Hollywood supports two kinds of transparency for its graphics objects: Mask transparency and alpha transparency. This section will explain the difference between the two.
Mask transparency knows only two settings per pixel: Visible and invisible. Alpha transparency, however, supports 256 different levels of transparency for every pixel. An alpha level of 0 means that the pixel is not visible, and a level of 255 means that the pixel is fully visible. A level of 128 thus means that a pixel is only 50% visible. Alpha transparency is very useful if you want to embed images that shall smoothly adapt to your background. For example, a brush with a shadow where the background shines through, or a brush with antialiased edges. For such purposes a mask is not enough.
Please note also that mask and alpha channel are mutually exclusive. That is, a brush cannot have a mask and an alpha channel but only one of the two.