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// ================================================================================================
// Example of how to draw while resizing / moving a window on Windows in C.
//
// Window contents are rendered via GDI, but the behavior is the same no matter what graphics API
// you use. The original version of this demo (on old-20260427 branch) used D3D11.
//
// Pretty much all modern Direct3D Windows apps use a render thread and therefore don't have to
// deal with this problem. That strategy doesn't work with APIs that have weird thread affinity
// issues (OpenGL), so this is one way of doing it without multithreading.
//
// Build (MSVC):
// > cl /W4 /Od /Zi win-resize-st.c /Fe:win-resize-st.exe
// Build (GCC/clang):
// $ cc -o win-resize-st.exe -Wall -Wextra -Wpedantic -O0 -g win-resize-st.c -ldwmapi -lgdi32
//
// Changelog:
// 5/15/2026: Initial release
//
// License:
// Copyright (c) 2026 Hunter Kvalevog
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE.
// ================================================================================================
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#include <dwmapi.h>
#include <assert.h>
#include <math.h>
#include <stdio.h>
#ifdef _MSC_VER
# pragma comment(lib, "dwmapi.lib")
# pragma comment(lib, "gdi32.lib")
# pragma comment(lib, "user32.lib")
#endif
#define UNUSED(Var) ((void)(Var))
static void GfxInit(HWND wnd);
static void GfxResize(UINT vpw, UINT vph);
static void GfxDraw(void);
//
static LRESULT CALLBACK WndProc(HWND wnd, UINT msg, WPARAM wparam, LPARAM lparam)
{
switch (msg)
{
case WM_DESTROY:
PostQuitMessage(0);
return 0;
case WM_SIZE:
GfxResize(LOWORD(lparam), HIWORD(lparam));
GfxDraw();
break;
case WM_ENTERSIZEMOVE:
SetTimer(wnd, 1, USER_TIMER_MINIMUM, 0);
break;
case WM_EXITSIZEMOVE:
KillTimer(wnd, 1);
break;
case WM_TIMER:
GfxDraw();
break;
}
return DefWindowProcW(wnd, msg, wparam, lparam);
}
//
int WinMain(HINSTANCE instance, HINSTANCE previnstance, LPSTR cmdline, int cmdshow)
{
UNUSED(previnstance); UNUSED(cmdline);
// Register window class
WNDCLASSEXW wc = { 0 };
wc.cbSize = sizeof(wc);
wc.hCursor = LoadCursor(0, IDC_ARROW);
wc.hInstance = instance;
wc.lpfnWndProc = WndProc;
wc.lpszClassName = L"win-resize-st";
ATOM atom = RegisterClassExW(&wc);
assert(atom && "Failed to register window class");
// Create window
HWND wnd = CreateWindowExW(WS_EX_APPWINDOW, wc.lpszClassName, wc.lpszClassName,
WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
CW_USEDEFAULT, 0, 0, wc.hInstance, 0);
assert(wnd && "Failed to create window");
//
GfxInit(wnd);
//
ShowWindow(wnd, cmdshow);
// Run a typical D3D-style window loop
BOOL quit = FALSE;
while (!quit)
{
// Pump the message loop
MSG msg;
while (PeekMessageW(&msg, 0, 0, 0, PM_REMOVE))
{
quit |= (msg.message == WM_QUIT);
TranslateMessage(&msg);
DispatchMessageW(&msg);
}
// Update the screen
GfxDraw();
}
return 0;
}
// ================================================================================================
static UINT g_vpw = 0;
static UINT g_vph = 0;
static HDC g_hdc = 0;
static HDC g_hdc_back = 0;
static HBITMAP g_bmp = 0;
static HBRUSH g_bgbr = 0;
static HBRUSH g_fgbr = 0;
static HPEN g_pen = 0;
static void GfxInit(HWND wnd)
{
g_hdc = GetDC(wnd);
g_hdc_back = CreateCompatibleDC(g_hdc);
g_bgbr = CreateSolidBrush(RGB(0x00, 0x20, 0x20));
g_fgbr = CreateSolidBrush(RGB(0xE0, 0xE0, 0xE0));
g_pen = CreatePen(PS_SOLID, 2, RGB(0xFF, 0x10, 0x10));
RECT rc;
GetClientRect(wnd, &rc);
GfxResize(rc.right, rc.bottom);
}
static void GfxResize(UINT vpw, UINT vph)
{
if (g_bmp)
{
DeleteObject(g_bmp);
g_bmp = 0;
}
if (vpw > 0 && vph > 0)
{
g_bmp = CreateCompatibleBitmap(g_hdc, vpw, vph);
SelectObject(g_hdc_back, g_bmp);
}
g_vpw = vpw;
g_vph = vph;
}
static void GfxDraw(void)
{
static UINT frame = 0;
++frame;
if (g_bmp && g_vpw > 0 && g_vph > 0)
{
HDC hdc = g_hdc_back;
// Clear
{
RECT rc = { 0, 0, g_vpw, g_vph };
FillRect(hdc, &rc, g_bgbr);
}
// Draw X
{
SelectObject(hdc, g_pen);
MoveToEx(hdc, 0, 0, 0);
LineTo(hdc, g_vpw, g_vph);
MoveToEx(hdc, 0, g_vph, 0);
LineTo(hdc, g_vpw, 0);
}
// Draw rotating triangle
{
int cx = g_vpw / 2;
int cy = g_vph / 2;
int r = (g_vpw < g_vph ? g_vpw : g_vph) / 3;
POINT verts[3];
for (int i = 0; i < 3; ++i)
{
float ang = ((float)frame / 100.0f) + (float)i * (2.0f * 3.14159265f / 3.0f);
verts[i].x = cx + (int)(cosf(ang) * r);
verts[i].y = cy + (int)(sinf(ang) * r);
}
SelectObject(hdc, g_pen);
SelectObject(hdc, g_fgbr);
Polygon(hdc, verts, ARRAYSIZE(verts));
}
// Draw text
{
WCHAR txtbuf[256];
_snwprintf_s(txtbuf, ARRAYSIZE(txtbuf), _TRUNCATE, L"win-resize-st (frame %u)", frame);
SIZE sz;
GetTextExtentPoint32W(hdc, txtbuf, (int)wcslen(txtbuf), &sz);
SetBkMode(hdc, TRANSPARENT);
SetTextColor(hdc, RGB(0xE0, 0xE0, 0xE0));
TextOutW(hdc, (g_vpw - sz.cx) / 2, 19, txtbuf, (int)wcslen(txtbuf));
}
// Present
BitBlt(g_hdc, 0, 0, g_vpw, g_vph, g_hdc_back, 0, 0, SRCCOPY);
// Wait for dwm to display a frame
DwmFlush();
}
}
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