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seems you are right. you can create an issue about this problem

TEST CODE

#include <opencv2/imgcodecs.hpp>
#include <opencv2/highgui.hpp>
#include <iostream>
#include <string>

using namespace cv;
using namespace std;

int main(int argc, char** argv)
{
    // Create mat with alpha channel
    Mat mat = imread("Astro_4D_stars_proper_radial_g_b_8mag_big.png");

    vector<int> compression_params;
    compression_params.push_back(IMWRITE_PNG_COMPRESSION);
    compression_params.push_back(0);
    compression_params.push_back(IMWRITE_PNG_STRATEGY);
    compression_params.push_back(IMWRITE_PNG_STRATEGY_DEFAULT);
    for (int i = 0; i < 10; i++)
        for (int j = 0; j < 5; j++)
        {
            compression_params[1] = i;
            compression_params[3] = j;
            imwrite(format("PNG_STRATEGY_%d_PNG_COMPRESSION_%d.png", j, i), mat, compression_params);
        }
    imwrite("PNG_SAVED_DEFAULT.png", mat);
    return 0;
}

here is the result sizes

Astro_4D_stars_proper_radial_g_b_8mag_big.png   1.966.656
PNG_SAVED_DEFAULT.png   3.245.449
PNG_STRATEGY_0_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_0_PNG_COMPRESSION_1.png    3.157.900
PNG_STRATEGY_0_PNG_COMPRESSION_2.png    3.039.178
PNG_STRATEGY_0_PNG_COMPRESSION_3.png    2.904.732
PNG_STRATEGY_0_PNG_COMPRESSION_4.png    2.637.388
PNG_STRATEGY_0_PNG_COMPRESSION_5.png    2.582.198
PNG_STRATEGY_0_PNG_COMPRESSION_6.png    2.529.017
PNG_STRATEGY_0_PNG_COMPRESSION_7.png    2.523.274
PNG_STRATEGY_0_PNG_COMPRESSION_8.png    2.503.723
PNG_STRATEGY_0_PNG_COMPRESSION_9.png    2.477.291
PNG_STRATEGY_1_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_1_PNG_COMPRESSION_1.png    3.157.900
PNG_STRATEGY_1_PNG_COMPRESSION_2.png    3.039.178
PNG_STRATEGY_1_PNG_COMPRESSION_3.png    2.904.732
PNG_STRATEGY_1_PNG_COMPRESSION_4.png    2.603.165
PNG_STRATEGY_1_PNG_COMPRESSION_5.png    2.566.294
PNG_STRATEGY_1_PNG_COMPRESSION_6.png    2.519.976
PNG_STRATEGY_1_PNG_COMPRESSION_7.png    2.515.584
PNG_STRATEGY_1_PNG_COMPRESSION_8.png    2.498.000
PNG_STRATEGY_1_PNG_COMPRESSION_9.png    2.475.941
PNG_STRATEGY_2_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_2_PNG_COMPRESSION_1.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_2.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_3.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_4.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_5.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_6.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_7.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_8.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_9.png    5.175.597
PNG_STRATEGY_3_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_3_PNG_COMPRESSION_1.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_2.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_3.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_4.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_5.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_6.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_7.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_8.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_9.png    2.762.428
PNG_STRATEGY_4_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_4_PNG_COMPRESSION_1.png    3.642.295
PNG_STRATEGY_4_PNG_COMPRESSION_2.png    3.492.898
PNG_STRATEGY_4_PNG_COMPRESSION_3.png    3.347.030
PNG_STRATEGY_4_PNG_COMPRESSION_4.png    3.058.305
PNG_STRATEGY_4_PNG_COMPRESSION_5.png    2.988.217
PNG_STRATEGY_4_PNG_COMPRESSION_6.png    2.915.866
PNG_STRATEGY_4_PNG_COMPRESSION_7.png    2.904.503
PNG_STRATEGY_4_PNG_COMPRESSION_8.png    2.876.399
PNG_STRATEGY_4_PNG_COMPRESSION_9.png    2.849.642

seems you are right. you can create an issue about this problem

TEST CODE

#include <opencv2/imgcodecs.hpp>
#include <opencv2/highgui.hpp>
#include <iostream>
#include <string>

using namespace cv;
using namespace std;

int main(int argc, char** argv)
{
    // Create mat with alpha channel
    Mat mat = imread("Astro_4D_stars_proper_radial_g_b_8mag_big.png");

    vector<int> compression_params;
    compression_params.push_back(IMWRITE_PNG_COMPRESSION);
    compression_params.push_back(0);
    compression_params.push_back(IMWRITE_PNG_STRATEGY);
    compression_params.push_back(IMWRITE_PNG_STRATEGY_DEFAULT);
    for (int i = 0; i < 10; i++)
        for (int j = 0; j < 5; j++)
        {
            compression_params[1] = i;
            compression_params[3] = j;
            imwrite(format("PNG_STRATEGY_%d_PNG_COMPRESSION_%d.png", j, i), mat, compression_params);
        }
    imwrite("PNG_SAVED_DEFAULT.png", mat);
    return 0;
}

here is the result sizes

Astro_4D_stars_proper_radial_g_b_8mag_big.png   1.966.656
PNG_SAVED_DEFAULT.png   3.245.449
PNG_STRATEGY_0_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_0_PNG_COMPRESSION_1.png    3.157.900
PNG_STRATEGY_0_PNG_COMPRESSION_2.png    3.039.178
PNG_STRATEGY_0_PNG_COMPRESSION_3.png    2.904.732
PNG_STRATEGY_0_PNG_COMPRESSION_4.png    2.637.388
PNG_STRATEGY_0_PNG_COMPRESSION_5.png    2.582.198
PNG_STRATEGY_0_PNG_COMPRESSION_6.png    2.529.017
PNG_STRATEGY_0_PNG_COMPRESSION_7.png    2.523.274
PNG_STRATEGY_0_PNG_COMPRESSION_8.png    2.503.723
PNG_STRATEGY_0_PNG_COMPRESSION_9.png    2.477.291
PNG_STRATEGY_1_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_1_PNG_COMPRESSION_1.png    3.157.900
PNG_STRATEGY_1_PNG_COMPRESSION_2.png    3.039.178
PNG_STRATEGY_1_PNG_COMPRESSION_3.png    2.904.732
PNG_STRATEGY_1_PNG_COMPRESSION_4.png    2.603.165
PNG_STRATEGY_1_PNG_COMPRESSION_5.png    2.566.294
PNG_STRATEGY_1_PNG_COMPRESSION_6.png    2.519.976
PNG_STRATEGY_1_PNG_COMPRESSION_7.png    2.515.584
PNG_STRATEGY_1_PNG_COMPRESSION_8.png    2.498.000
PNG_STRATEGY_1_PNG_COMPRESSION_9.png    2.475.941
PNG_STRATEGY_2_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_2_PNG_COMPRESSION_1.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_2.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_3.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_4.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_5.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_6.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_7.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_8.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_9.png    5.175.597
PNG_STRATEGY_3_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_3_PNG_COMPRESSION_1.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_2.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_3.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_4.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_5.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_6.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_7.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_8.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_9.png    2.762.428
PNG_STRATEGY_4_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_4_PNG_COMPRESSION_1.png    3.642.295
PNG_STRATEGY_4_PNG_COMPRESSION_2.png    3.492.898
PNG_STRATEGY_4_PNG_COMPRESSION_3.png    3.347.030
PNG_STRATEGY_4_PNG_COMPRESSION_4.png    3.058.305
PNG_STRATEGY_4_PNG_COMPRESSION_5.png    2.988.217
PNG_STRATEGY_4_PNG_COMPRESSION_6.png    2.915.866
PNG_STRATEGY_4_PNG_COMPRESSION_7.png    2.904.503
PNG_STRATEGY_4_PNG_COMPRESSION_8.png    2.876.399
PNG_STRATEGY_4_PNG_COMPRESSION_9.png    2.849.642

EDIT: i tested both images (original and saved by OpenCV) in https://compress-or-die.com/analyze

only difference : original image's ColorType is Palette and OpenCV's is RGB

i think the original image has only different tones of yellow and this is the reason for lesser size

image description

seems you are right. you can create an issue about this problem

TEST CODE

#include <opencv2/imgcodecs.hpp>
#include <opencv2/highgui.hpp>
#include <iostream>
#include <string>

using namespace cv;
using namespace std;

int main(int argc, char** argv)
{
    // Create mat with alpha channel
    Mat mat = imread("Astro_4D_stars_proper_radial_g_b_8mag_big.png");

    vector<int> compression_params;
    compression_params.push_back(IMWRITE_PNG_COMPRESSION);
    compression_params.push_back(0);
    compression_params.push_back(IMWRITE_PNG_STRATEGY);
    compression_params.push_back(IMWRITE_PNG_STRATEGY_DEFAULT);
    for (int i = 0; i < 10; i++)
        for (int j = 0; j < 5; j++)
        {
            compression_params[1] = i;
            compression_params[3] = j;
            imwrite(format("PNG_STRATEGY_%d_PNG_COMPRESSION_%d.png", j, i), mat, compression_params);
        }
    imwrite("PNG_SAVED_DEFAULT.png", mat);
    return 0;
}

here is the result sizes

Astro_4D_stars_proper_radial_g_b_8mag_big.png   1.966.656
PNG_SAVED_DEFAULT.png   3.245.449
PNG_STRATEGY_0_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_0_PNG_COMPRESSION_1.png    3.157.900
PNG_STRATEGY_0_PNG_COMPRESSION_2.png    3.039.178
PNG_STRATEGY_0_PNG_COMPRESSION_3.png    2.904.732
PNG_STRATEGY_0_PNG_COMPRESSION_4.png    2.637.388
PNG_STRATEGY_0_PNG_COMPRESSION_5.png    2.582.198
PNG_STRATEGY_0_PNG_COMPRESSION_6.png    2.529.017
PNG_STRATEGY_0_PNG_COMPRESSION_7.png    2.523.274
PNG_STRATEGY_0_PNG_COMPRESSION_8.png    2.503.723
PNG_STRATEGY_0_PNG_COMPRESSION_9.png    2.477.291
PNG_STRATEGY_1_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_1_PNG_COMPRESSION_1.png    3.157.900
PNG_STRATEGY_1_PNG_COMPRESSION_2.png    3.039.178
PNG_STRATEGY_1_PNG_COMPRESSION_3.png    2.904.732
PNG_STRATEGY_1_PNG_COMPRESSION_4.png    2.603.165
PNG_STRATEGY_1_PNG_COMPRESSION_5.png    2.566.294
PNG_STRATEGY_1_PNG_COMPRESSION_6.png    2.519.976
PNG_STRATEGY_1_PNG_COMPRESSION_7.png    2.515.584
PNG_STRATEGY_1_PNG_COMPRESSION_8.png    2.498.000
PNG_STRATEGY_1_PNG_COMPRESSION_9.png    2.475.941
PNG_STRATEGY_2_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_2_PNG_COMPRESSION_1.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_2.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_3.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_4.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_5.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_6.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_7.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_8.png    5.175.597
PNG_STRATEGY_2_PNG_COMPRESSION_9.png    5.175.597
PNG_STRATEGY_3_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_3_PNG_COMPRESSION_1.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_2.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_3.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_4.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_5.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_6.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_7.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_8.png    2.762.428
PNG_STRATEGY_3_PNG_COMPRESSION_9.png    2.762.428
PNG_STRATEGY_4_PNG_COMPRESSION_0.png    27.647.001
PNG_STRATEGY_4_PNG_COMPRESSION_1.png    3.642.295
PNG_STRATEGY_4_PNG_COMPRESSION_2.png    3.492.898
PNG_STRATEGY_4_PNG_COMPRESSION_3.png    3.347.030
PNG_STRATEGY_4_PNG_COMPRESSION_4.png    3.058.305
PNG_STRATEGY_4_PNG_COMPRESSION_5.png    2.988.217
PNG_STRATEGY_4_PNG_COMPRESSION_6.png    2.915.866
PNG_STRATEGY_4_PNG_COMPRESSION_7.png    2.904.503
PNG_STRATEGY_4_PNG_COMPRESSION_8.png    2.876.399
PNG_STRATEGY_4_PNG_COMPRESSION_9.png    2.849.642

EDIT: i tested both images (original and saved by OpenCV) in https://compress-or-die.com/analyze

only difference : original image's ColorType is Palette and OpenCV's is RGB

i think the original image has only different tones of yellow and this is the reason for lesser size

image description

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