Member Avatar for Trepach

Environment: MS Visual Studio 2008. (Please transfer this thread to where it belongs.)
Project type: Windows Forms Control Library.
Goal: Get the pixel color from a given process' main window.
Already achieved: I got the desired process's pointer by using

cli::array<Process ^>^ Proc = System::Diagnostics::Process::GetProcessesByName("Notepad");

I believe I got the window handler of the process' main window using:

IntPtr hWnd = Proc[0]->MainWindowHandle;

Do I need the process' hDC(Device Context Handler)? If so, how do I do it. Then How do I get the color of a pixel in a given location?
Thank you in advance!

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Dani AI

Generated

Good progress, — you already have the right idea (grab the HWND, get an HDC and sample). The confusion is that two different GetPixel functions exist: the managed one you see in System::Drawing (Bitmap::GetPixel) and the native Win32 one you want. The native prototype you need is:

// Win32 (native)
COLORREF GetPixel(HDC hdc, int x, int y);

GetPixel returns a COLORREF; convert that into a managed Color like this:

COLORREF cr = ::GetPixel(hdc, x, y);
if (cr == CLR_INVALID) { /* handle error */ }
System::Drawing::Color clr = System::Drawing::Color::FromArgb(
    GetRValue(cr), GetGValue(cr), GetBValue(cr));

To use the Win32 APIs from a C++/CLI Windows Forms project, include the headers in an unmanaged region or in the .cpp file (avoid putting raw Windows headers into headers that are compiled /clr by default). A safe include pattern:

#define WIN32_LEAN_AND_MEAN
#define NOMINMAX
#pragma managed(push, off)
#include <windows.h>
#pragma managed(pop)
#pragma comment(lib, "user32.lib")
#pragma comment(lib, "gdi32.lib")

Notes and troubleshooting: always check for CLR_INVALID from GetPixel; always release HDCs with ReleaseDC; some windows (layered, hardware-accelerated or DWM-composited) will not yield the expected pixels from a window DC — in those cases capture the composited image (PrintWindow / BitBlt from the screen DC or use a DWM-based approach). Also be aware of UIPI (integrity levels) and elevation: a lower-privilege app may be blocked from reading a higher-privilege window. Finally, #pragma once is simply a header guard to prevent multiple includes and is fine to leave as-is.

Credit to for pointing out the need for an HDC and to for the native API hint — you were very close; the remaining steps are the include/link setup and converting COLORREF into a managed Color.

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Member Avatar for caut_baia

You need a handle to a device context since GetPixel needs one as it's first argument.It returns a COLORREF object which consists of the amount of red green and blue that give the pixel it's color.

Edited by caut_baia because: n/a
Member Avatar for Trepach

Ok, but how do I "get" the hDc? I'm tired of searching. The google answers doesnt work on my compiler/version/project.
How do I point out which window/hWND do I want to get its hDC?
What type is the function that gets me the hDC?

Member Avatar for Duoas

Microsoft function names are not very creative (which is a good thing).

GetDC()

Hope this helps.

Member Avatar for caut_baia

If you got the window handle you can get the DC inside a WM_PAINT message using HDC BeginPaint(HWND,PAINTSTRUCT) or simply HDC GetDC(HWND)...else i might not get what your question is.

Edited by caut_baia because: n/a
Member Avatar for Trepach

GetDC is my function. How do I include the "Header: Winuser.h (include Windows.h)"?
Is the header the only thing I must include for the GetDC() function to work?
My project begins with

#pragma once


namespace MyProject{

	using namespace System;
	using namespace System::Diagnostics;
	using namespace System::ComponentModel;
	using namespace System::Collections;
	using namespace System::Windows::Forms;
	using namespace System::Data;
	using namespace System::Drawing;
	using namespace System::Drawing::Imaging;

	public ref class Form1 : public System::Windows::Forms::Form
	{
	public:
		Form1(void)
		{
			InitializeComponent();
			//
			//TODO: Add the constructor code here
			//
		}
          ...

I dont even know what "#pragma once" stands for. These Windows Forms are not like the projects I've been studying on.

Edited by Trepach because: formatting
Member Avatar for Trepach

Thank you very much. You helped me a lot.
From this code:

IntPtr hWnd = Proc[0]->MainWindowHandle;
				 void *handle = hWnd.ToPointer();
				 HWND hand = reinterpret_cast<HWND>(handle);

				 HDC h = GetDC(hand);
				 ReleaseDC(hand, h);
				 h = NULL;

				 HDC myHDC = GetDC(hand);
				 GetPixel(myHDC, 100, 100);

GetPixel() does not take 3 arguments. Whats the model of the GetPixel() that I need?
In the library for Windows Forms I find only GetPixel(POINT point) or GetPixel( int coordinate_1, int coordinate_2 )
You've been very helpful!

Edited by Trepach because: n/a
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