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5 Tips for Working with Masks in Apple Motion

Animating and manipulating image, text and vector masks By Dave Nagel
Apple Motion 1.0 has robust capabilities for creating, modifying and animating masks. But some of these capabilities can be a bit hard to find, and sometimes even confusing. So I've put together a collection of tips that should help you out in most situations--from simple adjustments you can use to create custom transitions to more complicated shape manipulations to using images and movie files as masks.

Tip 1: Mask properties and animation. If you create a mask on an object within a layer, it's properties are tied to those of the object itself, though the mask's properties (scale, shear, opacity and all of the other elements in the Properties tab in the Inspector palette) can be animated independently as well. If you move the object, the mask will move as well; if you shear the object, the mask will be sheared; essentially, all of the values of the basic properties of the mask are "relative" to the object being masked, and the settings are "cumulative" with the object's own settings. But, if you select the mask, rather than the object, and move that, it will move independent of the object it's masking. And that's how you can animate a mask without animating the properties of the object it masks.

For our project for this tip, we'll create a mask-based transition revealing a piece of footage using the Scale, Shear and Opacity properties. Click the Play button on the movie below to see an example.



Step 1. Select your image or footage in the Layers palette, and then select the Rectangle Mask tool (Option-R). Draw a mask on the Canvas that encompasses the entire viewable area of the mask. (Make sure you're on frame 1 when you create the mask.)



Step 2. In the Inspector palette, set keyframes for Scale X, Shear X and Opacity. Set Scale X to 0; Shear X to -90; and Opacity to 0.




Step 3. Move ahead to frame 10 in your timeline. Add another keyframe to each of those three parameters, and this time change the Opacity setting to 100. (Leave the other two parameters alone for now.)



Step 4. Now move ahead to frame 30. Add a keyframe to Scale X and Shear X. Set Scale X to 100 and Shear X to 0.



That's it.

Tip 2: Animating an image without moving its mask. But what if you want to do the opposite? What if you want to leave your mask in place while moving around the image that's being masked? For example, you might want to pan and zoom on a particular element in an image while leaving the area of focus motionless. The problem is that when you create a mask on an object, you can't do anything to the object without doing it to the mask as well, as we talked about above.

If you're used to Adobe After Effects, you know how this problem is solved. You set keyframes for both the mask and the object, then use the Pan Behind tool to move around the image while leaving the mask stationary. In Motion, it's a bit more simple.

For this project, we'll create a pan and zoom effect using two masks, though it can certainly be done with just one. The first mask will be there to reduce the opacity of the object in question (an image), while the second will define our area of focus. Click the Play button on the movie below to see this project in action.



Step 1. With your image or footage imported into your project, select the Layer in the Layers palette, rather than the footage itself.



Step 2. Then draw the first mask so that it encompasses the entire layer. Notice that the mask appears above the object in the layer rather than indented below it. This means that it's above the image object in the layer stacking order so that when you move the image, the mask won't be affected.



Step 3. Set the properties of this first mask. All we'll do here is select the mask, then change its Opacity to 25 percent in the Inspector palette. We can forget about this mask now. It won't be animated.



Step 4. Now select the Layer again in the Layers palette, and this time switch tot he Circle Mask tool (Option-C). Draw a circular mask on the center of your composition.



If you wish, you can also feather the mask a bit in the Dashboard to give it a softer edge. In the example below, I've feathered mine by 25 pixels.



Step 5. Select your image in the Layers palette, and turn on interactive animation by clicking the Record button below the Canvas window. Move ahead about 30 frames, and drag your image around a little to move the intended area of interest into the "spotlight" we've created with our mask. (If you wish, you can create something of a curve, rather than a straight line, in the motion of your image by moving it around to different positions prior to frame 30.)



Step 6. Then move ahead to frame 60 and zoom in on the object by dragging on a corner handle while holding down the Shift and Object keys. The Shift key constrains the scale, and the Option key forces the object to scale from the center, rather than from the edge of the object.



Because we didn't set a keyframe for the object's scale earlier in the timeline, it will scale up to this final value over the entire course of the animation. If you don't want it to do this, you can simply set a keyframe for the object's scale in the Properties palette back at frame 30.

Note: A mask you create on a layer in this manner will affect all objects on the layer. If you have a background image that you do not want to be affected by the mask, create a new layer in the Layers palette, and move the background footage to the new layer.

Tip 3: Text masks. In addition to the types of masks we've created so far, Motion will also let you use type as a mask. Actually, it will let you use pretty much anything as a mask, but we'll take a look at text as an example of a vector object that can be converted to two different types of masks--layer masks and object masks.

Step 1. With your footage in place, select the text tool and type whatever you want. I'll simply type "Space."



Step 2. We can quickly convert this into a layer mask--one that will affect all objects on the layer--by right-clicking on the text in the Layers palette (or Control-clicking), and switching the Blend Mode to "Stencil Alpha." (This can also be accomplished in the Properties tab in the Inspector palette, as seen below.)



And here's the result.



Step 3. But there are times when you don't want your text to be positioned as a layer mask. You might want to move it along with your object when you animate it; or you might have other elements on the layer that you don't want obscured by this mask. So now we'll convert this text into an object-specific mask. To do this, select the image in the Layers palette. Then choose Object > Add Image Mask (Shift-Command-M). an empty image mask beneath your object in the layer stacking order.



Step 4. Then simply drag your text element into the Image Mask slot. The text will become hidden (unchecked) in the Layers palette when you do this. This image will be masked off, and any other elements on the layer will become visible, as seen below.



These types of masks can be animated just as discussed in the first and second tips in this article.


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