How to make transparent window using UpdateLayeredWindow ?

Question

Hi, I hope you are doing good.
I am working in UnrealEngine and I want to make a transparent background by color, for example black color. I have tryed in two different ways.

1. I am using SetLayeredWindowAttributes, the black color disspear but there is some grey object that becomes semy transparent. As far as I understand it's because this uses 24bit colors. Please correct me if I am wrong. This is the code I have used: SetWindowLongPtr(hwnd, GWL_STYLE, WS_VISIBLE);
   SetWindowLongPtr(hwnd, GWL_EXSTYLE, WS_EX_LAYERED | WS_EX_TRANSPARENT);
   RECT rect;
   GetWindowRect(hwnd, &rect);
   SetWindowPos(hwnd, HWND_TOPMOST, rect.left, rect.top, rect.right - rect.left, rect.bottom - rect.top, SWP_SHOWWINDOW);
   SetLayeredWindowAttributes(hwnd, 0, 0, LWA_ALPHA);
   SetWindowLong(hwnd, -20, 524288 | 32);
   SetLayeredWindowAttributes(hwnd, RGB(0,0,0), 255, LWA_COLORKEY);
   const MARGINS margins{ -1 };
   DwmExtendFrameIntoClientArea(hwnd, &margins);
   HRGN rgn;
   DWM_BLURBEHIND blur;
   rgn = CreateRectRgn(rect.left, rect.top, rect.right, rect.bottom);
   blur.dwFlags = DWM_BB_ENABLE | DWM_BB_BLURREGION;
   blur.fEnable = true; //why the fuck else would i use this -.-
   blur.fTransitionOnMaximized = true; //why not...?
   blur.hRgnBlur = rgn;
   DwmEnableBlurBehindWindow(hwnd, &blur);
   DwmExtendFrameIntoClientArea(hwnd, &margins);
2. I am using UpdateLayeredWindow but I get a full transparent window like is getting no image.

   HDC hdc = GetDC(hwnd);
   HDC hdc1 = CreateCompatibleDC(hdc);
   HBITMAP hBitmap = CreateCompatibleBitmap(hdc, rect.right - rect.left, rect.bottom - rect.top);
   HBITMAP hBmpOld = (HBITMAP)SelectObject(hdc, hBitmap);
   SelectObject ( hdc, hBitmap );
   if (hdc)
   

   {
   HGDIOBJ hPrevObj = 0;
   POINT ptDest = {0, 0};
   POINT ptSrc = {0, 0};
   SIZE client = {rect.right - rect.left, rect.bottom - rect.top};
   BLENDFUNCTION blendFunc = {AC_SRC_OVER, 0, 255, AC_SRC_ALPHA};

    UpdateLayeredWindow(hwnd, hdc1, &ptDest, &client, hdc, &ptSrc, RGB(0, 0, 0), &blendFunc, ULW_COLORKEY);
    SelectObject(hdc, hBmpOld);
    DeleteObject(hBitmap);
    DeleteDC(hdc);
    ReleaseDC(NULL, hdc1);
   

   }

Any help is apreciated. Thankyou very much !

Answer 1

A test with a .JPG with UpdateLayeredWindow and white color as transparent color =>

Remove the space at S leep (bug in this editor...)

#include <windows.h>  
#include <tchar.h>  
  
#include "gdiplus.h"  
using namespace Gdiplus;  
#pragma comment(lib, "gdiplus.lib")  
  
#include <Urlmon.h> // URLDownloadToCacheFile  
#pragma comment (lib, "Urlmon")  
  
#include <shlwapi.h> // SHCreateStreamOnFile  
#pragma comment (lib, "shlwapi")  
  
#pragma comment(linker,"\"/manifestdependency:type='win32' \  
name='Microsoft.Windows.Common-Controls' version='6.0.0.0' \  
processorArchitecture='*' publicKeyToken='6595b64144ccf1df' language='*'\"")  
  
HINSTANCE hInst;  
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);  
int nWidth = 600, nHeight = 400;  
  
BOOL SetPicture(HWND hWnd, HBITMAP hBmp, COLORREF nColor);  
  
int APIENTRY wWinMain(_In_ HINSTANCE hInstance, _In_opt_ HINSTANCE hPrevInstance, _In_ LPWSTR lpCmdLine, _In_ int nCmdShow)  
{  
	GdiplusStartupInput gdiplusStartupInput;  
	ULONG_PTR gdiplusToken;  
	GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);  
  
	hInst = hInstance;  
	WNDCLASSEX wcex =  
	{  
		sizeof(WNDCLASSEX), CS_HREDRAW | CS_VREDRAW, WndProc, 0, 0, hInst, LoadIcon(NULL, IDI_APPLICATION),  
		LoadCursor(NULL, IDC_ARROW), (HBRUSH)(COLOR_WINDOW + 1), NULL, TEXT("WindowClass"), NULL,  
	};  
	if (!RegisterClassEx(&wcex))  
		return MessageBox(NULL, TEXT("Cannot register class !"), TEXT("Error"), MB_ICONERROR | MB_OK);  
	int nX = (GetSystemMetrics(SM_CXSCREEN) - nWidth) / 2, nY = (GetSystemMetrics(SM_CYSCREEN) - nHeight) / 2;  
	HWND hWnd = CreateWindowEx(WS_EX_NOREDIRECTIONBITMAP | WS_EX_LAYERED | WS_EX_TOPMOST | WS_EX_NOACTIVATE, wcex.lpszClassName, TEXT("Test"), WS_POPUP | WS_VISIBLE, nX, nY, nWidth, nHeight, NULL, NULL, hInst, NULL);  
	if (!hWnd)  
		return MessageBox(NULL, TEXT("Cannot create window !"), TEXT("Error"), MB_ICONERROR | MB_OK);  
	ShowWindow(hWnd, SW_SHOWNORMAL);  
	UpdateWindow(hWnd);  
	MSG msg;  
	while (GetMessage(&msg, NULL, 0, 0))  
	{  
		TranslateMessage(&msg);  
		DispatchMessage(&msg);  
	}  
	return (int)msg.wParam;  
}  
  
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)  
{  
	static HWND hWndButton = NULL, hWndStatic = NULL;  
	int wmId, wmEvent;  
	switch (message)  
	{  
	case WM_CREATE:  
	{	  
		WCHAR wsURL[MAX_PATH] = TEXT("https://image.ibb.co/buLv2e/Little_Girl3.jpg");  
		WCHAR wsFilename[MAX_PATH];  
		HRESULT hr = URLDownloadToCacheFile(NULL, wsURL, wsFilename, ARRAYSIZE(wsFilename), 0x0, NULL);  
		if (SUCCEEDED(hr))  
		{  
			IStream* pStream = NULL;  
			hr = SHCreateStreamOnFile(wsFilename, STGM_READ | STGM_SHARE_DENY_WRITE, &pStream);  
			if (SUCCEEDED(hr))  
			{  
				HBITMAP hBitmap = NULL;  
				Gdiplus::Bitmap* pBitmap = new Gdiplus::Bitmap(pStream);  
				if (pBitmap)  
				{  
					pBitmap->GetHBITMAP(Gdiplus::Color(255, 255, 255), &hBitmap);  
					SetPicture(hWnd, hBitmap, RGB(255, 255, 255));					  
				}  
			}  
		}		  
		return 0;  
	}  
	break;  
	case WM_NCHITTEST:  
	{  
		return HTCAPTION;  
	}  
	break;  
	case WM_NCRBUTTONDOWN:  
	{		  
		S leep(200);  
		DestroyWindow(hWnd);  
	}  
	break;  
	case WM_DESTROY:  
	{  
		PostQuitMessage(0);  
		return 0;  
	}  
	break;  
	default:  
		return DefWindowProc(hWnd, message, wParam, lParam);  
	}  
	return 0;  
}  
  
BOOL SetPicture(HWND hWnd, HBITMAP hBmp, COLORREF nColor)  
{  
	BITMAP bm;  
	GetObject(hBmp, sizeof(bm), &bm);  
	SIZE szBmp = { bm.bmWidth, bm.bmHeight };  
  
	HDC hDCScreen = GetDC(NULL);  
	HDC hDCMem = CreateCompatibleDC(hDCScreen);  
	HBITMAP hBmpOld = (HBITMAP)SelectObject(hDCMem, hBmp);  
  
	BLENDFUNCTION blend = { 0 };  
	blend.BlendOp = AC_SRC_OVER;  
	blend.SourceConstantAlpha = 255;  
	//blend.AlphaFormat = AC_SRC_OVER;  
	blend.AlphaFormat = AC_SRC_ALPHA;  
  
	RECT rc;  
	GetWindowRect(hWnd, &rc);  
  
	POINT ptSrc = { 0 };  
	POINT ptDest = { rc.left, rc.top };  
	BOOL bRet = UpdateLayeredWindow(  
		hWnd,  
		hDCScreen,  
		&ptDest,  
		&szBmp,  
		hDCMem,  
		&ptSrc,  
		nColor,  
		&blend,  
		//ULW_ALPHA);  
		ULW_COLORKEY);  
  
	SelectObject(hDCMem, hBmpOld);  
	DeleteDC(hDCMem);  
	ReleaseDC(NULL, hDCScreen);  
	return bRet;  
}