คัยเก่ง Graphics C# ช่วยอิธิบายกันใช้ GraphicsUnit.Millimeter ให้หน่อย

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;
using System.Drawing.Drawing2D;
using System.Collections;

namespace A10
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

      e.Graphics.PageUnit = GraphicsUnit.Millimeter;
      float dpix = e.Graphics.DpiX;
      float dpiy = e.Graphics.DpiY;
 
      Pen p = new Pen(Color.Black,3);
 
      Matrix mx=new Matrix(1f/dpix,0,0,1f/dpiy,0,0);
      p.Transform= mx;
 
      e.Graphics.DrawRectangle(p,0.5f,0.5f,1,1);
    }
}