how to draw constantly changing graphics

Question

Have not done this before (except in java, look how Steve McLeod fixed it), so obviously I suck at it. Here 64 pixels around current mouse position get drawn little bigger on a form. Problem is, that it's 'kind of' to slow, and I have no idea where to start fixing.

Besides that, I made a timer thread, that constantly calls update graphics when it's finished and a little fps like text, to show really how fast things are drawn.

Image example: (Image is from letter 'a' in "IntelliTrace" in Microsoft VS2010)

Source example:

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.Runtime.InteropServices;

namespace Zoom
{
    public partial class Form1 : Form
    {
        static class dllRef
        {
            [DllImport("user32.dll")]
            [return: MarshalAs(UnmanagedType.Bool)]
            static extern bool GetCursorPos(out Point lpPoint);

            [DllImport("user32.dll")]
            static extern IntPtr GetDC(IntPtr hwnd);

            [DllImport("user32.dll")]
            static extern Int32 ReleaseDC(IntPtr hwnd, IntPtr hdc);

            [DllImport("gdi32.dll")]
            static extern uint GetPixel(IntPtr hdc, int nXPos, int nYPos);

            // from http://www.pinvoke.net/default.aspx/gdi32/GetPixel.html
            static public System.Drawing.Color getPixelColor(int x, int y) {
                IntPtr hdc = GetDC(IntPtr.Zero);
                uint pixel = GetPixel(hdc, x, y);
                ReleaseDC(IntPtr.Zero, hdc);
                Color color = Color.FromArgb((int)(pixel & 0x000000FF),
                             (int)(pixel & 0x0000FF00) >> 8,
                             (int)(pixel & 0x00FF0000) >> 16);
                return color;
            }
            static public System.Drawing.Point getMousePosition() {
                Point p = new Point();
                GetCursorPos(out p); 
                return p;
            }
        }

        public Form1() {
            InitializeComponent();
            this.Size = new Size(400,400);
            this.Text="Image zoom";
            this.Location = new Point(640, 0);
            this.image = new Bitmap(320, 320);
            this.timeRef = DateTime.Now;
            this.BackColor = Color.White;
            Timer t = new Timer();
            t.Interval = 25;
            t.Tick += new EventHandler(Timer_Tick);
            t.Start();
        }

        public void Timer_Tick(object sender, EventArgs eArgs) {
            this.Form1_Paint(this, new PaintEventArgs(this.CreateGraphics(), new Rectangle(0, 0, this.Width, this.Height)));
        }

        private bool isdone = true;
        private int iter = 0;
        private Bitmap image;
        private DateTime timeRef;
        private void Form1_Paint(object sender, PaintEventArgs e) {
            if (isdone) {
                isdone = false;
                int step = 40;
                Point p = dllRef.getMousePosition();
                Pen myPen = new Pen(Color.Gray, 1);
                SolidBrush myBrush = null;
                Bitmap image2 = new Bitmap(320, 340);

                Graphics gc = Graphics.FromImage(image2);

                for (int x = 0; x < 8; x++) {
                    for (int y = 0; y < 8; y++) {
                        myBrush = new SolidBrush(dllRef.getPixelColor(p.X - 4 + x, p.Y - 4 + y));
                        gc.FillEllipse(myBrush, x * step, y * step, step - 3, step - 3);
                        gc.DrawEllipse(myPen, x * step, y * step, step - 3, step - 3);
                    }
                }
                StringBuilder sb = new StringBuilder();
                sb.Append(iter)
                        .Append(" frames in ")
                        .Append(String.Format("{0:0.###}", ((DateTime.Now-this.timeRef).TotalMilliseconds)/1000))
                        .Append("s.");
                gc.FillRectangle(new SolidBrush(this.BackColor), new Rectangle( 0, 320, 320, 40));
                gc.DrawString(sb.ToString(),new Font("Arial", 12),new SolidBrush(Color.Black), 10, 320);
                gc.Dispose();
                isdone = true;
                iter++;
                image = image2;
            }
            e.Graphics.DrawImage(image, 35f, 15f);
        }
    }
}

After changes i made, this one is ~98% faster:

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.Runtime.InteropServices;


namespace Zoom
{
    public partial class Form1 : Form
    {
        static class dllRef
        {
            [DllImport("user32.dll")]
            [return: MarshalAs(UnmanagedType.Bool)]
            static extern bool GetCursorPos(out Point lpPoint);

            [DllImport("user32.dll")]
            static extern IntPtr GetDC(IntPtr hwnd);

            [DllImport("user32.dll")]
            static extern Int32 ReleaseDC(IntPtr hwnd, IntPtr hdc);

            [DllImport("gdi32.dll")]
            static extern uint GetPixel(IntPtr hdc, int nXPos, int nYPos);

            // from http://www.pinvoke.net/default.aspx/gdi32/GetPixel.html
            static public System.Drawing.Color getPixelColor(int x, int y) {
                IntPtr hdc = GetDC(IntPtr.Zero);
                uint pixel = GetPixel(hdc, x, y);
                ReleaseDC(IntPtr.Zero, hdc);
                Color color = Color.FromArgb((int)(pixel & 0x000000FF),
                             (int)(pixel & 0x0000FF00) >> 8,
                             (int)(pixel & 0x00FF0000) >> 16);
                return color;
            }
            static public System.Drawing.Point getMousePosition() {
                Point p = new Point();
                GetCursorPos(out p); 
                return p;
            }
        }

        public Form1() {
            InitializeComponent();
            this.Size = new Size(400,400);
            this.Text="Image zoom";
            this.Location = new Point(640, 0);
            this.image = new Bitmap(320, 340);
            this.timeRef = DateTime.Now;
            this.BackColor = Color.White;
            Timer t = new Timer();
            t.Interval = 25;
            t.Tick += new EventHandler(Timer_Tick);
            t.Start();
        }

        public void Timer_Tick(object sender, EventArgs eArgs) {
            this.Form1_Paint(this, new PaintEventArgs(this.CreateGraphics(), new Rectangle(0, 0, this.Width, this.Height)));
        }

        private bool isdone = true;
        private int iter = 0;
        private Bitmap image;
        private DateTime timeRef;
        private void Form1_Paint(object sender, PaintEventArgs e) {
            if (isdone) {
                isdone = false;
                int step = 40;
                Point p = dllRef.getMousePosition();
                SolidBrush myBrush = null;
                Bitmap hc = new Bitmap(8, 8);

                using (Pen myPen = new Pen(Color.Gray, 1))
                using (Graphics gc = Graphics.FromImage(image))
                using (Graphics gf = Graphics.FromImage(hc))
                {
                    gf.CopyFromScreen(p.X - 4, p.Y - 4, 0, 0, new Size(8, 8),
                            CopyPixelOperation.SourceCopy);

                    for (int x = 0; x < 8; x++)
                    {
                        for (int y = 0; y < 8; y++)
                        {
                            myBrush = new SolidBrush(hc.GetPixel(x, y));
                            gc.FillEllipse(myBrush, x * step, y * step, step - 3, step - 3);
                            gc.DrawEllipse(myPen, x * step, y * step, step - 3, step - 3);
                        }
                    }

                    double ts = ((DateTime.Now - this.timeRef).TotalMilliseconds) / 1000;
                    StringBuilder sb = new StringBuilder();
                    sb.Append(++iter).Append(" frames in ").Append(String.Format("{0:0.###}", ts)).Append("s.");

                    gc.FillRectangle(new SolidBrush(this.BackColor), new Rectangle(0, 320, 320, 40));
                    gc.DrawString(sb.ToString(), new Font("Arial", 12), new SolidBrush(Color.Black), 10, 320);

                }

                isdone = true;
            }
            e.Graphics.DrawImage(image, 35f, 15f);
        }
    }
}

Answer 1

One thing that should speed things up is if you do the GetDC just once and get all of the pixels you need, then call ReleaseDC. So rather than:

for each pixel
  GetDC
  Read Pixel
  ReleaseDC

You have:

GetDC
for each pixel
  read pixel and store value
ReleaseDC

Then process the stored pixels.

That said, you're probably better off not using GetPixel at all, as I seem to remember it being terribly inefficient. I suspect you'd have better performance just grabbing the entire screen into a bitmap and getting the pixels from there. Perhaps the answer to this question will help you: http://stackoverflow.com/questions/362986/how-may-i-capture-the-screen-in-a-bitmap