ProVideo Coalition.com: Creating Motion Graphics: Keyframes by Chris & Trish Meyer

Here’s another one of our favorite tests to show the differences between gamma-corrected and linear light compositing:

In this case, you should note differences in the way the image blows out over time: The width of the bands in the transition area from somewhat normal color to blown-out color are broader when performed in linear space, looking subjectively more subtle or pleasing. Darker details such as the eyes and shadows in the creases of the face should persist longer in linear space, making the face recognizable longer during the blow-out (see the figures below). You will also find that you need to crank up the light much brighter - perhaps to 5000%, instead of 1000% - to completely blow out the image.

Overexposing an image with normal computer gamma causes it to blow out more or less evenly (left); applying the same illumination with linear light gamma creates a more subtle, pleasing effect (right). Original image courtesy Digital Vision.

These tests provide you a with blueprint of how to perform linear light compositing: Correct the sources to linear by applying a gamma of 0.4545, perform your treatments, and correct the result back to a 2.2 gamma. However, you cannot blindly apply this to any project. For one, some blending modes (such as Overlay, Soft Light, Hard Light, and to a degree Screen) will no longer work “properly” as they are based around the idea that a 50% pixel luminance value means 50% perceived brightness. You will also find that you will need to adjust or increase parameters such as light level, cone feather, and glow amount to achieve the same general results as you saw in the computer’s default space.

Nonlinear Thinking

This is all well and good if your goal is to mimic nature and reality...but that’s often not the goal of a motion graphics designer. For example, what if you want to favor the dark areas of an image instead of the light areas? Or what if you want a less (or more) severe effect? The simple answer is to experiment with gamma corrections other than 2.2. Here is one way to set up such a project with maximum flexibility in After Effects:

Then perform any treatments, effects, or compositing you like! For starters, try the simple crossfade you created in the first example. In this example, place the time marker in the middle of the transition, and then vary the RGB Gamma parameter for the Master Gamma layer. Note how the composite image changes, favoring light or dark areas depending if the gamma is above or below 1.0 (see below).

A normal computer crossfade (left), a “linear light” crossfade (center), and a creative fade using the opposite gamma as linear light (right).

Hopefully this will open up your mind to the potential in gamma-corrected compositing. It’s not something you need to employ on every job, but it’s an important tool to be familiar with, for both realistic and creative applications.

Postscript

As mentioned earlier, After Effects 7 and later now implement linear gamma as option in File > Project Settings, which eliminates the need for all of these extra layers, effects and expressions. The one downside is that you can’t set the gamma correction yourself (the last trick shown above); it is currently hardwired to the perceptually-correct 2.2 value.

It can be deceptive to set up 3D lighting in a scene (left) and then enable Linearize Working Space: The result will appear blown out (right) as the lighting will appear to be boosted.

In After Effects 7, there is only one choice: Linear Blending. It has the same effect as the Blend Colors Using 1.0 Gamma description above. We were greatly disappointed that we couldn’t achieve the lighting effect demonstrated earlier using this option in 7, but we now can using Linear Working Space in CS3.

Change these setting before you get too far into a project, as they will change how your composites look (observe the images above); you don’t want to select one of these modes just before blindly starting an overnight render!