Question

I’m actually trying to read a specific pixel on a window which is hidden by others. I want to use the GetPixel function from GDI library but it seems it only works with the global device context. I can’t read pixel from a specific window and I don’t understand why..
I found this article which uses the PrintWindow function to copy a specific window content to a temporary device context which can be read. But I can’t reproduce it.

EDIT

Thank you all my problem is solved 🙂
This script give you the RGB color of the pointer on the choosen window, even though the window is hidden. Remind that this program must be launch with admin privileges to get the pixels of processes launched with admin privileges.

#define STRICT
#define WINVER 0x0501
#define _WIN32_WINNT 0x0501
// 0x0501 for PrintWindow function
// You must be at least running Windows XP
// See http://msdn.microsoft.com/en-us/library/6sehtctf.aspx

#include <stdio.h>
#include <string.h>
#include <windows.h>

#define WINDOW_LIST_LIMIT 32
#define WINDOW_NAME_LIMIT 1024

void FatalError(char* error)
{
    printf("%s", error);
    exit(-1);
}

HWND window_list[WINDOW_LIST_LIMIT];
unsigned int window_list_index = 0;

BOOL EnumWindowsProc(HWND window_handle, LPARAM param)
{
    char window_title[WINDOW_NAME_LIMIT];

    if(!IsWindowVisible(window_handle)) return TRUE;

    RECT rectangle = {0};
    GetWindowRect(window_handle, &rectangle);
    if (IsRectEmpty(&rectangle)) return TRUE;

    GetWindowText(window_handle, window_title, sizeof(window_title));
    if(strlen(window_title) == 0) return TRUE;
    if(!strcmp(window_title, "Program Manager")) return TRUE;

    window_list[window_list_index] = window_handle;
    window_list_index++;

    printf("%u - %s\n", window_list_index, window_title);

    if(window_list_index == WINDOW_LIST_LIMIT) return FALSE;
    return TRUE;
}

int main(int argc, char** argv)
{
    unsigned int i, input;

    EnumWindows((WNDENUMPROC) EnumWindowsProc, (LPARAM) NULL);

    printf("\nChoose a window: ");
    scanf("%u", &input);
    printf("\n");
    if(input > window_list_index) FatalError("Bad choice..\n");

    HDC window_dc = GetWindowDC(window_list[input - 1]), global_dc = GetDC(0), temp_dc;
    if(!window_dc && !global_dc) FatalError("Fatal Error - Cannot get device context.\n");

    POINT cursor, previous_cursor;

    while(1)
    {
        temp_dc = CreateCompatibleDC(window_dc);
        if(!temp_dc) FatalError("Fatal Error - Cannot create compatible device context.\n");

        RECT window_rectangle;
        GetWindowRect(window_list[input - 1], &window_rectangle);

        HBITMAP bitmap = CreateCompatibleBitmap(window_dc,
            window_rectangle.right - window_rectangle.left,
            window_rectangle.bottom - window_rectangle.top);

        if (bitmap)
        {
            SelectObject(temp_dc, bitmap);
            PrintWindow(window_list[input - 1], temp_dc, 0);
            DeleteObject(bitmap);
        }

        GetCursorPos(&cursor);
        if(cursor.x != previous_cursor.x && cursor.y != previous_cursor.y)
        {
            COLORREF color = GetPixel(temp_dc, cursor.x - window_rectangle.left, cursor.y - window_rectangle.top);
            int red = GetRValue(color);
            int green = GetGValue(color);
            int blue = GetBValue(color);

            printf("\rRGB %02X%02X%02X", red, green, blue);

            cursor = previous_cursor;
        }

        DeleteDC(temp_dc);
        Sleep(50); // for lags
    }

    ReleaseDC(window_list[input - 1], window_dc);
    return 0;
}

I’ve changed some things, now User32 isn’t dynamically loaded.
It compiles with

gcc main.c -o main.exe -lGid32 -lUser32

Have a great day !

Answer 1

You are passing a process handle to GetDC. That’s not right. Processes don’t have device contexts, windows do. Remember a process can have many windows, or even none at all.

You need to get hold of the window handle, the HWND, for the window in question, and pass that to GetDC. I’d look to using FindWindow or EnumWindows to find your target top-level window.

Of course, there may be other problems with your code, but that’s the one that jumps out at me.