在linux下所有裝置都是檔案。所以對攝像頭的操作其實就是對檔案的操作。USB攝像頭的裝置檔案就是在/dev目錄下的video0(假如只有一個攝像頭)。在linux下操作攝像頭就是使用v4l2對攝像頭進行的操作，操作步驟如下

開啟裝置檔案。

int fd=open(”/dev/video0″,O_RDWR);

2. 取得裝置的capability，看看裝置具有什麼功能，比如是否具有輸入,或者音訊輸入輸出等。VIDIOC_QUERYCAP,struct v4l2_capability

v4l2_std_id std;

do {

ret= ioctl(fd, VIDIOC_QUERYSTD, std);

} while (ret == -1 errno == EAGAIN);

switch (std) {

case V4L2_STD_NTSC:

//……

case V4L2_STD_PAL:

//……

}

3. 選擇輸入，一個裝置可以有多個輸入。VIDIOC_S_INPUT,struct v4l2_input(可不要)

4. 設定的制式和幀格式，制式包括PAL，NTSC，幀的格式個包括寬度和高度等。

VIDIOC_S_STD,VIDIOC_S_FMT,struct v4l2_std_id,struct v4l2_format

struct v4l2_format fmt;

memset ( fmt, 0, sizeof(fmt) );

fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

fmt.fmt.pix.width = 320;

fmt.fmt.pix.height = 240;

fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_JPEG;

if (ioctl(fd, VIDIOC_S_FMT, fmt) < 0）

{

printf("set format failed\n");

//return 0;

}

5. 向驅動申請幀緩衝，一般不超過5個。struct v4l2_requestbuffers

struct v4l2_requestbuffers req;

memset(req, 0, sizeof (req));

req.count = 4;

req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

req.memory = V4L2_MEMORY_MMAP;

if (ioctl(fd,VIDIOC_REQBUFS,req) == -1)

{

perror("VIDIOC_REQBUFS error \n");

//return -1;

}

6.申請物理記憶體

將申請到的幀緩衝對映到使用者空間，這樣就可以直接操作採集到的幀了，而不必去複製。將申請到的幀緩衝全部入佇列，以便存放採集到的資料.VIDIOC_QBUF,struct v4l2_buffer

VideoBuffer* buffers = calloc( req.count, sizeof(VideoBuffer) );

printf("sizeof(VideoBuffer) is %d\n",sizeof(VideoBuffer));

struct v4l2_buffer buf;

for (numBufs = 0; numBufs < req.count; numBufs++)

{

memset( buf, 0, sizeof(buf) );

buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

buf.memory = V4L2_MEMORY_MMAP;

buf.index = numBufs;

if (ioctl(fd, VIDIOC_QUERYBUF, buf) < 0)

{

printf("VIDIOC_QUERYBUF error\n");

//return -1;

}

printf("buf len is %d\n",sizeof(buf));

//記憶體對映

buffers[numBufs].length = buf.length;

buffers[numBufs].offset = (size_t) buf.m.offset;

buffers[numBufs].start = mmap (NULL, buf.length,PROT_READ | PROT_WRITE, MAP_SHARED, fd, buf.m.offset);

printf("buffers.length = %d,buffers.offset = %d ,buffers.start[0] = %d\n",buffers[numBufs].length,buffers[numBufs].offset,buffers[numBufs].start[0]);

printf("buf2 len is %d\n",sizeof(buffers[numBufs].start));

if (buffers[numBufs].start == MAP_FAILED)

{

perror("buffers error\n");

//return -1;

}

if (ioctl (fd, VIDIOC_QBUF, buf) < 0)

{

printf("VIDIOC_QBUF error\n");

//return -1;

}

}

7. 開始的採集。

enum v4l2_buf_type type;

type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (ioctl (fd, VIDIOC_STREAMON, type) < 0)

{

printf("VIDIOC_STREAMON error\n");

// return -1;

}

8. 出佇列以取得已採集資料的幀緩衝，取得原始採集資料。VIDIOC_DQBUF, 將緩衝重新入佇列尾,這樣可以迴圈採集。VIDIOC_QBUF

if (ioctl(fd, VIDIOC_DQBUF, buf) < 0)

{

perror("VIDIOC_DQBUF failed.\n");

//return -1;

}

buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

buf.memory = V4L2_MEMORY_MMAP;

unsigned char *ptcur = buffers[numBufs].start;

DEBUG("buf.bytesused = %d \n",buf.bytesused);

int i1;

for(i1=0; i1<buf.bytesused; i1++)

{

if((buffers[numBufs].start[i1] == 0xFF) (buffers[numBufs].start[i1+1] == 0xC4))

{

DEBUG("huffman table finded! \nbuf.bytesused = %d\nFFC4 = %d \n",buf.bytesused,i1);

break;

}

}

if(i1 == buf.bytesused)printf("huffman table don"t exist! \n");

int i;

for(i=0; i<buf.bytesused; i++)

{

if((buffers[numBufs].start[i] == 0xFF) (buffers[numBufs].start[i+1] == 0xD8)) break;

ptcur++;

}

DEBUG("i=%d,FF=%02x,D8=%02x\n",i,buffers[numBufs].start[i],buffers[numBufs].start[i+1]);

int imagesize =buf.bytesused - i;

DEBUG("buf.bytesused = %d \n",buf.bytesused);

DEBUG ("imagesize = %d \n",imagesize);

9. 停止的採集。VIDIOC_STREAMOFF

10. 關閉裝置。close(fd);