Draw a US Flag using C# and GDI+

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;

namespace Example1_6
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            SetStyle(ControlStyles.ResizeRedraw, true);
            this.Width = 500;
            this.Height = 310;
            this.BackColor = Color.LightGreen;
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            Graphics g = e.Graphics;
            g.SmoothingMode = SmoothingMode.AntiAlias;
            DrawFlag(g, 20, 20, this.Width - 50);
            g.Dispose();
        }

        private void DrawFlag(Graphics g, float x0, float y0, float width)
        {
            SolidBrush whiteBrush = new SolidBrush(Color.White);
            SolidBrush blueBrush = new SolidBrush(Color.FromArgb(0, 0, 128));
            SolidBrush redBrush = new SolidBrush(Color.Red);

            float height = 10 * width / 19;

            // Draw white rectangle background:
            g.FillRectangle(whiteBrush, x0, y0, width, height);

            // Draw the seven red stripes.
            for (int i = 0; i < 7; i++)
            {
                g.FillRectangle(redBrush, x0, y0 + 2 * i * height / 13, width, height / 13);
            }

            // Draw blue box.
            // Size it so that it covers two fifths of the flag width and
            // the top four red stripes vertically.
            RectangleF blueBox = new RectangleF(x0, y0, 2 * width / 5, 7 * height / 13);
            g.FillRectangle(blueBrush, blueBox);

            // Draw fifty stars in the blue box.
            float offset = blueBox.Width / 40;
            // Divide the blue box into a grid of 11 x 9 squares and place a star
            // in every other square.

            float dx = (blueBox.Width - 2 * offset) / 11;
            float dy = (blueBox.Height - 2 * offset) / 9;

            for (int j = 0; j < 9; j++)
            {
                float yc = y0 + offset + j * dy + dy / 2;
                for (int i = 0; i < 11; i++)
                {
                    float xc = x0 + offset + i * dx + dx / 2;
                    if ((i + j) % 2 == 0)
                    {
                        DrawStar(g, this.Width/55, xc, yc);
                    }
                }
            }
            whiteBrush.Dispose();
            blueBrush.Dispose();
            redBrush.Dispose();
        }

        private void DrawStar(Graphics g, float r, float xc, float yc)
        {
            // r: determines the size of the star.
            // xc, yc: determine the location of the star.
            float sin36 = (float)Math.Sin(36.0 * Math.PI / 180.0);
            float sin72 = (float)Math.Sin(72.0 * Math.PI / 180.0);
            float cos36 = (float)Math.Cos(36.0 * Math.PI / 180.0);
            float cos72 = (float)Math.Cos(72.0 * Math.PI / 180.0);
            float r1 = r * cos72 / cos36;
            // Fill the star:
            PointF[] pts = new PointF[10];
            pts[0] = new PointF(xc, yc - r);
            pts[1] = new PointF(xc + r1 * sin36, yc - r1 * cos36);
            pts[2] = new PointF(xc + r * sin72, yc - r * cos72);
            pts[3] = new PointF(xc + r1 * sin72, yc + r1 * cos72);
            pts[4] = new PointF(xc + r * sin36, yc + r * cos36);
            pts[5] = new PointF(xc, yc + r1);
            pts[6] = new PointF(xc - r * sin36, yc + r * cos36);
            pts[7] = new PointF(xc - r1 * sin72, yc + r1 * cos72);
            pts[8] = new PointF(xc - r * sin72, yc - r * cos72);
            pts[9] = new PointF(xc - r1 * sin36, yc - r1 * cos36);
            g.FillPolygon(Brushes.White, pts);
        }  
    }
}