About framebuffer and overlay

Hi all,
    I've just got in to develop framebuffer programs, and I've read
though the Framebuffer-HOWTO, but it doesn't seem to help a lot.
    I wanna know if framebuffer is the abstraction of the graphic
device, what about the overlay? What's the relationship between
framebuffer and overlay?
   I've already enable frambuffer on my laptop, and the color-depth is
32-bit, which is BGRA. But is it possible to display YUV image on the
framebuffer directly without changing it to BGR?

  Thanks a lot!

-- 
Wu Yu

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