基于v4l2框架应用层的摄像头梳理
------------恢复内容开始------------
# 1.官方的测试程序 ### 代码路径 media/v4l/capture.c点击查看代码
/*
* V4L2 video capture example
*
* This program can be used and distributed without restrictions.
# *
* This program is provided with the V4L2 API
* see https://linuxtv.org/docs.php for more information
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <getopt.h> /* getopt_long() */
#include <fcntl.h> /* low-level i/o */
#include <unistd.h>
#include <errno.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
#include <linux/videodev2.h>
#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;
};
static char *dev_name;
static enum io_method io = IO_METHOD_MMAP;
static int fd = -1;
struct buffer *buffers;
static unsigned int n_buffers;
static int out_buf;
static int force_format;
static int frame_count = 70;
static void errno_exit(const char *s)
{
fprintf(stderr, "%s error %d, %s\\n", s, errno, strerror(errno));
exit(EXIT_FAILURE);
}
static int xioctl(int fh, int request, void *arg)
{
int r;
do {
r = ioctl(fh, request, arg);
} while (-1 == r && EINTR == errno);
return r;
}
static void process_image(const void *p, int size)
{
if (out_buf)
fwrite(p, size, 1, stdout);
fflush(stderr);
fprintf(stderr, ".");
fflush(stdout);
}
static 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;
}
static void mainloop(void)
{
unsigned int count;
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. */
}
}
}
static 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;
}
}
static 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;
}
}
static 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);
}
static void init_read(unsigned int buffer_size)
{
buffers = 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);
}
}
static 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", 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 = 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");
}
}
static 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 = 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);
}
}
}
static 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 = 640;
fmt.fmt.pix.height = 480;
fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
if (-1 == xioctl(fd, VIDIOC_S_FMT, &fmt))
errno_exit("VIDIOC_S_FMT");
/* Note VIDIOC_S_FMT may change width and height. */
} 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;
}
}
static void close_device(void)
{
if (-1 == close(fd))
errno_exit("close");
fd = -1;
}
static 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", dev_name);
exit(EXIT_FAILURE);
}
fd = open(dev_name, O_RDWR /* required */ | O_NONBLOCK, 0);
if (-1 == fd) {
fprintf(stderr, "Cannot open '%s': %d, %s\\n",
dev_name, errno, strerror(errno));
exit(EXIT_FAILURE);
}
}
static void usage(FILE *fp, int argc, char **argv)
{
fprintf(fp,
"Usage: %s [options]\\n\\n"
"Version 1.3\\n"
"Options:\\n"
"-d | --device name Video device name [%s]n"
"-h | --help Print this messagen"
"-m | --mmap Use memory mapped buffers [default]n"
"-r | --read Use read() callsn"
"-u | --userp Use application allocated buffersn"
"-o | --output Outputs stream to stdoutn"
"-f | --format Force format to 640x480 YUYVn"
"-c | --count Number of frames to grab [%i]n"
"",
argv[0], dev_name, frame_count);
}
static const char short_options[] = "d:hmruofc:";
static const struct option
long_options[] = {
{ "device", required_argument, NULL, 'd' },
{ "help", no_argument, NULL, 'h' },
{ "mmap", no_argument, NULL, 'm' },
{ "read", no_argument, NULL, 'r' },
{ "userp", no_argument, NULL, 'u' },
{ "output", no_argument, NULL, 'o' },
{ "format", no_argument, NULL, 'f' },
{ "count", required_argument, NULL, 'c' },
{ 0, 0, 0, 0 }
};
int main(int argc, char **argv)
{
dev_name = "/dev/video0";
for (;;) {
int idx;
int c;
c = getopt_long(argc, argv,
short_options, long_options, &idx);
if (-1 == c)
break;
switch (c) {
case 0: /* getopt_long() flag */
break;
case 'd':
dev_name = optarg;
break;
case 'h':
usage(stdout, argc, argv);
exit(EXIT_SUCCESS);
case 'm':
io = IO_METHOD_MMAP;
break;
case 'r':
io = IO_METHOD_READ;
break;
case 'u':
io = IO_METHOD_USERPTR;
break;
case 'o':
out_buf++;
break;
case 'f':
force_format++;
break;
case 'c':
errno = 0;
frame_count = strtol(optarg, NULL, 0);
if (errno)
errno_exit(optarg);
break;
default:
usage(stderr, argc, argv);
exit(EXIT_FAILURE);
}
}
open_device();
init_device();
start_capturing();
mainloop();-
stop_capturing();
uninit_device();
close_device();
fprintf(stderr, "\\n");
return 0;
}
打开视频设备
// 用非阻塞模式打开摄像头设备
cameraFd = open("/dev/video0", O_RDWR | O_NONBLOCK, 0);
// 如果用阻塞模式打开摄像头设备，上述代码变为：
cameraFd = open("/dev/video0", O_RDWR, 0);
关于阻塞模式和非阻塞模式：应用程序能够使用阻塞模式或非阻塞模式打开视频设备，如果使用非阻塞模式调用视频设备，即使尚未捕获到信息，驱动依旧会把缓存（DQBUFF）里的东西返回给应用程序。
设定属性及采集方式
int ioctl (int __fd, unsigned long int __request, ...) __THROW;
在进行V4L2开发中，一般会用到以下的命令标志符：
- VIDIOC_REQBUFS：分配内存
- VIDIOC_QUERYBUF：把VIDIOC_REQBUFS中分配的数据缓存转换成物理地址
- VIDIOC_QUERYCAP：查询驱动功能
- VIDIOC_ENUM_FMT：获取当前驱动支持的视频格式
- VIDIOC_S_FMT：设置当前驱动的频捕获格式
- VIDIOC_G_FMT：读取当前驱动的频捕获格式
- VIDIOC_TRY_FMT：验证当前驱动的显示格式
- VIDIOC_CROPCAP：查询驱动的修剪能力
- VIDIOC_S_CROP：设置视频信号的边框
- VIDIOC_G_CROP：读取视频信号的边框
- VIDIOC_QBUF：把数据从缓存中读取出来
- VIDIOC_DQBUF：把数据放回缓存队列
- VIDIOC_STREAMON：开始视频显示函数
- VIDIOC_STREAMOFF：结束视频显示函数
- VIDIOC_QUERYSTD：检查当前视频设备支持的标准，例如PAL或NTSC
检查当前视频设备支持的标准
ret = ioctl(fd, VIDIOC_QUERYSTD, &std);
} while (ret == -1 && errno == EAGAIN);
switch (std) {
case V4L2_STD_NTSC:
//……
case V4L2_STD_PAL:
//……
}
设置视频捕获格式
struct v4l2_format fmt;
memset ( &fmt, 0, sizeof(fmt) );
fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
fmt.fmt.pix.width = 720;
fmt.fmt.pix.height = 576;
fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
if (ioctl(fd, VIDIOC_S_FMT, &fmt) == -1) {
return -1;
}
v4l2_format结构体定义如下：
struct v4l2_format
{
enum v4l2_buf_type type; // 数据流类型，必须永远是V4L2_BUF_TYPE_VIDEO_CAPTURE
union
{
struct v4l2_pix_format pix;
struct v4l2_window win;
struct v4l2_vbi_format vbi;
__u8 raw_data[200];
} fmt;
};
struct v4l2_pix_format
{
__u32 width; // 宽，必须是16的倍数
__u32 height; // 高，必须是16的倍数
__u32 pixelformat; // 视频数据存储类型，例如是YUV4：2：2还是RGB
enum v4l2_field field;
__u32 bytesperline;
__u32 sizeimage;
enum v4l2_colorspace colorspace;
__u32 priv;
};
分配内存
接下来可以为视频捕获分配内存：
struct v4l2_requestbuffers req;
if (ioctl(fd, VIDIOC_REQBUFS, &req) == -1) {
return -1;
}
v4l2_requestbuffers定义如下：
struct v4l2_requestbuffers
{
__u32 count; // 缓存数量，也就是说在缓存队列里保持多少张照片
enum v4l2_buf_type type; // 数据流类型，必须永远是V4L2_BUF_TYPE_VIDEO_CAPTURE
enum v4l2_memory memory; // V4L2_MEMORY_MMAP 或 V4L2_MEMORY_USERPTR
__u32 reserved[2];
};
7 获取并记录缓存的物理空间
使用VIDIOC_REQBUFS，我们获取了req.count个缓存，下一步通过调用VIDIOC_QUERYBUF命令来获取这些缓存的地址，然后使用mmap函数转换成应用程序中的绝对地址，最后把这段缓存放入缓存队列：
typedef struct VideoBuffer {
void *start;
size_t length;
} VideoBuffer;
VideoBuffer* buffers = calloc( req.count, sizeof(*buffers) );
struct v4l2_buffer buf;
for (numBufs = 0; numBufs < req.count; numBufs++) {
memset( &buf, 0, sizeof(buf) );
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = numBufs;
// 读取缓存
if (ioctl(fd, VIDIOC_QUERYBUF, &buf) == -1) {
return -1;
}
buffers[numBufs].length = buf.length;
// 转换成相对地址
buffers[numBufs].start = mmap(NULL, buf.length,
PROT_READ | PROT_WRITE,
MAP_SHARED,
fd, buf.m.offset);
if (buffers[numBufs].start == MAP_FAILED) {
return -1;
}
// 放入缓存队列
if (ioctl(fd, VIDIOC_QBUF, &buf) == -1) {
return -1;
}
}
8、 关于视频采集方式
一共有三种视频采集方式：
1）使用read、write方式：直接使用 read 和 write 函数进行读写。这种方式最简单，但是这种方式会在 用户空间和内核空间不断拷贝数据 ，同时在用户空间和内核空间占用 了 大量内存，效率不高。
2）内存映射方式（mmap）：把设备里的内存映射到应用程序中的内存控件，直接处理设备内存，这是一种有效的方式。上面的mmap函数就是使用这种方式。
3）用户指针模式：内存由用户空间的应用程序分配，并把地址传递到内核中的驱动程序，然后由 v4l2 驱动程序直接将数据填充到用户空间的内存中。这点需要在v4l2_requestbuffers里将memory字段设置成V4L2_MEMORY_USERPTR。
第一种方式效率是最低的，后面两种方法都能提高执行的效率，但是对于mmap 方式，文档中有这样一句描述 --Remember the buffers are allocated in physical memory, as opposed to virtual memory which can be swapped out to disk. Applications should free the buffers as soon as possible with the munmap () function .(使用mmap方法的时候，buffers相当于是在内核空间中分配的，这种情况下，这些buffer是不能被交换到虚拟内存中，虽然这种方法不怎么影响读写效率，但是它一直占用着内核空间中的内存，当系统的内存有限的时候，如果同时运行有大量的进程，则对系统的整体性能会有一定的影响。)
所以，对于三种视频采集方式的选择，推荐的顺序是 userptr 、 mmap 、 read-write 。当使用 mmap 或 userptr 方式的时候，有一个环形缓冲队列的概念，这个队列中，有 n 个 buffer ，驱动程序采集到的视频帧数据，就是存储在每个 buffer 中。在每次用 VIDIOC_DQBUF 取出一个 buffer ，并且处理完数据后，一定要用 VIDIOC_QBUF 将这个 buffer 再次放回到环形缓冲队列中。环形缓冲队列，也使得这两种视频采集方式的效率高于直接 read/write 。
9、 处理采集数据
V4L2有一个数据缓存，存放req.count数量的缓存数据。数据缓存采用FIFO的方式，当应用程序调用缓存数据时，缓存队列将最先采集到的 视频数据缓存送出，并重新采集一张视频数据。这个过程需要用到两个ioctl命令,VIDIOC_DQBUF和VIDIOC_QBUF：
struct v4l2_buffer buf;
memset(&buf,0,sizeof(buf));
buf.type=V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory=V4L2_MEMORY_MMAP;
buf.index=0;
//读取缓存
if (ioctl(cameraFd, VIDIOC_DQBUF, &buf) == -1)
{
return -1;
}
//…………视频处理算法
//重新放入缓存队列
if (ioctl(cameraFd, VIDIOC_QBUF, &buf) == -1) {
return -1;
}
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