【学习笔记】Learning OpenCV3——Ch8 working with video
Reading Video with the cv::VideoCapture Object
对象创建的三种方法:
// 1. Input filename
cv::VideoCapture::VideoCapture(
const string& filename
);
// 2. Video capture device id
cv::VideoCapture::VideoCapture(
int device
);
// 3.
cv::VideoCapture::VideoCapture();
构造函数的第一种:
在第一种方法中,传入视频路径,创建对象后,需要加入视频是否成功打开:
// e.g.
cv::VideoCapture cap("1.mp4");
if (!cap.isOpen(www.quwanyule157.com)) {
std::cout << "Failed to open the video" << std::endl;
视频打不开的常见原因:
视频路径错误
视频编码格式未知
后一种原因的解决方案:
you will need to have the appropriate library already residing on your computer in order to successfully read the video file.
构造函数的第二种:
通常为设备的代号,整型
小tip:
cv::VideoCapture cap(-1);
1
可打开一个窗口供以选择捕获视频的设备
构造函数的第三种:
cv::VideoCapture cap;
cap.open("video.avi");
1
2
读取视频帧数据
// 1. read function
bool cv::VideoCapture::read(
cv::OutputArray image yongshiyule178.com/// Image into which to read data, return true for read frame successfully, false for failue
);
// 2. >> operator
cv::VideoCapture& cv:www.wanmeiyuele.cn:VideoCapture::operator>>( cv::Mat& image // Image into which to read data
);
// NOTE:
// << return a reference to the original object, should check the return array is not empty
// 3. grab && retrieve function
bool cv::VideoCapture::grab(www.mhylpt.com/ void ); // is designed simply to get it onto the computer as quickly as possible
bool cv::VideoCapture::retrieve(
cv::OutputArray image, // Image into which to read data
int channel = 0 // Used for multihead devices
);
// NOTE:
// can read images from mutiple cameras
// more general
设置&获取视频属性
double cv::VideoCapture::get(
int propid //www.mcyllpt.com Property identifier (see Table 8-4)
);
bool cv::VideoCapture::set(
int propid,
double value // Value to which to set the property
//Example 8-1. Unpacking a four-character code to identify a video codec
cv::VideoCapture cap( "my_video.avi" );
unsigned f = (unsigned)cap.get( cv::CAP_PROP_FOURCC );
char fourcc[] = {
(char) f, // First character is lowest bits
(char)(f >> 8), // Next character is bits 8-15
(char)(f >> 16), // Next character is bits 16-23
(char)(f >> 24), // Last character is bits 24-31
'\0' // and don't forget to terminate
Reading Video with the cv::VideoCapture Object
两种构造方式:
// 1. an uninitialized video object
cv::VideoWriter::VideoWriter();
// 2. an initialized video object
cv::VideoWriter::VideoWriter(
const string& filename, // output filename
int fourcc, // codec, use CV_FOURCC(www.michenggw.com/) macro
double fps, // Frame rate (stored in output file)
cv::Size frame_size, // Size of individual images
bool is_color = true // if false, you can pass gray frames
);
// e.g.
Don’t forget that you are providing characters (single quotes), not strings (double quotes).
同cv::VideoCapture,也有cv::VideoWriter::isOpened()方法暗示是否可以写视频,如果返回false,可能存在以下几点原因:
没有写权限
编码格式不可用
NOTES:
The codecs available to you depend on your operating system installation and the additional libraries you have installed.
For portable code, it is especially important to be able to gracefully handle the case in which the desired codec is not available on any particular machine.
写入视频的两个函数:
// 1. write function
cv::VideoWriter::write(
const Mat& image // Image to write as next frame
);
/*
NOTES:
- This image must have the same size as the size you gave to the writer when you configured it in the first place. - If you told the writer that the frames would be in color, this must also be a three-channel image.
*/
// 2. << Operator
my_video_writer << my_frame;
/*
NOTE:
once you have your video writer open, you can write images to the video stream in the same manner you would write to cout or a file ofstream object
简单实例
#include<opencv2/opencv.hpp>
#include<iostream>
using namespace cv;
int main() {
// params & input path && output path configure
int caseVideo = 6;
char pathInVideo[200], pathOutVideo[200];
sprintf(pathInVideo, "%d.avi", caseVideo);
sprintf(pathOutVideo, "%d_output.avi", caseVideo);
// read video
cv::VideoCapture cap(pathInVideo);
if (!cap.isOpened()) {
std::cerr << "Failed to open the video" << std::endl;
return -1;
}
int frameLength = cap.get(CAP_PROP_FRAME_COUNT);
// params for writting output video
cv::VideoWriter outputVideo;
cv::Size sWH = cv::Size((int)cap.get(CAP_PROP_FRAME_WIDTH),
(int)cap.get(CAP_PROP_FRAME_HEIGHT));
if (!outputVideo.open(pathOutVideo, cv::VideoWriter::fourcc('M', 'P', '4', '2'), cap.get(CAP_PROP_FPS), sWH)) {
std::cerr << "Failed to write video" << std::endl;
return -1;
}
// gauss filter frame by frame
for (int i = 0; i < frameLength; i++) {
std::cout << "Process the " << i << " frame" << std::endl;
cv::Mat inputImg, outputImg;
cap.read(inputImg);
if (!inputImg.empty()) {
cv::GaussianBlur(inputImg, outputImg, cv::Size(25, 25), 0.0, 0.0);
outputVideo.write(outputImg);
}
}
// release cap && outputVideo
cap.release();
outputVideo.release();
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