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Adobe After Effects CS5 : Levels: Histograms and Channels

11/15/2011 3:37:05 PM
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You might have noticed the odd appearance of the histogram 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 flat along the top with spikes protruding at regular intervals (Figure 1).
Figure 1. Strange-looking histograms: A colored solid (top) shows three spikes, one each for the red, green, and blue values, and nothing else. With Ramp (bottom) the distribution is even, but the spikes at the top are the result of the ramp not being an exact multiple of 255 pixels, causing certain pixels to recur more often than others.

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 flat 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 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.

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.

Tip

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 television amateurism.


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 . Occasionally a stylized look calls for crushed contrast, but generally speaking, this is bad form.

Tip

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.


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 film look like film, 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.

Notes

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.


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.

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 1).

Spikes at the end of the second histogram (which is there just to evaluate the adjustment of the first) indicate clipping at the ends of the spectrum, which seems necessary for the associated result. Clipping, then, is part of life.

Figure 2. 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 background (bottom).

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 cases, in which there is no data in between the spikes whatsoever, you may see a prime symptom of overadjustment, banding (Figure 3).

Figure 3. Push an adjustment far enough and you may see quantization, 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.


Banding is typically the result of limitations of 8-bpc color. 16-bpc color mode was added to After Effects 5.0 specifically to address this problem. You can switch to 16 bpc by Alt-clicking (Opt-clicking) on the bit-depth identifier along the bottom of the Project panel (Figure 4) or by changing it in File > Project Settings.

Figure 4. 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.

 
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