How to read the Color of a Screen Pixel

Question

Okay, I am looking for a function or something that will read the color of a certain pixel on my monitor, and when that color is detected, another function will be enabled. I figure using RGB. All help appreciated. Thank You.

Answer 1

This is the most efficient: It grabs a pixel at the location of the cursor, and doesn't rely on only having one monitor.

using System; using System.Drawing; using System.Drawing.Imaging; using System.Runtime.InteropServices; using System.Windows.Forms; using System.Diagnostics;  namespace FormTest {     public partial class Form1 : Form     {         [DllImport("user32.dll")]         static extern bool GetCursorPos(ref Point lpPoint);          [DllImport("gdi32.dll", CharSet = CharSet.Auto, SetLastError = true, ExactSpelling = true)]         public static extern int BitBlt(IntPtr hDC, int x, int y, int nWidth, int nHeight, IntPtr hSrcDC, int xSrc, int ySrc, int dwRop);          public Form1()         {             InitializeComponent();         }          private void MouseMoveTimer_Tick(object sender, EventArgs e)         {             Point cursor = new Point();             GetCursorPos(ref cursor);              var c = GetColorAt(cursor);             this.BackColor = c;              if (c.R == c.G && c.G < 64 && c.B > 128)             {                 MessageBox.Show("Blue");             }         }          Bitmap screenPixel = new Bitmap(1, 1, PixelFormat.Format32bppArgb);         public Color GetColorAt(Point location)         {             using (Graphics gdest = Graphics.FromImage(screenPixel))             {                 using (Graphics gsrc = Graphics.FromHwnd(IntPtr.Zero))                 {                     IntPtr hSrcDC = gsrc.GetHdc();                     IntPtr hDC = gdest.GetHdc();                     int retval = BitBlt(hDC, 0, 0, 1, 1, hSrcDC, location.X, location.Y, (int)CopyPixelOperation.SourceCopy);                     gdest.ReleaseHdc();                     gsrc.ReleaseHdc();                 }             }              return screenPixel.GetPixel(0, 0);         }     } } 

Now, obviously, you don't have to use the cursor's current location, but this is the general idea.

EDIT:

Given the above GetColorAt function you can poll a certain pixel on the screen in a safe, performance friendly way like this:

private void PollPixel(Point location, Color color) {     while(true)     {         var c = GetColorAt(location);          if (c.R == color.R && c.G == color.G && c.B == color.B)         {             DoAction();             return;         }          // By calling Thread.Sleep() without a parameter, we are signaling to the         // operating system that we only want to sleep long enough for other         // applications.  As soon as the other apps yield their CPU time, we will         // regain control.         Thread.Sleep()     } } 

You can wrap that in a Thread if you want, or execute it from a Console application. "Whatever suits your fancy," I guess.