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USB摄像头使用V4L2采集图像\视频

背景

V4L2（Video for Linux Two）是Linux内核自带的一部分，专门用于处理视频设备的管理和控制。‌ V4L2框架提供了统一的API和抽象层，使得开发者可以编写通用的视频驱动程序，同时使用户空间的应用程序能够轻松地访问和控制视频设备‌。在linux的开发板上，为了对符合UVC协议的摄像头进行视频或图像采集时，若不方便安装第三库，可以使用linux内核自带的V4L2框架进行处理。

V4L2视频采集

如下代码来自电子论坛，能够帮助我们采集uvc摄像头的图像，保存为.bmp。
在虚拟机中aarch64交叉编译得到可执行文件 V4L2_VideoCapture，不知道如何配置交叉编译的，可以查看我的其他博文。执行函数

#videoXX是你的设备号，我的是video36，使用命令gst-device-monitor-1.0可进行查询
./V4L2_VideoCapture /dev/videoXX

在这里插入图片描述
主要过程包含了

  1. v4l2设备的初始化。初始化中包括了打开相机，设置格式、分辨率、帧率，申请buffer空间等准备工作
  2. 数据采集。使用ioctl从队列中取出数据，拷贝副本，然后把原始数据再放回去
  3. 数据格式转换。如YUYV格式转换为RGB，源代码出处的转换公式有一处错误，本文已进行了修正。
  4. 保存图像
  5. 释放环境
// V4L2_VideoCapture.cpp
#include <fcntl.h>
#include <linux/videodev2.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

#define _UVC_CAM_HEIGHT (480)
#define _UVC_CAM_WIDTH  (640)
#define IMAGEHEIGHT     _UVC_CAM_HEIGHT
#define IMAGEWIDTH      _UVC_CAM_WIDTH

#define NB_BUFFER 4

struct vdIn {
    int   fd;
    char* videodevice;

    // v4l2
    struct v4l2_capability     cap;
    struct v4l2_format         fmt;
    struct v4l2_fmtdesc        fmtdesc;
    struct v4l2_streamparm     setfps;
    struct v4l2_requestbuffers rb;

    void* mem[NB_BUFFER];
    int   memlength[NB_BUFFER];

    unsigned char* framebuffer;
    int            framesizeIn;

    int width;
    int height;
    int fps;


    FILE* fp_bmp;
};


//14byte文件头
typedef struct
{
    unsigned char cfType[2];  //文件类型，"BM"(0x4D42)
    unsigned int  cfSize;     //文件大小（字节）
    unsigned int  cfReserved; //保留，值为0
    unsigned int  cfoffBits;  //数据区相对于文件头的偏移量（字节）
} __attribute__((packed)) BITMAPFILEHEADER;


//40byte信息头
typedef struct
{
    unsigned int       ciSize;            //BITMAPFILEHEADER所占的字节数
    unsigned int       ciWidth;           //宽度
    unsigned int       ciHeight;          //高度
    unsigned short int ciPlanes;          //目标设备的位平面数，值为1
    unsigned short int ciBitCount;        //每个像素的位数
    char               ciCompress[4];     //压缩说明
    unsigned int       ciSizeImage;       //用字节表示的图像大小，该数据必须是4的倍数
    unsigned int       ciXPelsPerMeter;   //目标设备的水平像素数/米
    unsigned int       ciYPelsPerMeter;   //目标设备的垂直像素数/米
    unsigned int       ciClrUsed;         //位图使用调色板的颜色数
    unsigned           intciClrImportant; //指定重要的颜色数，当该域的值等于颜色数时（或者等于0时），表示所有颜色都一样重要
} __attribute__((packed)) BITMAPINFOHEADER;

typedef struct
{
    unsigned char blue;
    unsigned char green;
    unsigned char red;
} __attribute__((packed)) PIXEL; //颜色模式RGB



/* Private function prototypes -----------------------------------------------*/
/* Private variables ---------------------------------------------------------*/
static struct vdIn   uvc_cam;
static unsigned char rgb888_buffer[IMAGEWIDTH * IMAGEHEIGHT * 3];

/* Global  variables ---------------------------------------------------------*/
/* Private functions ---------------------------------------------------------*/


void yuyv_to_rgb888(void)
{
    int            i, j;
    unsigned char  y1, y2, u, v;
    int            r1, g1, b1, r2, g2, b2;
    unsigned char* pointer;
    double         rbase = 0;
    double         gbase = 0;
    double         bbase = 0;


    pointer = uvc_cam.framebuffer;


    for (i = 0; i < IMAGEHEIGHT; i++) {
        for (j = 0; j < (IMAGEWIDTH / 2); j++) {
            y1 = *(pointer + ((i * (IMAGEWIDTH / 2) + j) << 2));
            u  = *(pointer + ((i * (IMAGEWIDTH / 2) + j) << 2) + 1);
            y2 = *(pointer + ((i * (IMAGEWIDTH / 2) + j) << 2) + 2);
            v  = *(pointer + ((i * (IMAGEWIDTH / 2) + j) << 2) + 3);


            rbase = 1.042 * (v - 128);
            gbase = 0.34414 * (u - 128) + 0.71414 * (v - 128);
            bbase = 1.772 * (u - 128);

            r1 = y1 + rbase;
            g1 = y1 - gbase;
            b1 = y1 + bbase;

            r2 = y2 + rbase;
            g2 = y2 - gbase;
            b2 = y2 + bbase;


            if (r1 > 255)
                r1 = 255;
            else if (r1 < 0)
                r1 = 0;

            if (b1 > 255)
                b1 = 255;
            else if (b1 < 0)
                b1 = 0;

            if (g1 > 255)
                g1 = 255;
            else if (g1 < 0)
                g1 = 0;

            if (r2 > 255)
                r2 = 255;
            else if (r2 < 0)
                r2 = 0;

            if (b2 > 255)
                b2 = 255;
            else if (b2 < 0)
                b2 = 0;

            if (g2 > 255)
                g2 = 255;
            else if (g2 < 0)
                g2 = 0;


            *(rgb888_buffer + ((IMAGEHEIGHT - 1 - i) * (IMAGEWIDTH / 2) + j) * 6)     = (unsigned char)b1;
            *(rgb888_buffer + ((IMAGEHEIGHT - 1 - i) * (IMAGEWIDTH / 2) + j) * 6 + 1) = (unsigned char)g1;
            *(rgb888_buffer + ((IMAGEHEIGHT - 1 - i) * (IMAGEWIDTH / 2) + j) * 6 + 2) = (unsigned char)r1;
            *(rgb888_buffer + ((IMAGEHEIGHT - 1 - i) * (IMAGEWIDTH / 2) + j) * 6 + 3) = (unsigned char)b2;
            *(rgb888_buffer + ((IMAGEHEIGHT - 1 - i) * (IMAGEWIDTH / 2) + j) * 6 + 4) = (unsigned char)g2;
            *(rgb888_buffer + ((IMAGEHEIGHT - 1 - i) * (IMAGEWIDTH / 2) + j) * 6 + 5) = (unsigned char)r2;
        }
    }

    printf("yuyv to rgb888 done\n");
}




int v4l2_init(void)
{
    int                i = 0;
    int                ret;
    struct v4l2_buffer buf;


    // 1. open cam
    if ((uvc_cam.fd = open(uvc_cam.videodevice, O_RDWR)) == -1) {
        printf("ERROR opening V4L interface\n");
        return -1;
    }

    // 2. querycap
    memset(&uvc_cam.cap, 0, sizeof(struct v4l2_capability));
    ret = ioctl(uvc_cam.fd, VIDIOC_QUERYCAP, &uvc_cam.cap);

    if (ret < 0) {
        printf("Error opening device %s: unable to query device.\n", uvc_cam.videodevice);
        return -1;
    }
    else {
        printf("driver:\t\t%s\n", uvc_cam.cap.driver);
        printf("card:\t\t%s\n", uvc_cam.cap.card);
        printf("bus_info:\t%s\n", uvc_cam.cap.bus_info);
        printf("version:\t%d\n", uvc_cam.cap.version);
        printf("capabilities:\t%x\n", uvc_cam.cap.capabilities);

        if ((uvc_cam.cap.capabilities & V4L2_CAP_VIDEO_CAPTURE) == V4L2_CAP_VIDEO_CAPTURE) {
            printf("%s: \tsupports capture.\n", uvc_cam.videodevice);
        }

        if ((uvc_cam.cap.capabilities & V4L2_CAP_STREAMING) == V4L2_CAP_STREAMING) {
            printf("%s: \tsupports streaming.\n", uvc_cam.videodevice);
        }
    }

    // 3. set format in

    // 3.1 enum fmt
    printf("\nSupport format:\n");

    memset(&uvc_cam.fmtdesc, 0, sizeof(struct v4l2_fmtdesc));
    uvc_cam.fmtdesc.index = 0;
    uvc_cam.fmtdesc.type  = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    while (ioctl(uvc_cam.fd, VIDIOC_ENUM_FMT, &uvc_cam.fmtdesc) != -1) {
        printf("\t%d.%s\n", uvc_cam.fmtdesc.index + 1, uvc_cam.fmtdesc.description);
        uvc_cam.fmtdesc.index++;
    }

    // 3.2 set fmt
    memset(&uvc_cam.fmt, 0, sizeof(struct v4l2_format));
    uvc_cam.fmt.type                = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    uvc_cam.fmt.fmt.pix.width       = uvc_cam.width;
    uvc_cam.fmt.fmt.pix.height      = uvc_cam.height;
    uvc_cam.fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
    uvc_cam.fmt.fmt.pix.field       = V4L2_FIELD_ANY;

    ret = ioctl(uvc_cam.fd, VIDIOC_S_FMT, &uvc_cam.fmt);
    if (ret < 0) {
        printf("Unable to set format\n");
        return -1;
    }

    // 3.3 get fmt
    ret = ioctl(uvc_cam.fd, VIDIOC_G_FMT, &uvc_cam.fmt);
    if (ret < 0) {
        printf("Unable to get format\n");
        return -1;
    }
    else {
        printf("\nfmt.type:\t\t%d\n", uvc_cam.fmt.type);
        printf("pix.pixelformat:\t%c%c%c%c\n", uvc_cam.fmt.fmt.pix.pixelformat & 0xFF, (uvc_cam.fmt.fmt.pix.pixelformat >> 8) & 0xFF, (uvc_cam.fmt.fmt.pix.pixelformat >> 16) & 0xFF, (uvc_cam.fmt.fmt.pix.pixelformat >> 24) & 0xFF);
        printf("pix.height:\t\t%d\n", uvc_cam.fmt.fmt.pix.height);
        printf("pix.width:\t\t%d\n", uvc_cam.fmt.fmt.pix.width);
        printf("pix.field:\t\t%d\n", uvc_cam.fmt.fmt.pix.field);
    }


    // 4. set fps
    memset(&uvc_cam.setfps, 0, sizeof(struct v4l2_streamparm));
    uvc_cam.setfps.type                                  = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    uvc_cam.setfps.parm.capture.timeperframe.numerator   = 1;
    uvc_cam.setfps.parm.capture.timeperframe.denominator = 25;

    ret = ioctl(uvc_cam.fd, VIDIOC_S_PARM, &uvc_cam.setfps);

    if (ret < 0) {
        printf("Unable to set frame rate\n");
        return -1;
    }
    else {
        printf("set fps OK!\n");
    }

    ret = ioctl(uvc_cam.fd, VIDIOC_G_PARM, &uvc_cam.setfps);

    if (ret < 0) {
        printf("Unable to get frame rate\n");
        return -1;
    }
    else {
        printf("get fps OK:\n");
        printf("timeperframe.numerator  : %d\n", uvc_cam.setfps.parm.capture.timeperframe.numerator);
        printf("timeperframe.denominator: %d\n", uvc_cam.setfps.parm.capture.timeperframe.denominator);
        printf("set fps : %d\n", 1 * uvc_cam.setfps.parm.capture.timeperframe.denominator / uvc_cam.setfps.parm.capture.timeperframe.numerator);
    }

    // 5. enum framesizes

    while (1) {
        struct v4l2_fmtdesc fmtdesc;
        memset(&fmtdesc, 0, sizeof(struct v4l2_fmtdesc));
        fmtdesc.index = i++;
        fmtdesc.type  = V4L2_BUF_TYPE_VIDEO_CAPTURE;

        if (ioctl(uvc_cam.fd, VIDIOC_ENUM_FMT, &fmtdesc) < 0) {
            break;
        }

        printf("Supported format: %s\n", fmtdesc.description);
        struct v4l2_frmsizeenum fsenum;
        memset(&fsenum, 0, sizeof(struct v4l2_frmsizeenum));
        fsenum.pixel_format = uvc_cam.fmtdesc.pixelformat;

        int j = 0;

        while (1) {
            fsenum.index = j;
            j++;

            if (ioctl(uvc_cam.fd, VIDIOC_ENUM_FRAMESIZES, &fsenum) == 0) {
                if (uvc_cam.fmt.fmt.pix.pixelformat == fmtdesc.pixelformat) {
                    printf("\tSupported size with the current format: %dx%d\n", fsenum.discrete.width, fsenum.discrete.height);
                }
                else {
                    printf("\tSupported size: %dx%d\n", fsenum.discrete.width, fsenum.discrete.height);
                }
            }
            else {
                break;
            }
        }
    }

    // 6. request buffers
    memset(&uvc_cam.rb, 0, sizeof(struct v4l2_requestbuffers));
    uvc_cam.rb.count  = NB_BUFFER;
    uvc_cam.rb.type   = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    uvc_cam.rb.memory = V4L2_MEMORY_MMAP;

    ret = ioctl(uvc_cam.fd, VIDIOC_REQBUFS, &uvc_cam.rb);

    if (ret < 0) {
        printf("Unable to allocate buffers\n");
        return -1;
    }

    // 6.1 map the buffers

    for (i = 0; i < NB_BUFFER; i++) {
        memset(&buf, 0, sizeof(struct v4l2_buffer));
        buf.index  = i;
        buf.type   = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;

        ret = ioctl(uvc_cam.fd, VIDIOC_QUERYBUF, &buf);
        if (ret < 0) {
            printf("Unable to query buffer\n");
            return -1;
        }

        uvc_cam.mem[i] = mmap(NULL, buf.length, PROT_READ | PROT_WRITE, MAP_SHARED, uvc_cam.fd, buf.m.offset);
        if (uvc_cam.mem[i] == MAP_FAILED) {
            printf("Unable to map buffer\n");
            return -1;
        }
        uvc_cam.memlength[i] = buf.length;
    }


    // 6.2 queue the buffers.
    for (i = 0; i < NB_BUFFER; i++) {
        memset(&buf, 0, sizeof(struct v4l2_buffer));
        buf.index  = i;
        buf.type   = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;
        ret        = ioctl(uvc_cam.fd, VIDIOC_QBUF, &buf);
        if (ret < 0) {
            printf("Unable to queue buffer\n");
            return -1;
        }
    }

    // 7. malloc yuyv buf
    uvc_cam.framesizeIn = uvc_cam.width * uvc_cam.height << 1; // w * h * 2
    uvc_cam.framebuffer = (unsigned char*)calloc(1, (size_t)uvc_cam.framesizeIn);

    if (uvc_cam.framebuffer == NULL) {
        printf("err calloc memory\n");
        return -1;
    }

    printf("init %s \t[OK]\n", uvc_cam.videodevice);

    return 0;
}

void v4l2_exit(void)
{
    free(uvc_cam.framebuffer);
    close(uvc_cam.fd);
}

int v4l2_enable(void)
{
    int type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    int ret;

    ret = ioctl(uvc_cam.fd, VIDIOC_STREAMON, &type);
    if (ret < 0) {
        printf("Unable to start capture\n");
        return ret;
    }

    printf("start capture\n");

    return 0;
}

int v4l2_disable(void)
{
    int type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    int ret;

    ret = ioctl(uvc_cam.fd, VIDIOC_STREAMOFF, &type);
    if (ret < 0) {
        printf("Unable to stop capture\n");
        return ret;
    }

    printf("stop capture\n");

    return 0;
}

int v4l2_uvc_grap(void)
{
    int                ret;
    struct v4l2_buffer buf;


    memset(&buf, 0, sizeof(struct v4l2_buffer));
    buf.type   = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    buf.memory = V4L2_MEMORY_MMAP;

    ret = ioctl(uvc_cam.fd, VIDIOC_DQBUF, &buf);
    if (ret < 0) {
        printf("Unable to dequeue buffer\n");
        exit(1);
    }

    memcpy(uvc_cam.framebuffer, uvc_cam.mem[buf.index], uvc_cam.framesizeIn);

    ioctl(uvc_cam.fd, VIDIOC_QBUF, &buf);

    printf("buf index: %d\n", buf.index);

    return 0;
}




int save_bmp(char* bmp_name)
{
    FILE*            fp;
    BITMAPFILEHEADER bf;
    BITMAPINFOHEADER bi;

    printf("save bmp  function\n");

    fp = fopen(bmp_name, "wb");
    if (fp == NULL) {
        printf("open errror\n");
        return (-1);
    }

    //Set BITMAPINFOHEADER
    memset(&bi, 0, sizeof(BITMAPINFOHEADER));
    bi.ciSize      = 40;
    bi.ciWidth     = IMAGEWIDTH;
    bi.ciHeight    = IMAGEHEIGHT;
    bi.ciPlanes    = 1;
    bi.ciBitCount  = 24;
    bi.ciSizeImage = IMAGEWIDTH * IMAGEHEIGHT * 3;


    //Set BITMAPFILEHEADER
    memset(&bf, 0, sizeof(BITMAPFILEHEADER));
    bf.cfType[0]  = 'B';
    bf.cfType[1]  = 'M';
    bf.cfSize     = 54 + bi.ciSizeImage;
    bf.cfReserved = 0;
    bf.cfoffBits  = 54;


    fwrite(&bf, 14, 1, fp);
    fwrite(&bi, 40, 1, fp);
    fwrite(rgb888_buffer, bi.ciSizeImage, 1, fp);

    printf("save %s done\n", bmp_name);

    fclose(fp);

    return 0;
}

int main(int argc, char const* argv[])
{
    char vdname[20];

    printf("\n----- v4l2 savebmp app start ----- \n");

    if (argc < 2) {

        printf("need:/dev/videox\n");
        printf("like:%s /dev/video1\n", argv[0]);

        printf("app exit.\n\n");
        exit(1);
    }

    snprintf(vdname, 20, argv[1]);

    memset(&uvc_cam, 0, sizeof(struct vdIn));
    uvc_cam.videodevice = vdname;
    printf("using: \t\t%s\n", uvc_cam.videodevice);


    uvc_cam.width  = _UVC_CAM_WIDTH;
    uvc_cam.height = _UVC_CAM_HEIGHT;

    // 1. init cam
    if (v4l2_init() < 0) {
        goto app_exit;
    }

    v4l2_enable();
    usleep(5 * 1000);

    // 2. grap uvc
    v4l2_uvc_grap();
    yuyv_to_rgb888();


    // 3. save bmp
    save_bmp("./uvc_grap.bmp");



app_exit:
    printf("app exit.\n\n");
    v4l2_exit();

    return 0;
}

悦读

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