[FFmpeg-devel] [PATCH] Set frame rate on v4l2 devices

[Måns Rullgård]

jose.goncalves at inov.pt writes:

> Using ffmpeg to stream a webcam MJPEG feed, I found that it can't set
> the webcam frame rate. For that we must use the VIDIOC_S_PARM
> ioctl. Please check the attached patch that fixes this.
>
> Jos? Gon?alves
>
> Index: libavdevice/v4l2.c
> ===================================================================
> --- libavdevice/v4l2.c	(revision 25082)
> +++ libavdevice/v4l2.c	(working copy)
> @@ -501,6 +501,8 @@
>      struct video_data *s = s1->priv_data;
>      struct v4l2_input input;
>      struct v4l2_standard standard;
> +    struct v4l2_streamparm streamparm;
> +    struct v4l2_fract *tpf;

Those can go inside the if() block.

>      int i;
>  
>      if(ap->channel>=0) {
> @@ -548,6 +550,33 @@
>          }
>      }
>  
> +    if(ap->time_base.num && ap->time_base.den) {
> +        av_log(s1, AV_LOG_DEBUG, "The V4L2 driver set time per frame: %d/%d\n",
> +               ap->time_base.num, ap->time_base.den);
> +        /* set frame rate */

Useless comment.  The log line above it says the same thing, and it's
pretty clear anyway.

> +        memset (&streamparm, 0, sizeof (streamparm));

Initialise the struct with = { 0 } instead.

> +        streamparm.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
> +        tpf = &streamparm.parm.capture.timeperframe;

Initialise tpf in the declaration.

> +        tpf->numerator = ap->time_base.num;
> +        tpf->denominator = ap->time_base.den;
> +        if (ioctl(s->fd, VIDIOC_S_PARM, &streamparm) < 0) {

ioctl() returns exactly -1 on error.

> +            av_log(s1, AV_LOG_ERROR,
> +                   "The V4L2 driver ioctl set time per frame(%d/%d) failed\n",
> +                   ap->time_base.num, ap->time_base.den);
> +            return AVERROR(EIO);

I'm not certain EIO is the best error code here.

> +        }
> +
> +        if (ap->time_base.den != tpf->denominator ||
> +            ap->time_base.num != tpf->numerator) {

Can the ioctl succeed even if a different frame rate was set?  Weird
interface...

> +            av_log(s1, AV_LOG_INFO,
> +                   "The V4L2 driver changed the time per frame from %d/%d to %d/%d\n",
> +                   ap->time_base.num, ap->time_base.den,
> +                   tpf->numerator, tpf->denominator);
> +            ap->time_base.num = tpf->numerator;
> +            ap->time_base.den = tpf->denominator;
> +        }
> +    }

There is no need to mention v4l2 in the messages as they will be
automatically tagged.

-- 
M?ns Rullg?rd
mans at mansr.com