There are two main features of using alpha factor in logiBITBLT.
If color format is defined as ARGB: ARGB8, ARGB16 or ARGB24, than logiBITBLT will consider that pixels in source and destination memory have alpha channel. Any operation in logiBITBLT can modify alpha factor. In this way logiBITBLT controls pixel alpha blending factors. Anti-aliased 8-bit font expansion operation modifies alpha factor in order to achieve anti-aliasing effect. However, ARGB8 or ARGB3332 format is not supported in this operation.
Porter-Duff composing technique uses alpha factors from source bitmap, destination bitmap and global alpha register (GLB_ALPHA_REG) to calculate composition of source and destination bitmap using Porter-Duff formulas for color and alpfa factor separately. In this way an alpha blending effect can be achieved between source and destination bitmap without multiple video layer support. In order to use global alpha factor in Porter-Duff composition, generic parameter C_USE_GLOBAL_ALPHA must be set prior to implementation.