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Asked: 2026-05-10T23:45:05+00:00

I have an HBITMAP containing alpha channel data. I can successfully render this using

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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;
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1 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.

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