Apple Motion 2 - Manual (Page 866)

866 Chapter 12 Using Shapes and Masks
The power of image masks is that they do not have to be drawn or animated. Instead,
you can use virtually any image or movie clip to create transparency in another object.
By default, movie clips create animated image masks, but you can also set an image
mask to use only a single frame.
Image masks can also be used to assign masks that were created in other applications.
For example, you can import an animated mask that was created in Shake and
exported as a QuickTime movie into your Motion project, and use it as an image mask.
When you use an object as an image mask, you can choose which of the object’s
channels to use to create transparency from the Source Channel pop-up menu in the
Image Mask tab of the Inspector. The choices include:
• Red
• Green
• Blue
• Alpha
• Luminance
Since alpha channels are basically 8-bit grayscale images, you can use any single color
channel as an image mask. You can also use another object’s alpha channel. Luminance
allows you to use the aggregate luminance from the red, green, and blue channels of
an image to create transparency. For all these options, color is ignored.
Assigning an image mask is a two-part process. First, you create a blank image mask
underneath an object. Then you assign the image you want to use to create
transparency.
To add an image mask to an object:
1 Select an object.
2 Choose Object > Add Image Mask (or press Shift+Command+M).
The image mask appears underneath the object in the Layers tab and Timeline.
3 Select the image mask in the Layers tab or Timeline.
01112.book Page 866 Sunday, March 13, 2005 10:36 PM