由于网上下载的V4L2的代码保存后的jpg图片打开后都描述不是jpg格式。所以下载了个YUYV转JPEG格式的代码,但是需要安装jpeglib。

问题1:

安装jpeglib参考《libjpeg 库的安装.txt》

问题2

安装完后gcc cam3 -o cam3.c进行编译报错
对jpeg_std_error未定义的引用
对jpeg_xxx未定义的引用
通过./configure –help找到安装路径默认为/usr/local/bin和/usr/local/lib
进入/usr/local/lib找到了libjpeg。通过grep ‘jpeg_std_error’ libjpeg.a没有找到。
通过nm -e libjpeg.a看到里面是有这个函数的。
说明的我编译的时候没有链接库导致报错。同时网上也搜索了下解决方法就是添加-ljpge即可

问题3

gcc cam3 -o cam3.c -ljpeg编译通过。
运行./cam3保存的test-mmap.jpg是一个完整的图像,可是看不到内容。
于是猜测分辨率问题,于是把高度和宽度改成640和480,能正常显示图片。

问题4

可是看起来模糊,于是我把质量参数80改为100稍微感觉效果好些。

学习的东东

  1. select函数使用
  2. calloc和malloc区别
  3. 图像基本格式YUV和RGB
  4. V4L2应用变成的步骤
    4.1 打开视频设备文件,进行视频采集的参数初始化,通过V4L2接口设置视频图像的采集窗口、采集的点阵大小和格式;
    4.2 申请若干视频采集的帧缓冲区,并将这些帧缓冲区从内核空间映射到用户空间,便于应用程序读取/处理视频数据;
    4.3 将申请到的帧缓冲区在视频采集输入队列排队,并启动视频采集;
    4.4 驱动开始视频数据的采集,应用程序从视频采集输出队列取出帧缓冲区,处理完后,将帧缓冲区重新放入视频采集输入队列,循环往复采集连续的视频数据;
    4.5 停止视频采集。

最后的代码cam3.c如下

#include 
#include 
#include 
#include 
#include             
#include              
#include 
#include 
#include 
#include 
#include 
#include 
#include  
#include 
#include          
#include 
#include 

 
#define OUTPUT_BUF_SIZE  4096
#define CLEAR(x) memset (&(x), 0, sizeof (x))
#define WIDTH 640//320
#define HEIGHT 480//240
 
struct buffer {
         void   *start;
         size_t length;
};
 
typedef struct {
  struct jpeg_destination_mgr pub; 
 
  JOCTET * buffer;  
 
  unsigned char *outbuffer;
  int outbuffer_size;
  unsigned char *outbuffer_cursor;
  int *written;
 
}mjpg_destination_mgr;
 
typedef mjpg_destination_mgr *mjpg_dest_ptr;
 
static char *           dev_name        = "/dev/video0";
static int              fd              = -1;
struct buffer *         buffers         = NULL;
static unsigned int     n_buffers       = 0;
FILE *file_fd;
static unsigned long file_length;
static unsigned char *file_name;
 
METHODDEF(void) init_destination(j_compress_ptr cinfo) {
  mjpg_dest_ptr dest = (mjpg_dest_ptr) cinfo->dest;
  dest->buffer = (JOCTET *)(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, OUTPUT_BUF_SIZE * sizeof(JOCTET));
  *(dest->written) = 0;
  dest->pub.next_output_byte = dest->buffer;
  dest->pub.free_in_buffer = OUTPUT_BUF_SIZE;
}
 
METHODDEF(boolean) empty_output_buffer(j_compress_ptr cinfo) {
  mjpg_dest_ptr dest = (mjpg_dest_ptr) cinfo->dest;
  memcpy(dest->outbuffer_cursor, dest->buffer, OUTPUT_BUF_SIZE);
  dest->outbuffer_cursor += OUTPUT_BUF_SIZE;
  *(dest->written) += OUTPUT_BUF_SIZE;
  dest->pub.next_output_byte = dest->buffer;
  dest->pub.free_in_buffer = OUTPUT_BUF_SIZE;
 
  return TRUE;
}
 
METHODDEF(void) term_destination(j_compress_ptr cinfo) {
  mjpg_dest_ptr dest = (mjpg_dest_ptr) cinfo->dest;
  size_t datacount = OUTPUT_BUF_SIZE - dest->pub.free_in_buffer;
  /* Write any data remaining in the buffer */
  memcpy(dest->outbuffer_cursor, dest->buffer, datacount);
  dest->outbuffer_cursor += datacount;
  *(dest->written) += datacount;
}
 
void dest_buffer(j_compress_ptr cinfo, unsigned char *buffer, int size, int *written) {
  mjpg_dest_ptr dest;
  if (cinfo->dest == NULL) {
    cinfo->dest = (struct jpeg_destination_mgr *)(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT, sizeof(mjpg_destination_mgr));
  }
 
  dest = (mjpg_dest_ptr)cinfo->dest;
  dest->pub.init_destination = init_destination;
  dest->pub.empty_output_buffer = empty_output_buffer;
  dest->pub.term_destination = term_destination;
  dest->outbuffer = buffer;
  dest->outbuffer_size = size;
  dest->outbuffer_cursor = buffer;
  dest->written = written;
}
 
//摄像头采集的YUYV格式转换为JPEG格式
int compress_yuyv_to_jpeg(unsigned char *buf, unsigned char *buffer, int size, int quality) {
  struct jpeg_compress_struct cinfo;
  struct jpeg_error_mgr jerr;
  JSAMPROW row_pointer[1];
  unsigned char *line_buffer, *yuyv;
  int z;
  static int written;
  //int count = 0;
  //printf("%sn", buf);
  line_buffer = calloc (WIDTH * 3, 1);
  yuyv = buf;//将YUYV格式的图片数据赋给YUYV指针
  printf("compress start...n");
  cinfo.err = jpeg_std_error (&jerr);
  jpeg_create_compress (&cinfo);
  /* jpeg_stdio_dest (&cinfo, file); */
  dest_buffer(&cinfo, buffer, size, &written);
 
  cinfo.image_width = WIDTH;
  cinfo.image_height = HEIGHT;
  cinfo.input_components = 3;
  cinfo.in_color_space = JCS_RGB;
 
  jpeg_set_defaults (&cinfo);
  jpeg_set_quality (&cinfo, quality, TRUE);
  jpeg_start_compress (&cinfo, TRUE);
 
  z = 0;
  while (cinfo.next_scanline > 8;
      g = (y - (88 * u) - (183 * v)) >> 8;
      b = (y + (454 * u)) >> 8;
 
      *(ptr++) = (r > 255) ? 255 : ((r  255) ? 255 : ((g  255) ? 255 : ((b > 8) & 0xFF,
  (fmt1.pixelformat >> 16) & 0xFF, (fmt1.pixelformat >> 24) & 0xFF,
   fmt1.description);
        }
 CLEAR (fmt);
 fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
 fmt.fmt.pix.width       = 640;
 fmt.fmt.pix.height      = 480;
 fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
 fmt.fmt.pix.field       = V4L2_FIELD_INTERLACED;
 
 ff = ioctl (fd, VIDIOC_S_FMT, &fmt); //设置图像格式
 if(ff

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