C++/Win32: How to get the alpha channel from an HBITMAP?

Question

I have an HBITMAP containing alpha channel data. I can successfully render this using the ::AlphaBlend GDI function.

However, when I call the ::GetPixel GDI function, I never get back values with an alpha component. The documentation does say that it returns the RGB value of the pixel.

Is there a way to retrieve the alpha channel values for pixels in an HBITMAP?

I want to be able to detect when to use ::AlphaBlend, and when to use an old-school method for treating a particular colour in the source HBITMAP as transparent.

---

HDC sourceHdc = ::CreateCompatibleDC(hdcDraw);
::SelectObject(sourceHdc, m_hbmp);

// This pixel has partial transparency, but ::GetPixel returns just RGB.
COLORREF c = ::GetPixel(sourceHdc, 20, 20);

// Draw the bitmap to hdcDraw
BLENDFUNCTION bf1;
bf1.BlendOp = AC_SRC_OVER;
bf1.BlendFlags = 0;
bf1.SourceConstantAlpha = 0xff;  
bf1.AlphaFormat = AC_SRC_ALPHA;            
::AlphaBlend(di.hdcDraw, x, 10, 64, 64, sourceHdc, 0, 0, 64, 64, bf1);

::DeleteDC(sourceHdc);

---

Answer

Use GetDIBits to retrieve the first (or more) scan line(s) of the image:

  byte* bits[1000];// = new byte[w * 4]; 
  BITMAPINFO bmi;
  memset(&bmi, 0, sizeof(BITMAPINFO)); 
  bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
  bmi.bmiHeader.biWidth = w;
  bmi.bmiHeader.biHeight = -h;
  bmi.bmiHeader.biBitCount = 32;
  bmi.bmiHeader.biPlanes = 1;
  bmi.bmiHeader.biCompression = BI_RGB;
  bmi.bmiHeader.biSizeImage     = 0;
  bmi.bmiHeader.biXPelsPerMeter = 0;
  bmi.bmiHeader.biYPelsPerMeter = 0;
  bmi.bmiHeader.biClrUsed       = 0;
  bmi.bmiHeader.biClrImportant  = 0;
  int rv = ::GetDIBits(sourceHdc1, m_hbmp, 0, 1, (void**)&bits, &bmi, DIB_RGB_COLORS);

  //bits[3] == alpha of topleft pixel;

  //delete[] bits;

Answer 1

Use GetDIBits. That way you get an array of RGBQUAD's which have as you can probably guess an alpha channel next to the R, G and B components.