the usual case, the foreground image is
black in those areas, and in that case the areas are fully transparent
-- the background shows through the icon.  But in general, a not
opaque pixel signifies that the background and foreground should be
merged as follows: The intensities of the color components in the
forgeground and background are represented as binary numbers, then
corresponding bits of the background and foreground intensities are
exlusive-or'ed together.  So there is a sort of reverse video effec