【发布时间】：2021-09-19 05:12:05
【问题描述】：
我有一个通过 USB 连接到 beaglebone 的网络摄像头。我正在用 C++ 编码，我的目标是从网络摄像头捕获原始未压缩图片。
首先我通过命令v4l2-ctl --list-formats检查了支持哪些格式，结果是：
Index : 0
Type : Video Capture
Pixel Format: 'MJPG' (compressed)
Name : Motion-JPEG
Index : 1
Type : Video Capture
Pixel Format: 'YUYV'
Name : YUYV 4:2:2
因此，我假设如果我尝试使用 YUYV 格式，它必须有可能获得未压缩的图片。
知道了这一点，我开始用 C++ 编写程序。我成功编写了一个程序来捕获压缩图片，但是当尝试使用 YUYV 格式捕获时它不起作用，我真的需要一些帮助才能完成这项工作。
这是我的代码：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <linux/videodev2.h>
#include <libv4l2.h>
template <typename typeXX>
void clear_memmory(typeXX* x) {
memset(x, 0, sizeof(*x));
}
void xioctl(int cd, int request, void *arg){
int response;
do{
//ensures we get the correct response.
response = v4l2_ioctl(cd, request, arg);
}
while (response == -1 && ((errno == EINTR) || (errno == EAGAIN)));
if (response == -1) {
fprintf(stderr, "error %d, %s\n", errno, strerror(errno));
exit(EXIT_FAILURE);
}
}
struct LMSBBB_buffer{
void* start;
size_t length;
};
int main(){
const char* dev_name = "/dev/video0";
int width=1920;
int height=1080;
int fd = v4l2_open(dev_name, O_RDWR | O_NONBLOCK, 0);
struct v4l2_format format = {0};
format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
format.fmt.pix.width = width;
format.fmt.pix.height = height;
format.fmt.pix.pixelformat = V4L2_PIX_FMT_RGB24;//V4L2_PIX_FMT_YUYV //V4L2_PIX_FMT_RGB24
format.fmt.pix.field = V4L2_FIELD_NONE; //V4L2_FIELD_NONE
xioctl(fd, VIDIOC_S_FMT, &format);
printf("Device initialized.\n");
///request buffers
struct v4l2_requestbuffers req = {0};
req.count = 2;
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_MMAP;
xioctl(fd, VIDIOC_REQBUFS, &req);
printf("Buffers requested.\n");
///mapping buffers
struct v4l2_buffer buf;
LMSBBB_buffer* buffers;
unsigned int i;
buffers = (LMSBBB_buffer*) calloc(req.count, sizeof(*buffers));
for (i = 0; i < req.count; i++) {
clear_memmory(&(buf));
(buf).type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
(buf).memory = V4L2_MEMORY_MMAP;
(buf).index = i;
xioctl(fd, VIDIOC_QUERYBUF, &buf);
buffers[i].length = (buf).length;
printf("A buff has a len of: %i\n",buffers[i].length);
buffers[i].start = v4l2_mmap(NULL, (buf).length, PROT_READ | PROT_WRITE, MAP_SHARED,fd, (buf).m.offset);
if (MAP_FAILED == buffers[i].start) {
perror("Can not map the buffers.");
exit(EXIT_FAILURE);
}
}
printf("Buffers mapped.\n");
for (i = 0; i < req.count; i++) {
clear_memmory(&(buf));
(buf).type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
(buf).memory = V4L2_MEMORY_MMAP;
(buf).index = i;
ioctl(fd,VIDIOC_QBUF, &(buf));
}
enum v4l2_buf_type type;
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
ioctl(fd,VIDIOC_STREAMON, &type);
printf("buffers queued and streaming.\n");
int pic_count=0;
///CAPTURE
fd_set fds;
struct timeval tv;
int r;
char out_name[256];
FILE* fout;
do {
FD_ZERO(&fds);
FD_SET(fd, &fds);
// Timeout.
tv.tv_sec = 2;
tv.tv_usec = 0;
r = select(fd + 1, &fds, NULL, NULL, &tv);
} while ((r == -1 && (errno = EINTR)));
if (r == -1) {
perror("select");
exit(EXIT_FAILURE);
}
clear_memmory(&(buf));
(buf).type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
(buf).memory = V4L2_MEMORY_MMAP;
xioctl(fd,VIDIOC_DQBUF, &(buf));
printf("Buff index: %i\n",(buf).index);
sprintf(out_name, "image%03d.ppm",pic_count);
fout = fopen(out_name, "w");
if (!fout) {
perror("Cannot open image");
exit(EXIT_FAILURE);
}
fprintf(fout, "P6\n%d %d 255\n",width, height);
fwrite(buffers[(buf).index].start, (buf).bytesused, 1, fout);
fclose(fout);
pic_count++;
clear_memmory(&(buf));
(buf).type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
(buf).memory = V4L2_MEMORY_MMAP;
xioctl(fd,VIDIOC_DQBUF, &(buf));
printf("Buff index: %i\n",(buf).index);
sprintf(out_name, "image%03d.ppm",pic_count);
fout = fopen(out_name, "w");
if (!fout) {
perror("Cannot open image");
exit(EXIT_FAILURE);
}
fprintf(fout, "P6\n%d %d 255\n",width, height);
fwrite(buffers[(buf).index].start, (buf).bytesused, 1, fout);
fclose(fout);
pic_count++;
///xioctl(fd,VIDIOC_QBUF, &(buf));
return 0;
}
在第 50 行，我可以在 V4L2_PIX_FMT_YUYV 和 V4L2_PIX_FMT_RGB24 之间选择格式。 对于 V4L2_PIX_FMT_RGB24 我得到了图片，但是当使用 V4L2_PIX_FMT_YUYV 我得到这个错误：
libv4l2: error dequeuing buf: Resource temporarily unavailable
libv4l2: error dequeuing buf: Resource temporarily unavailable
libv4l2: error dequeuing buf: Resource temporarily unavailable
libv4l2: error dequeuing buf: Resource temporarily unavailable
libv4l2: error dequeuing buf: Resource temporarily unavailable
错误行一直存在，直到我手动结束程序。
有人知道该怎么做吗？我花了两个多星期的时间，我不能从这里搬到任何地方。我真的很感激任何建议。
【问题讨论】：
标签： c++ camera v4l2 image-capture