TIL Day 21

2023.10.18

Today's agenda

1. Camera image processing
2. OpenCV drawing functions

1. Camera image processing

Using VideoCapture class, computer can do camera image processing

VideoCapture class

#include <iostream>
#include "opencv2/opencv.hpp"

using namespace std;
using namespace cv;

int main(){
	VideoCapture cap;
    cap.open(0);
    
    if(!cap.isopened()){
    	cerr << "Camera open failed" << "\n";
        return -1;
    }
    
    Mat frame;
    while(true){
    	cap >> frame;
        
        if(frame.empty()){
        	cerr << "Frame is empty" << "\n";
            break;
        }
    }
    
    imshow("frame", frame);
    if(waitKey(1) == 27){
    	break;
    }
    
    cap.release();
    destroyAllWindows();
}

VideoCapture class member functions

VideoCapture::VideoCapture(int index, int apiPreference = CAP_ANY);
bool VideoCapture::open(int index, int apiPreference = CAP_ANY);

parameters

• index : Camera id(if one camera connected, value will 0)
  Not the only camera, video file or image and video stream can be opened
• apiPreference : set prefer camera processing value

return value

• VideoCapture constructor returns VideoCapture object
• open() function returns true if success open camera, false if failed

bool VideoCapture::read(OutputArray image);
VideoCapture& VideoCapture::operator >> (Mat& image);

parameters

• image : current frame. if read fails, set to empty image

return value

• read() function returns true if success read, false if failed

** >> operator overloading is wrapper class of read()

double VideoCapture::get(int propId) const;
bool VideoCapture::set(int propId, double value);

parameters

• propId : property flag // defined cv::VideoCaptureProperties
  

VideoWriter class

Write image data to video class

#include <iostream>
#include "opencv2/opencv.hpp"

using namespace std;
using namespace cv;

int main(){
    VideoCapture cap(0);

    int fourcc = VideoWriter::fourcc('X', 'V', 'I', 'D');  
    double fps = 15;
    Size sz = Size((int)cap.get(CAP_PROP_FRAME_WIDTH), (int)cap.get(CAP_FRAME_HEIGHT));

    VideoWriter outputVideo("output.avi", fourcc, fps, sz);

    int delay = cvRound(1000 / fps);

    Mat frame;
    while(true){
        cap >> frame;

        outputViodeo << frame;
        imshow("frame", frame);

        if(waitKey(delay) == 27)
            break;
    }
}

VideoWriter class member functions

VideoWriter::VideoWriter(const String& filename, int fourcc, 
						double fps, Size frameSize, bool isColor = true);
bool Videowriter::open(const String& filename, int fourcc, 
						double fps, Size frameSize, bool isColor = true);

parameters

• filename : path to the file name for write video
• fourcc : video codec
  
• fps : frame per seconds
• frameSize : video frame size
• isColor : if false, write to the grey scale video

2. OpenCV drawing class

void line(InputOutputArray img, Point pt1, Point pt2, const Scalar& color, 
			int thickness = 1, int lineType = LINE_8, int shift = 0);

paramters

• img : input or output image(Mat type object)
• pt1 : point to start draw line
• pt2 : point to end draw line
• color : line color
• thickness : thickness of line
• lineType : line type
  LINE_4, LINE8, LINE_AA(Anti Aliasing, prefered options)

void rectangle(InputOutputArray img, Rect rec, const Scalar& color,
				int thickness = 1, int lineType = LINE_8, int shift = 0);

paramters

• img : input or output image(Mat type object)
• color : rectangle line color
• thickness : thickness of rectangle line
• lineType : rectangle line type
  LINE_4, LINE8, LINE_AA(Anti Aliasing, prefered options)
  

void circue(InputOutputArray img, Point center, int radius, const scalar& color,
		int thickness = 1, int lineType = LINE_8, int shift = 0);

paramters

• img : input or output image(Mat type object)
• center : point of center
• radius : radius of circle
• color : rectangle line color
• thickness : thickness of rectangle line
• lineType : rectangle line type
  LINE_4, LINE8, LINE_AA(Anti Aliasing, prefered options)
  

void putText(InputOutputArray img, const String& text, Point org, int fontFace,
			double fontScale, Scalar color, int thickness = 1,
            int lineType = LINE_8, bool bottonLeftOrigin = false);

• img : input or output image(Mat type object)
• text : text to present to the image
• org : point to text writing
• fontFace : type of font
• fontScale : size of font
• color : rectangle line color
• thickness : thickness of rectangle line
• lineType : rectangle line type
  LINE_4, LINE8, LINE_AA(Anti Aliasing, prefered options)