Error with WM_TIMER in WinAPI

Question

int wmId, wmEvent;
PAINTSTRUCT ps;
HDC hdc;
HBRUSH brush;
HBRUSH hBrush;
HPEN hPen;

static int dX[3] , dY[3] ;
static int x[3], y[3], oldX[3], oldY[3];
switch (message)
{
case WM_CREATE:
    SetTimer(hWnd, 1, 3, NULL);
    Beep(750, 300);
    for (int i = 0; i < 3; i++){
        dX[i] = rand() % 5 + 0;
        dY[i] = rand() % 5 + 0;
        x[i] = rand() % 5 + 0;
        y[i] = rand() % 5 + 0;
        oldX[i] = x[i];
        oldY[i] = y[i];
    }
    break;
case WM_TIMER:
    hdc = GetDC(hWnd);
    brush = (HBRUSH)SelectObject(hdc, GetStockObject(WHITE_BRUSH));

    RECT temp[3];
    RECT rect;
    GetClientRect(hWnd, &rect);
    for (int i = 0; i <3; i++){
        temp[i].left = oldX[i];
        temp[i].top = oldY[i];
        temp[i].right = oldX[i] + 30;
        temp[i].bottom = oldY[i] + 30;
        FillRect(hdc, &temp[i], brush); 
        brush = (HBRUSH)SelectObject(hdc, GetStockObject(GRAY_BRUSH));
        Ellipse(hdc, x[i], y[i], 30 + x[i], 30 + y[i]);
        oldX[i] = x[i];
        oldY[i] = y[i];

        x[i] += dX[i];
        y[i] += dY[i];


        if (x[i] + 30 > rect.right || x[i] < 0)
        {
            dX[i] = -dX[i];
        }
        if (y[i] + 30 > rect.bottom || y[i] < 0)
        {
            dY[i] = -dY[i];
        }
    }

    SelectObject(hdc, brush);

    ReleaseDC(hWnd, hdc);

    break;

If i draw > 2 circle . It blur streaks behind but i dont' want it . Please Help me . Thanks

Here is a picture of my problem:

enter image description here

The problem doesn't have anything to do with `WM_TIMER`. Not erasing the background properly would happen whether triggered by a timer or not. You drew the object with `Ellipse` so why wouldn't you erase it the same way? — Retired Ninja, Jul 13 '14 at 02:19
Thank you . You can tell me how to erase Ellipse . Thank you very much — user3883921, Jul 13 '14 at 03:17
@JonathanPotter But drawing with a white brush and pen is the same as erasing. — Retired Ninja, Jul 13 '14 at 03:41
@retired windows do not have persistent canvases. Erasing what was drawn last is not the solution. — David Heffernan, Jul 13 '14 at 07:11
@DavidHeffernan Oh, I totally agree, but in this particular case, erasing a circle with a square just seemed like a doubly wrong approach. — Retired Ninja, Jul 13 '14 at 08:57
Erasing is just wrong. You have to handle WM_PAINT come what may — David Heffernan, Jul 13 '14 at 13:14

score 2 · Answer 1 · answered Jul 13 '14 at 07:09

Your entire approach is broken. You are painting at the wrong time. You should perform your painting in response to WM_PAINT. The entire design of Win32 is based on programs responding to WM_PAINT by painting themselves. The framework will erase the background leaving your program to paint the foreground.

There are many reasons why a window might need to be repainted. You are concentrating on the regular pulse of your animation. But what if another program is dragged over yours?

Solve the problem like this:

Move the drawing code to a WM_PAINT handler.
Obtain the device context by calling BeginPaint.
Call InvalidateRect from your timer to mark the window as invalid and thereby force a paint cycle.

score 1 · Accepted Answer · answered Jul 13 '14 at 03:35

1

The first time you call FillRect you do it with a white brush. But on subsequent times through the loop the rectangle is drawn with the gray brush so you get the gray streaks. I suggest you create a white brush and a gray brush just once, before the loop. Then select the brush you want before each paint.

answered Jul 13 '14 at 03:35

ScottMcP-MVP

10,337
2
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I'd split it into two loops, one to erase all the items and one to draw all the items to minimize the brush swaps, but same difference in the end. – Retired Ninja Jul 13 '14 at 03:42
You can tell me how to handle collisions between the circle ? . Thank You very much – user3883921 Jul 13 '14 at 04:11

Error with WM_TIMER in WinAPI

2 Answers2