WS_EX_LAYERED from bitmap uses black as transparency mask

Question

I'm trying to display a transparent 32bit bitmap on a WS_EX_LAYERED hwnd and set the per-pixel transparency with it, but somehow it looks that the transparency is decided by black pixels instead of the bitmap's alpha value.
Here's the result with a black bitmap with few colored circles over it :
Code :
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
LPCWSTR szWindowClass = L"TransparentClass";
WNDCLASSEX wcex = { 0 };
wcex.cbSize = sizeof(WNDCLASSEX);
wcex.lpfnWndProc = DefWindowProc;
wcex.hInstance = hInstance;
wcex.lpszClassName = szWindowClass;
RegisterClassEx(&wcex);
HWND hWnd = CreateWindowEx(WS_EX_LAYERED, szWindowClass, 0, WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);
HBITMAP hbmp = (HBITMAP)LoadImage(GetModuleHandle(NULL), L"C:\\Users\\Domenico\\Desktop\\mask.bmp", IMAGE_BITMAP, 0, 0, LR_LOADFROMFILE);
//LPBYTE bits = (LPBYTE)hbmp;
//int sizet = 500 * 500;
//for (int pixel = 0; pixel != sizet; ++pixel)
// bits[0] = bits[0] * bits[3] / 255;
// bits[1] = bits[1] * bits[3] / 255;
// bits[2] = bits[2] * bits[3] / 255;
// bits += 4;
HDC hdcScreen = GetDC(0);
HDC hdc = CreateCompatibleDC(hdcScreen);
ReleaseDC(0, hdcScreen);
HBITMAP hbmpold = (HBITMAP)SelectObject(hdc, hbmp);
POINT dcOffset = { 0, 0 };
SIZE size = { 500, 500 };
BLENDFUNCTION bf;
bf.BlendOp = AC_SRC_OVER;
bf.BlendFlags = 0;
bf.SourceConstantAlpha = 255;
bf.AlphaFormat = AC_SRC_ALPHA;
UpdateLayeredWindow(hWnd, 0, 0, &size, hdc, &dcOffset, 0, &bf, ULW_ALPHA);
SelectObject(hdc, hbmpold);
DeleteDC(hdc);
DeleteObject(hbmp);
ShowWindow(hWnd, SW_SHOW);
MSG msg;
// Main message loop:
while (GetMessage(&msg, NULL, 0, 0))
TranslateMessage(&msg);
DispatchMessage(&msg);
return (int)msg.wParam;
Is it this the default behaviour to check for transparency ? I need black pixels to recreate a custom shadow-like effect.
I tried to multiply every bitmap's pixel as suggested in MDSN doc but I'm getting access violation while trying to do so.
Last question : Is it possible to repaint the whole window with a new bitmap at runtime ? Since you have call the updatelayeredwindow function before showing the window I'm thinking it should be impossible, right ?

Answer 1

I'm trying to display a transparent 32bit bitmap on a WS_EX_LAYERED hwnd and set the per-pixel transparency with it, but somehow it looks that the transparency is decided by black pixels instead of the bitmap's alpha value.
Here's the result with a black bitmap with few colored circles over it :
Code :
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
LPCWSTR szWindowClass = L"TransparentClass";
WNDCLASSEX wcex = { 0 };
wcex.cbSize = sizeof(WNDCLASSEX);
wcex.lpfnWndProc = DefWindowProc;
wcex.hInstance = hInstance;
wcex.lpszClassName = szWindowClass;
RegisterClassEx(&wcex);
HWND hWnd = CreateWindowEx(WS_EX_LAYERED, szWindowClass, 0, WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);
HBITMAP hbmp = (HBITMAP)LoadImage(GetModuleHandle(NULL), L"C:\\Users\\Domenico\\Desktop\\mask.bmp", IMAGE_BITMAP, 0, 0, LR_LOADFROMFILE);
//LPBYTE bits = (LPBYTE)hbmp;
//int sizet = 500 * 500;
//for (int pixel = 0; pixel != sizet; ++pixel)
// bits[0] = bits[0] * bits[3] / 255;
// bits[1] = bits[1] * bits[3] / 255;
// bits[2] = bits[2] * bits[3] / 255;
// bits += 4;
HDC hdcScreen = GetDC(0);
HDC hdc = CreateCompatibleDC(hdcScreen);
ReleaseDC(0, hdcScreen);
HBITMAP hbmpold = (HBITMAP)SelectObject(hdc, hbmp);
POINT dcOffset = { 0, 0 };
SIZE size = { 500, 500 };
BLENDFUNCTION bf;
bf.BlendOp = AC_SRC_OVER;
bf.BlendFlags = 0;
bf.SourceConstantAlpha = 255;
bf.AlphaFormat = AC_SRC_ALPHA;
UpdateLayeredWindow(hWnd, 0, 0, &size, hdc, &dcOffset, 0, &bf, ULW_ALPHA);
SelectObject(hdc, hbmpold);
DeleteDC(hdc);
DeleteObject(hbmp);
ShowWindow(hWnd, SW_SHOW);
MSG msg;
// Main message loop:
while (GetMessage(&msg, NULL, 0, 0))
TranslateMessage(&msg);
DispatchMessage(&msg);
return (int)msg.wParam;
Is it this the default behaviour to check for transparency ? I need black pixels to recreate a custom shadow-like effect.
I tried to multiply every bitmap's pixel as suggested in MDSN doc but I'm getting access violation while trying to do so.
Last question : Is it possible to repaint the whole window with a new bitmap at runtime ? Since you have call the updatelayeredwindow function before showing the window I'm thinking it should be impossible, right ?
1. Transparency in BGRA (32 bpp) bitmaps is controlled by the Alpha byte. When A is 0 the pixel is completely transparent, when A is 255 the pixel is completely opaque.
2. HBITMAP doesn't point to the pixels. Given an HBITMAP, there are APIs to get and set the pixels: GetDIBits, SetDIBits.
3. To repaint the window, call UpdateLayeredWindow again.