I have a work thread function which updates the value of a float called scale:

    DWORD WINAPI WorkThreadFn(LPVOID lpParam)
    MyWin* pWin = (MyWin*)lpParam;
    while (pWin->running)
        //pWin->scale = pWin->scale - 0.001;
        pWin->scale = 0.5;
        SendMessage(pWin->hwnd, WM_PAINT, NULL, NULL);
    return 0;

If I change the value of scale in this function, the WM_PAINT draws my triangle to the correct scale. But if I uncomment the section which increments the scale, then remove the line setting the scale, the scale of the triangle remains the same, despite the fact I can see from the window procedure that the value of scale is decreasing.

Code for Window procedure:

    LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
    HDC hdc;
    MyWin* pThis = NULL;
    if (uMsg == WM_CREATE)
        CREATESTRUCT* pCreate = (CREATESTRUCT*)lParam;
        pThis = (MyWin*)pCreate->lpCreateParams;
        SetWindowLongPtr(hwnd, GWLP_USERDATA, (LONG_PTR)pThis);
        pThis = (MyWin*)GetWindowLongPtr(hwnd, GWLP_USERDATA);
switch(uMsg) {
case WM_PAINT:
        hdc = BeginPaint(hwnd, &ps);
        RECT region = ps.rcPaint;
        glClearColor(0.62, 0.3, 0.5, 1.0);

        pThis->glUniform1f(pThis->scaleLoc, pThis->scale);
        glDrawArrays(GL_TRIANGLES, 0, 3);


And also here is the vertex shader code

in vec3 in_Position;
in vec3 in_Color;
out vec3 ex_Color;

uniform float scale;

void main(void)
    gl_Position = vec4(in_Position * scale, 1.0);
    ex_Color = in_Color;

I'm sure I'm doing something stupid here, can anybody help?

Edit (forgot to add the main function):

int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PWSTR pCmdLine, int nCmdShow)
    MyWin win(hInstance, nCmdShow);

    DWORD ThreadID;
    HANDLE hThread;
    hThread = CreateThread(NULL, 0, WorkThreadFn, &win, 0, &ThreadID);


    if (hThread != NULL)

    return 0;
  • $\begingroup$ Ok so, I discovered that if I resize the window the expected change in the triangle size takes place. Which I guess, means that windows does not like the way I am sending WM_PAINT messages to the window procedure. Still working on it. $\endgroup$
    – lewis
    Jan 17, 2023 at 2:43

1 Answer 1


I can send WM_PAINT messages with SendMessage or PostMessage, but the window wont be redrawn. To redraw the window I have to call InvalidateRect or InvalidateRgn. You can also use RedrawWindow.

In the end that wasn't the problem though. I was calling glViewport on WM_SIZE, which for some season unknown to me means that WM_PAINT wasn't being processed properly, but the window was drawing correctly during resizing.

I also changed my message loop, to clear the message queue before redrawing the window, which also seems to speed things up a lot.

Edit: Ok, I know why glViewport in WM_SIZE was causing issues now. I wasn't returning 0 after processing that message, and MSDN docs mentions this. After making this change, it all works as expected.


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