For xtow, I want to draw a top-level window with the standard non-client area and the client area filled with a bitmap which has an alpha channel.
I now discover the way I have implemented this works on Windows 7, but doesn't render correctly on Windows 8.1, leaving behind images of the window contents when it is moved or maximized.
To investigate, I made a simple test program alpha-test, which
- Uses DwmEnableBlurBehindWindow() to set a non-intersecting blur region, so that alpha values in the window are honoured, without blur.
- Uses BitBlt() to copy a bitmap with alpha into it.
//
// g++ alpha-test.cc -o alpha-test -mwindows -lgdiplus -ldwmapi
//
#define _WIN32_WINNT 0x0600
#include <assert.h>
#include <stdio.h>
#include <windows.h>
#include <gdiplus.h>
#include <dwmapi.h>
int width = 360;
int height = 360;
HBITMAP hBitmap;
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
switch (message)
{
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdcUpdate = BeginPaint(hWnd, &ps);
RECT rc;
GetClientRect(hWnd, &rc);
HBRUSH hbrush = CreateSolidBrush(RGB(0,0,0));
FillRect(hdcUpdate, &rc, hbrush);
DeleteObject(hbrush);
HDC hdcMem = CreateCompatibleDC(hdcUpdate);
HBITMAP hbmpold = (HBITMAP)SelectObject(hdcMem, hBitmap);
if (!BitBlt(hdcUpdate, 0, 0, ps.rcPaint.right, ps.rcPaint.bottom, hdcMem, 0, 0, SRCCOPY))
{
printf("BitBlt failed: 0x%08x\n", (int)GetLastError());
}
SelectObject(hdcMem, hbmpold);
DeleteDC(hdcMem);
EndPaint(hWnd, &ps);
}
return 0;
case WM_DESTROY:
PostQuitMessage(0);
return 0;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
}
}
int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPTSTR lpCmdLine, int nCmdShow)
{
ULONG_PTR gdiplusToken;
Gdiplus::GdiplusStartupInput gdiplusStartupInput;
GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);
LPCTSTR szWindowClass = "TransparentClass";
// Register class
WNDCLASSEX wcex = {0};
wcex.cbSize = sizeof(WNDCLASSEX);
wcex.style = CS_HREDRAW | CS_VREDRAW; // | CS_OWNDC;
wcex.lpfnWndProc = WndProc;
wcex.cbClsExtra = 0;
wcex.cbWndExtra = 0;
wcex.hInstance = hInstance;
wcex.hIcon = NULL;
wcex.hCursor = LoadCursor(NULL, IDC_ARROW);
wcex.lpszClassName = szWindowClass;
wcex.hIconSm = NULL;
wcex.hbrBackground = (HBRUSH)CreateSolidBrush(0x00000000);
RegisterClassEx(&wcex);
// Create window
HWND hWnd = CreateWindowEx(WS_EX_APPWINDOW,
szWindowClass,
"Transparent Window",
WS_OVERLAPPED | WS_SYSMENU,
CW_USEDEFAULT, CW_USEDEFAULT, width, height,
NULL, NULL, hInstance, NULL);
Gdiplus::Bitmap *m_pImage = Gdiplus::Bitmap::FromFile(L"sample.png", FALSE);
Gdiplus::Color bg(0,0,0,0);
m_pImage->GetHBITMAP(bg, &hBitmap);
assert(hBitmap);
DWM_BLURBEHIND blurBehind = { 0 };
blurBehind.dwFlags = DWM_BB_ENABLE | DWM_BB_BLURREGION;
blurBehind.hRgnBlur = CreateRectRgn(0, 0, -1, -1);
blurBehind.fEnable = TRUE;
blurBehind.fTransitionOnMaximized = FALSE;
DwmEnableBlurBehindWindow(hWnd, &blurBehind);
DeleteObject(blurBehind.hRgnBlur);
ShowWindow(hWnd, SW_SHOW);
// Main message loop
MSG msg;
while (GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return (int)msg.wParam;
}
Is this really broken? How can I fix my code? Is this a Windows bug/limitation?
Is there another way to achieve my goal of drawing a bitmap with alpha into a window with a border?
Update: I did some tests using Direct2D and Direct3D to fill the client area with the bitmaps, but they mis-rendered in the same way.