CVSROOT: /cvs/directfb
Module name: DirectFB
Changes by: dok 20050330 14:39:16
Modified files:
wm/default : default.c
Log message:
Handle (non)premultiplied window and layer surfaces.
Always use compliant Porter/Duff SRC_OVER.
Different compositing methods are used depending on the destination format.
DESTINATION WITH ALPHA
/*
Cd = destination color (non-premultiplied)
Ad = destination alpha
Cs = source color (non-premultiplied)
As = source alpha
Ac = color alpha
cd = Cd * Ad (premultiply destination)
cs = Cs * As (premultiply source)
The full equation to calculate resulting color and alpha (premultiplied):
cx = cd * (1-As*Ac) + cs * Ac
ax = Ad * (1-As*Ac) + As * Ac
*/
dfb_state_set_src_blend( state, DSBF_ONE );
/* Need to premultiply source? */
if (! (window->surface->caps & DSCAPS_PREMULTIPLIED))
. flags |= DSBLIT_SRC_PREMULTIPLY;
/* Need to premultiply/demultiply destination? */
if (! (state->destination->caps & DSCAPS_PREMULTIPLIED))
. flags |= DSBLIT_DST_PREMULTIPLY | DSBLIT_DEMULTIPLY;
DESTINATION WITHOUT ALPHA
/*
We can avoid DSBLIT_SRC_PREMULTIPLY for destinations without an alpha channel
by using another blending function, which is more likely that it's accelerated
than premultiplication at this point in time.
This way the resulting alpha (ax) doesn't comply with SRC_OVER,
but as the destination doesn't have an alpha channel it's no problem.
As the destination's alpha value is always 1.0 there's no need for
premultiplication. The resulting alpha value will also be 1.0 without
exceptions, therefore no need for demultiplication.
cx = Cd * (1-As*Ac) + Cs * As*Ac (still same effect as above)
ax = Ad * (1-As*Ac) + As * As*Ac (wrong, but discarded anyways)
*/
if (window->surface->caps & DSCAPS_PREMULTIPLIED)
. dfb_state_set_src_blend( state, DSBF_ONE );
else
. dfb_state_set_src_blend( state, DSBF_SRCALPHA );
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