opencv/samples/cpp/imgcodecs_jpeg.cpp

#include <opencv2/core.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/imgcodecs.hpp>
#include <iostream>
#include <vector>

using namespace std;
using namespace cv;

int main(int /*argc*/, const char** /* argv */ )
{
    Mat framebuffer( 160 * 2, 160 * 5, CV_8UC3, cv::Scalar::all(255) );

    Mat img( 160, 160, CV_8UC3, cv::Scalar::all(255) );

    // Create test image.
    {
        const Point center( img.rows / 2 , img.cols /2 );

        for( int radius = 5; radius < img.rows ; radius += 3.5 )
        {
            cv::circle( img, center, radius, Scalar(255,0,255) );
        }
        cv::rectangle( img, Point(0,0), Point(img.rows-1, img.cols-1), Scalar::all(0), 2 );
    }

    // Draw original image(s).
    int top = 0; // Upper images
    {
        for( int left = 0 ; left < img.rows * 5 ; left += img.rows ){
            Mat roi = framebuffer( Rect( left, top, img.rows, img.cols ) );
            img.copyTo(roi);

            cv::putText( roi, "original", Point(5,15), FONT_HERSHEY_SIMPLEX, 0.5, Scalar::all(0), 2, 4, false );
        }
    }

    // Draw lossy images
    top += img.cols; // Lower images
    {
        struct test_config{
            string comment;
            uint32_t sampling_factor;
        } config [] = {
            { "411", IMWRITE_JPEG_SAMPLING_FACTOR_411 },
            { "420", IMWRITE_JPEG_SAMPLING_FACTOR_420 },
            { "422", IMWRITE_JPEG_SAMPLING_FACTOR_422 },
            { "440", IMWRITE_JPEG_SAMPLING_FACTOR_440 },
            { "444", IMWRITE_JPEG_SAMPLING_FACTOR_444 },
        };

        const int config_num = 5;

        int left = 0;

        for ( int i = 0 ; i < config_num; i++ )
        {
            // Compress images with sampling factor parameter.
            vector<int> param;
            param.push_back( IMWRITE_JPEG_SAMPLING_FACTOR );
            param.push_back( config[i].sampling_factor );
            vector<uint8_t> jpeg;
            (void) imencode(".jpg", img, jpeg, param );

            // Decompress it.
            Mat jpegMat(jpeg);
            Mat lossy_img = imdecode(jpegMat, -1);

            // Copy into framebuffer and comment
            Mat roi = framebuffer( Rect( left, top, lossy_img.rows, lossy_img.cols ) );
            lossy_img.copyTo(roi);
            cv::putText( roi, config[i].comment, Point(5,155), FONT_HERSHEY_SIMPLEX, 0.5, Scalar::all(0), 2, 4, false );

            left += lossy_img.rows;
        }
    }

    // Output framebuffer(as lossless).
    imwrite( "imgcodecs_jpeg_samplingfactor_result.png", framebuffer );

    return 0;
}