Adobe After Effects 7.0 Studio Techniques

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response curve and the ability to capture even very bright light sources. No amount of exposure can

be expected to affect every single crystal.

As much as Cineon log is a great way to encode light captured by film, it should not be used for compositing

or other image transformations. This point is so important that it just has to be emphasized again:

Encoding color spaces are not compositing color spaces.

To illustrate this point, imagine you had a black pixel with Cineon value 95 next to an extremely bright pixel

with Cineon’s highest code value, 1023. If these two pixels were blended together (say, if the image was being

blurred), the result would be 559, which is somewhere around middle gray (0.37 to be precise). But when you

consider that the extremely bright pixel has a relative brightness of 13.5, that black pixel should only have

been able to bring it down to 6.75, which is still overbright white! Log space’s extra emphasis on darker values

causes standard image processing operations to give them extra weight, leading to an overall unpleasant and

inaccurate darkening of the image. So, final warning: If you’re working with a log source, don’t do image

processing in log space!

Video Gamma Space

Because log space certainly doesn’t look natural, it probably comes as no surprise that it is a bad color space to

work in. But there is another encoding color space that you have been intimately familiar with for your entire

computerusing life and no doubt have worked in directly at some point: 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 usually describe its behavior using the

gamma function, shown in Figure 11.9 and the equation

Output = input

gamma

0 <= input <= 1

Figure 11.9. 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.

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 multiply-

ing them together. Applying gamma and then 1/gamma has the net result of doing nothing. Gamma 1.0 is linear.

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Notes

The description of gamma in video is oversimplified here somewhat because the subject is complex

enough for a book of its own. An excellent one is Charles Poynton’s Digital Video and HDTV Algo-

rithms and Interfaces (Morgan Kaufmann Publishers).

Mac monitors usually have a gamma of 1.8, and the gamma value for PCs is 2.2. What this really says is that

the electronics in your screen are slow to react from lower levels of input voltage and everything is darkened

as a result.

The reason digital images do not appear dark, however, is that they have all been created with the inverse

gamma function baked in to pre-brighten pixels before they are displayed (Figure 11.10). Yes, all of them.

Figure 11.10. Offsetting gammas in the file and monitor 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 have extra weight. Video images need converting just as log Cineon files do.

Close-Up: Gamma-rama

In case all this gamma talk hasn’t already blown your mind, allow me to mention two other related

points.

First, you may be familiar with the standard photographic gray card, known as the 18% gray card.

But why not the 50% gray card?

Second, although I’ve mentioned that a monitor darkens everything on it using a 2.2 gamma, you may

wonder why a grayscale ramp doesn’t look skewed toward darkness. 50% gray on a monitor looks

like 50% gray.