v4l2 webcam program: cannot identify /dev/video

iochinome · 05-11-2009, 03:37 PM

ey guys, so i am trying to develop an application that uses webcameras (in jaunty), so i found this code here, which gives examples:

http://v4l2spec.bytesex.org/spec/capture-example.html

i compiled it on my computer (which has a built in webcam on top that i know works, tested it and it worked in luvcview) and then tried to run it. i get an error message that says:
Cannot identify '/dev/video': 2, No such file or directory

Cannot identify '/dev/video': 2, No such file or directory

i have located the part in the program where this comes from:

static void
open_device (void)
{
struct stat st;

if (-1 == stat (dev_name, &st)) {
/*dev_name is "/dev/video" btw, defined in main*/
fprintf (stderr, "Cannot identify '%s': %d, %s\n",
dev_name, errno, strerror (errno));
exit (EXIT_FAILURE);
}

so what is going on here? i do not have a ton of experience with linux, if you cant tell. thanks, much appreciated!

osor · 05-11-2009, 08:40 PM

The problem is that the test program hardcodes the file name into the code. Most likely you have no device node named /dev/video. It might be named /dev/video0 or /dev/bttv.

In the function main() on the first line, this string is hardcoded. You could just as well hardcode the correct name, create functionality to read the name as a command-line argument, or create a symlink /dev/video which points to the real device node.

Edit: it seems the second is already implemented. Just use the command-line switch “-d” followed by the real device name.

iochinome · 05-13-2009, 11:06 AM

oh, okay, i see- that worked! thanks!
so now my next step is taking whatever the camera 'reads' and displaying that on the screen. first off, how do i get a jpeg/other image type out of this program? i have never really done any work with images...

thanks, much appreciated!

osor · 05-15-2009, 05:00 PM

Quote:

Originally Posted by iochinome

first off, how do i get a jpeg/other image type out of this program? i have never really done any work with images...

The program you posted requests the device to go into this format state:

Code:

        fmt.type                = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        fmt.fmt.pix.width       = 640; 
        fmt.fmt.pix.height      = 480;
        fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
        fmt.fmt.pix.field       = V4L2_FIELD_INTERLACED;

If you device supports, you might be able to use V4L2_PIX_FMT_JPEG instead. More likely, you will have to use other libraries (e.g., libav and/or libjpeg) to decode the output into whatever format you want.