects in the world are, in fact darker than they appear—or, they become darker when we represent their true linear nature. The subjective observation that 18% gray, give or take a percentage point, appears to be the midpoint between black and white indicates that your eye (or more accurately, the human vision system) applies a gamma correction of 2.5, which can be calculated as follows: 0.5 = 0.18^(1/2.5). 6. Linearized Working Space In the real world, light intensity is linear. Double the wattage and the entire scene is twice as bright. Unless you work linear, with these scene-referred values, this is not how color intensity behaves on your computer’s monitor-referred values, which also clip at an intensity well below the whitest white your eye can see. If only your computer had the same color and light model as the real world, you could use light values the same way they are used in nature. Let’s take a trip now to a magical land where computer and real-world light values are one; to reach our destination, we need go no further than the Project Settings dialog. NotesTo follow this discussion, choose Decimal in the Info panel menu (this is the default for 32 bpc). 0.0 to 1.0 values are those falling in low dynamic range, or LDR—those values typically described in 8 bit as 0 to 255. Any values outside this range are HDR, 32 bpc only. Open Project Settings in any project and take a look at Color Settings. By default the dialog might look like that in Figure 11 (left). There are a couple of pull-down menus, a few checkboxes (one or two of which might be grayed out) and some complicated looking fine print below. Figure 11. This is how default Color Settings might look in the Project Settings dialog. If there’s one thing I know about visual artists, it’s that we’re generally impatient with fine print and are enamored of visual examples, so try the following: 1. Set a working space to enable color management. It doesn’t really matter for this exercise which you choose—more on the specific choice later—but if you want the most common and flexible one, choose sRGB (or, use its more complete and impossible-to-remember name, sRGB IEC61966-2.1), which is designed to correspond to your monitor itself. 2. Note that the checkboxes are now all live (none are grayed out), including Linearize Working Space. Enable that one, and leave Compensate for Scene-referred Profiles enabled, then click OK (Figure 11, right).3. For simplicity’s sake, make sure in Preferences > General that Use System Color Picker is disabled so that you’re working with the Adobe Color Picker.4. Create a new solid layer. Click on the Color swatch to open the Adobe Color Picker. Notice that it looks different—strange and unfamiliar, even (Figure 12). Set Brightness to 50% and notice a tone much brighter than what would be considered middle gray. Figure 12. When you set the working space to be linearized, the Adobe color picker responds in kind by presenting linear color values. Notice how far below center middle gray appears to be. 5. Now try switching that Brightness value to 18%. Aha. In a linearized working space, After Effects accommodates the 18% middle gray of real-world color. Now the question becomes, how does that help? This type of change should generally not be introduced to a project already in progress, since all of the solids and other color selections will shift and need to be reset, and any blends will also be linear, so even composites without color selections will shift. Plus, it’s confusing not to have middle gray at 50%, isn’t it?
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