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Adobe After Effects CS5 : The Timeline - Motion Blur

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6/6/2012 5:03:32 PM
Motion blur is clearly essential to a realistic shot with a good amount of motion. It is the natural result of movement that occurs while a camera shutter is open, causing objects in the image to be recorded at every point from the shutter opening to closing. The movement can be from individual objects or the camera itself. Although it essentially smears layers in a composition, motion blur is generally desirable; it adds to persistence of vision and relaxes the eye. Aesthetically, it can be quite beautiful.

Close-up: Blurred Vision

Motion blur occurs in your natural vision, although you might not realize it—stare at a ceiling fan in motion, and then try following an individual blade around instead and you will notice a dramatic difference. There is a trend in recent years to use extremely high-speed electronic shutters, which drastically reduce motion blur. It gives the psychological effect of excitement or adrenaline by making your eye feel as if it’s tracking motion with heightened awareness.

The idea with motion blur in a realistic visual effects shot is usually to match the amount of blur in the source shot, assuming you have a reference; if you lack visual reference, a camera report can also help you set this correctly. Any moving picture camera has a shutter speed setting that determines the amount of motion blur. This is not the camera’s frame rate, although the shutter does obviously have to be fast enough to accommodate the frame rate. A typical film camera shooting 24 fps (frames per second) has a shutter that is open half the time, or 1/48 of a second.

Decoding After Effects Motion Blur

The Advanced tab of Composition Settings (Ctrl+K/Cmd+K) contains Motion Blur settings (Figure 1):

  • Shutter Angle controls shutter speed, and thus the amount of blur.

  • Shutter Phase determines at what point the shutter opens.

  • Samples Per Frame applies to 3D motion blur and Shape layers; it sets the number of slices in time (samples), and thus, smoothness.

  • Adaptive Sample Limit applies only to 2D motion blur, which automatically uses as many samples as are needed up to this limit (Figure 2).

Figure 1. These are the default settings; 16 is really too low for good-looking blur at high speed, but a 180-degree shutter and –90 degree shutter angle match the look of a film camera. Any changes you make here stick and are passed along to the next composition, or even the next project, until you change them.

Figure 2. The low default 16 Samples Per Frame setting creates steppy-looking blur on a 3D layer only; the same animation and default settings in 2D use the higher default Adaptive Sample Limit of 128. The reason for the difference is simply performance; 3D blur is costlier, but like many settings it is conservative. Unless your machine is ancient, boost the number; the boosted setting will stay as a preference.

Here’s a bit of a gotcha: The default settings that you see in this panel are simply whatever was set the last time it was adjusted (unless it was never adjusted, in which case there are defaults). It’s theoretically great to reuse settings that work across several projects, but I’ve seen artists faked out by vestigial extreme settings like 2 Samples Per Frame or a 720-degree blur that may have matched perfectly in some unique case.

Shutter Angle refers to an angled mechanical shutter used in older film cameras; it is a hemisphere of a given angle that rotates on each frame. The angle corresponds to the radius of the open section—the wedge of the whole pie that exposes the frame (Figure 3). A typical film shutter is 180 degrees—open half the time, or 1/48 of a second at 24 frames per second.

Figure 3. The 180-degree mechanical shutter of a film camera prevents light from exposing film half the time, for an effective exposure of 1/48 of a second. In this abstraction the dark gray hemi-circular shutter spins to alternately expose and occlude the aperture, the circular opening in the light gray plate behind it.

Electronic shutters are variable but refer to shutter angle as a benchmark; they can operate down in the single digits or close to a full (mechanically impossible) 360 degrees. After Effects motion blur goes to 720 degrees simply because sometimes mathematical accuracy is not the name of the game, and you want more than 360 degrees.

If you don’t know how the shutter angle was set when the plate was shot, you can typically nail it by zooming in and matching background and foreground elements by eye (Figure 4). If your camera report includes shutter speed, you can calculate the Shutter Angle setting using the following formula:

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

Figure 4. The white solid tracked to the side of the streetcar has been eye-matched to have an equivalent blur by adjusting Shutter Angle; care is also taken to set Shutter Phase to –50% of Shutter Angle so that the layer stays centered on the track.

This isn’t as gnarly as it looks, but if you dislike formulas, think of it like this: If your camera takes 24 fps, but Shutter Angle is set at 180 degrees, then the frame is exposed half the time (180/360 = ½) or 1/48 of a second. However, if the shutter speed is 1/96 per second with this frame rate, Shutter Angle should be set to 90 degrees. A per second shutter would have a 9-degree shutter angle in order to obey this rule of thumb.

Shutter Phase determines how the shutter opens relative to the frame, which covers a given fraction of a second beginning at a given point in time. If the shutter is set to 0, it opens at that point in time, and the blur appears to extend forward through the frame, which makes it appear offset.

The default –90 Shutter Phase setting (with a 180-degree shutter angle) causes half the blur to occur before the frame so that blur extends in both directions from the current position. This is how blur appears when taken with a camera, so a setting that is –50% of shutter angle is essential when you’re adding motion blur to a motion-tracked shot. Otherwise, the track itself appears offset when motion blur is enabled.

Enhancement Easier Than Elimination

Although software may one day be developed to resolve a blurred image back to sharp detail, it is much, much harder to sharpen a blurred image elegantly than it is to add blur to a sharp image. Motion blur comes for free when you keyframe motion in After Effects; what about when there is motion but no blur and no keyframes, as can be the case in pre-existing footage?

If you have imported a 3D element with insufficient blur, or footage shot with too high a shutter speed, you have the options to add the effect of motion blur using

  • Directional Blur, which can mimic the blur of layers moving in some uniform X and Y direction

  • Radial Blur, which can mimic motion in Z depth (or spin)

  • Timewarp, which can add motion blur without any retiming whatsoever

Yes, you read that last one correctly.

  • set Speed to 100

  • toggle Enable Motion Blur

  • set Shutter Control to Manual

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