Motion Blur

Motion blur plays into many of the more advanced illusions you can create with After Effects, but it is fundamentally quite simple. If a layer animates at a sufficiently high velocity, and the Motion Blur toggle is enabled, for it and for the composition containing it, then for free you get this great illusion that the elements blur to match the apparent motion of the scene.

Motion blur is an artifact of motion occurring while a camera shutter is open. It is not something that you encounter with your naked eye, yet it looks very natural when you see it on film or video. It is a complement to your eye's persistence of vision that helps your eye to accept the low frame rate of film (or video). It also looks quite pretty even in still photos, which is part of the reason that efforts to eliminate motion blur from a sampled image have tended to fall flat.

The thing that some users don't realize is that After Effects gives you control over how long and when the camera shutter is open via the Composition Settings dialog's Advanced tab (Figure 2.28). Or, more typically, users encounter these settings without really understanding them.

Figure 2.28. The Shutter Angle and Shutter Phase settings on the Advanced tab of the Composition Settings dialog control the appearance of motion blur.

The Shutter Angle setting controls how long the shutter is open; a higher number means more blur. The Shutter Phase controls when, during a given frame, the shutter opens; at the default of 0, the shutter opens at the moment in time when the frame begins. See the section "What the Settings Mean" for an explanation of why you'd ever want it any other way.

What the Settings Mean

A physical film camera typically has an angled shutter that can open in a circular fashion anywhere between a few degrees and a full 360 degrees (an electronic shutter does not behave the same, but never mind about that in this context). Theoretically, 360 degrees would provide the maximum amount of blur a shot could have.

The Virtual Camera Shutter

With a real, physical camera, there is a good reason for how long the shutter is open: Along with the aperture setting, it controls how much light is passed on to the film or video pickup. Low-lit scenes tend to be blurrier as a result of the shutter remaining open longer to gather more light. Other scenes may deliberately be shot with a slow shutter (and the aperture closed down to compensate) just to get some nice, streaky blur.

After Effects is unconcerned with light gathering and does not emulate the differing amount of light that would pass through a lens with a high or low shutter angle setting (nor when adjusting the Aperture setting on a 3D camera, discussed in Chapter 9, "Virtual Cinematography"). Therefore, you only get the desirable partthe blur itself.

After Effects, however, cheats by offering you double the blur overhead of a fully open shutter. The maximum setting for Shutter Angle in the Composition Settings dialog is 720, an impossibility in the real world (because a fully open shutter only has 360 degrees, or a full circle, to reveal).

A camera report may help you adjust this setting, although it's often good enough to carefully eyeball it by zooming in on an area where your background and foreground element both have blur and matching them (Figures 2.29a and b). If you do have a camera report and it gives you the shutter speed, you can calculate the Shutter Angle setting with the following formula:

shutter speed = 1 / frame rate * (360 / shutter angle)

Figure 2.29a and b. In 2.29a, the default Shutter Angle setting of 180 degrees is too heavy for this white solid masked over the front hubcap, which has been tracked to match the shot motion. In 2.29b, Shutter Angle was cut down by 50% to 90 degrees. In this case, there was no camera report to guide the decisions; they are based on blur that can be observed on the rest of the moving truck.

This isn't as gnarly as it looks, but if you hate looking at formulas, try thinking about the relationship this way: If your camera shoots at 24 frames per second, but Shutter Angle is set at 180 degrees, then the camera is exposing half the total time of each frame (180/360 = ½), or 1/48 of a second. If your camera report tells you that the exposure was 1/96 of a second, the Shutter Angle setting should be 90 degrees.

When Blur Is Needed Where None Exists

Motion blur is cool and automatic, if you animated the layer to receive the blur in After Effects. However, if the element comes in already animated but without sufficient bluras can happen with 3D elements or when a scene is retimed to be fasteryou need a different solution, because After Effects has no motion to sample to generate blur.

The painful way to deal with this is to apply directional blur to the elements in question. The problem is that that natural motion blur is rarely unidirectional.

In such cases, and when there's no possibility of reshooting or re-rendering, it's worth considering an investment in Real Smart Motion Blur by RE: Vision Effects (www.revisionfx.com). This plug-in can sample motion from other layers and enhance the amount of blur created by that motion. It's render intensive and can require some tweaking, but it can be just the thing for such situations. A demo version is available on the book's CD-ROM.

The Shutter Phase setting determines when the shutter opens; in other words, the blur doesn't have to start at the beginning of the frame, the shutter could open between frames. Here's the trick: the Shutter Phase is meaningful as a fraction of the Shutter Angle.

In other words, a setting of 0 means the motion blur starts at the beginning of the given current frame. A 90 setting with a 180 Shutter Angle equates the blur starting 0.5 frames before the frame or in between this frame and the previous one. This setting is often used with motion tracking, where you are matching the middle of a motion-blurred target (not one end of it).

Consider a Shutter Phase setting of 50% of the Shutter Angle when working with motion-tracked plate (background) footage; otherwise, leave it at 0. The amount of blur won't change, only the timing of it.

Ricochet

One place a 50% Shutter Phase setting is used is when tracking in an object that ricochets, such as a bouncing ball, so that you can see the motion in both directions at the frame of impact. The thing is that this effect doesn't work by default in After Effects 6.5, because it averages the surround frames (Figure 2.30a). You have to trick it into making the V-shape of the ricochet by adding a very slight rotation to the ricochet layer. This forces After Effects into sampling all of the frames necessary to create the ricochet (Figure 2.30b).

Figure 2.30a and b. These two images should look the same, but you have to force After Effects into getting the result in 2.30b by adding a slight bit of rotation. The other key is setting the Shutter Phase to 50% of the Shutter Angle, which causes the motion blur to be sampled right at the middle of the motion.