Device context woes

My window procedure receives a WM_TIMER event, causing the window to animate (as some of you may remember from yesterday). But now I want to add the feature of drawing onto the game board while the animation is going on.

Although I know how to draw onto a window (thanks to a tutorial), I can't get it to happen here. I can write single pixels onto the window but I can't draw by clicking and dragging. To do multiple pixels you have to release the button and click again.

I know it's something to do with contention for the device context but I can't figure out a way around it. I even tried setting a flag on WM_LBUTTONDOWN so that the WM_TIMER code can read this and break out instead of attempting to take the DC but it still won't let me draw.

I know the draw code is fine because if I comment out the WM_TIMER clause in the switch block it works fine.

What is the problem?

Wouldn't it be better to do the drawing on WM_MOUSEMOVE?

I should clarify:

The drawing is done on both WM_MOUSEMOVE and WM_LBUTTONDOWN but it only has an effect in the latter.

Code:

---

case WM_LBUTTONDOWN:
                        hdc = GetDC (hwnd);
                       
                        if (hdc)
                        {
                                xp = LOWORD(lParam);
                                yp = HIWORD(lParam);
                                                       
                                SetPixel (hdc, xp, yp, RGB (255, 0, 0));
                                grid [currentGrid] [yp - 20] [xp - 20] = 1;
                        }
                break;

                case WM_LBUTTONUP:
                        if(hdc)
                                ReleaseDC (hwnd, hdc);
                               
                        hdc = NULL;
                break;

                case WM_MOUSEMOVE:
                        if(hdc)
                        {       
                                short xp = LOWORD (lParam);
                                short yp = HIWORD (lParam);

                                SetPixel (hdc, xp, yp, RGB (255, 0, 0));
                                grid [currentGrid] [yp - 20] [xp - 20] = 1;
                        }
                break;

---

Is hdc a static variable?

It's global.

I'm really tearing my hair out over this by the way if anyone can help...

38 views and nobody has any advice? Nobody has experienced similar problems in the past? I doubt that somehow...

Zip up and post a small one window app that demonstrates your woe's.....

With compilable code available to be debugged, plenty of folks with try to fix you up.

gg

OK, here's the code. It's severely abridged but it conveys the essence of my problem:

Code:

---

#include <time.h>
#include <stdlib.h>
#include <windows.h>

#define MAX 261

LRESULT CALLBACK MainWindowProc (HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);

int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
        WNDCLASS myWin;
        MSG msg;

        myWin.style = CS_VREDRAW | CS_HREDRAW;
        myWin.lpfnWndProc = MainWindowProc;
        myWin.cbClsExtra = 0;
        myWin.cbWndExtra = 0;
        myWin.hInstance = hInstance;
        myWin.hIcon = LoadIcon (NULL, IDI_APPLICATION);
        myWin.hCursor = LoadCursor (NULL, IDC_ARROW);
        myWin.hbrBackground = (HBRUSH) (COLOR_SCROLLBAR + 1);
        myWin.lpszMenuName = 0;
        myWin.lpszClassName = "My First App.";

       
        if (!RegisterClass (&myWin))
                return 0;

        HWND hwnd;

        hwnd = CreateWindow (        "My First App.",
                                                        "SamLife, v1.0",
                                                        WS_OVERLAPPEDWINDOW,
                                                        20,
                                                        20,
                                                        MAX + 40,
                                                        MAX + 60,
                                                        NULL,
                                                        NULL,
                                                        hInstance,
                                                        NULL);

        if (!hwnd)
                return 0;

        SetTimer(hwnd, 1, 1, NULL);
        ShowWindow (hwnd, nCmdShow);

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

    return 0;
}

HDC hdc = NULL;

LRESULT CALLBACK MainWindowProc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
        static int gen1 = 0;
        static int gen2 = 0;
        static int gen3 = 0;

        static short xp;
        static short yp;

    static int timerId = 3000;
        static int currentGrid = 0;

        int col = 0;

        int sum = 0;
        int i, j;
        int temp_i, temp_j;

        static unsigned char bias [8] = { 20, 50, 75, 15, 50, 10, 15, 20 };
        static unsigned char grid [2] [MAX] [MAX];
        register unsigned char temp;

        switch (msg)
        {
                case WM_CREATE:
                        srand ((unsigned) time (NULL));
                       
                        for (i = 0; i < MAX; i++)
                                for (j = 0; j < MAX; j++)
                                {
                                        grid [0] [i] [j] = 0;
                                        grid [1] [i] [j] = 0;
                                }

               
       
                        SetTimer (hwnd, timerId, 1, NULL);
                break;

                case WM_LBUTTONDOWN:
                        if (!hdc)
                                hdc = GetDC (hwnd);

                        if (hdc)
                        {
                                xp = LOWORD(lParam);
                                yp = HIWORD(lParam);
                                                       
                                SetPixel (hdc, xp, yp, RGB (255, 0, 0));
                                grid [currentGrid] [yp - 20] [xp - 20] = 1;
                        }
                break;

                case WM_LBUTTONUP:
                        if(hdc)
                        {
                                ReleaseDC (hwnd, hdc);
                                hdc = NULL;
                        }
                break;

                case WM_MOUSEMOVE:
                        if(hdc)
                        {       
                                short xp = LOWORD (lParam);
                                short yp = HIWORD (lParam);

                                SetPixel (hdc, xp, yp, RGB (255, 0, 0));
                                grid [currentGrid] [yp - 20] [xp - 20] = 1;
                        }
                break;

                case WM_MOVE:
                case WM_SIZE:
                case WM_PAINT:
                        hdc = GetDC (hwnd);

                        if (!hdc)
                                break;

                        for (i = 0; i < MAX; i++)
                                for (j = 0; j < MAX; j++)
                                {
                                        if (grid [currentGrid] [i] [j])
                                                SetPixel (hdc, j + 20, i + 20, RGB (255, 255, 255));

                                        else
                                                SetPixel (hdc, j + 20, i + 20, RGB (0, 0, 0));
                                }

                        ReleaseDC (hwnd, hdc);
                        hdc = NULL;
                break;

                case WM_CLOSE:
                        DestroyWindow (hwnd);
                break;

                case WM_DESTROY:
                        KillTimer (hwnd, timerId);
                        PostQuitMessage (0);;
                break;

        case WM_TIMER:       
                        if (!hdc)
                        hdc = GetDC (hwnd);

                        if (hdc)
                        {       
                                SetPixel (hdc, rand () % MAX, rand () % MAX, RGB (255, 0, 0));
                                ReleaseDC (hwnd, hdc);
                        hdc = NULL;
                        }
                       
                break;
        }
       
        return DefWindowProc (hwnd, msg, wParam, lParam);       
}

---

I've even put in code so that it only takes the HDC if it is free. But I still can't drag and draw unless I comment out the entire WM_TIMER clause.

OK, so now there are over 70 views and I've posted code that illustrates the problem completely and still nobody will help me. This seems like a pretty elementary problem to me - I'm sure that most of you know what the solution is but just have bigger fish to fry, so to speak...

Well it's not that no ones cares...
Maybe your problem has to do with WM_PAINT setting hdc to NULL which would stop WM_MOUSEMOVE from drawing untill WM_LBUTTONDOWN set it to the dc again?

It is recommended that you only actually paint when you receive a WM_PAINT message. If you want to paint in other messages, you should record where you want painted (in an array, a memory dc, etc) and then call InvalidateRect. This causes windows to send a WM_PAINT message. This is your program altered to use that approach:

Code:

---

#include <time.h>
#include <stdlib.h>
#include <windows.h>
#pragma comment(lib, "gdi32.lib")

#define MAX 261

LRESULT CALLBACK MainWindowProc (HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);

int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
        WNDCLASS myWin;
        MSG msg;

        myWin.style = CS_VREDRAW | CS_HREDRAW;
        myWin.lpfnWndProc = MainWindowProc;
        myWin.cbClsExtra = 0;
        myWin.cbWndExtra = 0;
        myWin.hInstance = hInstance;
        myWin.hIcon = LoadIcon (NULL, IDI_APPLICATION);
        myWin.hCursor = LoadCursor (NULL, IDC_ARROW);
        myWin.hbrBackground = (HBRUSH) (COLOR_SCROLLBAR + 1);
        myWin.lpszMenuName = 0;
        myWin.lpszClassName = "My First App.";

        if (!RegisterClass (&myWin))
                return 0;

        HWND hwnd;

        hwnd = CreateWindow (        "My First App.",
                                                        "SamLife, v1.0",
                                                        WS_OVERLAPPEDWINDOW & ~WS_THICKFRAME,
                                                        20,
                                                        20,
                                                        MAX + 40,
                                                        MAX + 60,
                                                        NULL,
                                                        NULL,
                                                        hInstance,
                                                        NULL);

        if (!hwnd)
                return 0;

        // SetTimer(hwnd, 1, 1, NULL);
        ShowWindow (hwnd, nCmdShow);

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

    return 0;
}


LRESULT CALLBACK MainWindowProc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
        static int gen1 = 0;
        static int gen2 = 0;
        static int gen3 = 0;

        static short xp;
        static short yp;

        static int timerId = 3000;
        static int currentGrid = 0;

        int col = 0;

        int sum = 0;
        int i, j;

        static unsigned char bias [8] = { 20, 50, 75, 15, 50, 10, 15, 20 };
        static unsigned char grid [2] [MAX] [MAX];

        static bool bDrawing;

        switch (msg)
        {
                case WM_CREATE:
                {
                        srand ((unsigned) time (NULL));
                       
                        for (i = 0; i < MAX; i++)
                                for (j = 0; j < MAX; j++)
                                {
                                        grid [0] [i] [j] = 0;
                                        grid [1] [i] [j] = 0;
                                }

                        SetTimer (hwnd, timerId, 10, NULL);
                }
                break;

                case WM_LBUTTONDOWN:
                {
                        bDrawing = TRUE;
                }
                break;

                case WM_LBUTTONUP:
                {
                        bDrawing = FALSE;
                }
                break;

                case WM_MOUSEMOVE:
                {
                        if (bDrawing)
                        {
                                xp = LOWORD(lParam) - 20;
                                yp = HIWORD(lParam) - 20;

                                if (xp > 0  && yp > 0 &&
                                    xp < MAX && yp < MAX)
                                {
                                        grid [currentGrid] [yp] [xp] = 1;
                                        InvalidateRect(hwnd, NULL, FALSE);
                                }
                        }
                }
                break;

                case WM_MOVE:
                case WM_SIZE:
                {
                        // InvalidateRect(hwnd, NULL, FALSE);
                }
                break;

                case WM_PAINT:
                {
                        PAINTSTRUCT ps;
                        HDC hdc = BeginPaint(hwnd, &ps);

                        for (i = 0; i < MAX; i++)
                                for (j = 0; j < MAX; j++)
                                {
                                        if (grid [currentGrid] [i] [j] == 1)
                                                SetPixel (hdc, j + 20, i + 20, RGB (255, 255, 255));
                                        else if (grid [currentGrid] [i] [j] == 2)
                                                SetPixel (hdc, j + 20, i + 20, RGB (255, 0, 0));
                                        else
                                                SetPixel (hdc, j + 20, i + 20, RGB (0, 0, 0));
                                }

                        EndPaint(hwnd, &ps);
                        return 0;
                }
                break;

                case WM_DESTROY:
                {
                        KillTimer (hwnd, timerId);
                        PostQuitMessage (0);;
                }
                break;

                case WM_TIMER:       
                {
                        grid[currentGrid][rand() % MAX][rand() % MAX] = 2;
                        InvalidateRect(hwnd, NULL, FALSE);
                }
                break;
        }
       
        return DefWindowProc (hwnd, msg, wParam, lParam);       
}

---

Things to note:

• I've used NULL with InvalidateRect to repaint the entire window. This is slow and causes flicker. If we are professional we should provide a rect to InvalidateRect that specifies only the area that needs repainting. Then in WM_PAINT, the contents of PAINTSTRUCT will tell us which area needs redrawing.
• You may notice that using SetPixel to draw does not provide particularly satisfactory results. You will want to look into the gdi drawing functions such as LineTo.
• When a thread yields, it will rarely be re-shcheduled in less than 10ms. Therefore, using a 1ms timeout with SetTimer will typically result in a timer that is triggered every 10ms or less.

That kind of works, but as you say it's slow. There must be a simpler way to be able to draw while animation is going on...

Try something like this....

Oppps.........


Forgot that you must 'clear' the lne DC on each mouse move. (note to self, don't post untested code)

BitBlt() the perm hdc to the line DC for the whole client rect, then draw the new line to the line DC.