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>Video Capture Devices</H1
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>Video Capture Device Types</H1
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> The video capture devices share the same interfaces as radio devices. In
order to explain the video capture interface I will use the example of a
camera that has no tuners or audio input. This keeps the example relatively
clean. To get both combine the two driver examples.
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> Video capture devices divide into four categories. A little technology
backgrounder. Full motion video even at television resolution (which is
actually fairly low) is pretty resource-intensive. You are continually
passing megabytes of data every second from the capture card to the display.
several alternative approaches have emerged because copying this through the
processor and the user program is a particularly bad idea .
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> The first is to add the television image onto the video output directly.
This is also how some 3D cards work. These basic cards can generally drop the
video into any chosen rectangle of the display. Cards like this, which
include most mpeg1 cards that used the feature connector, aren't very
friendly in a windowing environment. They don't understand windows or
clipping. The video window is always on the top of the display.
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> Chroma keying is a technique used by cards to get around this. It is an old
television mixing trick where you mark all the areas you wish to replace
with a single clear colour that isn't used in the image - TV people use an
incredibly bright blue while computing people often use a particularly
virulent purple. Bright blue occurs on the desktop. Anyone with virulent
purple windows has another problem besides their TV overlay.
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> The third approach is to copy the data from the capture card to the video
card, but to do it directly across the PCI bus. This relieves the processor
from doing the work but does require some smartness on the part of the video
capture chip, as well as a suitable video card. Programming this kind of
card and more so debugging it can be extremely tricky. There are some quite
complicated interactions with the display and you may also have to cope with
various chipset bugs that show up when PCI cards start talking to each
other.
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> To keep our example fairly simple we will assume a card that supports
overlaying a flat rectangular image onto the frame buffer output, and which
can also capture stuff into processor memory.
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