MainForm.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace FirstTest
{
    public partial class MainForm : Form
    {
        public MainForm()
        {
            InitializeComponent();
        }

        private void BrightnessButtonClick(object sender, EventArgs e)
        {
            var bitmap = new Bitmap(picture.Image);
            var graphics = picture.CreateGraphics();
            var colors = new List<Color>();
            var points = new List<Point>();
            var pen = new Pen(Color.Black, 2);
            
            for (var i = 0; i < bitmap.Width; i++)
            {
                var point = new Point(i, 255 - bitmap.GetPixel(i, 0).R);
                points.Add(point);
            }
            
            graphics.DrawLines(pen, points.ToArray());
        }

        private void ThresholdingButtonClick(object sender, EventArgs e)
        {
            var bitmap = new Bitmap(picture.Image);
            var total = 0;
            var sum = bitmap.Width * bitmap.Height;
            var average = 0.0;

            for (var i = 0; i < bitmap.Width; i++)
            {
                for (var j = 0; j < bitmap.Height; j++)
                {
                    total += bitmap.GetPixel(i, j).R;
                }
            }

            average = (float) total / (float) sum;

            for (var i = 0; i < bitmap.Width; i++)
            {
                for (var j = 0; j < bitmap.Height; j++)
                {
                    bitmap.SetPixel(i, j, bitmap.GetPixel(i, j).R < average ? Color.Black : Color.White);
                }
            }

            picture.Image = bitmap;
            picture.Refresh();
        }
    }
}