Programming implementation of V4L2 camera - LCD display of GEC6818

Camera under Linux - V4L2 (complete code implementation at the end)

 

The V4L2 driver framework accesses the system IO interface—the ioctl function (ioctl is dedicated to the hardware control system IO interface). Its role is to make the hardware work normally and provide an interface for the upper application program.

First of all, I first determine the camera device and check it in SecureCRT

Connect the camera to the USB interface of the development board. If the device is normal, the driver framework should output prompt information, such as usb2.0 pc camera, etc.

Find the device by command

ls /dev/video*  

You can see that the extra /dev/video7 is our target device. 

1. Open the camera access of V4L2

Required header file: #include <linux/video2.h>

    int fd_v4l2 = open("/dev/video7", O_RDWR);

2. Obtain the function parameters of the camera

It can be found in videodev2.h, here you only need to set the parameters related to yourself

/**
  * struct v4l2_capability - Describes V4L2 device caps returned by VIDIOC_QUERYCAP
  *
  * @driver:	   name of the driver module (e.g. "bttv")
  * @card:	   name of the card (e.g. "Hauppauge WinTV")
  * @bus_info:	   name of the bus (e.g. "PCI:" + pci_name(pci_dev) )
  * @version:	   KERNEL_VERSION
  * @capabilities: capabilities of the physical device as a whole
  * @device_caps:  capabilities accessed via this particular device (node)
  * @reserved:	   reserved fields for future extensions
  */
struct v4l2_capability {
	__u8	driver[16];
	__u8	card[32];
	__u8	bus_info[32];
	__u32   version;         //内核版本
	__u32	capabilities;    //功能参数（设置这里）    
	__u32	device_caps;
	__u32	reserved[3];
};

Code:

// 获取功能参数
    struct v4l2_capability cap = {};
    int res = ioctl(fd_v4l2, VIDIOC_QUERYCAP, &cap);
    if (res == -1)
    {
        perror("ioctl cap");
        exit(-1);
    }

3. Obtain the format supported by the camera (YUYV)

/*
 *	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];
};

Code:

//有个定义格式的宏
#define v4l2_fourcc(a,b,c,d) \ ((__u32)(a) | (__u32)(b) << 8 | (__u32)(c) << 16) | (__u32)(d) << 16 | (__u32)(d) << 24)



// 获取摄像头支持的格式
    struct v4l2_fmtdesc fmt = {};
    fmt.index = 0;
    fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; // 获取摄像头格式


    ioctl(fd,VIDIOC_ENUM_FMT,&fmt);

The LCD screen of GEC6818 is RGB, and the data collected by V4L2 is in YUYV format. Here you need to convert YUYV to RGB. I found a high-porting function on GitHub.

unsigned int sign3 = 0;

int yuyv2rgb(int y, int u, int v)
{
    unsigned int pixel24 = 0;
    unsigned char *pixel = (unsigned char *)&pixel24;
    int r, g, b;
    static int ruv, guv, buv;

    if (sign3)
    {
        sign3 = 0;
        ruv = 1159 * (v - 128);
        guv = 380 * (u - 128) + 813 * (v - 128);
        buv = 2018 * (u - 128);
    }

    r = (1164 * (y - 16) + ruv) / 1000;
    g = (1164 * (y - 16) - guv) / 1000;
    b = (1164 * (y - 16) + buv) / 1000;

    if (r > 255)
        r = 255;
    if (g > 255)
        g = 255;
    if (b > 255)
        b = 255;
    if (r < 0)
        r = 0;
    if (g < 0)
        g = 0;
    if (b < 0)
        b = 0;

    pixel[0] = r;
    pixel[1] = g;
    pixel[2] = b;

    return pixel24;
}

//YUYV转rgb
int yuyv2rgb0(unsigned char *yuv, unsigned char *rgb, unsigned int width, unsigned int height)
{
    unsigned int in, out;
    int y0, u, y1, v;
    unsigned int pixel24;
    unsigned char *pixel = (unsigned char *)&pixel24;
    unsigned int size = width * height * 2;

    for (in = 0, out = 0; in < size; in += 4, out += 6)
    {
        y0 = yuv[in + 0];
        u = yuv[in + 1];
        y1 = yuv[in + 2];
        v = yuv[in + 3];

        sign3 = 1;
        pixel24 = yuyv2rgb(y0, u, v);
        rgb[out + 0] = pixel[0];
        rgb[out + 1] = pixel[1];
        rgb[out + 2] = pixel[2];

        pixel24 = yuyv2rgb(y1, u, v);
        rgb[out + 3] = pixel[0];
        rgb[out + 4] = pixel[1];
        rgb[out + 5] = pixel[2];
    }

    return 0;
}

4. Set the access camera channel

// 设置采集通道
    int index = 0; // 使用通道0
    res = ioctl(fd_v4l2, VIDIOC_S_INPUT, &index);
    if (res == -1)
    {
        perror("ioctl_s_input");
        exit(-1);
    }

5. Set the acquisition format and parameters of the acquisition camera

/**
 * 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;
};

This is a union, just identify as a device

Code:

// 设置摄像头采集格式
    struct v4l2_format format = {};
    format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    format.fmt.pix.width = 640;
    format.fmt.pix.height = 480;
    format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV; // YUYV
    format.fmt.pix.field = V4L2_FIELD_NONE;
    res = ioctl(fd_v4l2, VIDIOC_S_FMT, &format);
    if (res == -1)
    {
        perror("ioctl s_fmt");
        exit(-1);
    }

Sixth, apply for cache space, map to user space, and design the buffer as a queue

// 申请缓存空间
    struct v4l2_requestbuffers req = {};
    req.count = 4;
    req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    req.memory = V4L2_MEMORY_MMAP;
    res = ioctl(fd_v4l2, VIDIOC_REQBUFS, &req);
    if (res == -1)
    {
        perror("ioctl reqbufs");
        exit(-1);
    }

    // 分配映射入队
    size_t i, max_len = 0;
    for (i = 0; i < 4; i++)
    {
        struct v4l2_buffer buf = {};
        buf.index = i; // 0~3展现4帧图片
        buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;
        res = ioctl(fd_v4l2, VIDIOC_QUERYBUF, &buf);
        if (res == -1)
        {
            perror("ioctl querybuf");
            exit(-1);
        }

        // 判读并记录最大长度，以适配各个图帧
        if (buf.length > max_len)
            max_len = buf.length;

        // 映射
        buffer[i].length = buf.length;
        buffer[i].start = mmap(NULL, buf.length, PROT_READ | PROT_WRITE, MAP_SHARED, fd_v4l2, buf.m.offset);
        if (buffer[i].start == MAP_FAILED)
        {
            perror("mmap");
            exit(-1);
        }

        // 入队
        res = ioctl(fd_v4l2, VIDIOC_QBUF, &buf);
        if (res == -1)
        {
            perror("ioctl qbuf");
            exit(-1);
        }
    }

Seven, start the camera acquisition

// 启动摄像头
    enum v4l2_buf_type buf_type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    res = ioctl(fd_v4l2, VIDIOC_STREAMON, &buf_type);
    if (res == -1)
    {
        perror("ioctl streamon");
        exit(-1);
    }

8. Obtain camera data from the collection queue

while (1)
    {
        struct v4l2_buffer buf = {};
        buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;

        // 出队
        res = ioctl(fd_v4l2, VIDIOC_DQBUF, &buf);
        if (res == -1)
        {
            perror("ioctl dqbuf");
        }

        // 拷贝数据
        memcpy(current.start, buffer[buf.index].start, buf.bytesused);
        current.length = buf.bytesused;

        // 入队
        res = ioctl(fd_v4l2, VIDIOC_QBUF, &buf);
        if (res == -1)
        {
            perror("ioctl qbuf");
        }

Nine, close the camera collection

// 关闭摄像头采集
    buf_type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    res = ioctl(fd_v4l2, VIDIOC_STREAMOFF, &buf_type);
    if (res == -1)
    {
        perror("ioctl streamoff");
        exit(-1);
    }

fully realized

/*************************************************************************
	> File Name: V4L2.c
	> Author: volcano.eth
	> Mail: [email protected]
	> Created Time: 2023年05月10日 星期四 15时46分08秒
 ************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <linux/videodev2.h>
#include <netinet/in.h>        //  TCP/IP协议所需头文件
#include <sys/socket.h>


typedef struct
{
    char *start;
    size_t length;
} buffer_t;

buffer_t buffer[4]; // 映射所需的素材缓存在数组中
buffer_t current;   // 保存当前输出的一帧

int *plcd;  // 用于存储屏幕缓冲区的首地址,以便后续对屏幕进行读写操作
int *lcd_p; // 指向屏幕缓冲区的特定位置
int lcd_fd; // 用于存储屏幕设备文件的文件描述符

void *lcd_init()
{
    lcd_fd = open("/dev/fb0", O_RDWR);

    if (lcd_fd == -1)
    {
        perror("open lcd_file error\n");
        return MAP_FAILED;
    }

    plcd = (int *)mmap(NULL, 800 * 480 * 4, PROT_READ | PROT_WRITE, MAP_SHARED, lcd_fd, 0);
    return plcd;
}

int uninit_lcd()
{
    close(lcd_fd);

    if (munmap(lcd_p, 800 * 480 * 4) == -1)
    {
        return -1;
    }
    return 0;
}

unsigned int sign3 = 0;
//YUYV转rgb
int yuyv2rgb(int y, int u, int v)
{
    unsigned int pixel24 = 0;
    unsigned char *pixel = (unsigned char *)&pixel24;
    int r, g, b;
    static int ruv, guv, buv;

    if (sign3)
    {
        sign3 = 0;
        ruv = 1159 * (v - 128);
        guv = 380 * (u - 128) + 813 * (v - 128);
        buv = 2018 * (u - 128);
    }

    r = (1164 * (y - 16) + ruv) / 1000;
    g = (1164 * (y - 16) - guv) / 1000;
    b = (1164 * (y - 16) + buv) / 1000;

    if (r > 255)
        r = 255;
    if (g > 255)
        g = 255;
    if (b > 255)
        b = 255;
    if (r < 0)
        r = 0;
    if (g < 0)
        g = 0;
    if (b < 0)
        b = 0;

    pixel[0] = r;
    pixel[1] = g;
    pixel[2] = b;

    return pixel24;
}

//YUYV转rgb（调用yuyv2rgb函数）
int yuyv2rgb0(unsigned char *yuv, unsigned char *rgb, unsigned int width, unsigned int height)
{
    unsigned int in, out;
    int y0, u, y1, v;
    unsigned int pixel24;
    unsigned char *pixel = (unsigned char *)&pixel24;
    unsigned int size = width * height * 2;

    for (in = 0, out = 0; in < size; in += 4, out += 6)
    {
        y0 = yuv[in + 0];
        u = yuv[in + 1];
        y1 = yuv[in + 2];
        v = yuv[in + 3];

        sign3 = 1;
        pixel24 = yuyv2rgb(y0, u, v);
        rgb[out + 0] = pixel[0];
        rgb[out + 1] = pixel[1];
        rgb[out + 2] = pixel[2];

        pixel24 = yuyv2rgb(y1, u, v);
        rgb[out + 3] = pixel[0];
        rgb[out + 4] = pixel[1];
        rgb[out + 5] = pixel[2];
    }

    return 0;
}

int main()
{
    lcd_init();

    // 打开摄像头
    int fd_v4l2 = open("/dev/video7", O_RDWR); // 根据secureCRT确定设备
    if (fd_v4l2 == -1)
    {
        perror("open");
        exit(-1);
    }

    // 获取功能参数
    struct v4l2_capability cap = {};
    int res = ioctl(fd_v4l2, VIDIOC_QUERYCAP, &cap);
    if (res == -1)
    {
        perror("ioctl cap");
        exit(-1);
    }

    // 先确定摄像头功能可以使用
    if (cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)
    {
        printf("is a capture device!\n");
    }
    else
    {
        printf("is not a capture device!\n");
        exit(-1);
    }

    // 获取摄像头支持的格式
    struct v4l2_fmtdesc fmt = {};
    fmt.index = 0;
    fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; // 获取摄像头格式

    while ((res = ioctl(fd_v4l2, VIDIOC_ENUM_FMT, &fmt)) == 0)
    {
        printf("pixformat = %c %c %c %c,description = %s\n",
               fmt.pixelformat & 0xff,
               (fmt.pixelformat >> 8) & 0xff,
               (fmt.pixelformat >> 16) & 0xff,
               (fmt.pixelformat >> 24) & 0xff,
               fmt.description);
        fmt.index++;
    }

    // 设置采集通道
    int index = 0; // 使用通道0
    res = ioctl(fd_v4l2, VIDIOC_S_INPUT, &index);
    if (res == -1)
    {
        perror("ioctl_s_input");
        exit(-1);
    }

    // 设置摄像头采集格式
    struct v4l2_format format = {};
    format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    format.fmt.pix.width = 640;
    format.fmt.pix.height = 480;
    format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV; // YUYV
    format.fmt.pix.field = V4L2_FIELD_NONE;
    res = ioctl(fd_v4l2, VIDIOC_S_FMT, &format);
    if (res == -1)
    {
        perror("ioctl s_fmt");
        exit(-1);
    }

    // 申请缓存空间
    struct v4l2_requestbuffers req = {};
    req.count = 4;
    req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    req.memory = V4L2_MEMORY_MMAP;
    res = ioctl(fd_v4l2, VIDIOC_REQBUFS, &req);
    if (res == -1)
    {
        perror("ioctl reqbufs");
        exit(-1);
    }

    // 分配映射入队
    size_t i, max_len = 0;
    for (i = 0; i < 4; i++)
    {
        struct v4l2_buffer buf = {};
        buf.index = i; // 0~3展现4帧图片
        buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;
        res = ioctl(fd_v4l2, VIDIOC_QUERYBUF, &buf);
        if (res == -1)
        {
            perror("ioctl querybuf");
            exit(-1);
        }

        // 判读并记录最大长度，以适配各个图帧
        if (buf.length > max_len)
            max_len = buf.length;

        // 映射
        buffer[i].length = buf.length;
        buffer[i].start = mmap(NULL, buf.length, PROT_READ | PROT_WRITE, MAP_SHARED, fd_v4l2, buf.m.offset);
        if (buffer[i].start == MAP_FAILED)
        {
            perror("mmap");
            exit(-1);
        }

        // 入队
        res = ioctl(fd_v4l2, VIDIOC_QBUF, &buf);
        if (res == -1)
        {
            perror("ioctl qbuf");
            exit(-1);
        }
    }

    // 申请临时缓冲区
    current.start = malloc(max_len);
    if (current.start == NULL)
    {
        perror("malloc");
        exit(-1);
    }

    // 启动摄像头
    enum v4l2_buf_type buf_type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    res = ioctl(fd_v4l2, VIDIOC_STREAMON, &buf_type);
    if (res == -1)
    {
        perror("ioctl streamon");
        exit(-1);
    }

    // 延时
    sleep(1);

    // RGB缓冲区
    char rgb[640 * 480 * 3];


    // 采集数据
    while (1)
    {
        struct v4l2_buffer buf = {};
        buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;

        // 出队
        res = ioctl(fd_v4l2, VIDIOC_DQBUF, &buf);
        if (res == -1)
        {
            perror("ioctl dqbuf");
        }

        // 拷贝数据
        memcpy(current.start, buffer[buf.index].start, buf.bytesused);
        current.length = buf.bytesused;

        // 入队
        res = ioctl(fd_v4l2, VIDIOC_QBUF, &buf);
        if (res == -1)
        {
            perror("ioctl qbuf");
        }

        // YUYV转RGB
        yuyv2rgb0(current.start, rgb, 640, 480);

        // 显示LCD
        int x, y;
        for (y = 0; y < 480; y++)
        {
            for (x = 0; x < 640; x++)
            {
                *(plcd + y*800 + x) = rgb[3 * (y*640 + x)] << 16 | rgb[3 * (y*640 + x) + 1] << 8 | rgb[3 * (y*640 + x) + 2];
            }
        }
    }

    // 关闭摄像头采集
    buf_type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    res = ioctl(fd_v4l2, VIDIOC_STREAMOFF, &buf_type);
    if (res == -1)
    {
        perror("ioctl streamoff");
        exit(-1);
    }

    // 解除映射
    for (i = 0; i < 4; i++)
    {
        munmap(buffer[i].start, buffer[i].length);
    }
    free(current.start);

    sleep(1); // 延时一下

    close(fd_v4l2);
    uninit_lcd();
    return 0;
}

The final effect is as shown in the figure