Linux下V4L2实时显示摄像头捕捉画面

た入场券 2022-10-30 01:22 1481阅读 0赞

> Video for Linuxtwo(Video4Linux2)简称V4L2，是V4L的改进版。V4L2是linux操作系统下用于采集图片、视频和音频数据的API接口，配合适当的视频采集设备和相应的驱动程序，可以实现图片、视频、音频等的采集。在远程会议、可视电话、视频监控系统和嵌入式多媒体终端中都有广泛的应用。

在Linux下，所有外设都被看成一种特殊的文件，成为“设备文件”，可以象访问普通文件一样对其进行读写。一般来说，采用V4L2驱动的摄像头设备文件是/dev/video0。

连接摄像头设备到虚拟机。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0NnczcyMw_size_16_color_FFFFFF_t_70]  
之后会在设备里面看到Video0设备。  
![在这里插入图片描述][2021021617563898.png]

1.  打开video0设备

int fd = open("/dev/video0", O_RDWR);
    if (fd < 0) { 
       perror("Open video0");
       return -1;
    }

1.  获取摄像头的相关信息

struct v4l2_fmtdesc v4fmt;
    v4fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    v4fmt.index = 1;//0 MJPEG 1 YUYV
    int ret = ioctl(fd, VIDIOC_ENUM_FMT, &v4fmt);
    if (ret < 0) { 
        perror("Get inf failed:");
        return -1;
    }
    printf("index:%d\n", v4fmt.index);
    printf("flags:%d\n", v4fmt.flags);
    printf("description:%s\n", v4fmt.description);
    unsigned char *p =(unsigned char *)(&v4fmt.pixelformat);
    printf("pixelformat:%c%c%c%c\n", p[0], p[1], p[2], p[3]);

v4fmt.index的值会决定获取的每一帧图片的格式，需要的可以设置其他数字看看自己的摄像头支持什么格式，我这里0代表的是MJPEG格式、1代表YUYV格式，下面我是用的都是YUYV格式。  
![在这里插入图片描述][20210216181113451.png]  
![在这里插入图片描述][20210216181228110.png]

1.  设置视频格式

struct v4l2_format vfmt;
    vfmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    vfmt.fmt.pix.width = 640;
    vfmt.fmt.pix.height = 480;
    vfmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;//设置视频采集格式YUYV
    ret = ioctl(fd, VIDIOC_S_FMT, &vfmt);
    if (ret < 0) { 
        perror("Set format failed");
        return -1;
    }
    memset(&vfmt, 0, sizeof(vfmt));
    vfmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    ret = ioctl(fd, VIDIOC_G_FMT, &vfmt);
    if (ret < 0) { 
        perror("Get format failed");
        return -1;
    }
    
    if (vfmt.fmt.pix.width == 640 && vfmt.fmt.pix.height == 480 && vfmt.fmt.pix.pixelformat == V4L2_PIX_FMT_YUYV) { 
        printf("Set successful!\n");
    }
    else { 
        printf("Set failed!\n");
        return -1;
    }

1.  申请内核缓冲区队列

struct v4l2_requestbuffers reqbuffer;
    reqbuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    reqbuffer.count = 4;
    reqbuffer.memory = V4L2_MEMORY_MMAP;//映射方式
    ret = ioctl(fd, VIDIOC_REQBUFS, &reqbuffer);
    if (ret < 0) { 
        perror("Request Queue space failed");
        return -1;
    }

1.  映射到用户空间

struct v4l2_buffer mapbuffer;
    int i;
    unsigned char *mptr[4];
    unsigned int size[4];//存储大小，方便释放
    mapbuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    for (i = 0; i < 4; i++) { 
        mapbuffer.index = i;
        ret = ioctl(fd, VIDIOC_QUERYBUF, &mapbuffer);
        if (ret < 0) { 
            perror("Kernel space queue failed");
            return -1;
        }
        mptr[i] = (unsigned char *)mmap(NULL, mapbuffer.length, PROT_READ | PROT_WRITE, MAP_SHAR    ED, fd, mapbuffer.m.offset);
        size[i] = mapbuffer.length;
        //使用完毕
        ret = ioctl(fd, VIDIOC_QBUF, &mapbuffer);
        if (ret < 0) { 
            perror("Return failed");
            return -1;
        }
    }

1.  开始采集数据

打开fb设备，获取虚拟的LCD屏幕宽和高，地址映射到用户空间。

lcdfd = open("/dev/fb0", O_RDWR);
    if (lcdfd < 0) { 
        perror("LCD open failed:");
    }
    /*获取LCD信息*/
    struct fb_var_screeninfo info;
    int lret = ioctl(lcdfd, FBIOGET_VSCREENINFO, &info);
    if (lret < 0) { 
        perror("get info failed:");
    }
    //获取虚拟LCD屏幕宽和高
    lcd_w = info.xres_virtual;
    lcd_h = info.yres_virtual;
    //开发板用物理宽和高;
    //lcd_w = info.xres;
    //lcd_h = info.yres;
    lcdptr = (int *)mmap(NULL, lcd_w*lcd_h*4,PROT_READ | PROT_WRITE, MAP_SHARED, lcdfd, 0);
    if (lcdptr == NULL) { 
        perror("lcd mmap failed:");
    }

YUYV格式转RGB格式函数

void yuyv_to_rgb(unsigned char *yuyvdata, unsigned char * rgbdata, int w, int h)
    { 
       int r1, g1, b1;
       int r2, g2, b2;
       for (int i = 0; i < w*h/2; i++) { 
           char data[4];
           memcpy(data, yuyvdata + i*4, 4);
           unsigned char Y0 = data[0];
           unsigned char U0 = data[1];
           unsigned char Y1 = data[2];
           unsigned char V1 = data[3];
           r1 = Y0 + 1.4075*(V1 - 128);
           if (r1 > 255)
               r1 = 255;
           if (r1 < 0)
               r1 = 0;
           g1 = Y0 - 0.3455*(U0 - 128) - 0.7169*(V1 - 128);
           if (g1 > 255)
               g1 = 255;
           if (g1 < 0)
               g1 = 0;
           b1 = Y0 + 1.779*(U0 - 128);
           if (b1 > 255)
               b1 = 255;
           if (b1 < 0)
               b1 = 0;
    
           r2 = Y1 + 1.4075*(V1 - 128);
           if (r2 > 255)
               r2 = 255;
           if (r2 < 0)
               r2 = 0;
           g2 = Y1 - 0.3455*(U0 - 128) - 0.7169*(V1 - 128);
           if (g2 > 255)
               g2 = 255;
           if (g2 < 0)
               g2 = 0;
           b2 = Y1 + 1.779*(U0 - 128);
           if (b2 > 255)
               b2 = 255;
           if (b2 < 0)
               b2 = 0;
    
            rgbdata[i*6 + 0] = r1;
            rgbdata[i*6 + 1] = g1;
            rgbdata[i*6 + 2] = b1;
            rgbdata[i*6 + 3] = r2;
            rgbdata[i*6 + 4] = g2;
            rgbdata[i*6 + 5] = b2;
        }
    }

将RGB格式的数据显示在LCD屏幕上。

void lcd_show_rgb(unsigned char *rgbdata, int w, int h)
    { 
        unsigned int *ptr = lcdptr;
        for (int i = 0; i < h; i++) { 
            for (int j = 0; j < w; j++) { 
                memcpy(ptr + j, rgbdata + j*3, 3);
            }
            ptr += lcd_w;//偏移一行
            rgbdata += w*3;//偏移一行
        }
    }

开始采集数据。

int type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    ret = ioctl(fd, VIDIOC_STREAMON, &type);
    if (ret < 0) { 
         perror("Start failed:");
         return -1;
    }
    //从队列中提取一帧数据
    unsigned char rgbdata[640*480*3];
    while (1) { 
        struct v4l2_buffer readbuffer;
        readbuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        ret = ioctl(fd, VIDIOC_DQBUF, &readbuffer);
        if (ret < 0) { 
            perror("Capture failed:");
        }
        //显示在LCD上
        yuyv_to_rgb(mptr[readbuffer.index], rgbdata, 640, 480);
        lcd_show_rgb(rgbdata, 640, 480);
        //通知内核已经使用完毕
        ret = ioctl(fd, VIDIOC_QBUF, &readbuffer);
        if (ret < 0) { 
            perror("return failed:");
        }
    }

1.  停止采集数据，释放映射空间。

ret = ioctl(fd, VIDIOC_STREAMOFF, &type);
    if (ret < 0) { 
        perror("Stop failed:");
        return -1;
    }
    
    //释放映射空间
    for (i = 0; i < 4; i++) { 
        munmap(mptr[i], size[i]);
    }
    close(fd);

完整代码

#include <stdio.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fcntl.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <sys/ioctl.h>
    #include <linux/videodev2.h>
    #include <sys/mman.h>
    #include <string.h>
    #include <linux/fb.h>
    #include <memory.h>
    
    int lcdfd = 0;
    int *lcdptr = NULL;
    int lcd_w = 800, lcd_h = 480;
    
    void yuyv_to_rgb(unsigned char *yuyvdata, unsigned char * rgbdata, int w, int h)
    { 
    	int r1, g1, b1;
    	int r2, g2, b2;
    	for (int i = 0; i < w*h/2; i++) { 
    		char data[4];
    		memcpy(data, yuyvdata + i*4, 4);
    		unsigned char Y0 = data[0];
    		unsigned char U0 = data[1];
    		unsigned char Y1 = data[2];
    		unsigned char V1 = data[3];
    		r1 = Y0 + 1.4075*(V1 - 128);
    		if (r1 > 255)
    			r1 = 255;
    		if (r1 < 0)
    			r1 = 0;
    		g1 = Y0 - 0.3455*(U0 - 128) - 0.7169*(V1 - 128);
    		if (g1 > 255)
    			g1 = 255;
    		if (g1 < 0)
    			g1 = 0;
    		b1 = Y0 + 1.779*(U0 - 128);
    		if (b1 > 255)
    			b1 = 255;
    		if (b1 < 0)
    			b1 = 0;
    
    		r2 = Y1 + 1.4075*(V1 - 128);
    		if (r2 > 255)
    			r2 = 255;
    		if (r2 < 0)
    			r2 = 0;
    		g2 = Y1 - 0.3455*(U0 - 128) - 0.7169*(V1 - 128);
    		if (g2 > 255)
    			g2 = 255;
    		if (g2 < 0)
    			g2 = 0;
    		b2 = Y1 + 1.779*(U0 - 128);
    		if (b2 > 255)
    			b2 = 255;
    		if (b2 < 0)
    			b2 = 0;
    
    		rgbdata[i*6 + 0] = r1;
    		rgbdata[i*6 + 1] = g1;
    		rgbdata[i*6 + 2] = b1;
    		rgbdata[i*6 + 3] = r2;
    		rgbdata[i*6 + 4] = g2;
    		rgbdata[i*6 + 5] = b2;
    	}
    }
    
    
    
    void lcd_show_rgb(unsigned char *rgbdata, int w, int h)
    { 
    	unsigned int *ptr = lcdptr;
    	for (int i = 0; i < h; i++) { 
    		for (int j = 0; j < w; j++) { 
    			memcpy(ptr + j, rgbdata + j*3, 3);
    		}
    		ptr += lcd_w;//偏移一行
    		rgbdata += w*3;//偏移一行
    	}
    }
    
    int main()
    { 
    	lcdfd = open("/dev/fb0", O_RDWR);
    	if (lcdfd < 0) { 
    		perror("LCD open failed:");
    	}
    	/*获取LCD信息*/
    	struct fb_var_screeninfo info;
    	int lret = ioctl(lcdfd, FBIOGET_VSCREENINFO, &info);
    	if (lret < 0) { 
    		perror("get info failed:");
    	}
    	//获取虚拟LCD屏幕宽和高
    	lcd_w = info.xres_virtual;
    	lcd_h = info.yres_virtual;
    	//开发板用物理宽和高;
    	//lcd_w = info.xres;
    	//lcd_h = info.yres;
    	lcdptr = (int *)mmap(NULL, lcd_w*lcd_h*4,PROT_READ | PROT_WRITE, MAP_SHARED, lcdfd, 0);
    	if (lcdptr == NULL) { 
    		perror("lcd mmap failed:");
    	}
    	/*打开设备*/
    	int fd = open("/dev/video0", O_RDWR);
    	if (fd < 0) { 
    		perror("Open video0:");
    		return -1;
    	}
    
    	/*获取设备相关信息*/
    	struct v4l2_fmtdesc v4fmt;
    	v4fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    	v4fmt.index = 1;//0 MJPEG 1 YUYV
    	int ret = ioctl(fd, VIDIOC_ENUM_FMT, &v4fmt);
    	if (ret < 0) { 
    		perror("Get inf failed:");
    		return -1;
    	}
    	printf("index:%d\n", v4fmt.index);
    	printf("flags:%d\n", v4fmt.flags);
    	printf("description:%s\n", v4fmt.description);
    	unsigned char *p =(unsigned char *)(&v4fmt.pixelformat);
    	printf("pixelformat:%c%c%c%c\n", p[0], p[1], p[2], p[3]);
    	
    	/*设置视频格式*/
    	struct v4l2_format vfmt;
    	vfmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    	vfmt.fmt.pix.width = 640;
    	vfmt.fmt.pix.height = 480;
    	vfmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;//设置视频采集格式YUYV
    	ret = ioctl(fd, VIDIOC_S_FMT, &vfmt);
    	if (ret < 0) { 
    		perror("Set format failed:");
    		return -1;
    	}
    	memset(&vfmt, 0, sizeof(vfmt));
    	vfmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    	ret = ioctl(fd, VIDIOC_G_FMT, &vfmt);
    	if (ret < 0) { 
    		perror("Get format failed:");
    		return -1;
    	}
    	
    	if (vfmt.fmt.pix.width == 640 && vfmt.fmt.pix.height == 480 && vfmt.fmt.pix.pixelformat == V4L2_PIX_FMT_YUYV) { 
    		printf("Set successful!\n");
    	}
    	else { 
    		printf("Set failed!\n");
    		return -1;
    	}
    
    	/*申请内核缓冲区队列*/
    	struct v4l2_requestbuffers reqbuffer;
    	reqbuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    	reqbuffer.count = 4;
    	reqbuffer.memory = V4L2_MEMORY_MMAP;//映射方式
    	ret = ioctl(fd, VIDIOC_REQBUFS, &reqbuffer);
    	if (ret < 0) { 
    		perror("Request Queue space failed:");
    		return -1;
    	}
    	
    	/*映射到用户空间*/
    	struct v4l2_buffer mapbuffer;
    	int i;
    	unsigned char *mptr[4];
    	unsigned int size[4];//存储大小，方便释放
    	mapbuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    	for (i = 0; i < 4; i++) { 
    		mapbuffer.index = i;
    		ret = ioctl(fd, VIDIOC_QUERYBUF, &mapbuffer);
    		if (ret < 0) { 
    			perror("Kernel space queue failed:");
    			return -1;
    		}
    		mptr[i] = (unsigned char *)mmap(NULL, mapbuffer.length, PROT_READ | PROT_WRITE, MAP_SHARED, fd, mapbuffer.m.offset);
    		size[i] = mapbuffer.length;
    		//使用完毕
    		ret = ioctl(fd, VIDIOC_QBUF, &mapbuffer);
    		if (ret < 0) { 
    			perror("Return failed:");
    			return -1;
    		}
    	}
    
    	/*开始采集数据*/
    	int type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    	ret = ioctl(fd, VIDIOC_STREAMON, &type);
    	if (ret < 0) { 
    		perror("Start failed:");
    		return -1;
    	}
    	//从队列中提取一帧数据
    	unsigned char rgbdata[640*480*3];
    	while (1) { 
    		struct v4l2_buffer readbuffer;
    		readbuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    		ret = ioctl(fd, VIDIOC_DQBUF, &readbuffer);
    		if (ret < 0) { 
    			perror("Capture failed:");
    		}
    		//显示在LCD上
    		yuyv_to_rgb(mptr[readbuffer.index], rgbdata, 640, 480);
    		lcd_show_rgb(rgbdata, 640, 480);
    		//通知内核已经使用完毕
    		ret = ioctl(fd, VIDIOC_QBUF, &readbuffer);
    		if (ret < 0) { 
    			perror("return failed:");
    		}
    	}
    	//停止采集数据
    	ret = ioctl(fd, VIDIOC_STREAMOFF, &type);
    	if (ret < 0) { 
    		perror("Stop failed:");
    		return -1;
    	}
    
    	//释放映射空间
    	for (i = 0; i < 4; i++) { 
    		munmap(mptr[i], size[i]);
    	}
    	close(fd);
    	return 0;
    }

代码测试  
CTRL+ALT+F1进入字符界面  
进入代码目录![在这里插入图片描述][20210216184254965.png]

能够看见屏幕中显示摄像头实时捕捉的画面。  
（显示出来的画面有点偏蓝，解码函数可能不是很准确。。。）

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0NnczcyMw_size_16_color_FFFFFF_t_70]: /images/20221024/3dac7c4512aa469b8fb668d7610f5e03.png
[2021021617563898.png]: /images/20221024/fe2ecbbefaa54ff4b3b3eba1428029eb.png
[20210216181113451.png]: /images/20221024/47bb26a66bff4e2ea4d592573b4410e1.png
[20210216181228110.png]: /images/20221024/3f04b173bbdc4c4a9d0c9a0724d9a0bf.png
[20210216184254965.png]: /images/20221024/7be45d0a1b9747c4a64e68c6681c5ac8.png