Getting pixel color in C++ - c++

Question

I would like to get the RGB values of a pixel at different x, y coordinates on the screen.
How would I go about this in C++?
I'm trying to create my own gaussian blur effect.
This would be in Windows 7.
Edit
What libraries need to be included for this to run?
What I have going:
#include <iostream>
using namespace std ;
int main(){
HDC dc = GetDC(NULL);
COLORREF color = GetPixel(dc, 0, 0);
ReleaseDC(NULL, dc);
cout << color;
}

Answer 1

You can use GetDC on the NULL window to get a device context for the whole screen, and can follow that up with a call to GetPixel:
HDC dc = GetDC(NULL);
COLORREF color = GetPixel(dc, x, y);
ReleaseDC(NULL, dc);
Of course, you'd want to only acquire and release the device context once while doing all the pixel-reading for efficiency.

Answer 2

As mentioned in a previous post, you want the GetPixel function from the Win32 API.
GetPixel sits inside gdi32.dll, so if you have a proper environment setup, you should be able to include windows.h (which includes wingdi.h) and you should be golden.
If you have a minimal environment setup for whatever reason, you could also use LoadLibrary on gdi32.dll directly.
The first parameter to GetPixel is a handle to the device context, which can be retrieved by calling the GetDC function(which is also available via <windows.h>).
A basic example that loads GetPixel from the dll and prints out the color of the pixel at your current cursor position is as follows.
#include<windows.h>
#include<stdio.h>
typedef WINAPI COLORREF (*GETPIXEL)(HDC, int, int);
int main(int argc, char** argv)
{
HINSTANCE _hGDI = LoadLibrary("gdi32.dll");
if(_hGDI)
{
while(true) {
GETPIXEL pGetPixel = (GETPIXEL)GetProcAddress(_hGDI, "GetPixel");
HDC _hdc = GetDC(NULL);
if(_hdc)
{
POINT _cursor;
GetCursorPos(&_cursor);
COLORREF _color = (*pGetPixel) (_hdc, _cursor.x, _cursor.y);
int _red = GetRValue(_color);
int _green = GetGValue(_color);
int _blue = GetBValue(_color);
printf("Red: 0x%02x\n", _red);
printf("Green: 0x%02x\n", _green);
printf("Blue: 0x%02x\n", _blue);
}
FreeLibrary(_hGDI);
}
}
return 0;
}