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
主要过程包含了
- 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
- 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
- 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。
- 保存图像
- 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 4struct vdIn {int fd;char* videodevice;// v4l2struct 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0unsigned 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1unsigned 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 camif ((uvc_cam.fd = open(uvc_cam.videodevice, O_RDWR)) == -1) {printf("ERROR opening V4L interface\n");return -1;}// 2. querycapmemset(&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 fmtprintf("\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 fmtmemset(&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 fmtret = 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 fpsmemset(&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 framesizeswhile (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 buffersmemset(&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 buffersfor (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 bufuvc_cam.framesizeIn = uvc_cam.width * uvc_cam.height << 1; // w * h * 2uvc_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 BITMAPINFOHEADERmemset(&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 BITMAPFILEHEADERmemset(&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 camif (v4l2_init() < 0) {goto app_exit;}v4l2_enable();usleep(5 * 1000);// 2. grap uvcv4l2_uvc_grap();yuyv_to_rgb888();// 3. save bmpsave_bmp("./uvc_grap.bmp");app_exit:printf("app exit.\n\n");v4l2_exit();return 0;
}