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Tuesday, November 04, 2003

Managing Moving Masks

Bringing some predictability and control to animating Mask Shapes in After Effects.

Might Makes Right

The title “Smart Mask Interpolation” might make you think this is an option to make masks in After Effects interpolate...well, more smartly. In truth, SMI does not change the way After Effects interpolates mask vertices. What it does is create a lot of additional mask vertices, as well as Mask Shape keyframes, to force After Effects to “interpolate” in the desired manner.

Before using SMI, make sure you have set up good First Vertex points for your shapes, and that the shape isn’t inverting upon itself during normal interpolation (which would mean the path direction of one needs to be reversed). It is a good idea to duplicate the layer at this point, so you always have your original to go back to if SMI doesn’t help. Turn off the Video switch for the original, and select the Mask Shape keyframes for your duplicate.

Open Window > Smart Mask Interpolation; you will see the panel shown at the top of this page. Click Apply. At its default settings, SMI will create new Mask Shape keyframes for every frame between your existing keyframes. If you move the Time Marker to any of these new keyframes, you will see your Mask Shapes also have a large number of new vertices (shown at left). This gives After Effects more “correspondence points” to preserve a good mask outline during interpolation.

In many cases, the default settings will work fine. If you feel there’s room for improvement, Undo back to your original Mask Shape keyframes, and get ready to dive into those optional settings.

The simplest setting is to increase the number of mask vertices created. To do this, make sure the Add Mask Shape Vertices option is enabled, and change the popup underneath to Total Vertices. We often use a value of 100 or 200 Total Vertices with SMI.

Tweaking the other parameters will take a bit of thought as to what you want your mask shape to do, as well as some trial and error. Here’s an overview of what they are and how to use them:

• Keyframe Rate controls how often new keyframes are created. More keyframes mean a more controlled interpolation; a lower rate creates a looser interpolation. If you’re field rendering, set Keyframe Rate to the frame rate used for rendering, and enable Keyframe Fields. Otherwise, you will get odd shape jumps at the start and end of interpolation. Regardless of this setting, SMI will also always create additional keyframes just before and after the ones you set, which it uses to establish its additional mask shape vertices.
• Use Linear Vertex Paths causes SMI to move all of the mask points in a straight line between its first and last keyframes - good for very geometric transformation; not so good for more organic interpolations. Disable this option to allow curved paths, including allowing shapes to appear to rotate. Be warned, however, that disabling it can cause wild swings in the mask shape.
• Bending Resistance decides how fluid the mask shape is during interpolation. Lower settings allow the shapes to bend more; higher settings maintain more rigid, geometric shapes. The default of 50% is a good compromise. Don’t go above 99%; setting it to 100% will occasionally yield odd results, regardless of how the other parameters are set.
• Matching Method controls the inner logic SMI follows. Leaving this set to Auto works most of the time; change it only if you’re having problems. Use the Curve setting for organic interpolations; Polyline for more rigid ones.
• First Vertices Match is crucial for having interpolations proceed in a predictable manner (especially after you’ve gone through the trouble to set them up properly in the first place) - keep this enabled.
• Quality and Use 1:1 Vertex Matches decide how the rest of the mask vertices interpolate. Usually, you want to let SMI decide how to move one vertex to another; that means disabling Use 1:1 Vertex Matches, and if necessary, increasing Quality.

Motion Graphics •

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