Geek asks and answers

Where does the countdown v4l2_buffer-> timestamp start?

I am trying to use the value of timestamp v4l2_buffer (type timeval) to synchronize images captured from a UVC webcam to external events.

However, the timestamp does not match the system time or timeout, etc .:

printf("image captured at %ld, %ld\n",
   buffer->timestamp.tv_sec, 
   buffer->timestamp.tv_usec);

struct timeval tv;
gettimeofday(&tv, 0);
printf("current time %ld, %ld\n", tv.tv_sec, tv.tv_usec);

Results in

image captured at 367746, 476270
current time 1335083395, 11225

My downtime is 10 days.

+3
source share
2 answers

http://comments.gmane.org/gmane.linux.drivers.video-input-infrastructure/39892 v4l2 ( UVC) ( ), , . Linux ( ) ( , ) , (.. ).

+5

OP, , .

#include <time.h>
#include <math.h>

//////////////////////
//setup: 

    long getEpochTimeShift(){
        struct timeval epochtime;
        struct timespec  vsTime;

        gettimeofday(&epochtime, NULL);
        clock_gettime(CLOCK_MONOTONIC, &vsTime);

        long uptime_ms = vsTime.tv_sec* 1000 + (long)  round( vsTime.tv_nsec/ 1000000.0);
        long epoch_ms =  epochtime.tv_sec * 1000  + (long) round( epochtime.tv_usec/1000.0);
        return epoch_ms - uptime_ms;
    }

    //stick this somewhere so that it runs once, on the startup of your capture process
    //  noting, if you hibernate a laptop, you might need to recalc this if you don't restart 
    //  the process after dehibernation
    long toEpochOffset_ms = getEpochTimeShift();


//////////////////////
//...somewhere in your capture loop: 

    struct v4l2_buffer buf;

    //make the v4l call to  xioctl(fd, VIDIOC_DQBUF, &buf)

    //then: 
    long temp_ms = 1000 * buf.timestamp.tv_sec + (long) round(  buf.timestamp.tv_usec / 1000.0);
    long epochTimeStamp_ms = temp_ms + toEpochOffset_ms ;

    printf( "the frame timestamp in epoch ms is: %ld", epochTimeStamp_ms);
+2

More articles:


All Articles