Thread: How can I draw directly on the Desktop?

I've done my research on this, and discovered that my two options are either to hook into the explorer.exe process or create a transparent window. I'd rather not create a transparent window, because I'm doing this for fun and to learn more about .DLL functionality. That being said, although I understand the MSDN documentation of the SetWindowHookEx() function, I'm not exactly sure of the lineage of what I need to do. The main part that I'm not sure about is creating a .DLL, unless I'm just creating a .DLL that would hook into a specific function that updates the screen. Even so, I don't see the point in doing that. If my understanding is correct, I would need to save my drawing objects and just update them when the application I hooked into receives a WM_PAINT/WM_ERASEBKGND message. I would appreciate if someone cleared up the exact line of calls and code I need to make.

Originally Posted by HelpfulPerson

I've done my research on this, and discovered that my two options are either to hook into the explorer.exe process or create a transparent window. I'd rather not create a transparent window, because I'm doing this for fun and to learn more about .DLL functionality. That being said, although I understand the MSDN documentation of the SetWindowHookEx() function, I'm not exactly sure of the lineage of what I need to do. The main part that I'm not sure about is creating a .DLL, unless I'm just creating a .DLL that would hook into a specific function that updates the screen. Even so, I don't see the point in doing that. If my understanding is correct, I would need to save my drawing objects and just update them when the application I hooked into receives a WM_PAINT/WM_ERASEBKGND message. I would appreciate if someone cleared up the exact line of calls and code I need to make.

It's been a good long while since I've done any Win32 GDI, but I do remember being able to do this by simply grabbing a NULL device context (ie: GetDC(NULL)) and drawing to it. Worked back then, no idea if it works on current Windows machines though...

Originally Posted by Sebastiani

It's been a good long while since I've done any Win32 GDI, but I do remember being able to do this by simply grabbing a NULL device context (ie: GetDC(NULL)) and drawing to it. Worked back then, no idea if it works on current Windows machines though...

I've done that too, I created an SDL GUI to display pixels where the mouse cursor is on the screen, but I ran into lots of problems with it. First, SDL isn't exactly ideal since when it loses focus it no longer recieves events. Second, even with constantly getting the mouse cursor position eventually the code to get the pixel glitched up and the whole application broke for some reason. Which I end up having to restart it to fix it. I'm not sure if the cause is because the device context doesn't stay updated with the screen or if it's because of me closing and opening applications. Then, when I did research on it, apparently you have to constantly update the drawing you made for it to stick on the screen. It makes sense because it probably updates just like any SDL GUI I've made. My problem is, I've never done GDI before and just recently got interested in trying to do something in it. As I said, I'm confused to how to hook into explorer.exe ( e.g Do I make my own .DLL file?, How do I call it?, Do I need to do anything else to save the user's drawing on the screen? ). I'm hoping someone will know more about this than I do.

Edit : SirPrattlepod, your story was hilarious

Code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>


#include <windows.h>


#define VKEY_IS_PRESSED(vk)    ( GetAsyncKeyState(vk) & 0x8000 )


enum
{
    TIMER_START,
    TIMER_STOP,
    TIMER_GETDIFF
};


typedef struct t_timer
{
    clock_t start_time;
    clock_t end_time;
} t_timer;


clock_t timer( int command, t_timer * timer1 )
{
    switch ( command )
    {
        case TIMER_START :
            timer1->start_time = clock( );
        break;


        case TIMER_STOP :
            timer1->end_time = clock( );
        break;


        case TIMER_GETDIFF :
            return ( ( timer1->end_time - timer1->start_time ) / ( CLOCKS_PER_SEC / 1000 ));
        break;


        default : break;
    }


    return -1;
}


void win_error( char * message, BOOL exit )
{
    char buffer[BUFSIZ] = { 0 };
    DWORD error_code = GetLastError( );


    FormatMessage
    (
        FORMAT_MESSAGE_FROM_SYSTEM,
        NULL,
        error_code,
        MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
        (LPTSTR) buffer,
        BUFSIZ,
        NULL
    );


    MessageBox(NULL, buffer, message, MB_ICONWARNING | MB_OK );


    if ( exit ) ExitProcess( error_code );


    return;
}


void draw_box( COLORREF color, int x, int y, int w, int h )
{
    RECT box_shape = { x, y, x + w, y + h };
    HDC device_context = GetDC( NULL );
    HBRUSH brush = 0;




    if ( !( brush = CreateSolidBrush( color ) ) )
    {
        win_error( "CreateSolidBrush( )", FALSE );
        return;
    }


    if ( !( FillRect( device_context, &box_shape, brush ) ) )
    {
        win_error( "FillRgn( )", FALSE );
        return;
    }


    if ( !( DeleteObject( brush ) ) )
    {
        win_error( "DeleteObject( )", FALSE );
        return;
    }


    ReleaseDC( NULL, device_context );


    return;
}


int main( )
{
    HDC screen_context = GetDC( NULL );
    POINT pos1 = { 0 }, pos2 = { 0 };
    POINT mouse_cursor_pos = { 0 };
    COLORREF color = { 0 };
    size_t brush_size = 25;


    for ( ; ; )
    {
        GetCursorPos( &mouse_cursor_pos );


        if ( VKEY_IS_PRESSED( VK_ESCAPE ) )
            break;


        if ( VKEY_IS_PRESSED( VK_LBUTTON ) )
            color = GetPixel( screen_context, mouse_cursor_pos.x, mouse_cursor_pos.y );
        if ( VKEY_IS_PRESSED( VK_UP ) )
            draw_box( color, mouse_cursor_pos.x, mouse_cursor_pos.y, brush_size, brush_size );
        if ( VKEY_IS_PRESSED( VK_LEFT ) )
            pos1 = mouse_cursor_pos;
        if ( VKEY_IS_PRESSED( VK_RIGHT ) )
            pos2 = mouse_cursor_pos;


        if ( VKEY_IS_PRESSED( VK_SPACE ) )
            if ( pos1.x && pos1.y && pos2.x && pos2.y )
                draw_box( color, pos1.x, pos1.y, fabs( pos2.x - pos1.x ), fabs( pos2.y - pos1.y ) );


        if ( VKEY_IS_PRESSED( VK_ADD ) )
            brush_size += 5;
        if ( VKEY_IS_PRESSED( VK_SUBTRACT ) )
            brush_size = fabs( brush_size - 5 );


        Sleep( 20 );
    }


    ReleaseDC( NULL, screen_context );


    return 0;
}

The program I made above works, but any slight update to the screen clears out any drawing I did.

Well, you probably need to set up a message loop to process things like WM_ERASEBKGND, to begin with.

I spent quite a while trying to experiment with this sort of thing a few years ago.

I just did some digging and found the code, it's for Windows XP no idea if it will work on anything else.

From the looks of it it seems that on Windows XP you can get the HDC of the display with disp = CreateDC( "DISPLAY", NULL, NULL, NULL );.

This won't compile btw because it's missing the resource file and stuff, but it's pretty simple so it might give you ideas. Sorry for the code quality it's from years ago and I was never really good with Windows API anyways. Looks like the most relevant code is in WM_TIMER, knock yourself out.

Code:

/*A desktop draw POC*/

#include <windows.h>
#include "DeskDraw.h"

int CurrentSelected = MOUSESELECT;

HINSTANCE AppInstance;
int MidPressed = NOTPRESSED;

HBITMAP Pens;
HBITMAP PensSel;
HCURSOR SystemCursor, CpySystemCursor;


void DrawButtons( HWND WndHwnd )
{
    HDC whDC, NewDC;

    whDC = GetDC( WndHwnd );
    NewDC = CreateCompatibleDC( whDC );

    SelectObject( NewDC, Pens );

    if( !BitBlt( whDC, 0, 0, 150, 50, NewDC, 0, 0, SRCCOPY ))
    {
        MessageBox( NULL, "Failed to Blit src to dest", "Failed", MB_OK );
    }

    SelectObject( NewDC, PensSel );

    BitBlt( whDC, CurrentSelected * 50, 0, 50, 50, NewDC, CurrentSelected * 50, 0, SRCCOPY );

    DeleteDC( NewDC );
    DeleteDC( whDC );
}

LRESULT CALLBACK WndProc( HWND WndHwnd, UINT Msg, WPARAM wParam, LPARAM lParam )
{
    static HDC disp;
    SHORT MidKeyState = 0;
    POINT mousepos;
    int penstyle, width;
    COLORREF PenCol;

    switch( Msg )
    {
        case WM_CREATE:
            SetTimer( WndHwnd, 1, 80, NULL );
            disp = CreateDC( "DISPLAY", NULL, NULL, NULL );

            Pens = LoadBitmap( AppInstance, "PENS1" );
            PensSel = LoadBitmap( AppInstance, "PENSPUSHED2" );

            if( Pens == NULL || PensSel == NULL )
            {
                MessageBox( NULL, "Could not load Bitmaps", "Failed", MB_OK );
                DestroyWindow( WndHwnd );
            }

            SystemCursor = LoadCursor( NULL, IDC_ARROW );
            CpySystemCursor = CopyCursor( SystemCursor );
            DestroyCursor( SystemCursor );
            break;

        case WM_TIMER:
            if( CurrentSelected != MOUSESELECT )
            {
                if( !MidPressed )
                {
                    GetCursorPos( &mousepos );
                    MoveToEx( disp, mousepos.x, mousepos.y, NULL );
                    DrawButtons( WndHwnd );
                    UpdateWindow( WndHwnd );
                }
                if( (MidKeyState = GetAsyncKeyState( VK_MBUTTON )) != 0 )
                {
                    MidPressed = PRESSED;
                    if( MidKeyState & 1 )
                    {
                        //If Mid mouse pressed since last call
                        GetCursorPos( &mousepos );
                        MoveToEx( disp, mousepos.x, mousepos.y, NULL );
                    }

                    else
                    {
                        GetCursorPos( &mousepos );
                        LineTo( disp, mousepos.x, mousepos.y );
                    }
                }
            }
            break;

        case WM_LBUTTONDOWN:
            if( (lParam & 0xFFFF ) < 50 )
            {
                SystemCursor = CopyCursor( CpySystemCursor );
                SetSystemCursor( CpySystemCursor, OCR_NORMAL );
                CpySystemCursor = CopyCursor( SystemCursor );
                DestroyCursor( SystemCursor );

                PenCol = 0;
                CurrentSelected = MOUSESELECT;
            }
            else if( (lParam & 0xFFFF ) < 100 )
            {
                SetSystemCursor( LoadCursor( AppInstance, "BLUECUR" ), OCR_NORMAL );

                PenCol = (0xFF << 16);
                CurrentSelected = BLUESELECT;
            }
            else
            {
                SetSystemCursor( LoadCursor( AppInstance, "REDCUR" ), OCR_NORMAL );

                PenCol = 0xFF;
                CurrentSelected = REDSELECT;
            }

            penstyle = PS_SOLID;
            width = 2;

            SelectObject( disp, CreatePen( penstyle, width, PenCol ) );

            DrawButtons( WndHwnd );
            UpdateWindow( WndHwnd );
            break;

        case WM_MOVE:
            DrawButtons( WndHwnd );
            UpdateWindow( WndHwnd );
            break;

        case WM_CLOSE:
            DestroyWindow( WndHwnd );
            break;

        case WM_DESTROY:
            KillTimer( WndHwnd, 1 );
            DeleteDC( disp );
            DeleteObject( Pens );
            DeleteObject( PensSel );
            SetSystemCursor( CpySystemCursor, OCR_NORMAL );
            DestroyCursor( CpySystemCursor );
            PostQuitMessage( 0 );
            break;

        default:
            return DefWindowProc( WndHwnd, Msg, wParam, lParam );
            break;
    }
    return 0;
}


int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR CmdArgs, int nCmdShow )
{
    WNDCLASSEX WndCls;
    HWND WndHwnd;
    MSG Msg;
    int captionh, borderw;

    AppInstance = hInstance;

    WndCls.cbSize = sizeof( WNDCLASSEX );
    WndCls.style = 0;
    WndCls.lpfnWndProc = WndProc;
    WndCls.cbClsExtra = 0;
    WndCls.cbWndExtra = 0;
    WndCls.hInstance = hInstance;
    WndCls.hIcon = LoadIcon( hInstance, "APPICON" );
    WndCls.hCursor = LoadCursor( NULL, IDC_ARROW );
    WndCls.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
    WndCls.lpszMenuName = NULL;
    WndCls.lpszClassName = "NewClass";
    WndCls.hIconSm = NULL;

    if( !RegisterClassEx( &WndCls ) )
    {
        MessageBox( NULL, "Class registration failed", "Failed", MB_OK );
        return 0;
    }

    captionh = GetSystemMetrics( SM_CYCAPTION );
    borderw = GetSystemMetrics( SM_CXBORDER );

    WndHwnd = CreateWindowEx( WS_EX_TOPMOST, "NewClass", "ToolBox", WS_SYSMENU, 0, 0, borderw * 2 + 150, captionh + borderw * 2 + 50, NULL, NULL, hInstance, NULL );

    if( WndHwnd == NULL )
    {
        MessageBox( NULL, "Could not create window", "Failed", MB_OK );
        UnregisterClass( "NewClass", hInstance );
        return 0;
    }

    ShowWindow( WndHwnd, SW_SHOW );
    UpdateWindow( WndHwnd );

    while( GetMessage( &Msg, WndHwnd, 0, 0 ) > 0 )
    {
        TranslateMessage( &Msg );
        DispatchMessage( &Msg );
    }
    UnregisterClass( "NewClass", hInstance );
    return 0;
}

Originally Posted by DeadPlanet

I spent quite a while trying to experiment with this sort of thing a few years ago.

I just did some digging and found the code, it's for Windows XP no idea if it will work on anything else.

From the looks of it it seems that on Windows XP you can get the HDC of the display with disp = CreateDC( "DISPLAY", NULL, NULL, NULL );.

This won't compile btw because it's missing the resource file and stuff, but it's pretty simple so it might give you ideas. Sorry for the code quality it's from years ago and I was never really good with Windows API anyways. Looks like the most relevant code is in WM_TIMER, knock yourself out.

Code:

/*A desktop draw POC*/

#include <windows.h>
#include "DeskDraw.h"

int CurrentSelected = MOUSESELECT;

HINSTANCE AppInstance;
int MidPressed = NOTPRESSED;

HBITMAP Pens;
HBITMAP PensSel;
HCURSOR SystemCursor, CpySystemCursor;


void DrawButtons( HWND WndHwnd )
{
    HDC whDC, NewDC;

    whDC = GetDC( WndHwnd );
    NewDC = CreateCompatibleDC( whDC );

    SelectObject( NewDC, Pens );

    if( !BitBlt( whDC, 0, 0, 150, 50, NewDC, 0, 0, SRCCOPY ))
    {
        MessageBox( NULL, "Failed to Blit src to dest", "Failed", MB_OK );
    }

    SelectObject( NewDC, PensSel );

    BitBlt( whDC, CurrentSelected * 50, 0, 50, 50, NewDC, CurrentSelected * 50, 0, SRCCOPY );

    DeleteDC( NewDC );
    DeleteDC( whDC );
}

LRESULT CALLBACK WndProc( HWND WndHwnd, UINT Msg, WPARAM wParam, LPARAM lParam )
{
    static HDC disp;
    SHORT MidKeyState = 0;
    POINT mousepos;
    int penstyle, width;
    COLORREF PenCol;

    switch( Msg )
    {
        case WM_CREATE:
            SetTimer( WndHwnd, 1, 80, NULL );
            disp = CreateDC( "DISPLAY", NULL, NULL, NULL );

            Pens = LoadBitmap( AppInstance, "PENS1" );
            PensSel = LoadBitmap( AppInstance, "PENSPUSHED2" );

            if( Pens == NULL || PensSel == NULL )
            {
                MessageBox( NULL, "Could not load Bitmaps", "Failed", MB_OK );
                DestroyWindow( WndHwnd );
            }

            SystemCursor = LoadCursor( NULL, IDC_ARROW );
            CpySystemCursor = CopyCursor( SystemCursor );
            DestroyCursor( SystemCursor );
            break;

        case WM_TIMER:
            if( CurrentSelected != MOUSESELECT )
            {
                if( !MidPressed )
                {
                    GetCursorPos( &mousepos );
                    MoveToEx( disp, mousepos.x, mousepos.y, NULL );
                    DrawButtons( WndHwnd );
                    UpdateWindow( WndHwnd );
                }
                if( (MidKeyState = GetAsyncKeyState( VK_MBUTTON )) != 0 )
                {
                    MidPressed = PRESSED;
                    if( MidKeyState & 1 )
                    {
                        //If Mid mouse pressed since last call
                        GetCursorPos( &mousepos );
                        MoveToEx( disp, mousepos.x, mousepos.y, NULL );
                    }

                    else
                    {
                        GetCursorPos( &mousepos );
                        LineTo( disp, mousepos.x, mousepos.y );
                    }
                }
            }
            break;

        case WM_LBUTTONDOWN:
            if( (lParam & 0xFFFF ) < 50 )
            {
                SystemCursor = CopyCursor( CpySystemCursor );
                SetSystemCursor( CpySystemCursor, OCR_NORMAL );
                CpySystemCursor = CopyCursor( SystemCursor );
                DestroyCursor( SystemCursor );

                PenCol = 0;
                CurrentSelected = MOUSESELECT;
            }
            else if( (lParam & 0xFFFF ) < 100 )
            {
                SetSystemCursor( LoadCursor( AppInstance, "BLUECUR" ), OCR_NORMAL );

                PenCol = (0xFF << 16);
                CurrentSelected = BLUESELECT;
            }
            else
            {
                SetSystemCursor( LoadCursor( AppInstance, "REDCUR" ), OCR_NORMAL );

                PenCol = 0xFF;
                CurrentSelected = REDSELECT;
            }

            penstyle = PS_SOLID;
            width = 2;

            SelectObject( disp, CreatePen( penstyle, width, PenCol ) );

            DrawButtons( WndHwnd );
            UpdateWindow( WndHwnd );
            break;

        case WM_MOVE:
            DrawButtons( WndHwnd );
            UpdateWindow( WndHwnd );
            break;

        case WM_CLOSE:
            DestroyWindow( WndHwnd );
            break;

        case WM_DESTROY:
            KillTimer( WndHwnd, 1 );
            DeleteDC( disp );
            DeleteObject( Pens );
            DeleteObject( PensSel );
            SetSystemCursor( CpySystemCursor, OCR_NORMAL );
            DestroyCursor( CpySystemCursor );
            PostQuitMessage( 0 );
            break;

        default:
            return DefWindowProc( WndHwnd, Msg, wParam, lParam );
            break;
    }
    return 0;
}


int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR CmdArgs, int nCmdShow )
{
    WNDCLASSEX WndCls;
    HWND WndHwnd;
    MSG Msg;
    int captionh, borderw;

    AppInstance = hInstance;

    WndCls.cbSize = sizeof( WNDCLASSEX );
    WndCls.style = 0;
    WndCls.lpfnWndProc = WndProc;
    WndCls.cbClsExtra = 0;
    WndCls.cbWndExtra = 0;
    WndCls.hInstance = hInstance;
    WndCls.hIcon = LoadIcon( hInstance, "APPICON" );
    WndCls.hCursor = LoadCursor( NULL, IDC_ARROW );
    WndCls.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
    WndCls.lpszMenuName = NULL;
    WndCls.lpszClassName = "NewClass";
    WndCls.hIconSm = NULL;

    if( !RegisterClassEx( &WndCls ) )
    {
        MessageBox( NULL, "Class registration failed", "Failed", MB_OK );
        return 0;
    }

    captionh = GetSystemMetrics( SM_CYCAPTION );
    borderw = GetSystemMetrics( SM_CXBORDER );

    WndHwnd = CreateWindowEx( WS_EX_TOPMOST, "NewClass", "ToolBox", WS_SYSMENU, 0, 0, borderw * 2 + 150, captionh + borderw * 2 + 50, NULL, NULL, hInstance, NULL );

    if( WndHwnd == NULL )
    {
        MessageBox( NULL, "Could not create window", "Failed", MB_OK );
        UnregisterClass( "NewClass", hInstance );
        return 0;
    }

    ShowWindow( WndHwnd, SW_SHOW );
    UpdateWindow( WndHwnd );

    while( GetMessage( &Msg, WndHwnd, 0, 0 ) > 0 )
    {
        TranslateMessage( &Msg );
        DispatchMessage( &Msg );
    }
    UnregisterClass( "NewClass", hInstance );
    return 0;
}

Thanks, I'll definitely play around with it some. I've never personally messed with Windows GUI objects, so it will be nice to use that code to set up a base for learning. I hope I can learn more about the WinAPI from it.

t is not easy or advisable to draw directly to the desktop, as this tends to annoy users and to slow down windows. Not to mention that the desktop screen changes, as apps start, move close, etc.

I suggest you first create an app that allows you to draw on its window and make your drawing persist (even when another window covers yours).

Once you have that then you can consider making it draw to the desktop.

Transparent Bitmaps

>>The program I made above works, but any slight update to the screen clears out any drawing I did.

You need a 'back buffer'.

You 'save' all your drawing to this back buffer.

This is a compatible DC with a compatible bitmap the same size as the whole screen.

When you get a message that requires some drawing, you draw to the back buffer, then copy (bitblt) the back buffer to a copy of the screen DC and then generate a paint message.

In the paint handler you bitblt the minimum area to the screen (not the whole screen everytime as most new coders do).

>>CreateDC( "DISPLAY", NULL, NULL, NULL );.

This is the same as GetDC(NULL), except you DeleteDC() if you 'create' and ReleaseDC() if you 'get'.

Rememeber you need to save the return from SelectObject() so you can put the DC back to its default state prior to calling DeleteDC().

ie when you CreateCompatibleDC() it already contains a set of default GDI objects (pen, brush, font, etc) and a 1x1 monochrome bitmap.
A GDI object may not delete if it is currently selected into a DC, (this is why you other app probably crashed, as it ran your PC out of GDI memory).

You can view the GDI objects an app is using in TaskManager (View -> Select Columns and add 'GDI Objects')