Problems using GetPixel() in VB.net 2005

[phillydub]

Ok, I am trying to read a single pixel's color from another application.

I have used GetPixel before (way back with VB6). However, this time around, I can't get it to read correctly!

Here is what I am TRYING to do:

1. Get target window handle (hWnd) using FindWindow()
2. Get device context (hDC) of window using GetDC(hwnd)
3. Get Pixel color using GetPixel(hdc, x, y)
4. ReleaseDC()


However, GetPixel never returns a color. It either returns 'FFFFFF', 0, or -1.

I don't know if it's my declarations, or what I'm doing wrong. I have been able to declare and use other API functions just fine- I just can't figure out how to get GetPixel() to work properly!

I'm using VB.net / Visual Studio 2005. Here is the code that is in question:
Code:

    Declare Auto Function FindWindow Lib "user32" ( _
      ByVal lpClassName As String, _
      ByVal lpWindowName As String) As IntPtr

    Declare Function GetDC Lib "user32" (ByVal hWnd As IntPtr) As IntPtr
    Declare Function ReleaseDC Lib "user32" (ByVal hwnd As IntPtr, ByVal hdc As IntPtr) As IntPtr

    Private Declare Function GetPixel Lib "gdi32" (ByVal hdc As IntPtr, ByVal X As Int32, ByVal Y As Int32) As Int32

    Public Function GetColorAt(ByVal X As Int32, ByVal Y As Int32) As Int32

        Dim hWnd As IntPtr
        Dim hDC As IntPtr

        hWnd = FindWindow(vbNullString, "RagII")

        hDC = GetDC(hWnd)

        Dim lColor As Int32 = GetPixel(hDC, X, Y)  

        ReleaseDC(hWnd, hDC)

        Return lColor
    End Function

Does anyone know what I'm doing wrong?

Thanks in advance,
P.dub

[schoolbusdriver]

You could have problems if the pixel is part of a video. This may give you ideas - it displays the colour under the cursor and an RGB value as the forms' caption. Just put a timer on the form.

Option Strict Off
Option Explicit On
Imports VB = Microsoft.VisualBasic
Imports System.Runtime.InteropServices

Friend Class Form1
   Inherits System.Windows.Forms.Form

<StructLayout(LayoutKind.Sequential)> Private Structure POINTAPI
   Dim x As Integer
   Dim y As Integer
End Structure

Private Declare Function GetCursorPos Lib "user32" (ByRef lpPoint As POINTAPI) As Integer
Private Declare Function GetDC Lib "user32" Alias "GetDC" (ByVal hwnd As Integer) As Integer
Private Declare Function GetPixel Lib "gdi32" (ByVal hdc As Integer, ByVal x As Integer, ByVal y As Integer) As Integer

Private Sub Form1_Load(ByVal eventSender As System.Object, ByVal eventArgs As System.EventArgs) Handles MyBase.Load
   Timer1.Interval = 100
   Timer1.Enabled = True
End Sub
	
Private Sub Timer1_Tick(ByVal eventSender As System.Object, ByVal eventArgs As System.EventArgs) Handles Timer1.Tick
   Dim structCursorPosition As POINTAPI
   Call GetCursorPos(structCursorPosition)
   Dim lColor As Integer = GetPixel(GetDC(0), structCursorPosition.x, structCursorPosition.y)
   BackColor = System.Drawing.ColorTranslator.FromOle(lColor)
   Dim strRgb As String = VB.Right("000000" & Hex(lColor), 6)
   Text = "R:" & VB.Right(strRgb, 2) & " G:" & Mid(strRgb, 3, 2) & " B:" & VB.Left(strRgb, 2)
End Sub
End Class