Qt浅谈之四十Centos下Qt结合v4l2实现的视频显示

一、简介

       v4l2是针对uvc免驱usb设备的编程框架，主要用于采集usb摄像头。 可从网上下载最新的源码（包括v4l2.c和v4l2.h两个文件），本文中修改过。
      Qt运行界面如下（动态变化的）：

二、详解

1、准备

（1）插入usb摄像头，检测设备文件/dev/video0

与代码中的pd.dev_name = "/dev/video0";保持一致。
（2）检测颜色编码
安装包#yum install v4l-utils，然后执行命令#v4l2-ctl -d /dev/video0 --list-formats

颜色编码格式为YUYV，与代码中的s->fmt.fmt.pix.pixelformat    = V4L2_PIX_FMT_YUYV;保持一致。
（3）遇到错误
在centos6.6中，遇到了错误：

VIDIOC_STREAMON error 28, 设备上没有空间

没有查到原因，不清楚什么问题，试了如下方法也不行：

#rmmod uvcvideo
#modprobe uvcvideo quirks=128

于是切换到centos6.3上，能顺利的运行（在虚拟机中也是可以正常运行的）。

2、主要点

（1）将YUYV转换成rgb显示在界面，以前使用MPEG没有显示

convert_yuv_to_rgb_buffer((unsigned char *)pd.buffers[pd.buf.index].start,bufrgb,640,480);
QImage image(bufrgb,640,480,QImage::Format_RGB888);
ui.displayLabel->setPixmap(QPixmap::fromImage(image));

（2）将视频流保存到本地文件（最小单位为秒，需要更快的可以调整到毫米）

if (bufrgb > 0 && strlen((char *)bufrgb) > 0) {
    tm_time = localtime(&now);
    char filename[30] = {0};
    sprintf(filename,"%4d-%02d-%02d_%02d.%02d.%02d.png",1900+tm_time->tm_year,1+tm_time->tm_mon,tm_time->tm_mday,
            tm_time->tm_hour,tm_time->tm_min,tm_time->tm_sec);
   QImage image(bufrgb,640,480,QImage::Format_RGB888);
   image.save(filename);
}

3、完整代码

（1）v4l2.h

#ifndef		__V4L2_H__
#define		__V4L2_H__
#include <linux/types.h>
#include <linux/videodev2.h>

#ifdef __cplusplus
extern "C" {
#endif

typedef	struct
{
	void	*start;
	size_t	length;
}buffer;

typedef	struct
{
	int			fd;
	int			n_buffers;
	char			*dev_name;
	buffer			*buffers;
	struct v4l2_buffer	buf;
	struct v4l2_format 	fmt;
}pass_data;

int init_dev (pass_data *s);
void stop_dev (pass_data *s);
void read_frame(pass_data *s);
void return_data (pass_data *s);

void init_mmap(pass_data *s);
void init_device(pass_data *s);
int open_device(pass_data *s);
void start_capturing(pass_data *s);
void close_device(pass_data *s);
void stop_capturing(pass_data *s);
void stop_device(pass_data *s);
void errno_exit(const char *s);
int xioctl(int fd, int request, void *arg);
void process_image(void *p, pass_data *s, int i);

int convert_yuv_to_rgb_buffer(unsigned char *yuv, unsigned char *rgb, unsigned int width, unsigned int height);

#ifdef __cplusplus
}
#endif

#endif

（2）v4l2.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <errno.h>

#include <fcntl.h>
#include <unistd.h>
#include <malloc.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
#include "v4l2.h"

#define	CLEAR(x)	memset (&x, 0, sizeof(x))
char *mpeg[] = {"./1.mpeg", "./2.mpeg", "./3.mpeg", "./4.mpeg", "./5.mpeg"};

int init_dev (pass_data *s)
{
    int flag = open_device(s);
    if (flag != 0) {
        return flag;
    }
	init_device(s);
	init_mmap(s);
	start_capturing(s);
	fprintf(stdout, "'%s' initialize finish ...\n", s->dev_name);
    return 0;
}

void stop_dev (pass_data *s)
{
	stop_capturing(s);
	stop_device(s);
	close_device(s);
	fprintf(stdout, "close '%s' ...\n", s->dev_name);
}

void process_image(void *p, pass_data *s, int i)
{
	fputc ('.', stdout);
	fflush (stdout);

	fprintf (stderr, "%s", mpeg[i]);
	
	int fd;	
	if ((fd = open (mpeg[i], O_RDWR | O_CREAT | O_TRUNC, 0644)) == -1)
		errno_exit("open");
	if ((write (fd, (struct v4l2_buffer *)p, s->fmt.fmt.pix.sizeimage)) == -1)
		errno_exit("write");

	close (fd);
}

void read_frame(pass_data *s)
{
	CLEAR (s->buf);
	s->buf.type	=	V4L2_BUF_TYPE_VIDEO_CAPTURE;
	s->buf.memory	= 	V4L2_MEMORY_MMAP;

	if (xioctl (s->fd, VIDIOC_DQBUF, &s->buf) == -1)
	{
		switch (errno)
		{
			case EAGAIN:
			  errno_exit ("VIDIOC_DQBUF");
			case EIO:
			  /* could ignore EIO, see spec. */
			default:
			  errno_exit ("VIDIOC_DQBUF");
		}

	}
	assert (s->buf.index < s->n_buffers);
}

void return_data (pass_data *s)
{
	if (xioctl (s->fd, VIDIOC_QBUF, &s->buf) == -1)	errno_exit ("VIDIOC_QBUF");
}

void start_capturing(pass_data *s)
{
	unsigned int i;
	enum v4l2_buf_type type;

	for (i = 0; i < s->n_buffers; ++ i)
	{
		struct v4l2_buffer buf;
		CLEAR (buf);

		buf.type	= 	V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory	=	V4L2_MEMORY_MMAP;
		buf.index	=	i;

		if (xioctl (s->fd, VIDIOC_QBUF, &buf) == -1)
			errno_exit("VIDIOC_QBUF");
	}
	type	=	V4L2_BUF_TYPE_VIDEO_CAPTURE;

    if (xioctl (s->fd, VIDIOC_STREAMON, &type))
        errno_exit("VIDIOC_STREAMON");
}

void stop_capturing(pass_data *s)
{
	enum v4l2_buf_type type;

	type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

	if (xioctl (s->fd, VIDIOC_STREAMOFF, &type))
		errno_exit("VIDIOC_STREAMOFF");
}

void init_mmap(pass_data *s)
{
	struct v4l2_requestbuffers req;
	CLEAR (req);
    req.count	=	20;
	req.type	=	V4L2_BUF_TYPE_VIDEO_CAPTURE;
	req.memory	=	V4L2_MEMORY_MMAP;

	if (xioctl (s->fd, VIDIOC_REQBUFS, &req))
	{
		if (EINVAL == errno)
		{
			fprintf(stderr, "%s does not support 'memory mapping'\n", s->dev_name);
			exit (EXIT_FAILURE);
		}
		else
		{
			errno_exit ("VIDIOC_REQBUFS");
		}
	}

	if (req.count < 2)
	{
		fprintf(stderr, "Insufficient buffer memory on %s\n", s->dev_name);
		exit (EXIT_FAILURE);
	}	

	if ((s->buffers = (buffer *)calloc (req.count, sizeof (*s->buffers))) == NULL)
	{
		fprintf(stderr, "Out of memory\n");
		exit ( EXIT_FAILURE);
	}

	for (s->n_buffers = 0; s->n_buffers < req.count; ++ s->n_buffers)
	{
		struct v4l2_buffer buf;
		CLEAR (buf);
	
		buf.type	=	V4L2_BUF_TYPE_VIDEO_CAPTURE;
		buf.memory	=	V4L2_MEMORY_MMAP;
		buf.index	=	s->n_buffers;

		if (xioctl (s->fd, VIDIOC_QUERYBUF, &buf) == -1)
			errno_exit("VIDIOC_QUERYBUF");

		s->buffers[s->n_buffers].length	=	buf.length;
		s->buffers[s->n_buffers].start	=
			mmap(   NULL,
				buf.length,
				PROT_READ | PROT_WRITE,
				MAP_SHARED,
				s->fd,
				buf.m.offset	
			    );

		if (s->buffers[s->n_buffers].start == MAP_FAILED)
			errno_exit ("mmap");
#if 	_DEBUG_
	fprintf(stdout, "%d -> %p\n", s->n_buffers, s->buffers[s->n_buffers].start);
#endif
	}
}

void init_device(pass_data* s)
{
	struct v4l2_capability cap;
	struct v4l2_cropcap cropcap;
	struct v4l2_crop crop;
	unsigned int min;

	if (xioctl (s->fd, VIDIOC_QUERYCAP, &cap) == -1)
	{
		if (EINVAL == errno)
		{
			fprintf (stderr, "%s is no V4L2 device\n", s->dev_name);
			exit (EXIT_FAILURE);
		}
		else
		{
			errno_exit ("VIDIOC_QUERYCAP");
		}
	}
	
	if (! (cap.capabilities & V4L2_CAP_VIDEO_CAPTURE))
	{
		fprintf(stderr, "%s is no video captrue device\n", s->dev_name);
		exit(EXIT_FAILURE);
	}

	if (! (cap.capabilities & V4L2_CAP_STREAMING))
	{
		fprintf(stderr, "%s does not support straming I/O\n", s->dev_name);
		exit(EXIT_FAILURE);
	}	

	CLEAR(cropcap);
	
	cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
	
	if (xioctl (s->fd, VIDIOC_CROPCAP, &cropcap) == 0)
	{
		crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
		crop.c = cropcap.defrect;

		if (xioctl (s->fd, VIDIOC_S_CROP, &crop))
		{
			switch (errno)
			{
				case EINVAL:
				  break;
				default:
				  break;
			}
		}
		else
		{
			/* Errors ignored */
		}
	}
	CLEAR (s->fmt);
	
	s->fmt.type		= V4L2_BUF_TYPE_VIDEO_CAPTURE;
    s->fmt.fmt.pix.width	= 640;
    s->fmt.fmt.pix.height	= 480;
    s->fmt.fmt.pix.pixelformat	= V4L2_PIX_FMT_YUYV;
	s->fmt.fmt.pix.field	= V4L2_FIELD_INTERLACED;

	if (xioctl (s->fd, VIDIOC_S_FMT, &s->fmt) == -1)
		errno_exit("VIDIOC_S_FMT");

	min = s->fmt.fmt.pix.width * 2;
	if (s->fmt.fmt.pix.bytesperline < min)
		s->fmt.fmt.pix.bytesperline = min;

	min = s->fmt.fmt.pix.bytesperline * s->fmt.fmt.pix.height;
	if (s->fmt.fmt.pix.sizeimage < min)
		s->fmt.fmt.pix.sizeimage = min;

}

void stop_device(pass_data *s)
{
	unsigned int i;
	
	for (i = 0; i < s->n_buffers; ++i)
		if (munmap (s->buffers[i].start, s->buffers[i].length) == -1)
			errno_exit("munmap");
}

int open_device(pass_data *s)
{
	struct stat st;
	
	if (stat (s->dev_name, &st) == -1)
	{
		fprintf(stderr, "Can't identify '%s':[%d] %s\n", s->dev_name, errno, strerror(errno));
        return -1;
	}
	
	if (!S_ISCHR (st.st_mode))
	{
		fprintf(stderr, "%s is no device\n", s->dev_name);
        return -2;
	}

	if ((s->fd = open (s->dev_name, O_RDWR, 0)) == -1 )
	{
		fprintf(stderr, "Can't oprn '%s': error %d, %s\n", s->dev_name, errno, strerror(errno));
        return -2;
	}
    return 0;
}

void close_device(pass_data *s)
{
	close (s->fd);
}

int xioctl(int fd, int request, void *arg)
{
	int r;
	
	do r = ioctl(fd, request, arg);
	while(r == -1 && EINTR == errno);

	return r;
}

void errno_exit(const char *s)
{
	fprintf(stderr, "%s error %d, %s\n", s, errno, strerror(errno));
	exit(EXIT_FAILURE);
}

static int convert_yuv_to_rgb_pixel(int y, int u, int v)
{
    unsigned int pixel32 = 0;
    unsigned char *pixel = (unsigned char *)&pixel32;
    int r, g, b;
    r = y + (1.370705 * (v-128));
    g = y - (0.698001 * (v-128)) - (0.337633 * (u-128));
    b = y + (1.732446 * (u-128));
    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 * 220 / 256;
    pixel[1] = g * 220 / 256;
    pixel[2] = b * 220 / 256;
    return pixel32;
}

int convert_yuv_to_rgb_buffer(unsigned char *yuv, unsigned char *rgb, unsigned int width, unsigned int height)
{
    unsigned int in, out = 0;
    unsigned int pixel_16;
    unsigned char pixel_24[3];
    unsigned int pixel32;
    int y0, u, y1, v;
    for(in = 0; in < width * height * 2; in += 4) {
        pixel_16 = yuv[in + 3] << 24 |
                   yuv[in + 2] << 16 |
                   yuv[in + 1] <<  8 |
                   yuv[in + 0];
        y0 = (pixel_16 & 0x000000ff);
        u  = (pixel_16 & 0x0000ff00) >>  8;
        y1 = (pixel_16 & 0x00ff0000) >> 16;
        v  = (pixel_16 & 0xff000000) >> 24;
        pixel32 = convert_yuv_to_rgb_pixel(y0, u, v);
        pixel_24[0] = (pixel32 & 0x000000ff);
        pixel_24[1] = (pixel32 & 0x0000ff00) >> 8;
        pixel_24[2] = (pixel32 & 0x00ff0000) >> 16;
        rgb[out++] = pixel_24[0];
        rgb[out++] = pixel_24[1];
        rgb[out++] = pixel_24[2];
        pixel32 = convert_yuv_to_rgb_pixel(y1, u, v);
        pixel_24[0] = (pixel32 & 0x000000ff);
        pixel_24[1] = (pixel32 & 0x0000ff00) >> 8;
        pixel_24[2] = (pixel32 & 0x00ff0000) >> 16;
        rgb[out++] = pixel_24[0];
        rgb[out++] = pixel_24[1];
        rgb[out++] = pixel_24[2];
    }
    return 0;
}

（3）videodisplay.h

#ifndef VIDEODISPLAY_H
#define VIDEODISPLAY_H

#include <QtGui>
#include "ui_dialog.h"
#include "v4l2.h"

class VideoDisplay : public QDialog
{
    Q_OBJECT

public:
    VideoDisplay(QWidget *parent = 0);
    ~VideoDisplay();

private slots:
    void beginCapture();
    void flushBuff();
    void savebmpData();

private:
    Ui::Dialog ui;
    pass_data pd;
    QTimer *timer;
    unsigned char *bufrgb;
};

#endif // VIDEODISPLAY_H

（4）videodisplay.cpp

#include <string.h>
#include "videodisplay.h"

VideoDisplay::VideoDisplay(QWidget *parent)
    : QDialog(parent)
    , bufrgb(NULL)
{
    ui.setupUi(this);
    connect(ui.beginButton, SIGNAL(clicked()), this, SLOT(beginCapture()));
    connect(ui.saveButton, SIGNAL(clicked()), this, SLOT(savebmpData()));
    connect(ui.exitButton, SIGNAL(clicked()), this, SLOT(reject()));

    timer = new QTimer(this);
    timer->setInterval(10);
    connect(timer, SIGNAL(timeout()), this, SLOT(flushBuff()));

    pd.dev_name = "/dev/video0";
}

VideoDisplay::~VideoDisplay()
{
    if (timer->isActive()) {
        timer->stop();
    }
}

void VideoDisplay::beginCapture()
{
    int flag = init_dev(&pd);
    if (flag == -1) {
        QMessageBox::information(this,tr("Tip"),tr("no device"));
        exit(1);
    }
    else if (flag == -2) {
        QMessageBox::information(this,tr("Tip"),tr("device is wrong"));
        exit(2);
    }
    else if (flag == -3) {
        QMessageBox::information(this,tr("Tip"),tr("can not open device"));
        exit(3);
    }
    timer->start();
    ui.beginButton->setDisabled(TRUE);
}

void VideoDisplay::flushBuff()
{
    read_frame (&pd);
    if (!bufrgb) {
        bufrgb = (unsigned char *)malloc(640 * 480* 3);
    }
    memset(bufrgb, 0, 640 * 480* 3);

    convert_yuv_to_rgb_buffer((unsigned char *)pd.buffers[pd.buf.index].start,bufrgb,640,480);
    QImage image(bufrgb,640,480,QImage::Format_RGB888);
    ui.displayLabel->setPixmap(QPixmap::fromImage(image));
    return_data(&pd);
}

void VideoDisplay::savebmpData()
{
    FILE *fp;
    time_t now;
    struct tm *tm_time;
    time(&now);

    if (bufrgb > 0 && strlen((char *)bufrgb) > 0) {
        tm_time = localtime(&now);
        char filename[30] = {0};
        sprintf(filename,"%4d-%02d-%02d_%02d.%02d.%02d.png",1900+tm_time->tm_year,1+tm_time->tm_mon,tm_time->tm_mday,
                tm_time->tm_hour,tm_time->tm_min,tm_time->tm_sec);
       QImage image(bufrgb,640,480,QImage::Format_RGB888);
       image.save(filename);
    }
}

（5）编译运行
 
点击BEGIN开始显示，点击SAVE会在当前的可执行程序目录下生成界面显示的视频的png的文件：2016-01-20_17.28.36.png、2016-01-20_17.36.08.png

三、总结

（1）不同的系统中v4l2的问题不一样，通过调整参数可以解决部分，但有些因能力有限实在无法解决。
（2）该文仅是以前毕业设计的一个开头，还有图像的转化和处理等一系列的自动识别的功能，接着可以将图片存放到数据库。
（3）若有问题或建议，请留言，在此感谢！