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    最近在做视频编码，经过一段时间的折腾，终于可以把V4L2采集到的图片数据编码成H264视频，并且成功将工程移植到了ARM开发板上。无奈开发板性能太低，每秒钟只能编码一帧数据，查看CPU已经跑到100%，计划换另外一种编码方式，先把遇到的问题和开发过程记录下来，做个备忘，也可以给大家一个参考。

   我的整个开发流程是：

    (1)通过V4L2采集摄像头数据，保存成图片

    (2)通过V4l2采集摄像头数据，保存成yuv视频

    (3)下载编译X264库，将该库移植到ARM开发板

    (4)通过V4L2采集摄像头YUV数据，调用X264库将数据编码成H264视频

    在整个开发过程中，不少代码是参考网上资料，自己也做了不少的优化处理。

    首先就是V4L2数据采集，确保linux的驱动和摄像头是可以正常工作的。V4L2编程的一般流程是：

    我一般是先在PC机上测试代码，代码没有问题之后再移植到ARM开发板上，这样可以保证应用层的代码没有没有问题，如果出问题，那就是开发板驱动的问题比较大。下面直接上代码。我在PC机上使用的是中星微的USB摄像头，在我的PC机上，它只支持JPEG格式。代码如下：

/*=============================================================================#     FileName: v4l2.c#         Desc: this program aim to get image from USB camera,#               used the V4L2 interface.#       Author: Licaibiao#      Version: #   LastChange: 2016-12-10 #      History:=============================================================================*/#include 
   
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                  #define FILE_VIDEO "/dev/video0"#define JPG "./out/image%d.jpg"typedef struct{ void *start; int length;}BUFTYPE;BUFTYPE *usr_buf;static unsigned int n_buffer = 0;/*set video capture ways(mmap)*/int init_mmap(int fd){ /*to request frame cache, contain requested counts*/ struct v4l2_requestbuffers reqbufs; memset(&reqbufs, 0, sizeof(reqbufs)); reqbufs.count = 1; /*the number of buffer*/ reqbufs.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; reqbufs.memory = V4L2_MEMORY_MMAP; if(-1 == ioctl(fd,VIDIOC_REQBUFS,&reqbufs)) { perror("Fail to ioctl 'VIDIOC_REQBUFS'"); exit(EXIT_FAILURE); } n_buffer = reqbufs.count; printf("n_buffer = %d\n", n_buffer); //usr_buf = calloc(reqbufs.count, sizeof(usr_buf)); usr_buf = calloc(reqbufs.count, sizeof(BUFTYPE));/*** (4) ***/ if(usr_buf == NULL) { printf("Out of memory\n"); exit(-1); } /*map kernel cache to user process*/ for(n_buffer = 0; n_buffer < reqbufs.count; ++n_buffer) { //stand for a frame struct v4l2_buffer buf; memset(&buf, 0, sizeof(buf)); buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; buf.memory = V4L2_MEMORY_MMAP; buf.index = n_buffer; /*check the information of the kernel cache requested*/ if(-1 == ioctl(fd,VIDIOC_QUERYBUF,&buf)) { perror("Fail to ioctl : VIDIOC_QUERYBUF"); exit(EXIT_FAILURE); } usr_buf[n_buffer].length = buf.length; usr_buf[n_buffer].start = (char *)mmap(NULL,buf.length,PROT_READ | PROT_WRITE,MAP_PRIVATE, fd,buf.m.offset); if(MAP_FAILED == usr_buf[n_buffer].start) { perror("Fail to mmap"); exit(EXIT_FAILURE); } }}int open_camera(void){ int fd; /*open video device with block */ fd = open(FILE_VIDEO, O_RDONLY); if(fd < 0) { fprintf(stderr, "%s open err \n", FILE_VIDEO); exit(EXIT_FAILURE); }; return fd;}int init_camera(int fd){ struct v4l2_capability cap; /* decive fuction, such as video input */ struct v4l2_format tv_fmt; /* frame format */ struct v4l2_fmtdesc fmtdesc; /* detail control value */ struct v4l2_control ctrl; int ret; /*show all the support format*/ memset(&fmtdesc, 0, sizeof(fmtdesc)); fmtdesc.index = 0 ; /* the number to check */ fmtdesc.type=V4L2_BUF_TYPE_VIDEO_CAPTURE; /* check video decive driver capability */ if(ret=ioctl(fd, VIDIOC_QUERYCAP, &cap)<0) { fprintf(stderr, "fail to ioctl VIDEO_QUERYCAP \n"); exit(EXIT_FAILURE); } /*judge wherher or not to be a video-get device*/ if(!(cap.capabilities & V4L2_BUF_TYPE_VIDEO_CAPTURE)) { fprintf(stderr, "The Current device is not a video capture device \n"); exit(EXIT_FAILURE); } /*judge whether or not to supply the form of video stream*/ if(!(cap.capabilities & V4L2_CAP_STREAMING)) { printf("The Current device does not support streaming i/o\n"); exit(EXIT_FAILURE); } printf("\ncamera driver name is : %s\n",cap.driver); printf("camera device name is : %s\n",cap.card); printf("camera bus information: %s\n",cap.bus_info); /*display the format device support*/ while(ioctl(fd,VIDIOC_ENUM_FMT,&fmtdesc)!=-1) { printf("\nsupport device %d.%s\n\n",fmtdesc.index+1,fmtdesc.description); fmtdesc.index++; } /*set the form of camera capture data*/ tv_fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; /*v4l2_buf_typea,camera must use V4L2_BUF_TYPE_VIDEO_CAPTURE*/ tv_fmt.fmt.pix.width = 640; tv_fmt.fmt.pix.height = 480; tv_fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_JPEG; /*V4L2_PIX_FMT_YYUV*/ tv_fmt.fmt.pix.field = V4L2_FIELD_NONE; /*V4L2_FIELD_NONE*/ if (ioctl(fd, VIDIOC_S_FMT, &tv_fmt)< 0) { fprintf(stderr,"VIDIOC_S_FMT set err\n"); exit(-1); close(fd); } init_mmap(fd);}int start_capture(int fd){ unsigned int i; enum v4l2_buf_type type; /*place the kernel cache to a queue*/ for(i = 0; i < n_buffer; i++) { struct v4l2_buffer buf; memset(&buf, 0, sizeof(buf)); buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; buf.memory = V4L2_MEMORY_MMAP; buf.index = i; if(-1 == ioctl(fd, VIDIOC_QBUF, &buf)) { perror("Fail to ioctl 'VIDIOC_QBUF'"); exit(EXIT_FAILURE); } } type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if(-1 == ioctl(fd, VIDIOC_STREAMON, &type)) { printf("i=%d.\n", i); perror("VIDIOC_STREAMON"); close(fd); exit(EXIT_FAILURE); } return 0;}int process_image(void *addr, int length){ FILE *fp; static int num = 0; char image_name[20]; sprintf(image_name, JPG, num++); if((fp = fopen(image_name, "w")) == NULL) { perror("Fail to fopen"); exit(EXIT_FAILURE); } fwrite(addr, length, 1, fp); usleep(500); fclose(fp); return 0;}int read_frame(int fd){ struct v4l2_buffer buf; unsigned int i; memset(&buf, 0, sizeof(buf)); buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; buf.memory = V4L2_MEMORY_MMAP; //put cache from queue if(-1 == ioctl(fd, VIDIOC_DQBUF,&buf)) { perror("Fail to ioctl 'VIDIOC_DQBUF'"); exit(EXIT_FAILURE); } assert(buf.index < n_buffer); //read process space's data to a file process_image(usr_buf[buf.index].start, usr_buf[buf.index].length); if(-1 == ioctl(fd, VIDIOC_QBUF,&buf)) { perror("Fail to ioctl 'VIDIOC_QBUF'"); exit(EXIT_FAILURE); } return 1;}int mainloop(int fd){ int count = 10; while(count-- > 0) { for(;;) { fd_set fds; struct timeval tv; int r; FD_ZERO(&fds); FD_SET(fd,&fds); /*Timeout*/ tv.tv_sec = 2; tv.tv_usec = 0; r = select(fd + 1,&fds,NULL,NULL,&tv); if(-1 == r) { if(EINTR == errno) continue; perror("Fail to select"); exit(EXIT_FAILURE); } if(0 == r) { fprintf(stderr,"select Timeout\n"); exit(-1); } if(read_frame(fd)) break; } } return 0;}void stop_capture(int fd){ enum v4l2_buf_type type; type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if(-1 == ioctl(fd,VIDIOC_STREAMOFF,&type)) { perror("Fail to ioctl 'VIDIOC_STREAMOFF'"); exit(EXIT_FAILURE); }}void close_camera_device(int fd){ unsigned int i; for(i = 0;i < n_buffer; i++) { if(-1 == munmap(usr_buf[i].start,usr_buf[i].length)) { exit(-1); } } free(usr_buf); if(-1 == close(fd)) { perror("Fail to close fd"); exit(EXIT_FAILURE); }}void main(void){ int fd; fd = open_camera(); init_camera(fd); start_capture(fd); mainloop(fd); stop_capture(fd); close_camera_device(fd);}
                 
                
               
              
             
            
           
          
         
        
       
      
     
    
   

    程序编译运行结果如下：

root@ubuntu:/home/share/v4l2_test# gcc v4l2.c -o v4l2_testroot@ubuntu:/home/share/v4l2_test# ./v4l2_test camera driver name is : gspca_zc3xxcamera device name is : PC Cameracamera bus information: usb-0000:02:00.0-2.1support device 1.JPEGn_buffer = 1root@ubuntu:/home/share/v4l2_test# lsout   v4l2.c  v4l2_testroot@ubuntu:/home/share/v4l2_test# cd out/root@ubuntu:/home/share/v4l2_test/out# lsimage0.jpg  image1.jpg  image2.jpg  image3.jpg  image4.jpg  image5.jpg  image6.jpg  image7.jpg  image8.jpg  image9.jpgroot@ubuntu:/home/share/v4l2_test/out# root@ubuntu:/home/share/v4l2_test/out#

    (1)在初始化摄像头函数init_camera 里面的tv_fmt.fmt.pix.pixelformat 参数一定是需要该摄像头所支持的。

    (2)变量 tv_fmt.fmt.pix.field 是摄像头field的设置，如果设置不正确会出现花屏的现象。field 理解可以参考的：

    (3)我这里采用了内存映射mmap, mmap的失败与camera的打开方式有关。我在PC机上，camera以阻塞方式打开，在内存映射的时候，mmap 的flag 参数需要填写MAP_PRIVATE，但是我在开发板上时，flag只能是MAP_SHARED，这个时候，我的camera只能是以O_NONBLOCK非阻塞方式打开，具体的原因有待研究。

    (4)上面的代码，如果reqbufs.count 的值设置大于1的话，编码的时候，编出来的图片有一些是空的，如果是设置为1就不会出现这个问题。这个有待研究。(该问题已经定位到，并且已更新上面的代码，详见)

    最后，打开保存的图片就是个人的自拍照了，从师弟那拿的一个摄像头，不知道像素是多少，拍的照片噪点多多，清晰度差。

    本文参考文档：

    http://blog.csdn.net/zgyulongfei/article/details/7526249

    http://www.cnblogs.com/emouse/archive/2013/03/04/2943243.html

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