Video4Linux2(Video for Linux Two, 简称V4L2)是Linux中关于视频设备的驱动框架,为上层访问底层的视频设备提供统一接口。V4L2主要支持三类设备:视频输入输出设备、VBI设备和Radio设备,分别会在/dev目录下产生videoX、vbiX和radioX设备节点,其中X是0,1,2等的数字。如USB摄像头是我们常见的视频输入设备。FFmpeg和OpenCV对V4L2均支持。
在使用USB摄像头获取视频时,外部调用V4L2 API的包含的主要头文件为videodev2.h,在/usr/include/linux目录下。
V4L2获取USB视频流执行流程如下:代码参考:https://linuxtv.org/downloads/v4l-dvb-apis-new/uapi/v4l/capture.c.html
1. 打开设备调用open_device函数:
(1). 调用stat函数,通过设备名字获取设备信息,并将结果保存在结构体stat中,结构体stat内容如下:
struct stat {
dev_t st_dev; /* ID of device containing file */
ino_t st_ino; /* inode number */
mode_t st_mode; /* protection */
nlink_t st_nlink; /* number of hard links */
uid_t st_uid; /* user ID of owner */
gid_t st_gid; /* group ID of owner */
dev_t st_rdev; /* device ID (if special file) */
off_t st_size; /* total size, in bytes */
blksize_t st_blksize; /* blocksize for file system I/O */
blkcnt_t st_blocks; /* number of 512B blocks allocated */
time_t st_atime; /* time of last access */
time_t st_mtime; /* time of last modification */
time_t st_ctime; /* time of last status change */
};
(2). 调用S_ISCHR函数判断是否是字符设备(character device);
(3). 调用open函数打开设备,以可读可写方式(O_RDWR)和无阻塞方式(O_NONBLOCK)打开,open函数返回一个文件(或设备)描述符。
2. 初始化设备调用init_device函数:
(1). 调用ioctl函数,对设备的I/O通道进行管理,查询设备能力,并将结果保存在结构体v4l2_capability中,结构体v4l2_capability内容如下:
struct v4l2_capability {
__u8 driver[16]; // name of the driver module (e.g. "bttv")
__u8 card[32]; // name of the card (e.g. "Hauppauge WinTV")
__u8 bus_info[32]; // name of the bus (e.g. "PCI:" + pci_name(pci_dev) )
__u32 version; // KERNEL_VERSION
__u32 capabilities; // capabilities of the physical device as a whole
__u32 device_caps; // capabilities accessed via this particular device (node)
__u32 reserved[3]; // reserved fields for future extensions
};
(2). 判断是否是视频捕获设备:V4L2_CAP_VIDEO_CAPTURE;
(3). 判断是否支持流I/O(streaming I/O):V4L2_CAP_STREAMING;
(4). 调用ioctl函数,查询设备视频裁剪和缩放功能信息,并将结果保存在结构体v4l2_cropcap中,结构体v4l2_cropcap内容如下:
enum v4l2_buf_type {
V4L2_BUF_TYPE_VIDEO_CAPTURE = 1,
V4L2_BUF_TYPE_VIDEO_OUTPUT = 2,
V4L2_BUF_TYPE_VIDEO_OVERLAY = 3,
V4L2_BUF_TYPE_VBI_CAPTURE = 4,
V4L2_BUF_TYPE_VBI_OUTPUT = 5,
V4L2_BUF_TYPE_SLICED_VBI_CAPTURE = 6,
V4L2_BUF_TYPE_SLICED_VBI_OUTPUT = 7,
#if 1
/* Experimental */
V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY = 8,
#endif
V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE = 9,
V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE = 10,
/* Deprecated, do not use */
V4L2_BUF_TYPE_PRIVATE = 0x80,
};
struct v4l2_cropcap {
__u32 type; /* enum v4l2_buf_type */
struct v4l2_rect bounds;
struct v4l2_rect defrect;
struct v4l2_fract pixelaspect;
};
(5). 调用ioctl函数,设置当前裁剪矩形,并将结果保存在结构体v4l2_crop中,结构体v4l2_crop内容如下:
struct v4l2_crop {
__u32 type; /* enum v4l2_buf_type */
struct v4l2_rect c;
};
(6). 调用ioctl函数,设置流数据格式,包括宽、高、像素格式,并将结果保存在结构体v4l2_format中,结构体v4l2_format内容如下:
struct v4l2_pix_format {
__u32 width;
__u32 height;
__u32 pixelformat;
__u32 field; /* enum v4l2_field */
__u32 bytesperline; /* for padding, zero if unused */
__u32 sizeimage;
__u32 colorspace; /* enum v4l2_colorspace */
__u32 priv; /* private data, depends on pixelformat */
};
struct v4l2_format { // stream data format
__u32 type; // enum v4l2_buf_type; type of the data stream
union {
struct v4l2_pix_format pix; // V4L2_BUF_TYPE_VIDEO_CAPTURE, definition of an image format
struct v4l2_pix_format_mplane pix_mp; // V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE, definition of a multiplanar image format
struct v4l2_window win; // V4L2_BUF_TYPE_VIDEO_OVERLAY, definition of an overlaid image
struct v4l2_vbi_format vbi; // V4L2_BUF_TYPE_VBI_CAPTURE, raw VBI capture or output parameters
struct v4l2_sliced_vbi_format sliced; // V4L2_BUF_TYPE_SLICED_VBI_CAPTURE, sliced VBI capture or output parameters
__u8 raw_data[200]; // user-defined, placeholder for future extensions and custom formats
} fmt;
};
3. 因为io采用的是MMAP即内存映射方式,因此调用init_mmap函数:
(1). 调用ioctl函数,设置内存映射I/O,并将结果保存在结构体v4l2_requestbuffers中,结构体v4l2_requestbuffers内容如下:
enum v4l2_memory {
V4L2_MEMORY_MMAP = 1,
V4L2_MEMORY_USERPTR = 2,
V4L2_MEMORY_OVERLAY = 3,
V4L2_MEMORY_DMABUF = 4,
};
struct v4l2_requestbuffers {
__u32 count;
__u32 type; /* enum v4l2_buf_type */
__u32 memory; /* enum v4l2_memory */
__u32 reserved[2];
};
(2). 调用ioctl函数,查询缓冲区状态,并将结果保存在结构体v4l2_buffer中,结构体v4l2_buffer内容如下:
/**
* struct v4l2_buffer - video buffer info
* @index: id number of the buffer
* @type: enum v4l2_buf_type; buffer type (type == *_MPLANE for
* multiplanar buffers);
* @bytesused: number of bytes occupied by data in the buffer (payload);
* unused (set to 0) for multiplanar buffers
* @flags: buffer informational flags
* @field: enum v4l2_field; field order of the image in the buffer
* @timestamp: frame timestamp
* @timecode: frame timecode
* @sequence: sequence count of this frame
* @memory: enum v4l2_memory; the method, in which the actual video data is
* passed
* @offset: for non-multiplanar buffers with memory == V4L2_MEMORY_MMAP;
* offset from the start of the device memory for this plane,
* (or a "cookie" that should be passed to mmap() as offset)
* @userptr: for non-multiplanar buffers with memory == V4L2_MEMORY_USERPTR;
* a userspace pointer pointing to this buffer
* @fd: for non-multiplanar buffers with memory == V4L2_MEMORY_DMABUF;
* a userspace file descriptor associated with this buffer
* @planes: for multiplanar buffers; userspace pointer to the array of plane
* info structs for this buffer
* @length: size in bytes of the buffer (NOT its payload) for single-plane
* buffers (when type != *_MPLANE); number of elements in the
* planes array for multi-plane buffers
* @input: input number from which the video data has has been captured
*
* Contains data exchanged by application and driver using one of the Streaming
* I/O methods.
*/
struct v4l2_buffer {
__u32 index;
__u32 type;
__u32 bytesused;
__u32 flags;
__u32 field;
struct timeval timestamp;
struct v4l2_timecode timecode;
__u32 sequence;
/* memory location */
__u32 memory;
union {
__u32 offset;
unsigned long userptr;
struct v4l2_plane *planes;
__s32 fd;
} m;
__u32 length;
__u32 reserved2;
__u32 reserved;
};
(3). 调用mmap函数,应用程序通过内存映射将帧缓冲区地址映射到用户空间;通常在需要对文件进行频繁读写时使用,这样用内存读写取代I/O读写,以获得较高的性能。
4. 开始捕捉,调用start_capturing函数:
(1). 调用ioctl函数,VIDIOC_QBUF,并将结果保存在结构体v4l2_buffer中;
(2). 调用ioctl函数,启动流I/O,并将结果保存在结构体v4l2_buf_type中;
5. 主循环,调用mainloop函数:
(1). 清空集合调用FD_ZERO函数,将一个给定的文件描述符加入到集合中调用FD_SET函数;
(2). 调用select函数;
(3). 调用ioctl函数,VIDIOC_DQBUF,并将结果保存在结构体v4l2_buffer中;
(4). 将帧数据写入指定的文件;
(5). 调用ioctl函数,VIDIOC_QBUF,并将结果保存在结构体v4l2_buffer中。
6. 停止捕捉,调用stop_capturing函数:
(1). 调用ioctl函数,停止流I/O,并将结果保存在结构体v4l2_buf_type中。
7. 调用uninit_device函数:
(1). 调用munmap函数,取消映射设备内存;
(2). 调用free函数,删除calloc申请的内存。
8. 关闭设备,调用close_device函数:
(1). 调用close函数关闭设备。
这里通过ffmpeg查看usb摄像头支持的编码类型,执行结果如下图所示,编码类型支持Raw和Mjpeg两种,对应的像素格式为V4L2_PIX_FMT_YUYV和V4L2_PIX_FMT_MJPEG:
测试代码如下:
#include "funset.hpp"
#include
#include
#include
#ifndef _MSC_VER
#include
#include
#include
#include
#include
#include
#include
#include
#include
namespace {
#define CLEAR(x) memset(&(x), 0, sizeof(x))
enum io_method {
IO_METHOD_READ,
IO_METHOD_MMAP,
IO_METHOD_USERPTR,
};
struct buffer {
void *start;
size_t length;
};
char* dev_name;
enum io_method io = IO_METHOD_MMAP;
int fd = -1;
struct buffer* buffers;
unsigned int n_buffers;
int out_buf;
int force_format = 1;
int frame_count = 10;
int width = 640;
int height = 480;
FILE* f;
void errno_exit(const char *s)
{
fprintf(stderr, "%s error %d, %s\n", s, errno, strerror(errno));
exit(EXIT_FAILURE);
}
int xioctl(int fh, int request, void *arg)
{
int r;
do {
r = ioctl(fh, request, arg);
} while (-1 == r && EINTR == errno);
return r;
}
void process_image(const void *p, int size)
{
/*if (out_buf)
fwrite(p, size, 1, stdout);
fflush(stderr);
fprintf(stderr, ".");
fflush(stdout);*/
fwrite(p, size, 1, f);
}
int read_frame(void)
{
struct v4l2_buffer buf;
unsigned int i;
switch (io) {
case IO_METHOD_READ:
if (-1 == read(fd, buffers[0].start, buffers[0].length)) {
switch (errno) {
case EAGAIN:
return 0;
case EIO:
// Could ignore EIO, see spec. fall through
default:
errno_exit("read");
}
}
process_image(buffers[0].start, buffers[0].length);
break;
case IO_METHOD_MMAP:
CLEAR(buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
if (-1 == xioctl(fd, VIDIOC_DQBUF, &buf)) {
switch (errno) {
case EAGAIN:
return 0;
case EIO:
// Could ignore EIO, see spec. fall through
default:
errno_exit("VIDIOC_DQBUF");
}
}
assert(buf.index < n_buffers);
process_image(buffers[buf.index].start, buf.bytesused);
if (-1 == xioctl(fd, VIDIOC_QBUF, &buf))
errno_exit("VIDIOC_QBUF");
break;
case IO_METHOD_USERPTR:
CLEAR(buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_USERPTR;
if (-1 == xioctl(fd, VIDIOC_DQBUF, &buf)) {
switch (errno) {
case EAGAIN:
return 0;
case EIO:
// Could ignore EIO, see spec. fall through
default:
errno_exit("VIDIOC_DQBUF");
}
}
for (i = 0; i < n_buffers; ++i)
if (buf.m.userptr == (unsigned long)buffers[i].start && buf.length == buffers[i].length)
break;
assert(i < n_buffers);
process_image((void *)buf.m.userptr, buf.bytesused);
if (-1 == xioctl(fd, VIDIOC_QBUF, &buf))
errno_exit("VIDIOC_QBUF");
break;
}
return 1;
}
void mainloop(void)
{
unsigned int count = frame_count;
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;
errno_exit("select");
}
if (0 == r) {
fprintf(stderr, "select timeout\n");
exit(EXIT_FAILURE);
}
if (read_frame())
break;
// EAGAIN - continue select loop
}
}
}
void stop_capturing(void)
{
enum v4l2_buf_type type;
switch (io) {
case IO_METHOD_READ:
// Nothing to do
break;
case IO_METHOD_MMAP:
case IO_METHOD_USERPTR:
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == xioctl(fd, VIDIOC_STREAMOFF, &type))
errno_exit("VIDIOC_STREAMOFF");
break;
}
}
void start_capturing(void)
{
unsigned int i;
enum v4l2_buf_type type;
switch (io) {
case IO_METHOD_READ:
// Nothing to do
break;
case IO_METHOD_MMAP:
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 == xioctl(fd, VIDIOC_QBUF, &buf))
errno_exit("VIDIOC_QBUF");
}
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == xioctl(fd, VIDIOC_STREAMON, &type))
errno_exit("VIDIOC_STREAMON");
break;
case IO_METHOD_USERPTR:
for (i = 0; i < n_buffers; ++i) {
struct v4l2_buffer buf;
CLEAR(buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_USERPTR;
buf.index = i;
buf.m.userptr = (unsigned long)buffers[i].start;
buf.length = buffers[i].length;
if (-1 == xioctl(fd, VIDIOC_QBUF, &buf))
errno_exit("VIDIOC_QBUF");
}
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == xioctl(fd, VIDIOC_STREAMON, &type))
errno_exit("VIDIOC_STREAMON");
break;
}
f = fopen("usb.yuv", "w");
if (!f) {
errno_exit("fail to open file");
}
}
void uninit_device(void)
{
unsigned int i;
switch (io) {
case IO_METHOD_READ:
free(buffers[0].start);
break;
case IO_METHOD_MMAP:
for (i = 0; i < n_buffers; ++i)
if (-1 == munmap(buffers[i].start, buffers[i].length))
errno_exit("munmap");
break;
case IO_METHOD_USERPTR:
for (i = 0; i < n_buffers; ++i)
free(buffers[i].start);
break;
}
free(buffers);
fclose(f);
}
void init_read(unsigned int buffer_size)
{
buffers = (struct buffer*)calloc(1, sizeof(*buffers));
if (!buffers) {
fprintf(stderr, "Out of memory\n");
exit(EXIT_FAILURE);
}
buffers[0].length = buffer_size;
buffers[0].start = malloc(buffer_size);
if (!buffers[0].start) {
fprintf(stderr, "Out of memory\n");
exit(EXIT_FAILURE);
}
}
void init_mmap(void)
{
struct v4l2_requestbuffers req;
CLEAR(req);
req.count = 4;
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_MMAP;
if (-1 == xioctl(fd, VIDIOC_REQBUFS, &req)) {
if (EINVAL == errno) {
fprintf(stderr, "%s does not support memory mappingn\n", dev_name);
exit(EXIT_FAILURE);
} else {
errno_exit("VIDIOC_REQBUFS");
}
}
if (req.count < 2) {
fprintf(stderr, "Insufficient buffer memory on %s\n", dev_name);
exit(EXIT_FAILURE);
}
buffers = (struct buffer*)calloc(req.count, sizeof(*buffers));
if (!buffers) {
fprintf(stderr, "Out of memory\n");
exit(EXIT_FAILURE);
}
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 == xioctl(fd, VIDIOC_QUERYBUF, &buf))
errno_exit("VIDIOC_QUERYBUF");
buffers[n_buffers].length = buf.length;
buffers[n_buffers].start =
mmap(NULL, // start anywhere
buf.length,
PROT_READ | PROT_WRITE, // required
MAP_SHARED, // recommended
fd, buf.m.offset);
if (MAP_FAILED == buffers[n_buffers].start)
errno_exit("mmap");
}
}
void init_userp(unsigned int buffer_size)
{
struct v4l2_requestbuffers req;
CLEAR(req);
req.count = 4;
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_USERPTR;
if (-1 == xioctl(fd, VIDIOC_REQBUFS, &req)) {
if (EINVAL == errno) {
fprintf(stderr, "%s does not support user pointer i/on", dev_name);
exit(EXIT_FAILURE);
} else {
errno_exit("VIDIOC_REQBUFS");
}
}
buffers = (struct buffer*)calloc(4, sizeof(*buffers));
if (!buffers) {
fprintf(stderr, "Out of memory\n");
exit(EXIT_FAILURE);
}
for (n_buffers = 0; n_buffers < 4; ++n_buffers) {
buffers[n_buffers].length = buffer_size;
buffers[n_buffers].start = malloc(buffer_size);
if (!buffers[n_buffers].start) {
fprintf(stderr, "Out of memory\n");
exit(EXIT_FAILURE);
}
}
}
void init_device(void)
{
struct v4l2_capability cap;
struct v4l2_cropcap cropcap;
struct v4l2_crop crop;
struct v4l2_format fmt;
unsigned int min;
if (-1 == xioctl(fd, VIDIOC_QUERYCAP, &cap)) {
if (EINVAL == errno) {
fprintf(stderr, "%s is no V4L2 device\n", dev_name);
exit(EXIT_FAILURE);
} else {
errno_exit("VIDIOC_QUERYCAP");
}
}
if (!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)) {
fprintf(stderr, "%s is no video capture device\n", dev_name);
exit(EXIT_FAILURE);
}
switch (io) {
case IO_METHOD_READ:
if (!(cap.capabilities & V4L2_CAP_READWRITE)) {
fprintf(stderr, "%s does not support read i/o\n", dev_name);
exit(EXIT_FAILURE);
}
break;
case IO_METHOD_MMAP:
case IO_METHOD_USERPTR:
if (!(cap.capabilities & V4L2_CAP_STREAMING)) {
fprintf(stderr, "%s does not support streaming i/o\n", dev_name);
exit(EXIT_FAILURE);
}
break;
}
// Select video input, video standard and tune here
CLEAR(cropcap);
cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (0 == xioctl(fd, VIDIOC_CROPCAP, &cropcap)) {
crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
crop.c = cropcap.defrect; // reset to default
if (-1 == xioctl(fd, VIDIOC_S_CROP, &crop)) {
switch (errno) {
case EINVAL:
// Cropping not supported
break;
default:
// Errors ignored
break;
}
}
} else {
// Errors ignored
}
CLEAR(fmt);
fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (force_format) {
fmt.fmt.pix.width = width;
fmt.fmt.pix.height = height;
fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
// Note VIDIOC_S_FMT may change width and height
if (-1 == xioctl(fd, VIDIOC_S_FMT, &fmt))
errno_exit("VIDIOC_S_FMT");
} else {
// Preserve original settings as set by v4l2-ctl for example
if (-1 == xioctl(fd, VIDIOC_G_FMT, &fmt))
errno_exit("VIDIOC_G_FMT");
}
// Buggy driver paranoia
min = fmt.fmt.pix.width * 2;
if (fmt.fmt.pix.bytesperline < min)
fmt.fmt.pix.bytesperline = min;
min = fmt.fmt.pix.bytesperline * fmt.fmt.pix.height;
if (fmt.fmt.pix.sizeimage < min)
fmt.fmt.pix.sizeimage = min;
switch (io) {
case IO_METHOD_READ:
init_read(fmt.fmt.pix.sizeimage);
break;
case IO_METHOD_MMAP:
init_mmap();
break;
case IO_METHOD_USERPTR:
init_userp(fmt.fmt.pix.sizeimage);
break;
}
}
void close_device(void)
{
if (-1 == close(fd))
errno_exit("close");
fd = -1;
}
void open_device(void)
{
struct stat st;
if (-1 == stat(dev_name, &st)) {
fprintf(stderr, "Cannot identify '%s': %d, %s\n", dev_name, errno, strerror(errno));
exit(EXIT_FAILURE);
}
if (!S_ISCHR(st.st_mode)) {
fprintf(stderr, "%s is no devicen\n", dev_name);
exit(EXIT_FAILURE);
}
fd = open(dev_name, O_RDWR | O_NONBLOCK, 0); // O_RDWR: required
if (-1 == fd) {
fprintf(stderr, "Cannot open '%s': %d, %s\n", dev_name, errno, strerror(errno));
exit(EXIT_FAILURE);
}
}
} // namespace
int test_v4l2_usb_stream()
{
// reference: https://linuxtv.org/downloads/v4l-dvb-apis-new/uapi/v4l/capture.c.html
dev_name = "/dev/video0";
open_device();
init_device();
start_capturing();
mainloop();
stop_capturing();
uninit_device();
close_device();
fprintf(stdout, "test finish\n");
return 0;
}
#else
int test_v4l2_usb_stream()
{
fprintf(stderr, "Error: this test code only support linux platform\n");
return -1;
}
#endif
通过ffplay播放生成的usb.yuv文件,执行结果如下:
GitHub:https://github.com//fengbingchun/OpenCV_Test