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打开设备–> 检查和设备设备属性–>设置帧格式–>设置一种输入输出的方法（缓冲区管理)–>循环获取数据–>关闭设备
1. 打开视频设备
fd = open("/dev/video0",O_RDWR);2.查询视频设备能力 VIDIOC_QUERYCAP
ioctl_ops(fd,VIDIOC_QUERYCAP,&cap);
if(!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)){
perror("this is not cap");
return -1;
}
struct v4l2_capability cap;
struct v4l2_capability {
__u8 driver[16]; 驱动名词
__u8 card[32]; 设备名字
__u8 bus_info[32]; 设备在系统中的位置
__u32 version; 驱动版本号
__u32 capabilities; 设置支持的操作
__u32 device_caps;
__u32 reserved[3];
};
/* Values for 'capabilities' field */
#define V4L2_CAP_VIDEO_CAPTURE 0x00000001 /* Is a video capture device */
#define V4L2_CAP_VIDEO_OUTPUT 0x00000002 /* Is a video output device */
#define V4L2_CAP_VIDEO_OVERLAY 0x00000004 /* Can do video overlay */
#define V4L2_CAP_VBI_CAPTURE 0x00000010 /* Is a raw VBI capture device */
#define V4L2_CAP_VBI_OUTPUT 0x00000020 /* Is a raw VBI output device */
#define V4L2_CAP_SLICED_VBI_CAPTURE 0x00000040 /* Is a sliced VBI capture device */
#define V4L2_CAP_SLICED_VBI_OUTPUT 0x00000080 /* Is a sliced VBI output device */
#define V4L2_CAP_RDS_CAPTURE 0x00000100 /* RDS data capture */
#define V4L2_CAP_VIDEO_OUTPUT_OVERLAY 0x00000200 /* Can do video output overlay */
#define V4L2_CAP_HW_FREQ_SEEK 0x00000400 /* Can do hardware frequency seek */
#define V4L2_CAP_RDS_OUTPUT 0x00000800 /* Is an RDS encoder */
一般我们需要的是 V4L2_CAP_VIDEO_CAPTURE
调用底层 例如：
static int vidioc_querycap(struct file *file, void *priv,
struct v4l2_capability *cap)
{
struct vivid_dev *dev = video_drvdata(file);
strcpy(cap->driver, "vivid");
strcpy(cap->card, "vivid");
snprintf(cap->bus_info, sizeof(cap->bus_info),
"platform:%s", dev->v4l2_dev.name);
cap->capabilities = dev->vid_cap_caps | dev->vid_out_caps |
dev->vbi_cap_caps | dev->vbi_out_caps |
dev->radio_rx_caps | dev->radio_tx_caps |
dev->sdr_cap_caps | V4L2_CAP_DEVICE_CAPS;
return 0;
}
3.查询视频设备输出的格式 // 一般可以不做直接设置即可
VIDIOC_ENUM_FMT
int ioctl(int fd, int request, struct v4l2_fmtdesc *argp);
/*
* F O R M A T E N U M E R A T I O N
*/
struct v4l2_fmtdesc {
__u32 index; /* Format number */
__u32 type; /* enum v4l2_buf_type */
__u32 flags;
__u8 description[32]; /* Description string */
__u32 pixelformat; /* Format fourcc */
__u32 reserved[4];
};
4.设置视频采集格式
1. 设置摄像头的格式
2. 设置图片的长度，宽度，图片格式
//VIDIOC_S_FMT
my_fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE ;
my_fmt.fmt.pix.width = 720;
my_fmt.fmt.pix.height = 576;
my_fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
ioctl(fd, VIDIOC_S_FMT, &my_fmt);
/**
* struct v4l2_format - stream data format
* @type: enum v4l2_buf_type; type of the data stream
* @pix: definition of an image format
* @pix_mp: definition of a multiplanar image format
* @win: definition of an overlaid image
* @vbi: raw VBI capture or output parameters
* @sliced: sliced VBI capture or output parameters
* @raw_data: placeholder for future extensions and custom formats
*/
struct v4l2_format {
__u32 type;
union {
struct v4l2_pix_format pix; /* V4L2_BUF_TYPE_VIDEO_CAPTURE */
struct v4l2_pix_format_mplane pix_mp; /* V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE */
struct v4l2_window win; /* V4L2_BUF_TYPE_VIDEO_OVERLAY */
struct v4l2_vbi_format vbi; /* V4L2_BUF_TYPE_VBI_CAPTURE */
struct v4l2_sliced_vbi_format sliced; /* V4L2_BUF_TYPE_SLICED_VBI_CAPTURE */
struct v4l2_sdr_format sdr; /* V4L2_BUF_TYPE_SDR_CAPTURE */
struct v4l2_meta_format meta; /* V4L2_BUF_TYPE_META_CAPTURE */
__u8 raw_data[200]; /* user-defined */
} fmt;
};
/*
* V I D E O I M A G E F O R M A T
*/
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 */
__u32 flags; /* format flags (V4L2_PIX_FMT_FLAG_*) */
union {
/* enum v4l2_ycbcr_encoding */
__u32 ycbcr_enc;
/* enum v4l2_hsv_encoding */
__u32 hsv_enc;
};
__u32 quantization; /* enum v4l2_quantization */
__u32 xfer_func; /* enum v4l2_xfer_func */
};
/* RGB formats */
#define V4L2_PIX_FMT_RGB332 v4l2_fourcc('R', 'G', 'B', '1') /* 8 RGB-3-3-2 */
#define V4L2_PIX_FMT_RGB444 v4l2_fourcc('R', '4', '4', '4') /* 16 xxxxrrrr ggggbbbb */
#define V4L2_PIX_FMT_ARGB444 v4l2_fourcc('A', 'R', '1', '2') /* 16 aaaarrrr ggggbbbb */
#define V4L2_PIX_FMT_XRGB444 v4l2_fourcc('X', 'R', '1', '2') /* 16 xxxxrrrr ggggbbbb */
#define V4L2_PIX_FMT_RGB555 v4l2_fourcc('R', 'G', 'B', 'O') /* 16 RGB-5-5-5 */
#define V4L2_PIX_FMT_ARGB555 v4l2_fourcc('A', 'R', '1', '5') /* 16 ARGB-1-5-5-5 */
#define V4L2_PIX_FMT_XRGB555 v4l2_fourcc('X', 'R', '1', '5') /* 16 XRGB-1-5-5-5 */
#define V4L2_PIX_FMT_RGB565 v4l2_fourcc('R', 'G', 'B', 'P') /* 16 RGB-5-6-5 */
....
/* Grey formats */
#define V4L2_PIX_FMT_GREY v4l2_fourcc('G', 'R', 'E', 'Y') /* 8 Greyscale */
#define V4L2_PIX_FMT_Y4 v4l2_fourcc('Y', '0', '4', ' ') /* 4 Greyscale */
#define V4L2_PIX_FMT_Y6 v4l2_fourcc('Y', '0', '6', ' ') /* 6 Greyscale */
#define V4L2_PIX_FMT_Y10 v4l2_fourcc('Y', '1', '0', ' ') /* 10 Greyscale */
#define V4L2_PIX_FMT_Y12 v4l2_fourcc('Y', '1', '2', ' ') /* 12 Greyscale */
....
调用底层： 例如
static int vidioc_s_fmt_vid_cap(struct file *file, void *priv,
struct v4l2_format *f)
{
struct comp_fh *fh = priv;
struct most_video_dev *mdev = fh->mdev;
return comp_set_format(mdev, VIDIOC_S_FMT, f);
}
static int comp_set_format(struct most_video_dev *mdev, unsigned int cmd,
struct v4l2_format *format)
{
if (format->fmt.pix.pixelformat != V4L2_PIX_FMT_MPEG)
return -EINVAL;
if (cmd == VIDIOC_TRY_FMT)
return 0;
comp_set_format_struct(format);
return 0;
}
static void comp_set_format_struct(struct v4l2_format *f)
{
f->fmt.pix.width = 8;
f->fmt.pix.height = 8;
f->fmt.pix.pixelformat = V4L2_PIX_FMT_MPEG;
f->fmt.pix.bytesperline = 0;
f->fmt.pix.sizeimage = 188 * 2;
f->fmt.pix.colorspace = V4L2_COLORSPACE_REC709;
f->fmt.pix.field = V4L2_FIELD_NONE;
f->fmt.pix.priv = 0;
}
5.获取视频采集格式 // 查看一下是否自己设置成功，当然也可以不设置
VIDOC_G_FMT
调用底层： 例如
static int vidioc_g_fmt_vid_cap(struct file *file, void *priv,
struct v4l2_format *f)
{
comp_set_format_struct(f);
return 0;
}
这里又设置了一遍 ！！！6.申请内核空间的视频缓冲区
1. 需要申请的缓冲区个数
2.申请的格式及映射方式
//VIDIOC_REQBUFS
my_requestbuffer.count = 4;
my_requestbuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
my_requestbuffer.memory = V4L2_MEMORY_MMAP;
ioctl(fd, VIDIOC_REQBUFS, &my_requestbuffer);
struct v4l2_requestbuffers v4l2_buf;
struct v4l2_requestbuffers {
__u32 count;
__u32 type; /* enum v4l2_buf_type */
__u32 memory; /* enum v4l2_memory */
__u32 reserved[2];
};
enum v4l2_buf_type { / 看之前的结构体
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,
V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY = 8,
V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE = 9,
V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE = 10,
V4L2_BUF_TYPE_SDR_CAPTURE = 11,
V4L2_BUF_TYPE_SDR_OUTPUT = 12,
V4L2_BUF_TYPE_META_CAPTURE = 13,
/* Deprecated, do not use */
V4L2_BUF_TYPE_PRIVATE = 0x80,
};
enum v4l2_memory { // 看之前的结构体
V4L2_MEMORY_MMAP = 1,
V4L2_MEMORY_USERPTR = 2,
V4L2_MEMORY_OVERLAY = 3,
V4L2_MEMORY_DMABUF = 4,
};
调用底层 ： 例如
/* vb2 ioctl helpers */
int vb2_ioctl_reqbufs(struct file *file, void *priv,
struct v4l2_requestbuffers *p)
{
struct video_device *vdev = video_devdata(file);
int res = vb2_verify_memory_type(vdev->queue, p->memory, p->type);
if (res)
return res;
if (vb2_queue_is_busy(vdev, file))
return -EBUSY;
res = vb2_core_reqbufs(vdev->queue, p->memory, &p->count);
/* If count == 0, then the owner has released all buffers and he
is no longer owner of the queue. Otherwise we have a new owner. */
if (res == 0)
vdev->queue->owner = p->count ? file->private_data : NULL;
return res;
}
EXPORT_SYMBOL_GPL(vb2_ioctl_reqbufs);7.查询内核空间视频缓冲区信息
ioctl_ops(fd,VIDIOC_QUERYBUF,&v_buffer);
struct v4l2_buffer v_buffer;
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;
};
/**
* 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
*
* Contains data exchanged by application and driver using one of the Streaming
* I/O methods.
*/
调用底层： 如下
int vb2_ioctl_querybuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
struct video_device *vdev = video_devdata(file);
/* No need to call vb2_queue_is_busy(), anyone can query buffers. */
return vb2_querybuf(vdev->queue, p);
}8.根据查询的缓冲区信息进行内存映射mmap
struct v4l2_buffer my_buffer; // 系统给的结构体
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; // 偏移量 （4个缓冲区）
unsigned long userptr;
struct v4l2_plane *planes;
__s32 fd;
} m;
__u32 length; // 长度
__u32 reserved2;
__u32 reserved;
};
struct mmap_addr{
void *addr;
int length;
};
struct mmap_addr *addr;
addr = calloc(4, sizeof(struct mmap_addr));
for(i = 0; i < 4; i++)
{
my_buffer.index = i;
my_buffer.type = V4L2_CAP_VIDEO_CAPTURE;
my_buffer.memory = V4L2_MEMORY_MMAP;
ioctl(fd, VIDIOC_QUERYBUF, &my_buffer); //查询内核缓冲区信息
//mmap
addr[i].length = my_buffer.length;
addr[i].addr = mmap(NULL, my_buffer.length, PROT_READ | PROT_WRITE, MAP_SHARED, fd, my_buffer.m.offset);
}
ioctl(fd, VIDIOC_QBUF,&my_buffer); // 投放一个空的视频缓冲区
9.投放一个空的视频缓冲区视频
ioctl(fd, VIDIOC_QBUF,&my_buffer); // 投放一个空的视频缓冲区
调用底层： 例如
int vb2_ioctl_qbuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
struct video_device *vdev = video_devdata(file);
if (vb2_queue_is_busy(vdev, file))
return -EBUSY;
return vb2_qbuf(vdev->queue, p);
}
int vb2_qbuf(struct vb2_queue *q, struct v4l2_buffer *b)
{
int ret;
if (vb2_fileio_is_active(q)) {
dprintk(1, "file io in progress\n");
return -EBUSY;
}
ret = vb2_queue_or_prepare_buf(q, b, "qbuf");
return ret ? ret : vb2_core_qbuf(q, b->index, b);
}
10.启动视频采集
//VIDIOC_STREAMON
int v4l2_type;
v4l2_type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
ioctl(fd, VIDIOC_STREAMON, &v4l2_type);
调用底层： 例如
int vb2_ioctl_streamon(struct file *file, void *priv, enum v4l2_buf_type i)
{
struct video_device *vdev = video_devdata(file);
if (vb2_queue_is_busy(vdev, file))
return -EBUSY;
return vb2_streamon(vdev->queue, i);
}
int vb2_core_streamon(struct vb2_queue *q, unsigned int type)：
if (q->queued_count >= q->min_buffers_needed) {
ret = v4l_vb2q_enable_media_source(q);
if (ret)
return ret;
ret = vb2_start_streaming(q);
if (ret)
return ret;
}
static int vb2_start_streaming(struct vb2_queue *q) ：
list_for_each_entry(vb, &q->queued_list, queued_entry)
__enqueue_in_driver(vb);
/* Tell the driver to start streaming */
q->start_streaming_called = 1;
ret = call_qop(q, start_streaming, q,
atomic_read(&q->owned_by_drv_count));
/*
* Forcefully reclaim buffers if the driver did not
* correctly return them to vb2.
*/
for (i = 0; i < q->num_buffers; ++i) {
vb = q->bufs[i];
if (vb->state == VB2_BUF_STATE_ACTIVE)
vb2_buffer_done(vb, VB2_BUF_STATE_QUEUED);
}11.等待视频采集完成 select
FD_ZERO(&rfd);
FD_SET(fd, &rfd);
ret = select(fd+1, &rfd, NULL, NULL, NULL);
if(ret > 0){
get_buffer(fd);
}else{
return -1;
}12.从视频缓冲区获取一个已有视频数据的buffer， 进行数据处理
memeset(&my_buffer, 0, sizeof(my_buffer));
my_buffer.type = V4L2_CAP_VIDEO_CAPTURE;
my_buffer.memory = V4L2_MEMORY_MMAP;
ioctl(fd, VIDIOC_DQBUF, &my_buffer);
这里就可以对数据进行处理，
例如写入到文本中。
在投放一个空的缓冲区入内核。
ioctl(fd, VIDIOC_QBUF, &my_buffer);
调用底层： 例如
int vb2_ioctl_dqbuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
struct video_device *vdev = video_devdata(file);
if (vb2_queue_is_busy(vdev, file))
return -EBUSY;
return vb2_dqbuf(vdev->queue, p, file->f_flags & O_NONBLOCK);
}
int vb2_dqbuf(struct vb2_queue *q, struct v4l2_buffer *b, bool nonblocking)
{
int ret;
if (vb2_fileio_is_active(q)) {
dprintk(1, "file io in progress\n");
return -EBUSY;
}
if (b->type != q->type) {
dprintk(1, "invalid buffer type\n");
return -EINVAL;
}
ret = vb2_core_dqbuf(q, NULL, b, nonblocking);
/*
* After calling the VIDIOC_DQBUF V4L2_BUF_FLAG_DONE must be
* cleared.
*/
b->flags &= ~V4L2_BUF_FLAG_DONE;
return ret;
}
EXPORT_SYMBOL_GPL(vb2_dqbuf);13.关闭视频设备
//free
v4l2_type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
ioctl(fd, VIDIOC_STREAMOFF, &v4l2_type);
for(i = 0; i < 4; i++){
munmap(addr[i].addr, addr[i].length);
}
free(addr);
close(fd);其他的相关的 ：
设置视频的制式和帧格式 : 制式有 PAL, NTSC 帧格式包括宽度和高度等。
int ioctl( int fd, int request, struct v4l2_ftmdesc *argp）;
int ioctl(int fd, int request, struct v4l2_format *argp);
v4l2_cropcap 结构体用来设置摄像头捕捉能力，在捕捉视频时应先设置v4l2_cropcap的type域：
在通过VIDIO_CROPCAP操作命令获取设备的捕捉能力的参数，保存在v4l2_cropcap结构体中，包括bounds(最大捕捉方框的左上角坐标宽高），defrect默认捕捉方框的左上角坐标和宽高）等
v4l2_format结构体用来设置摄像头的视频制式，帧格式等。用VIDIO_S_FMT来设置
图像的缩放VIDIO_CROPCAP
int ioctl(int fd, int request, struct v4l2_cropcap *argp);
参考来源： 思睿
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