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
- select函数使用
- calloc和malloc区别
- 图像基本格式YUV和RGB
- 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 <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <getopt.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <malloc.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
#include <asm/types.h>
#include <linux/videodev2.h>
#include <jpeglib.h>
#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("%s\n", 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 < HEIGHT) {
int x;
unsigned char *ptr = line_buffer;
for (x = 0; x < WIDTH; x++) {
int r, g, b;
int y, u, v;
if (!z)
y = yuyv[0] << 8;
else
y = yuyv[2] << 8;
u = yuyv[1] - 128;
v = yuyv[3] - 128;
r = (y + (359 * v)) >> 8;
g = (y - (88 * u) - (183 * v)) >> 8;
b = (y + (454 * u)) >> 8;
*(ptr++) = (r > 255) ? 255 : ((r < 0) ? 0 : r);
*(ptr++) = (g > 255) ? 255 : ((g < 0) ? 0 : g);
*(ptr++) = (b > 255) ? 255 : ((b < 0) ? 0 : b);
if (z++) {
z = 0;
yuyv += 4;
}
}
row_pointer[0] = line_buffer;
jpeg_write_scanlines (&cinfo, row_pointer, 1);
}
jpeg_finish_compress (&cinfo);
jpeg_destroy_compress (&cinfo);
free (line_buffer);
return (written);
}
//读取一帧的内容
static int read_frame (void)
{
struct v4l2_buffer buf;
int ret;
unsigned int i;
CLEAR (buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
int ff = ioctl (fd, VIDIOC_DQBUF, &buf); //出列采集的帧缓冲
if(ff<0)
printf("failture\n");
unsigned char src[buf.length+1];
unsigned char dest[buf.length+1];
assert (buf.index < n_buffers);
printf ("buf.index dq is %d,\n",buf.index);
memcpy(src, buffers[buf.index].start, buf.length);
ret = compress_yuyv_to_jpeg(src, dest,(WIDTH * HEIGHT), 100);//数据转换
fwrite(dest, ret, 1, file_fd);//转换后的数据写入
ff=ioctl (fd, VIDIOC_QBUF, &buf); //重新入列
if(ff<0)
printf("failture VIDIOC_QBUF\n");
return 1;
}
int main(void)
{
struct v4l2_capability cap;
struct v4l2_format fmt;
unsigned int i;
enum v4l2_buf_type type;
file_fd = fopen("test-mmap.jpg", "w");
fd = open (dev_name, O_RDWR | O_NONBLOCK, 0);
int ff=ioctl(fd, VIDIOC_QUERYCAP, &cap);//获取摄像头参数
if(ff<0)
printf("failture VIDIOC_QUERYCAP\n");
struct v4l2_fmtdesc fmt1;
int ret;
memset(&fmt1, 0, sizeof(fmt1));
fmt1.index = 0;
fmt1.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
while ((ret = ioctl(fd, VIDIOC_ENUM_FMT, &fmt1)) == 0) //查看摄像头所支持的格式
{
fmt1.index++;
printf("{ pixelformat = '%c%c%c%c', description = '%s' }\n",
fmt1.pixelformat & 0xFF, (fmt1.pixelformat >> 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<0)
printf("failture VIDIOC_S_FMT\n");
file_length = fmt.fmt.pix.bytesperline * fmt.fmt.pix.height; //计算图片大小
struct v4l2_requestbuffers req;
CLEAR (req);
req.count = 1;
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_MMAP;
ioctl (fd, VIDIOC_REQBUFS, &req); //申请缓冲，count是申请的数量
if(ff<0)
printf("failture VIDIOC_REQBUFS\n");
if (req.count < 1)
printf("Insufficient buffer memory\n");
buffers = calloc (req.count, sizeof (*buffers));//内存中建立对应空间
for (n_buffers = 0; n_buffers < req.count; ++n_buffers)
{
struct v4l2_buffer buf;
CLEAR (buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = n_buffers;
if (-1 == ioctl (fd, VIDIOC_QUERYBUF, &buf)) //映射用户空间
printf ("VIDIOC_QUERYBUF error\n");
buffers[n_buffers].length = buf.length;
buffers[n_buffers].start=mmap(NULL,buf.length,PROT_READ|PROT_WRITE,MAP_SHARED,fd,buf.m.offset); //通过mmap建立映射关系
if (MAP_FAILED == buffers[n_buffers].start)
printf ("mmap failed\n");
}
for (i = 0; i < n_buffers; ++i)
{
struct v4l2_buffer buf;
CLEAR (buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = i;
if (-1 == ioctl (fd, VIDIOC_QBUF, &buf))//申请到的缓冲进入列队
printf ("VIDIOC_QBUF failed\n");
}
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == ioctl (fd, VIDIOC_STREAMON, &type)) //开始捕捉图像数据
printf ("VIDIOC_STREAMON failed\n");
for (;;) //这一段涉及到异步IO
{
fd_set fds;
struct timeval tv;
int r;
FD_ZERO (&fds);//将指定的文件描述符集清空
FD_SET (fd, &fds);//在文件描述符集合中增加一个新的文件描述符
/* Timeout. */
tv.tv_sec = 2;
tv.tv_usec = 0;
r = select (fd + 1, &fds, NULL, NULL, &tv);//判断是否可读（即摄像头是否准备好），tv是定时
if (-1 == r)
{
if (EINTR == errno)
continue;
printf ("select err\n");
}
if (0 == r)
{
fprintf (stderr, "select timeout\n");
exit (EXIT_FAILURE);
}
if (read_frame ())//如果可读，执行read_frame函数
break;
}
unmap:
for (i = 0; i < n_buffers; ++i)
if (-1 == munmap (buffers[i].start, buffers[i].length))
printf ("munmap error");
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == ioctl(fd, VIDIOC_STREAMOFF, &type))
printf("VIDIOC_STREAMOFF");
close (fd);
fclose (file_fd);
exit (EXIT_SUCCESS);
return 0;
}
