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Chapter 8. Natural Video Coding > Coding of Arbitrarily Shaped Video Objects

8.3. Coding of Arbitrarily Shaped Video Objects

As described in Section 8.1.2, each VOP of an arbitrarily shaped video object is represented by its YUV components and, in case of nonrectangular VOPs, by a shape component. In MPEG-4, the shape information is represented by means of alpha masks.

An alpha mask defines the level of transparency of a VOP. In a gray-level alpha mask, transparency is represented by an 8-bit integer, resulting in a range of possible values between 0 and 255. The 0 corresponds to completely transparent pixels and the 255 to completely opaque pixels; in-between values correspond to pixels that are more or less transparent but neither completely transparent nor opaque. In a binary alpha mask, only two values are allowed: completely transparent (0) and completely opaque (255). A video object is only defined for pels whose alpha value (A) is larger than 0—that is, for those pels of the VOP that are not completely transparent.


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