2

I'm trying to use V4L2 api to enumertate supported frame size and frame rate of webcam device. I try to do it with the following code without success. The ioctl function always returns -1.

#include <stdio.h>
#include <fcntl.h>
#include "linux/videodev2.h"
int main(int argc, char **argv) {
    int fd = open("/dev/video0", O_RDWR);
    struct v4l2_frmivalenum frmsize;
    memset(&frmsize, 0, sizeof (struct v4l2_frmsizeenum));
    ioctl(fd, VIDIOC_ENUM_FRAMEINTERVALS , &frmsize);
    printf("%u \n", frmsize.he);
    return 0;
}
Meti
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  • Does the `open()` return -1 as well? –  Feb 27 '13 at 15:02
  • No, it doesn't. Actually ioctl works with command VIDIOC_QUERYCAP. I guess maybe I use the API in the wrong way. Maybe I should set pixel format before using VIDIOC_ENUM_FRAMEINTERVALS. – Meti Feb 27 '13 at 22:51

2 Answers2

5

You should set the in parameter before ioctl VIDIOC_ENUM_FRAMESIZES to get frame sizes :

Table A.34. struct v4l2_frmivalenum
__u32   index               IN: Index of the given frame interval in the enumeration.
__u32   pixel_format        IN: Pixel format for which the frame intervals are enumerated.
__u32   width               IN: Frame width for which the frame intervals are enumerated.
__u32   height              IN: Frame height for which the frame intervals are enumerated.

and ioctl VIDIOC_ENUM_FRAMEINTERVALS to get frame intervals :

Table A.31. struct v4l2_frmsizeenum
__u32   index               IN: Index of the given frame size in the enumeration.
__u32   pixel_format        IN: Pixel format for which the frame sizes are enumerated.

Then an implementation that enumerates frame sizes and frame intervals could be :

#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include "linux/videodev2.h"
void printFrameInterval(int fd, unsigned int fmt, unsigned int width, unsigned int height)
{
    struct v4l2_frmivalenum frmival;
    memset(&frmival,0,sizeof(frmival));
    frmival.pixel_format = fmt;
    frmival.width = width;
    frmival.height = height;
    while (ioctl(fd, VIDIOC_ENUM_FRAMEINTERVALS, &frmival) == 0) 
    {
        if (frmival.type == V4L2_FRMIVAL_TYPE_DISCRETE) 
            printf("[%dx%d] %f fps\n", width, height, 1.0*frmival.discrete.denominator/frmival.discrete.numerator);
        else
            printf("[%dx%d] [%f,%f] fps\n", width, height, 1.0*frmival.stepwise.max.denominator/frmival.stepwise.max.numerator, 1.0*frmival.stepwise.min.denominator/frmival.stepwise.min.numerator);
        frmival.index++;    
    }
}

int main(int argc, char **argv) {
    unsigned int width=0, height=0;;
    int fd = open("/dev/video0", O_RDWR);
    struct v4l2_frmsizeenum frmsize;
    memset(&frmsize,0,sizeof(frmsize));
    frmsize.pixel_format = V4L2_PIX_FMT_JPEG;
    while (ioctl(fd, VIDIOC_ENUM_FRAMESIZES, &frmsize) == 0)
    {
        if (frmsize.type == V4L2_FRMSIZE_TYPE_DISCRETE) 
        {
            printFrameInterval(fd, frmsize.pixel_format, frmsize.discrete.width, frmsize.discrete.height);
        }
        else
        {
            for (width=frmsize.stepwise.min_width; width< frmsize.stepwise.max_width; width+=frmsize.stepwise.step_width)
                for (height=frmsize.stepwise.min_height; height< frmsize.stepwise.max_height; height+=frmsize.stepwise.step_height)
                    printFrameInterval(fd, frmsize.pixel_format, width, height);

        }
        frmsize.index++;
    }
    return 0;
}
mpromonet
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1

I think the driver you used doesn't implement vidioc_enum_frameintervals.

Note that every video driver registers v4l2_ioctl_ops.

For example, in kernel 3.7, drivers/media/usb/s2255/s2255drv.c implements it. drivers/staging/media/go7007/go7007-v4l2.c also implements it.

However, the virtual video driver, drivers/media/platform/vivi.c, doesn't implement it.

Undo
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hazem
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