video4linux

Sam Revitch samr7 at cs.washington.edu
Fri Feb 9 15:34:45 PST 2001


On Fri, 9 Feb 2001, Mike wrote:


> I have some *expletive deleted* questions about changes in the video4linux

> driver (or something) between kernel 2.4.0-test11 and 2.4.1. I bought a

> cheapo USB IBM cam ($20 at best buy) since there's a module which

> purportedly supports this cam (ibmcam.o) (verified by FCC identifier

> listed in kernel docs). I've got video4linux set up as a module

> (videodev.o). When I try to use the cam with 'xawtv' in 2.4.0-test11, the

> application starts (but the cam doesn't work; a problem to be worked out

> later). When I run 'xawtv' in 2.4.1 I get an error stating that

> /dev/video doesn't exist. But it does! It's a symlink to /dev/video0

> which has the correct major/minor numbers. I assume that this is a

> problem with v4l, but I don't know how to approach the problem. I'm new

> to working with video/framegrabbers and USB in linux, so I don't know what

> questions to be asking...


Mike,

I'm not sure exactly how v4l treats plug-in cameras, but I can share some
experience using it with bttv-based TV tuner cards.

The first thing is, if /dev/video doesn't exist, then the v4l subsystem
driver for the device you're using isn't loaded or configured. videodev.o
is sort of the video4linux device manager, and its major task is to
multiplex char-major-81. Device drivers plug in and get assigned the next
available /dev/video* subdevice. If you loaded the ibmcam.o module,
followed all the directions, etc., and it gives you this error, check the
system logs, v4l device drivers tend to leave many droppings.

While in theory it might work, I would not suggest using xawtv or any of
the other TV tuner apps for anything other than TV tuner cards. TV tuner
apps exclusively use mmap() for I/O, along with other tricks to get
digitized TV video directly to the screen, and video4linux doesn't require
all device drivers to support all tricks.

Does anybody know of a good frame grabber app for webcams? I know
there's at least one good v4l cgi-bin program for setting up a webcam, and
sane apparently has a v4l device driver, so you could in theory use XSane
or the Gimp for capturing frames. Also, take a look at the avifile
package, which uses Win32 video codecs and comes with a sample v4l video
capture and compression application.

Thanks,
-Sam Revitch



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