Write Image & Video to File

In this lesson, I am going to show you how to write images and videos to a file using OpenCV.

If you have not install and configure OpenCV yet, please refer to Installing & Configuring with Visual Studio.

Write Image to File

In the following example, a yellow image is created and written into a file. Let's see how to do it with OpenCV.

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

using namespace cv;
using namespace std;

int main( int argc, const char** argv )
{
  Mat img(650, 600, CV_16UC3, Scalar(0,50000, 50000)); //create an image ( 3 channels, 16 bit image depth, 650 high, 600 wide, (0, 50000, 50000) assigned for Blue, Green and Red plane respectively. )

if (img.empty()) //check whether the image is loaded or not
{
cout << "ERROR : Image cannot be loaded..!!" << endl;
          //system("pause"); //wait for a key press
return -1;
}

     vector<int> compression_params; //vector that stores the compression parameters of the image

     compression_params.push_back(CV_IMWRITE_JPEG_QUALITY); //specify the compression technique

     compression_params.push_back(98); //specify the compression quality

     bool bSuccess = imwrite("D:/TestImage.jpg", img, compression_params); //write the image to file

     if ( !bSuccess )

    {

cout << "ERROR : Failed to save the image" << endl;

         //system("pause"); //wait for a key press

    }

namedWindow("MyWindow", CV_WINDOW_AUTOSIZE); //create a window with the name "MyWindow"
imshow("MyWindow", img); //display the image which is stored in the 'img' in the "MyWindow" window

waitKey(0);  //wait for a keypress

     destroyWindow("MyWindow"); //destroy the window with the name, "MyWindow"

return 0;
}

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You can download this OpenCV visual c++ project from here. (The downloaded file is a compressed .rar folder. So, you have to extract it using Winrar or other suitable software)

The above program is very much similar to the program under 'Create a Blank Image & Display' section in the lesson of Read & Display Image. If you need further clarifications of any OpenCV functions which are not explained here, please refer to Read & Display Image lesson.

New OpenCV functions which are not found earlier

• bool imwrite( const string& filename, InputArray img, const vector<int>& params=vector<int>())

The function saves the image in the variable 'img' to a file, specified by 'filename' . If this function fails to save the image, it will return false. On success of writing the file to the harddisk, it will return true.

Parameters -

• filename - specify the location and name of the file to be saved
• img - hold the image which is going to save
• params - This is a int vector to which you have to insert some int parameters specifying the format of the image
• JPEG format - You have to puch_back CV_IMWRITE_JPEG_QUALITY first and then a number between 0 and 100 (higher is the better). If you want the best quality output, use 100. I have used 98 in the above sample program. But higher the value, it will take longer time to write the image
• PNG format - You have to puch_back CV_IMWRITE_PNG_COMPRESSION first and then a number between 0 and 9 (higher is the better compression, but slower).

The image format is chosen depending on the file name extension. Only images with 8 bit or 16 bit unsigned  single channel or 3 channel ( CV_8UC1, CV_8UC3, CV_8SC1, CV_8SC3, CV_16UC1, CV_16UC3) with 'BGR' channel order, can be saved. If the depth or channel order of the image is different, use 'Mat::convertTo()' or 'cvtColor' functions to convert the image to supporting format before using imwrite function.

The above program is very much similar to the program under 'Create a Blank Image & Display' section in the lesson of Read & Display Image. If you need further clarifications of any OpenCV functions which are not explained here, please refer to Read & Display Image lesson.

Summary

This program creates an yellowish image ( 3 channels, 16 bit image depth, 650 high, 600 wide, (0, 50,000, 50,000) assigned for BGR channels). Because the image has 16 bit depth, you can use values between 0 and 65535 for each element in each channel. I have used 50,000 for each element in the green and red planes which gives the yellowish color. You can try different values. 
Then it specifies the compressing technique. I have used JPEG as the compression technique in the above example. Then it saves the image in the "D:/TestImage.jpg" location. Then it displays the newly created image in a window and wait indefinitely until an user presses a key.