Introduction
I have just started programming in VC++, and this is my first article on CodeProject. I have created this project after reading the materials from CodeProject itself.
Everybody knows about the Screen Capture (Prt Scr Button) utility that comes with each and every Microsoft Windows. So what is the necessity of another Screen Capture application. But the Screen Capture utility that ships with Windows is very simple, you can only capture the whole screen. But what if you have to capture the part of the screen or say only a particular control on other window. Don't worry; here is the application for that. Using this application, you can capture the Selected area rather than the whole window. You can also capture any window or control by just clicking on it. (You can even capture only the start button or clock on task bar). Isn't it good! And it uses a simple Win32 API function for that.
Background
This article uses the two main functions of the Win32 API:
SetWindowsHookEx()
: To Set the global Mouse Hook (see MSDN for more help about this.) Global Mouse Hook is necessary to capture the Mouse Event from outside of our window.BitBlt()
(pronounced as Bitblit): Used to transfer color data between source device context and destination device context. It is used here to Capture the Screen.
Using the Code
MyMouseHook.cpp
Below is the code from MyMouseHook.cpp. Here we are sending MOUSE_DOWN
message when user presses the left Mouse Button and MOUSE_UP
when user leaves the left Mouse Button. Both of these are user messages.
LRESULT CALLBACK MyMouseHookProc(int nCode, WPARAM wParam, LPARAM lParam)
{
HWND hWndApp = FindWindow(0,L"Capture");
if(wParam== WM_LBUTTONDOWN || wParam == WM_NCLBUTTONDOWN)
{
PostMessage(hWndApp,MOUSE_DOWN,0,0);
}
if(wParam == WM_LBUTTONUP)
{
PostMessage(hWndApp,MOUSE_UP,0,0);
}
}
ScreenCaptureDlg.cpp
Below is the part of the code used in ScreenCaptureDlg.cpp File. When the user first presses the Mouse Button after pressing the Capture button, we take that point as the starting point in Begpt
as a starting point. And after dragging the Mouse when the user releases the button, we take the End points of the Selected Rectangle in Endpt
. Then we capture the area between this Begpt
and Endpt
from Desktop to MemDC
and send it to the SaveBitmap()
function which will save DC into .bmp file. And then we will create a process to show the .bmp file into PAIN application. Don't worry about the other function I haven't explain here. I have explained all the things well in the code:
case MOUSE_DOWN:
if(CaptureActWnd == TRUE)
{
GetCursorPos(&Begpt); hWndActWnd= WindowFromPoint(Begpt); CaptureActiveWindow(); EndDialog(hWndDlg,LOWORD(lParam)); PostQuitMessage(0); }
OutputDebugString( L"Desktop -GDI, MOUSE_DOWN");
GetCursorPos(&Begpt); break;
case MOUSE_UP:
if(CaptureActWnd == FALSE)
{
GetCursorPos(&Endpt); DrawBox(); MemDC = CreateCompatibleDC(DlgDC);
hBitmap = CreateCompatibleBitmap(DlgDC,width,height);
SelectObject(MemDC,hBitmap);
BitBlt(MemDC,0,0,width,height ,DeskDC,Begpt.x,Begpt.y,SRCCOPY);
SaveBitmap(MemDC, hBitmap,"Sample.bmp");
ShowImage();
SendMessage(hWndDlg,WM_COMMAND,(WPARAM)IDC_Exit,0);
}
break;
Points of Interest
I tried to capture WM_MOUSEMOVE
event to draw the rectangle on screen rather than capture only WM_LBUTTONDOWN
and WM_LBUTTONUP
but it was making the application very sluggish. So I left it and took only Starting Point and End Points to draw the rectangle and capture the screen. So you will not be able to see the rectangle when you will be selecting the area, but the rectangle will appear when you will complete your selection.
Special Thanks
To Arun Krishnan for his nice article on Desktop Calendar from where I came to know how to save the DC(Device Context)in .bmp file.
To Mumtaz Zaheer for his nice project about Sweep the Minesweeper from where I came to know about global hook.
History
There was a little flaw in the previous version of the application. When it was capturing the Window, it was failing to capture the Title bar which is also important. I have fixed the problem in the updated version.