winapi How to draw opaque text on a transparent window background? - winapi

Question

Using the Windows API, I'm trying to draw opaque text on a semi-transparent background. Using SetLayeredWindowAttributes(hWnd, RGB(0, 0, 0), 128, LWA_ALPHA); and a window style of WS_EX_LAYERED, I've managed to make the entire window semi-transparent, but that also includes the text.
How do I keep the text opaque and the background translucent?

Answer 1

In order to do "proper" alpha in a layered window you need to supply the window manager with a PARGB bitmap by a call to UpdateLayeredWindow.
Try the code below, it works for me.
#include <Windows.h>
#include <stdio.h>
#include <iostream>
#include <ObjIdl.h>
#include <gdiplus.h>
#include <gdiplusheaders.h>
using namespace Gdiplus;
#pragma comment (lib,"Gdiplus.lib")
#define MAX_WIDTH 800
#define MAX_HEIGHT 600
using namespace std;
void Drawtext(HWND hwnd, HDC hdc);
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam) {
if (message == WM_DESTROY) {
PostQuitMessage(0);
}
return DefWindowProc(hwnd, message, wParam, lParam);
};
HINSTANCE hinst;
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevinstance, PSTR szCmdLine, int iCmdShow) {
HWND hWnd;
GdiplusStartupInput gdiplusStartupInput;
ULONG_PTR gdiplusToken;
//Initialize GDI+
GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);
hinst = GetModuleHandle(NULL);
// create a window class:
WNDCLASS wc = {};
wc.lpfnWndProc = WndProc;
wc.hInstance = hinst;
wc.lpszClassName = L"win32";
// register class with operating system:
RegisterClass(&wc);
// create and show window:
hWnd = CreateWindowExW(
WS_EX_LAYERED | WS_EX_TOPMOST,
L"win32",
L"WinSoup",
WS_POPUP,
0, 0, 1000, 500,
nullptr,
nullptr,
hInstance,
nullptr
);
if (hWnd == NULL) {
return 0;
}
Drawtext(hWnd, GetDC(hWnd));
ShowWindow(hWnd, SW_SHOW);
MSG msg = {};
while (GetMessage(&msg, NULL, 0, 0)) {
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
void Drawtext(HWND hwnd, HDC hdc)
{
FontFamily fontFamily(L"Times New Roman");
Font font(&fontFamily, 32, FontStyleRegular, UnitPixel);
PointF pointF(30.0f, 10.0f);
SolidBrush solidBrush(Color(255, 0, 0, 0));
Bitmap softwareBitmap(MAX_WIDTH, MAX_HEIGHT, PixelFormat32bppARGB);
Graphics g(&softwareBitmap);
g.Clear(Gdiplus::Color(30, 0, 0, 0)); // 30: alpha value
g.DrawString(L"Hello Hello Hello Hello Hello Hello Hello Hello", -1, &font, pointF, &solidBrush);
HBITMAP bmp;
softwareBitmap.GetHBITMAP(Color(0, 0, 0, 0), &bmp);
HDC memdc = CreateCompatibleDC(hdc);
HGDIOBJ original = SelectObject(memdc, bmp);
BLENDFUNCTION blend = { 0 };
blend.BlendOp = AC_SRC_OVER;
blend.SourceConstantAlpha = 255;
blend.AlphaFormat = AC_SRC_ALPHA;
POINT ptLocation = { 200, 300 };
SIZE szWnd = { MAX_WIDTH, MAX_HEIGHT };
POINT ptSrc = { 0, 0 };
UpdateLayeredWindow(hwnd, hdc, &ptLocation, &szWnd, memdc, &ptSrc, 0, &blend, ULW_ALPHA);
SelectObject(hdc, original);
DeleteObject(bmp);
DeleteObject(memdc);
}
Debug:
For more details, you can refer to #Koro's answer.