Contrast: Input and Output Levels

Contrast: Input and Output Levels

Four of the fi ve controls—Input Black, Input White,
Output Black, and Output White (Figure 5.1)—determine
brightness and contrast, and combined with the fi fth,
Gamma, they offer more precision than is possible with the
effect called Brightness & Contrast.
Figure 5.2 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.

Move the Input White carat 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.
Either adjustment effectively increases contrast, but note
that the midpoint of the gradient also changes if one is
adjusted further than the other. In  Input Black
has been adjusted more heavily than Input White, causing
the horizon of the gradient to move closer to white and
more of the image to turn black. You can re-create this
adjustment with Brightness & Contrast (Figure 5.4 on the
next page), but to do so you must adjust both contrast

and brightness, with no direct control of the midpoint
(gamma) of the image .
Reset Levels (click Reset at the top of the Levels effect
controls) and try the same experiment with Output Black
and Output White, whose controls sit below the little
gradient. Output Black specifi es the darkest black that can
appear in the image; adjust it upwards and the minimum
value is raised.
Similarly, lowering Input White is something like dimming
the image, cutting off the maximum possible white value at
the given threshold. Adjust both and you effectively reduce
contrast in the image; with them close together, the gradient
becomes a solid gray .
Evidently the Input and Output controls have the opposite
effect on their respective black and white values, when
examined in this straightforward fashion. However, there
are even situations where you would use them together.

As is the case throughout After Effects, the controls are
operating in the order listed in the interface. In other
words, raising the Input Black level fi rst crushes the blacks,
and then a higher Output Black level raises all of those
pure black levels as one . It
does not restore the black detail in the original pixels; the
blacks remain crushed, they all just become lighter.
If you’re thinking, “So what?” at this point, just stay with
this—the controls are being broken down to build up an
understanding.