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Adobe After Effects CS5 : Color Correction for Image Optimization (part 1) - Levels

11/5/2011 4:41:21 PM
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What constitutes an “optimized” clip? What makes a color-corrected image correct? Let’s look at what is typically “wrong” with source footage levels and the usual methods for correcting them, in order to lay the groundwork for color matching. As an example, let’s look at brightness and contrast of a plate image, with no foreground layers to match.

Notes

The term plate stretches back to the earliest days of optical compositing (and indeed, of photography itself) and refers to the source footage, typically the background onto which foreground elements will be composited. A related term, clean plate, refers to the background with any moving foreground elements removed.


Levels

Levels may be the most-used tool in After Effects. It consists of five basic controls—Input Black, Input White, Output Black, Output White, and Gamma—each of which can be adjusted in five separate contexts (the four individual image channels R, G, B, and A, as well as all three color channels, RGB, at once). There are two different ways to adjust these controls: via their numerical sliders or by dragging their respective caret sliders on the histogram (which is the more typical method).

Contrast: Input and Output Levels

Four of the five controls—Input Black, Input White, Output Black, and Output White (Figure 1)—determine brightness and contrast, and combined with the fifth, Gamma, they offer more precision than is possible with the Brightness & Contrast effect.

Figure 1. The CS5 version of Levels displays each channel of the histogram in color. The small round icons at the right of the histogram (left) toggle between this and the traditional black and white histogram, which can be easier to read.
(Image from the film Dopamine, courtesy of Mark Decena, Kontent Films.)


Figure 2 shows a Ramp effect applied to a solid using the default settings, followed by the Levels effect. Move the black caret at the lower left of the histogram—the Input Black level—to the right, and values below its threshold (the numerical Input Black setting, which changes as you move the caret) are pushed to black. The further you move the caret, the more values are “crushed” to pure black.

Figure 2. Levels is applied to a layer containing a Ramp effect at the default settings, which creates a smooth gradient from black to white. The spikes occur simply because the gradient height does not have an exact multiple of 256 pixels.

Move the Input White caret at the right end of the histogram to the left, toward the Input Black caret. The effect is similar to Input Black’s but inverted: More and more white values are “blown out” to pure white (Figure 3).

Figure 3. Raising Input Black and lowering Input White has the effect of increasing contrast at either end of the scale; at an extreme adjustment like this, many pixels in an 8-bpc or 16-bpc project are pushed to full white or black or “crushed.”

Either adjustment effectively increases contrast, but note that the midpoint of the gradient also changes as each endpoint is moved in. In Figure 3, Input Black has been adjusted more heavily than Input White, causing the horizon of the gradient to move closer to white and the shadows to darken. You can re-create this adjustment with Brightness & Contrast (Figure 4), but there’s no direct control of the midpoint (gamma) of the image (Figure 5).

Figure 4. This gradient with Brightness & Contrast applied shows the midpoint clearly sliding toward brighter values, and the effect contains no gamma control to influence this side effect.

Figure 5. Interiors with exterior windows present a classic lighting and color challenge (left). A well-shot image from a powerful enough camera can resolve detail in both. However, Brightness & Contrast doesn’t let you adjust the midpoint (gamma) and thus forces you to choose between resolving the background (Brightnesss –45, Contrast –8; middle) and the foreground (Brightness 30, Contrast 30; right).


Reset Levels (click Reset at the top of the Levels effect controls) and try adjusting Output Black and Output White, whose controls sit below the gradient. Output Black specifies the darkest black that can appear in the image; adjust it upward and the minimum value is raised.

Tip

You can reset any individual effect control by context-clicking it and choosing Reset. You know it’s an individual effect if it has its own stopwatch.


Lowering Input White is something like dimming the image, cutting off the maximum white value at the given threshold. Adjust both and you effectively reduce contrast in the image. Bring them alongside one another, and the gradient becomes a solid gray (Figure 6).

Figure 6. Raising Output Black and lowering Output White reduces contrast in the dark and light areas of the image, respectively; it doesn’t produce such a beautiful image in this case, but comes into play in the Matching section.

So Input and Output controls have inverse effects. But you will find situations where you might use them together, first balancing the image, then reducing contrast in the whites, blacks, or both.

As is the case throughout After Effects, the controls operate in the order listed in the interface. In other words, raising the Input Black level first raises black density, and a higher Output Black level raises all of the resulting black levels together (Figure 7). If you’ve crushed the blacks with Input Black they remain crushed, and they all just appear lighter.

Figure 7. Black and white levels crushed by adjusting the Input controls aren’t then brought back by the Output controls. Instead, Output simply limits the overall dynamic range of the image (bottom), raising the darkest possible black level and lowering the brightest possible white.

If you’re thinking, “So what?” at this point, just stay with this until we move to a situation in which to apply it.

Brightness: Gamma

As you adjust the Input Black and White values, you may have noticed the third caret that maintains its place between them. This is the Gamma control, affecting midtones (the middle gray point in the gradient) without touching the white and black points. Adjust gamma of the gradient image and notice that you can push the grays in the image brighter (by moving it to the left) or darker (by moving it to the right) without changing the black and white levels.

Many images have healthy contrast, but a gamma boost gives them extra punch. Similarly, an image that looks a bit too “hot” may be instantly adjusted simply by lowering gamma.

In most cases, the image itself rather than the histogram offers the best clue as to whether the gamma needs adjustment. So what is your guideline for how much to adjust gamma, if at all? I first learned always to go too far before dialing back, which is especially helpful when learning. An even more powerful gamma adjustment tool that scares novices away is Curves (coming up).

Figure 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.

Close-up: What Is Gamma, Anyway?

It would be nice but inaccurate simply to say, “Gamma is the midpoint of your color range” and leave it at that. The more accurate the discussion of gamma becomes, the more purely mathematical it becomes. Plenty of artists out there understand gamma intuitively and are able to work with it without knowing the math behind it—but here it is anyway.

Gamma adjustment shifts the midpoint of a color range without affecting the black or white points. This is done by taking a pixel value and raising it to the inverse power of the gamma value:

newPixel = pixel (1/gamma)

You’re probably used to thinking of pixel values as fitting into the range 0 to 255, but this formula works with values normalized to 1. 0 is 0, 255 is 1, and 128 is 0.5—which is how the math “normally” operates behind the scenes in computer graphics.

Gamma operates according to the magic of logarithms: Any number to the power of 0 is 1, any number to the power of 1 is itself, and any fractional value (less than 1) raised to a higher power approaches 0 without ever reaching it. Lower the power closer to 0 and the value approaches 1, again without ever reaching it. Not only that, but the values distribute proportionally, along a curve, so the closer an initial value is to pure black (0) or pure white (1) the less it is affected by a gamma adjustment.


By mixing these five controls together, have we covered Levels? No—because there are not, in fact, five basic controls in Levels (Input and Output White and Black plus Gamma), but instead, five times five (RGB, Red, Green, Blue, and Alpha).

 
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