我在Linux上使用OpenCv 2.4.8，并获取USB网络摄像头（logitech C200）捕获以进行进一步处理。我已经确定捕获使用cap_libv4.cpp（存在于modlules / highgui中）来捕获图像。我不得不修改所述文件以添加对时间戳的支持。
我已尝试使用多个网络摄像头，并注意到如果结构v4l2_buffer中的flags字段为0，它只给出正确的时间戳。如果它等于2，对应于V4L2_BUF_FLAG_TIMESTAMP MONOTONIC，则它给出0.我是想知道我的代码修改或libv4l驱动程序是否有问题。
函数read_frame_v4l2读取缓冲区，并更新时间戳。可以使用icvGetPropertyCAM_V4L函数读取时间戳。这两个函数的修改代码如下：
icvGetPropertyCAM_V4L：
static double icvGetPropertyCAM_V4L (CvCaptureCAM_V4L* capture,
int property_id ) {
char name[32];
int is_v4l2_device = 0;
/* initialize the control structure */
switch (property_id) {
case CV_CAP_PROP_FRAME_WIDTH:
case CV_CAP_PROP_FRAME_HEIGHT:
CLEAR (capture->form);
capture->form.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == xioctl (capture->deviceHandle, VIDIOC_G_FMT, &capture->form)) {
/* display an error message, and return an error code */
perror ("VIDIOC_G_FMT");
if (v4l1_ioctl (capture->deviceHandle, VIDIOCGWIN, &capture->captureWindow) < 0) {
fprintf (stderr, "HIGHGUI ERROR: V4L: Unable to determine size of incoming image\n");
icvCloseCAM_V4L(capture);
return -1;
} else {
int retval = (property_id == CV_CAP_PROP_FRAME_WIDTH)?capture->captureWindow.width:capture->captureWindow.height;
return retval / 0xFFFF;
}
}
return (property_id == CV_CAP_PROP_FRAME_WIDTH)?capture->form.fmt.pix.width:capture->form.fmt.pix.height;
case CV_CAP_PROP_BRIGHTNESS:
sprintf(name, "Brightness");
capture->control.id = V4L2_CID_BRIGHTNESS;
break;
case CV_CAP_PROP_CONTRAST:
sprintf(name, "Contrast");
capture->control.id = V4L2_CID_CONTRAST;
break;
case CV_CAP_PROP_SATURATION:
sprintf(name, "Saturation");
capture->control.id = V4L2_CID_SATURATION;
break;
case CV_CAP_PROP_HUE:
sprintf(name, "Hue");
capture->control.id = V4L2_CID_HUE;
break;
case CV_CAP_PROP_GAIN:
sprintf(name, "Gain");
capture->control.id = V4L2_CID_GAIN;
break;
case CV_CAP_PROP_EXPOSURE:
sprintf(name, "Exposure");
capture->control.id = V4L2_CID_EXPOSURE;
break;
case CV_CAP_PROP_POS_MSEC:
if (capture->FirstCapture) {
return 0;
} else {
return 1000 * capture->timestamp.tv_sec + ((double) capture->timestamp.tv_usec) / 1000;
}
break;
default:
sprintf(name, "<unknown property string>");
capture->control.id = property_id;
}
if(v4l2_ioctl(capture->deviceHandle, VIDIOC_G_CTRL, &capture->control) == 0) {
/* all went well */
is_v4l2_device = 1;
} else {
fprintf(stderr, "HIGHGUI ERROR: V4L2: Unable to get property %s(%u) - %s\n", name, capture->control.id, strerror(errno));
}
if (is_v4l2_device == 1) {
/* get the min/max values */
int v4l2_min = v4l2_get_ctrl_min(capture, capture->control.id);
int v4l2_max = v4l2_get_ctrl_max(capture, capture->control.id);
if ((v4l2_min == -1) && (v4l2_max == -1)) {
fprintf(stderr, "HIGHGUI ERROR: V4L2: Property %s(%u) not supported by device\n", name, property_id);
return -1;
}
/* all was OK, so convert to 0.0 - 1.0 range, and return the value */
return ((float)capture->control.value - v4l2_min) / (v4l2_max - v4l2_min);
} else {
/* TODO: review this section */
int retval = -1;
switch (property_id) {
case CV_CAP_PROP_BRIGHTNESS:
retval = capture->imageProperties.brightness;
break;
case CV_CAP_PROP_CONTRAST:
retval = capture->imageProperties.contrast;
break;
case CV_CAP_PROP_SATURATION:
retval = capture->imageProperties.colour;
break;
case CV_CAP_PROP_HUE:
retval = capture->imageProperties.hue;
break;
case CV_CAP_PROP_GAIN:
fprintf(stderr, "HIGHGUI ERROR: V4L: Gain control in V4L is not supported\n");
return -1;
break;
case CV_CAP_PROP_EXPOSURE:
fprintf(stderr, "HIGHGUI ERROR: V4L: Exposure control in V4L is not supported\n");
return -1;
break;
}
if (retval == -1) {
/* there was a problem */
return -1;
}
/* all was OK, so convert to 0.0 - 1.0 range, and return the value */
return float (retval) / 0xFFFF;
}
}
read_frame_v4l2：
static CvCaptureCAM_V4L * icvCaptureFromCAM_V4L (int index)
{
static int autoindex;
autoindex = 0;
char deviceName[MAX_DEVICE_DRIVER_NAME];
if (!numCameras)
icvInitCapture_V4L(); /* Havent called icvInitCapture yet - do it now! */
if (!numCameras)
return NULL; /* Are there any /dev/video input sources? */
//search index in indexList
if ( (index>-1) && ! ((1 << index) & indexList) )
{
fprintf( stderr, "HIGHGUI ERROR: V4L: index %d is not correct!\n",index);
return NULL; /* Did someone ask for not correct video source number? */
}
/* Allocate memory for this humongus CvCaptureCAM_V4L structure that contains ALL
the handles for V4L processing */
CvCaptureCAM_V4L * capture = (CvCaptureCAM_V4L*)cvAlloc(sizeof(CvCaptureCAM_V4L));
if (!capture) {
fprintf( stderr, "HIGHGUI ERROR: V4L: Could not allocate memory for capture process.\n");
return NULL;
}
#ifdef USE_TEMP_BUFFER
capture->buffers[MAX_V4L_BUFFERS].start = NULL;
#endif
/* Select camera, or rather, V4L video source */
if (index<0) { // Asking for the first device available
for (; autoindex<MAX_CAMERAS;autoindex++)
if (indexList & (1<<autoindex))
break;
if (autoindex==MAX_CAMERAS)
return NULL;
index=autoindex;
autoindex++;// i can recall icvOpenCAM_V4l with index=-1 for next camera
}
/* Print the CameraNumber at the end of the string with a width of one character */
sprintf(deviceName, "/dev/video%1d", index);
/* w/o memset some parts arent initialized - AKA: Fill it with zeros so it is clean */
memset(capture,0,sizeof(CvCaptureCAM_V4L));
/* Present the routines needed for V4L funtionality. They are inserted as part of
the standard set of cv calls promoting transparency. "Vector Table" insertion. */
capture->FirstCapture = 1;
/* set the default size */
capture->width = DEFAULT_V4L_WIDTH;
capture->height = DEFAULT_V4L_HEIGHT;
if (_capture_V4L2 (capture, deviceName) == -1) {
icvCloseCAM_V4L(capture);
capture->is_v4l2_device = 0;
if (_capture_V4L (capture, deviceName) == -1) {
icvCloseCAM_V4L(capture);
return NULL;
}
} else {
capture->is_v4l2_device = 1;
}
return capture;
}; /* End icvOpenCAM_V4L */
#ifdef HAVE_CAMV4L2
static int read_frame_v4l2(CvCaptureCAM_V4L* capture) {
struct v4l2_buffer buf;
CLEAR (buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
if (-1 == xioctl (capture->deviceHandle, VIDIOC_DQBUF, &buf)) {
switch (errno) {
case EAGAIN:
return 0;
case EIO:
/* Could ignore EIO, see spec. */
/* fall through */
default:
/* display the error and stop processing */
perror ("VIDIOC_DQBUF");
return -1;
}
}
assert(buf.index < capture->req.count);
#ifdef USE_TEMP_BUFFER
memcpy(capture->buffers[MAX_V4L_BUFFERS].start,
capture->buffers[buf.index].start,
capture->buffers[MAX_V4L_BUFFERS].length );
capture->bufferIndex = MAX_V4L_BUFFERS;
//printf("got data in buff %d, len=%d, flags=0x%X, seq=%d, used=%d)\n",
// buf.index, buf.length, buf.flags, buf.sequence, buf.bytesused);
#else
capture->bufferIndex = buf.index;
#endif
if (-1 == xioctl (capture->deviceHandle, VIDIOC_QBUF, &buf))
perror ("VIDIOC_QBUF");
//set timestamp in capture struct to be timestamp of most recent frame
/** where timestamps refer to the instant the field or frame was received by the driver, not the capture time*/
capture->timestamp = buf.timestamp; //
printf("Flags %0x.X, timestamp = %ld, %ld", buf.flags, buf.timestamp.tv_sec, buf.timestamp.tv_usec);
//prints 0 if the flags is of the 4th nibble from LSB is 2 i.e. of the form .....2xxx H
return 1;
}
我需要时间戳以确定我在图像中检测到的物体的速度。我搜索了很多，但没有找到任何解决方案。