AlphaBlend() Failing with error code 87 Winapi

[Tyler T]

I have a layered window and I want to draw some text with transparency on it. I followed the guide here: https://theartofdev.com/2013/10/24/transparent-text-rendering-with-gdi/ and wrote this code:

	LPCTSTR lpTxt = _T("hello world");
//hdcWnd is the dc of the layered window
//Create temporary dc for the text
	HDC hdcMem = CreateCompatibleDC(hdcWnd);
	SetTextAlign(hdcMem, TA_LEFT | TA_TOP);
//Get the length and width of the text block
	SIZE szTxt;
	GetTextExtentPoint32(hdcMem, lpTxt, lstrlen(lpTxt), &szTxt);
//Create a bitmap
	BITMAPINFOHEADER bih;
	ZeroMemory(&bih, sizeof(BITMAPINFOHEADER));
	bih.biSize = sizeof(BITMAPINFOHEADER);
	bih.biPlanes = 1;
	bih.biWidth = szTxt.cx;
	bih.biHeight = szTxt.cy;
	bih.biBitCount = 24;
	bih.biCompression = BI_RGB;
	bih.biSizeImage = 0;
	bih.biClrImportant = 0;
	bih.biClrUsed = 0;
	BITMAPINFO bi;
	ZeroMemory(&bi, sizeof(BITMAPINFO));
	bi.bmiHeader = bih;
	VOID *dibBitVal;
	HBITMAP hTextBmp = CreateDIBSection(hdcMem, &bi, DIB_RGB_COLORS, &dibBitVal, NULL, NULL);
//Select into temporary dc
	HBITMAP hOldMemBmp = (HBITMAP)SelectObject(hdcMem, hTextBmp);
//hdcImg is the DC i want the text to be drawn to
//Copy background and draw text over it
	BitBlt(hdcMem, 0, 0, szTxt.cx, szTxt.cy, hdcImg, szWnd.cx / 2 - szTxt.cx / 2, 100, SRCCOPY);
	SetBkMode(hdcMem, TRANSPARENT);
	TextOut(hdcMem, 0, 0, lpTxt, lstrlen(lpTxt));
//Call AlphaBlend() to transfer image from temporary dc to target
	BLENDFUNCTION bfAlpha;
	bfAlpha.BlendFlags = 0;
	bfAlpha.BlendOp = AC_SRC_OVER;
	bfAlpha.AlphaFormat = AC_SRC_ALPHA;
	bfAlpha.SourceConstantAlpha = 0xFF;
	AlphaBlend(hdcImg, szWnd.cx / 2 - szTxt.cx / 2, 100, szTxt.cx, szTxt.cy, hdcMem, 0, 0, szTxt.cx, szTxt.cy, bfAlpha);

//Cleanup not shown here 

When I run it, nothing shows up. Further debugging shows that AlphaBlend() failed with the error code 87 (ERR_INVALID_PARAMETER). All the previous function call have succeeded. Can anyone tell me what's wrong with my code? Thanks in advance.

[Tyler T]

Switched to BitBlt() instead of AlphaBlend() to see if the bitmap was working. The bitmap was completely fine. Here's the line for BitBlt():

BitBlt(hdcImg, szWnd.cx / 2 - szTxt.cx / 2, 100, szTxt.cx, szTxt.cy, hdcMem, 0, 0, SRCCOPY);

The exact same parameters are used for this as they are for AlphaBlend(), except for my BLENDFUNCTION, but I'm pretty sure my BLENDFUNCTION is fine as I've seen examples online that worked using the exact same BLENDFUNCTION.

[Tyler T]

Played around with the parameters... When I set the AlphaFormat of the BLENDFUNCTION to 0 it seems to solve the problem. Still have no idea why though. Can someone explain?

[coder777]

See this:

https://msdn.microsoft.com/en-us/library/windows/desktop/dd183393(v=vs.85).aspx

particularly:

When the AlphaFormat member is AC_SRC_ALPHA, the source bitmap must be 32 bpp. If it is not, the AlphaBlend function will fail.

What effect are you trying to achieve?