Qt浅谈之四十Centos下Qt结合v4l2实现的视频显示
本文共 11827 字，大约阅读时间需要 39 分钟。
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#include #ifdef __cplusplusextern "C" {#endiftypedef 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#include #include #include #include #include #include #include #include #include #include #include #include #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#include "ui_dialog.h"#include "v4l2.h"class VideoDisplay : public QDialog{ Q_OBJECTpublic: 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#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！
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