Thread: Help. Drawing program

Ims tarting with Windows programming with C++. I did this program with the help of my book, but i still have one problem. While im hlding the mouse Lbutton, if i want to continue draging it to the right or bottom sides out of the window i can, but if i want to do it to the top or left side i cant, my line instantly goes to the opposite side limit. i captured the mouse in the window so i can do this, but i still have that problem. Whats weong with my code?

Code:

#include <windows.h>
#include <string>
#include <vector>
using namespace std;

//=========================================================
// Globals.

HWND ghMainWnd = 0;
HINSTANCE ghAppInst = 0;

struct Line
{
POINT p0;
POINT p1;
};

vector<Line> gLines;
Line gLine;

bool gMouseDown = false;

// Step 1: Define and implement the window procedure.

LRESULT CALLBACK
WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{

	// Objects for painting.

	HDC hdc = 0;
	PAINTSTRUCT ps;

	switch( msg )
	{

		// Handle left mouse button click message.

	case WM_LBUTTONDOWN:

		// Capture the mouse (we still get mouse input
		// even after the mouse cursor moves off the client area.

		SetCapture(hWnd);
		gMouseDown = true;

		// Point that was clicked is stored in the lParam.

		gLine.p0.x = LOWORD(lParam);
		gLine.p0.y = HIWORD(lParam);

		return 0;

		// Message sent whenever the mouse moves.

	case WM_MOUSEMOVE:
		if( gMouseDown )
		{
			// Current mouse position is stored in the lParam.

			gLine.p1.x = LOWORD(lParam);
			gLine.p1.y = HIWORD(lParam);
			InvalidateRect(hWnd, 0, true);
		}
		return 0;

	case WM_LBUTTONUP:
		// Release the captured mouse when the left mouse
		// button is lifted.

		ReleaseCapture();
		gMouseDown = false;

		// Current mouse position is stored in the lParam.

		gLine.p1.x = LOWORD(lParam);
		gLine.p1.y = HIWORD(lParam);
		gLines.push_back( gLine );
		InvalidateRect(hWnd, 0, true);
		return 0;

		// Handle paint message.

	case WM_PAINT:
		hdc = BeginPaint(hWnd, &ps);
		if( gMouseDown )
		{
			MoveToEx(hdc, gLine.p0.x, gLine.p0.y, 0);
			LineTo(hdc, gLine.p1.x, gLine.p1.y);
		}
		
		for(int i = 0; i < gLines.size(); ++i)
		{
			MoveToEx(hdc, gLines[i].p0.x, gLines[i].p0.y, 0);
			LineTo(hdc, gLines[i].p1.x, gLines[i].p1.y);
		}
		EndPaint(hWnd, &ps);

		return 0;

// Handle key down message.

	case WM_KEYDOWN:

		if( wParam == VK_ESCAPE )
			DestroyWindow(ghMainWnd);

		return 0;

		// Handle destroy window message.

	case WM_DESTROY:
		PostQuitMessage(0);
		return 0;
	}

	// Forward any other messages we didn't handle to the
	// default window procedure.
	
	return DefWindowProc(hWnd, msg, wParam, lParam);
}

// WinMain: Entry point for a Windows application.

int WINAPI
WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
PSTR cmdLine, int showCmd)
{

	// Save handle to application instance.

	ghAppInst = hInstance;

	// Step 2: Fill out a WNDCLASS instance.

	WNDCLASS wc;
	wc.style = CS_HREDRAW | CS_VREDRAW;
	wc.lpfnWndProc = WndProc;
	wc.cbClsExtra = 0;
	wc.cbWndExtra = 0;
	wc.hInstance = ghAppInst;
	wc.hIcon = ::LoadIcon(0, IDI_APPLICATION);
	wc.hCursor = ::LoadCursor(0, IDC_ARROW);
	wc.hbrBackground = (HBRUSH)::GetStockObject(WHITE_BRUSH);
	wc.lpszMenuName = 0;
	wc.lpszClassName = "MyWndClassName";

	// Step 3: Register the WNDCLASS instance with Windows.

	RegisterClass( &wc );

	// Step 4: Create the window, and save handle in globla
	// window handle variable ghMainWnd.

	ghMainWnd = ::CreateWindow("MyWndClassName", "MyWindow",
		WS_OVERLAPPEDWINDOW, 200, 200, 640, 480, 0, 0,
		ghAppInst, 0);
	
	if(ghMainWnd == 0)
	{
		::MessageBox(0, "CreateWindow - Failed", 0, 0);
		return false;
	}

	// Step 5: Show and update the window.

	ShowWindow(ghMainWnd, showCmd);
	UpdateWindow(ghMainWnd);

	// Step 6: Enter the message loop and don't quit until
	// a WM_QUIT message is received.

	MSG msg;
	ZeroMemory(&msg, sizeof(MSG));

	while( GetMessage(&msg, 0, 0, 0) )
	{
		TranslateMessage(&msg);
		DispatchMessage(&msg);
	}

	// Return exit code back to operating system.

	return (int)msg.wParam;
}

Code:

#include <windows.h>
#include <string>
#include <vector>
using namespace std;

//=========================================================
// Globals.

HWND ghMainWnd = 0;
HINSTANCE ghAppInst = 0;


POINT p1;

struct Line
{
    POINT p0;
    POINT p1;
};

RECT rcClient;
vector<Line> gLines;
Line gLine;

bool gMouseDown = false;

// Step 1: Define and implement the window procedure.
LRESULT CALLBACK
    WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    int i;   
 // Objects for painting.
    HDC hdc = 0;
    PAINTSTRUCT ps;
    switch( msg )
    {
            // Handle left mouse button click message.
        case WM_LBUTTONDOWN:
            // Capture the mouse (we still get mouse input
            // even after the mouse cursor moves off the client area.
            SetCapture(hWnd);
            GetWindowRect(hWnd, &rcClient );
            // Restrict the cursor
            ClipCursor( &rcClient );            
            gMouseDown = true;
            // Point that was clicked is stored in the lParam.
            gLine.p0.x = LOWORD(lParam);
            gLine.p0.y = HIWORD(lParam);
            return 0;

            // Message sent whenever the mouse moves.
        case WM_MOUSEMOVE:
            if( gMouseDown )
            {
                gLine.p1.x = LOWORD(lParam);
                gLine.p1.y = HIWORD(lParam);
                if (gLine.p1.x > 32768)
                    gLine.p1.x -= 65536;
                if (gLine.p1.y > 32768)
                    gLine.p1.y -= 65536; 
               InvalidateRect(hWnd, 0, true);
            }
            return 0;

        case WM_LBUTTONUP:
            // Release the captured mouse when the left mouse
            // button is lifted.
            ReleaseCapture();
            gMouseDown = false;
            // Free the cursor.
            ClipCursor( 0 );            
            gLine.p1.x = LOWORD(lParam);
            gLine.p1.y = HIWORD(lParam);
            if (gLine.p1.x > 32768)
                gLine.p1.x  -= 65536;
            if (gLine.p1.y > 32768)
                gLine.p1.y -= 65536;            
           gLines.push_back( gLine );
            InvalidateRect(hWnd, 0, true);
            return 0;

            // Handle paint message.
        case WM_PAINT:
            hdc = BeginPaint(hWnd, &ps);
            if( gMouseDown )
            {
                MoveToEx(hdc, gLine.p0.x, gLine.p0.y, 0);
                LineTo(hdc, gLine.p1.x, gLine.p1.y);
            }
            for(i = 0; i < gLines.size(); ++i)
            {
                MoveToEx(hdc, gLines[i].p0.x, gLines[i].p0.y, 0);
                LineTo(hdc, gLines[i].p1.x, gLines[i].p1.y);
            }
            EndPaint(hWnd, &ps);
            return 0;

            // Handle key down message.
        case WM_KEYDOWN:

            if( wParam == VK_ESCAPE )
                DestroyWindow(ghMainWnd);
            return 0;

            // Handle destroy window message.
        case WM_DESTROY:
            PostQuitMessage(0);
            return 0;
    }

    // Forward any other messages we didn't handle to the
    // default window procedure.
    return DefWindowProc(hWnd, msg, wParam, lParam);
}

// WinMain: Entry point for a Windows application.

int WINAPI
WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
PSTR cmdLine, int showCmd)
{
    // Save handle to application instance.
    ghAppInst = hInstance;

    // Step 2: Fill out a WNDCLASS instance.
    WNDCLASS wc;
    wc.style = CS_HREDRAW | CS_VREDRAW;
    wc.lpfnWndProc = WndProc;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.hInstance = ghAppInst;
    wc.hIcon = ::LoadIcon(0, IDI_APPLICATION);
    wc.hCursor = ::LoadCursor(0, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)::GetStockObject(WHITE_BRUSH);
    wc.lpszMenuName = 0;
    wc.lpszClassName = "MyWndClassName";

    // Step 3: Register the WNDCLASS instance with Windows.
    RegisterClass( &wc );

    // Step 4: Create the window, and save handle in globla
    // window handle variable ghMainWnd.
    ghMainWnd = ::CreateWindow("MyWndClassName", "MyWindow",
        WS_OVERLAPPEDWINDOW, 200, 200, 640, 480, 0, 0,
        ghAppInst, 0);

    if(ghMainWnd == 0)
    {
        ::MessageBox(0, "CreateWindow - Failed", 0, 0);
        return false;
    }

    // Step 5: Show and update the window.
    ShowWindow(ghMainWnd, showCmd);
    UpdateWindow(ghMainWnd);

    // Step 6: Enter the message loop and don't quit until
    // a WM_QUIT message is received.
    MSG msg;
    ZeroMemory(&msg, sizeof(MSG));

    while( GetMessage(&msg, 0, 0, 0) )
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    // Return exit code back to operating system.
    return (int)msg.wParam;
}

Originally Posted by Cherry65

can you explain me why you used the numbers 32768 and 65536?

Because 32768 is the maximum positive number of a signed short, and 65536 is 1 more than the maximum value of an unsigned short. I'm not sure why a simple cast to signed wouldn't have worked here, but too lazy to examine the code in detail right now. *grin*

but if i want to do it to the top or left side i cant, my line instantly goes to the opposite side limit

can you explain me why you used the numbers 32768 and 65536?

The upper left corner of your client area is the origin, x= 0 any y= 0. Thus, when your original code was going to the left or above the origin point, x and/or y was going from zero to 65536 and counting down. The if statements just mapped the area outside of the client window to the client window. That is, when the mouse curser moved to the left of origin, x took on a negative value starting at -1 and on which kept your line on the x axis and to the left of origin. With your original code, once you let go of the mouse button, your line would be mapped to the far right because the x value was something along the lines of 65536 which would incorrectly map the line to the far right even though you expected it to map to the far left.

In a nutshell, I'm changing the value of x any y when they go the left and/or above the origin to a negative number as opposed to a high positive number.