Re: V4l programming problem while grabbing frames in sequence...

[Justin Schoeman <justin@xxxxxxxxxxxxxxxxxxxx>]

Heavystation System Manager wrote:

> Hi all,
>
> I'm learning to use v4l module tring to write an application that captures
> more frames in sequence (like xawtv)... but, i noticed that the ioctl,
> with VIDIOCMCAPTURE as argument, needs too much time for its execution!
> bringing to an excessive time between two different grabbed frames. I
> don't know if it's a problem of my cheap webcam (a Trust SpaceC@M 300 with
> ov511+ chipset) or my early experience of v4l programming (i think my
> early experience, because this webcam works very well with xawtv)...
>
> The function grab() is inspired by a procedure explained in a previous
> mail sent from "Nemosoft Unv." with subject "RE:  sync: input/output
> error" that i took as example form this experiment...
>
> The function looks like this:
>
> int grab(int video_fd,int w,int h,int format,int nframes)
> {
>         int frmbuf,frames = 0;
>         struct video_mbuf buffer;
>         struct video_mmap map;
>         void * grabarea;
>
>         if( ioctl(video_fd,VIDIOCGMBUF,&buffer) == -1 ) return (-1);
>
>         grabarea = mmap(NULL,buffer.size,PROT_READ,MAP_SHARED,video_fd,0);
>
>         if( grabarea == MAP_FAILED ) return (-2);
>
>         map.frame       = 0;
>         map.width       = w;
>         map.height      = h;
>         map.format      = format;
>
>         // MCAPTURE(0)
>         if( ioctl(video_fd,VIDIOCMCAPTURE,&map) == -1 ) return (-4);
>
>         do
>         {
>                 // SYNC(0)
>                 frmbuf = 0;
>                 if( ioctl(video_fd,VIDIOCSYNC,&frmbuf) == -1 )
>                  return (-5);
>
>                 // MCAPTURE(1)
>                 map.frame       = 1;
>                 if( ioctl(video_fd,VIDIOCMCAPTURE,&map) == -1 )
>                  return (-4);
>
>                 // Do something with frame 0...
>                 // ...
>
>                 // SYNC(1)
>                 frmbuf = 1;
>                 if( ioctl(video_fd,VIDIOCSYNC,&frmbuf) == -1 )
>                  return (-5);
>
>                 // MCAPTURE(0)
>                 map.frame       = 0;
>                 if( ioctl(video_fd,VIDIOCMCAPTURE,&map) == -1 )
>                   return (-4);
>
>                 // Do something with frame 1...
>                 // ...
>
>                 ++frames;
>
>         }while( frames < nframes );
>
>         // SYNC(0)
>         frmbuf = 0;
>         if( ioctl(video_fd,VIDIOCSYNC,&frmbuf) == -1 ) return (-5);
>
>         munmap(grabarea,buffer.size);
>
>         return (0);
> }
>
> What's wrong???
>
> Thanks in advance!!


To get high throughput, you must queue them up as fast as possible:

int grab(int video_fd,int w,int h,int format,int nframes)
{
        int frmbuf,frames = 0;
        struct video_mbuf buffer;
        struct video_mmap map;
        void * grabarea;

        if( ioctl(video_fd,VIDIOCGMBUF,&buffer) == -1 ) return (-1);

        grabarea = mmap(NULL,buffer.size,PROT_READ,MAP_SHARED,video_fd,0);

        if( grabarea == MAP_FAILED ) return (-2);

        map.frame       = 0;
        map.width       = w;
        map.height      = h;
        map.format      = format;

        // MCAPTURE(0)
        if( ioctl(video_fd,VIDIOCMCAPTURE,&map) == -1 ) return (-4);
	// MCAPTURE(1)
	map.frame	= 1;
	if( ioctl(video_fd,VIDIOCMCAPTURE,&map) == -1 ) return (-4);

        do
        {
                // SYNC(0)
                frmbuf = 0;
                if( ioctl(video_fd,VIDIOCSYNC,&frmbuf) == -1 )
                 return (-5);

		// Do something with frame 0...
		// ...

                // MCAPTURE(0)
                map.frame       = 0;
                if( ioctl(video_fd,VIDIOCMCAPTURE,&map) == -1 )
                 return (-4);


                // SYNC(1)
                frmbuf = 1;
                if( ioctl(video_fd,VIDIOCSYNC,&frmbuf) == -1 )
                 return (-5);

		// Do something with frame 1...
		// ...

                // MCAPTURE(1)
                map.frame       = 1;
                if( ioctl(video_fd,VIDIOCMCAPTURE,&map) == -1 )
                  return (-4);

                ++frames;

        }while( frames < nframes );

        // SYNC(0)
        frmbuf = 0;
        if( ioctl(video_fd,VIDIOCSYNC,&frmbuf) == -1 ) return (-5);

        munmap(grabarea,buffer.size);

        return (0);
}

This way the buffers are queued as far in advance as possible, and you 
should have the lowest possible latency.

-justin