Draw Something To Screen

[serpentinez]

How would you draw something to your screen? Not in one specific window that you make but, on top of everything you have currently open? Like for example a dot or circle?

[srelu]

Call GetDCEx with hwnd = NULL. You'll get a HDC for the entire screen. You'll be able to draw anywhere on the screen regardless what windows are there.

[serpentinez]

Sorry I am pretty new to win32 could you please elaborate?

[MrViggy]

Device Contexts Overview

Viggy

[srelu]

Here's a sample that uses a thread.

Put the following function in any Win32 application:

DWORD WINAPI MyThread(LPVOID lpParameter)
{
  HDC hdc;
  SIZE s;
  s.cx = ::GetSystemMetrics(SM_CXSCREEN);
  s.cy = ::GetSystemMetrics(SM_CYSCREEN);
  int x, y, z, r, g, b;
  HBRUSH hbr, hbrOld;
  while(TRUE)
  {
    x = s.cx * rand() / RAND_MAX; // position x
    y = s.cy * rand() / RAND_MAX; // position y
    z = 100 * rand() / RAND_MAX; // radius
    r = 255 * rand() / RAND_MAX; // red color componennt
    g = 255 * rand() / RAND_MAX;// green color component
    b = 255 * rand() / RAND_MAX;// blue color component
    hbr = ::CreateSolidBrush(RGB(r,g,b));
    hdc = ::GetDCEx(NULL, 0, 0);
    hbrOld = (HBRUSH) ::SelectObject(hdc, hbr);
    ::Ellipse(hdc, x - z, y - z, x + z, y + z);
    ::SelectObject(hdc, hbrOld);
    ::DeleteObject(hbr);
    ::ReleaseDC(NULL, hdc);
    ::Sleep(20);
  }
}

You start the thread using the following code:

::CreateThread(0, 0, MyThread, 0, 0, 0);

This line can be antwhere in the Win32 application, just make sure it's in a place where it will be excuted exactly once (you can place it in the InitInstance function).

It will fill the whole screen with colorfull circles.

[serpentinez]

That will draw on my desktop not just one specific window?

[srelu]

That will draw on my desktop not just one specific window?

I repeat: "It will fill the *W.H.O.L.E. S.C.R.E.E.N.* with colorful circles".

Instead of asking, why don't you try it ?

[serpentinez]

Yay, it works! Thank you soo much!!!
BTW I wasn't home so I just wanted to make sure! Thanks again!
EDIT: Okay, now I have this code:

#include <windows.h>
#include "wtypes.h"
#include <iostream>

const char g_szClassName[] = "testWindowClass";


// Step 4: the Window Procedure
LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
	switch(msg)
	{
	case WM_CLOSE:
		DestroyWindow(hwnd);
		break;
	case WM_DESTROY:
		PostQuitMessage(0);
		break;
	default:
		return DefWindowProc(hwnd, msg, wParam, lParam);
	}
	return 0;
}

DWORD WINAPI MyThread(LPVOID lpParameter)
{
	HDC hdc;
	SIZE s;
	s.cx = ::GetSystemMetrics(SM_CXSCREEN);
	s.cy = ::GetSystemMetrics(SM_CYSCREEN);
	int x, y, z, r, g, b;
	int horizontal = 0;
	int vertical = 0;
	RECT desktop;
	// Get a handle to the desktop window
	const HWND hDesktop = GetDesktopWindow();
	// Get the size of screen to the variable desktop
	GetWindowRect(hDesktop, &desktop);
	// The top left corner will have coordinates (0,0)
	// and the bottom right corner will have coordinates
	// (horizontal, vertical)
	horizontal = desktop.right;
	vertical = desktop.bottom;
	HBRUSH hbr, hbrOld;
	x = horizontal / 4; //s.cx * rand() / RAND_MAX;  // position x
	y = vertical / 4; //s.cy * rand() / RAND_MAX;  // position y
	z = 30; // radius
	r = 255; // red color componennt
	g = 200;// green color component
	b = 0;// blue color component
	hbr = ::CreateSolidBrush(RGB(r,g,b));
	hdc = ::GetDCEx(NULL, 0, 0);
	hbrOld = (HBRUSH) ::SelectObject(hdc, hbr);
	::Ellipse(hdc, x - z, y - z, x + z, y + z);
	::SelectObject(hdc, hbrOld);
	::DeleteObject(hbr);
	::ReleaseDC(NULL, hdc);
	::Sleep(20);
	return 0;
}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
				   LPSTR lpCmdLine, int nCmdShow)
{
	::CreateThread(0, 0, MyThread, 0, 0, 0);
	WNDCLASSEX wc;
	HWND hwnd;
	MSG Msg;

	//Step 1: Registering the Window Class
	wc.cbSize        = sizeof(WNDCLASSEX);
	wc.style         = 0;
	wc.lpfnWndProc   = WndProc;
	wc.cbClsExtra    = 0;
	wc.cbWndExtra    = 0;
	wc.hInstance     = hInstance;
	wc.hIcon         = LoadIcon(NULL, IDI_APPLICATION);
	wc.hCursor       = LoadCursor(NULL, IDC_ARROW);
	wc.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
	wc.lpszMenuName  = NULL;
	wc.lpszClassName = g_szClassName;
	wc.hIconSm       = LoadIcon(NULL, IDI_APPLICATION);

	if(!RegisterClassEx(&wc))
	{
		MessageBox(NULL, "Cannot Register Window!", "Error!",
			MB_ICONEXCLAMATION | MB_OK);
		return 0;
	}

	// Step 2: Creating the Window
	hwnd = CreateWindowEx(
		WS_EX_CLIENTEDGE,
		g_szClassName,
		"Test Window",
		WS_OVERLAPPEDWINDOW,
		CW_USEDEFAULT, CW_USEDEFAULT, 240, 120,
		NULL, NULL, hInstance, NULL);

	if(hwnd == NULL)
	{
		MessageBox(NULL, "Cannot Create Window!", "Error!",
			MB_ICONEXCLAMATION | MB_OK);
		return 0;
	}

	ShowWindow(hwnd, nCmdShow);
	UpdateWindow(hwnd);

	// Step 3: The Message Loop
	while(GetMessage(&Msg, NULL, 0, 0) > 0)
	{
		TranslateMessage(&Msg);
		DispatchMessage(&Msg);
	}
	return Msg.wParam;
}

But every time I minimize then maximize a window the yellow circle disappears even while I have the application running (not that I would want it to stay after I close it but just to tell you that I am running the application). I have tried getting rid of the 'return 0;' at the end of the function but then I get an error saying that the function needs to return a value. Could anyone help please?