(forwarded to users list only)
Hi Craig,
just as a clarification to the thread on the dev-list.
The general explanation is as follows.
A layer, if you want to display it, must at least have one buffer in
video memory.
FRONTONLY has 1 buffer in video memory.
BACKVIDEO has 2 buffers in video memory.
BACKSYSTEM has 1 buffer in video and 1 buffer in system memory.
a buffer is an area of memory as big as the layer.
"video memory" and "system memory" are concepts which are not fixed in
their implementation; in general, "video memory" relates to the RAM on
your video card, and "system memory" to the DRAM on your motherboard.
However, for e.g. unified memory architectures (such as are often seen
in embedded systems) there is no need for a difference, although some
implementations might do it anyway, e.g. for memory bookkeeping.
If you have a unified memory architecture, there is no performance
distinction between BACKVIDEO and BACKSYSTEM. If you have to perform a
lot of software operations (such as software blending) before calling
Flip(), BACKSYSTEM might be best, while if you normally perform
accelerated operations BACKVIDEO will be the better choice.
The explanation that you give below, is the way that the dfb-system
implements the above rules.
hth,
Niels
Craig Matsuura wrote:
I want to get some clarification on the BACKSYSTEM, BACKVIDEO and
FRONTONLY.
Here is my understanding:
BACKSYSTEM uses a virtual framebuffer of the screen (Some place in
RAM) and DirectFB draws on this screen (surface), once a Flip is
called the virtual framebuffer is copied to the Real/Visible framebuffer.
BACKVIDEO use the off screen video memory and draws in the screen
(surface) that is off screen. When a Flip is called the framebuffer
starting position is changed to the off screen position to make the
screen visible. The next frame is draw on the off screen and the
process is repeated.
FRONTONLY, draw directly to the visible framebuffer.
Thanks,
Craig
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Control4
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Salt Lake City, UT 84020-6432
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