谁在linux下用了usb摄像头

在linux下所有设备都是文件。所以对摄像头的操作其实就是对文件的操作。USB摄像头的设备文件就是在/dev目录下的video0(假如只有一个摄像头)。在linux下操作摄像头就是使用v4l2对摄像头进行视频的操作，操作步骤如下
1. 打开设备文件。
int fd=open(”/dev/video0″,O_RDWR);
2. 取得设备的capability，看看设备具有什么功能，比如是否具有视频输入,或者音频输入输出等。VIDIOC_QUERYCAP,struct v4l2_capability

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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. 选择视频输入，一个视频设备可以有多个视频输入。VIDIOC_S_INPUT,struct v4l2_input(可不要)
4. 设置视频的制式和帧格式，制式包括PAL，NTSC，帧的格式个包括宽度和高度等。
VIDIOC_S_STD,VIDIOC_S_FMT,struct v4l2_std_id,struct v4l2_format

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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. 向驱动申请帧缓冲，一般不超过5个。struct v4l2_requestbuffers

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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.申请物理内存
将申请到的帧缓冲映射到用户空间，这样就可以直接操作采集到的帧了，而不必去复制。将申请到的帧缓冲全部入队列，以便存放采集到的数据.VIDIOC_QBUF,struct v4l2_buffer

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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));//内存映射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. 开始视频的采集。

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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. 出队列以取得已采集数据的帧缓冲，取得原始采集数据。VIDIOC_DQBUF, 将缓冲重新入队列尾,这样可以循环采集。VIDIOC_QBUF

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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
9. 停止视频的采集。VIDIOC_STREAMOFF
10. 关闭视频设备。close(fd);