V4L2数据获取流程
整个过程相关的数据结构有如下几个:
struct v4l2_capability m_cap; /* 驱动能力 */
struct v4l2_format m_fmt; /* 数据格式 */
struct v4l2_fmtdesc m_desc_fmt; /* 格式描述 */
struct v4l2_requestbuffers m_rb; /* 请求帧缓冲 */
struct v4l2_buffer m_buf; /* 数据缓冲 */
struct v4l2_frmsizeenum m_frmsize; /* 帧大小描述 */
1. 打开设备节点
#define DEFAULT_CAMERA_PATH "/dev/video0"
m_fd = open(DEFAULT_CAMERA_PATH, O_RDWR);
if(m_fd < 0) {
perror("open failed.");
return -1;
}
2. 可使用ioctl VIDIOC_QUERYCAP 查询当前驱动能力。
memset(&m_cap, 0, sizeof(m_cap));
if(ioctl(m_fd, VIDIOC_QUERYCAP, &m_cap) < 0) {
perror("ioctl failed");
return -1;
}
if(m_cap.capabilities & V4L2_CAP_VIDEO_CAPTURE) {
printf("V4L2_CAP_VIDEO_CAPTURE supportted\n");
}
if(m_cap.capabilities & V4L2_CAP_STREAMING) {
printf("V4L2_CAP_STREAMING supportted\n");
}
if(m_cap.capabilities & V4L2_CAP_READWRITE) {
printf("V4L2_CAP_READWRITE supportted\n");
}
3. 设置数据格式
/* set data format */
memset(&m_fmt, 0x0, sizeof(m_fmt));
m_fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
m_fmt.fmt.pix.width = 640;
m_fmt.fmt.pix.height = 480;
m_fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_MJPEG;
m_fmt.fmt.pix.field = V4L2_FIELD_ANY;
if(ioctl(m_fd, VIDIOC_S_FMT, &m_fmt) < 0) {
perror("ioctl VIDIOC_S_FMT failed!");
return -1;
}
4. 请求帧缓冲
/* request framebuffer */
memset(&m_rb, 0x0, sizeof(m_rb));
m_rb.count = DEFAULT_BUFFER_COUNT;
m_rb.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
m_rb.memory = V4L2_MEMORY_MMAP;
if(ioctl(m_fd, VIDIOC_REQBUFS, &m_rb) < 0) {
perror("ioctl VIDIOC_REQBUFS failed!");
return -1;
}
5. 映射buffer到用户空间
m_rb.count上一步请求的BUFFER数量，即v4l2_requestbuffers中定义
video_buffer用户空间的buffer结构体，用来存放mmap相关信息，定义如下
struct video_buffer {
unsigned int length;
unsigned int offset;
unsigned char *start;
};
m_video_buffers = (struct video_buffer *)calloc(m_rb.count, sizeof(struct video_buffer));
/* map the framebuffer to userspace */
for(int index_buf = 0; index_buf < m_rb.count; index_buf++) {
/* calloc the userspace buffer */
memset(&m_buf, 0x0, sizeof(m_buf));
m_buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
m_buf.memory = V4L2_MEMORY_MMAP;
m_buf.index = index_buf;
if(ioctl(m_fd, VIDIOC_QUERYBUF, &m_buf) < 0) {
perror("ioctl VIDIOC_QUERYBUF failed!");
return -1;
}
m_video_buff_size = m_buf.length;
/* map to userspace */
m_video_buffers[index_buf].start = (unsigned char *)mmap(NULL,
m_buf.length,
PROT_READ | PROT_WRITE,
MAP_SHARED,
m_fd,
m_buf.m.offset);
if(m_video_buffers[index_buf].start == MAP_FAILED) {
perror("map video buf failed!");
return -1;
}
/* queue the buffer */
memset(&m_buf, 0x0, sizeof(m_buf));
m_buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
m_buf.memory = V4L2_MEMORY_MMAP;
m_buf.index = index_buf;
if(ioctl(m_fd, VIDIOC_QBUF, &m_buf) < 0) {
perror("ioctl VIDIOC_QBUF failed!");
return -1;
}
}
printf("v4l2_buf size: %d\n", m_buf.length);
6.启动数据流
enum v4l2_buf_type type;
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if(ioctl(m_fd, VIDIOC_STREAMON, &type) < 0) {
perror("ioctl VIDIOC_STREAMON failed!");
return -1;
}
7.获取帧数据（写入到文件）
int ret;
static int frame_count=1;
struct v4l2_buffer v4lbuffer;
/* pop fb from queue */
memset(&v4lbuffer, 0x0, sizeof(v4lbuffer));
v4lbuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
v4lbuffer.memory = V4L2_MEMORY_MMAP;
if(ioctl(m_fd, VIDIOC_DQBUF, &v4lbuffer) < 0) {
perror("ioctl VIDIOC_DQBUF failed!");
return ret;
}
m_rb_current = v4lbuffer.index;
m_total_bytes = v4lbuffer.bytesused;
int out_fd;
unsigned char *file_base;
char out_fname[50];
sprintf(out_fname, "./abc_%04d.jpeg", frame_count);
LOG_DEBUG("***save_fbdata_to_file by mmap.");
out_fd = open(out_fname, O_RDWR | O_CREAT | O_TRUNC, 0755);
if(out_fd < 0) {
perror("open path failed!");
return out_fd;
}
/* make file length */
lseek(out_fd, m_total_bytes - 1, SEEK_END);
write(out_fd, "", 1); /* because of copy on write? */
printf("total_bytes:%d\n", m_total_bytes);
printf("current:%d\n", m_rb_current);
/* write operation here */
file_base = (unsigned char *)mmap(NULL, m_total_bytes,
PROT_READ | PROT_WRITE,
MAP_SHARED,
out_fd, 0);
if(file_base == MAP_FAILED) {
perror("mmap file failed!");
return -1;
}
LOG_DEBUG("memcpying...");
memcpy(file_base, m_video_buffers[m_rb_current].start, m_total_bytes);
/* release the resource */
munmap(file_base, m_total_bytes);
close(out_fd);
frame_count++;
/* push back fb to queue */
LOG_DEBUG("push back fb to queue");
memset(&v4lbuffer, 0x0, sizeof(v4lbuffer));
v4lbuffer.index = m_rb_current;
v4lbuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
v4lbuffer.memory = V4L2_MEMORY_MMAP;
if(ioctl(m_fd, VIDIOC_QBUF, &v4lbuffer) < 0) {
perror("ioctl VIDIOC_QBUF failed!");
return -1;
}
//if(frame_count > 24)exit(1);
return frame_count;
8.停止数据流
enum v4l2_buf_type type;
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if(ioctl(m_fd, VIDIOC_STREAMOFF, &type) < 0) {
perror("ioctl VIDIOC_STREAMOFF failed!");
return -1;
}
最后附上main.cpp
#include <iostream>
#include <time.h>
#include <signal.h>
#include "v4l2_capturer.h"
v4l2_capturer capturer;
void capturer_sigint_handler(int signal)
{
capturer.stop();
exit(1);
}
/* TODO: listen SIGINT and call capturer.stop() when it come. */
int main(int argc, char const *argv[])
{
int frame_count=0;
std::cout << "Hello World!" << std::endl;
/* register signal handler function. */
signal(SIGINT, capturer_sigint_handler);
capturer.init();
capturer.query_supported_format_new();
capturer.start();
int i=25;
char filename[10];
while(1){
frame_count=capturer.get_frame();
if(frame_count==25*10)break;
}
/*capturer.get_frame();
capturer.save_fbdata_to_file_by_mmap("./out.jpg");*/
capturer.stop();
return 0;
}