The alpha channel is used only in B_RGBA32 bitmap (not in B_RGB32 where
the byte is unused). Depdending on the drawing mode, various things can
happen:
- In B_OP_ALPHA: the alpha channel is handled according to the blending
mode set with SetBlendingMode.
- In other modes: the alpha channel is handled with the same algorithm
as the color channels. For example, with B_OP_COPY, the alpha value
for each pixel is copied to the destination.
These are the same for Haiku and BeOS.
In Haiku, there are also some improvements:
- ClipToPicture now uses the alpha channel of the given picture, making
it possible to clip to a partially opaque picture, or clip to an
antialiased shape. In BeOS, any nonzero pixel is considered to be
fully opaque (IOW, the picture was rendered to an 1-bit bitmap for
masking). Additionally we have helper methods ClipToShape, ClipToRect,
ClipToInverseShape, ClipToInverseRect (accepting a BShape and a BRect
respectively), also with alpha clipping support. This also plays
nicely with transforms, so even a rectangle clipping can have
antialiased edges (because it was rotated or the edges are not on
pixel boundaries).
- A new feature in Haiku (introduced recently) is
BView::BeginLayer/EndLayer, which allows to perform a set of
operations, then put the end result onto the view while applying a
global alpha modifier to it.
--
Adrien.
