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korea

https://en.wikipedia.org/wiki/Flag_of_South_Korea#/media/File:Flag_of_South_Korea_(construction_sheet).svg
#include <stdio.h>
#include <gd.h>
#include <math.h>

// Function prototype
void draw_korean_flag(gdImagePtr img);

// Declare the rotation function
void rotatePoints(int x_orig, int y_orig, double rotation_ang, gdPoint *points, int num_points) {
    int i;
    double angle_rad = rotation_ang * M_PI / 180.0;

    for (i = 0; i < num_points; i++) {
        int x = points[i].x - x_orig;
        int y = points[i].y - y_orig;

        points[i].x = x_orig + (int)(x * cos(angle_rad) - y * sin(angle_rad));
        points[i].y = y_orig + (int)(x * sin(angle_rad) + y * cos(angle_rad));
    }
}

int main() {
    // width 3: height 2
    int width = 1200;
    int height = (int)(width * 2.0 / 3.0);

    gdImagePtr img = gdImageCreateTrueColor(width, height);
    gdImageAlphaBlending(img, 0);

    draw_korean_flag(img);

    FILE *outputFile = fopen("korea_gd.png", "wb");
    if (outputFile == NULL) {
        fprintf(stderr, "Error opening the output file.\n");
        return 1;
    }
    gdImagePngEx(img, outputFile, 9);
    fclose(outputFile);
    gdImageDestroy(img);
    return 0;
}

void draw_korean_flag(gdImagePtr img) {
    int width = gdImageSX(img);
    int height = gdImageSY(img);
    int red, blue, black, grey, white;
    // according to https://en.wikipedia.org/wiki/Flag_of_South_Korea#/media/File:Flag_of_South_Korea_(construction_sheet).svg
    double scale = width/72;
    int big_circle_dia = (int)24*scale;
    int small_circle_dia = (int) big_circle_dia/2;
    // left small circle rotated angle is rot_ang
    double rot_ang = atan2(48.0, 72.0);
    // rectangle rotated angle is rot_ang2;
    double rot_ang2 = atan2(72.0, 48.0);
    double degree = M_PI/180.0;
    int center_x = (int)(width / 2);
    int center_y = (int)(height / 2);
    gdPoint points[4];
    // for upper rectangle
    int rect_lower_left_x = center_x - (int)6*scale;
    int rect_lower_right_x = center_x + (int)6*scale;
    int rect_upper_left_x = rect_lower_left_x;
    int rect_upper_right_x = rect_lower_right_x;
    int rect_lower_left_y = center_y - (int)18*scale;
    int rect_lower_right_y = rect_lower_left_y;
    int rect_upper_left_y = rect_lower_left_y - (int)8*scale;
    int rect_upper_right_y = rect_upper_left_y;
    // get points
    points[0].x = rect_lower_left_x;
    points[0].y = rect_lower_left_y;
    points[1].x = rect_upper_left_x;
    points[1].y = rect_upper_left_y;
    points[2].x = rect_upper_right_x;
    points[2].y = rect_upper_right_y;
    points[3].x = rect_lower_right_x;
    points[3].y = rect_lower_right_y;

    printf("small_circle_dia is %d\n", small_circle_dia);
    int left_center_x = center_x - (small_circle_dia/2)*cos(rot_ang);
    printf("center_x is %d\n", center_x);
    printf("atan2(48.0, 72.0) is %f\n", atan2(48.0, 72.0));
    printf("left_center_x is %d\n", left_center_x);
    int left_center_y = center_y - (small_circle_dia/2)*sin(rot_ang);
    int right_center_x = center_x + (small_circle_dia/2)*cos(rot_ang);
    int right_center_y = center_y + (small_circle_dia/2)*sin(rot_ang);

    red = gdImageColorAllocate(img, 255, 0, 0);      // Red
    blue = gdImageColorAllocate(img, 0, 0, 255);     // Blue
    black = gdImageColorAllocate(img, 0, 0, 0);      // Black
    grey = gdImageColorAllocate(img, 125, 125, 125); // grey
    white = gdImageColorAllocate(img, 255, 255, 255); // white

    gdImageFilledRectangle(img, 0, 0, width, height, white);
    gdImageRectangle(img, 0, 0, width, height, black);
    
    // rectangle rotate to upper right
    rotatePoints(center_x, center_y, rot_ang2/degree, points, 4);
    gdImageFilledPolygon(img, points, 4, grey);
    //printf("rot_ang/degree is %f\n", rot_ang2/degree);
    // rectangle rotate to upper left
    rotatePoints(center_x, center_y, -2*rot_ang2/degree, points, 4);
    gdImageFilledPolygon(img, points, 4, grey);
    // rectangle rotate to lower ritht
    rotatePoints(center_x, center_y, - 2*rot_ang/degree, points, 4);
    gdImageFilledPolygon(img, points, 4, grey);
    // rectangle rotate to lower left
    rotatePoints(center_x, center_y, -2*rot_ang2/degree, points, 4);
    gdImageFilledPolygon(img, points, 4, grey);
    // Draw upper rectagle four lines
    /*
    gdImageLine(img, rect_lower_left_x, rect_lower_left_y, rect_upper_left_x, rect_upper_left_y, grey);
    gdImageLine(img, rect_lower_right_x, rect_lower_right_y, rect_upper_right_x, rect_upper_right_y, grey);
    gdImageLine(img, rect_lower_left_x, rect_lower_left_y, rect_lower_right_x, rect_lower_right_y, grey);
    gdImageLine(img, rect_upper_left_x, rect_upper_left_y, rect_upper_left_x, rect_upper_left_y, grey);
    */
    // Draw the big circle outline
    int circle_radius = (int)(height * 2.0 / 5.0);
    gdImageEllipse(img, center_x, center_y, big_circle_dia, big_circle_dia, black);
    // left small circle
    gdImageEllipse(img, left_center_x, left_center_y, small_circle_dia, small_circle_dia, red);
    // right small circle
    gdImageEllipse(img, right_center_x, right_center_y, small_circle_dia, small_circle_dia, blue);
    gdImageFill(img, left_center_x, left_center_y, red); // Fill left small circle with red
    gdImageFill(img, right_center_x, right_center_y - small_circle_dia/2-10, red);
    gdImageFill(img, right_center_x, right_center_y, blue); // Fill right small circle with blue
    gdImageFill(img, left_center_x, left_center_y + small_circle_dia/2+10, blue);
    gdImageEllipse(img, center_x, center_y, big_circle_dia, big_circle_dia, white);
    
    gdImageEllipse(img, center_x + circle_radius / 2, center_y, circle_radius * 2, circle_radius * 2, black);

    // Flood fill the left area with red
    gdImageFill(img, center_x - circle_radius + 1, center_y, red);

    // Flood fill the right area with blue
    gdImageFill(img, center_x + circle_radius + 1, center_y, blue);

    // Flood fill the intersection area with yellow
    gdImageFill(img, center_x, center_y, yellow);
    */
}
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