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

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  1. POC++/Win32: How to get the alpha channel from an HBITMAP?
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    <p>I have an <code>HBITMAP</code> containing alpha channel data. I can successfully render this using the <code>::AlphaBlend</code> GDI function. </p> <p>However, when I call the <code>::GetPixel</code> GDI function, I never get back values with an alpha component. The documentation does say that it returns the RGB value of the pixel.</p> <p>Is there a way to retrieve the alpha channel values for pixels in an <code>HBITMAP</code>?</p> <p>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.</p> <hr/> <pre><code>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); </code></pre> <hr/> <h2>Answer</h2> <p>Use GetDIBits to retrieve the first (or more) scan line(s) of the image:</p> <pre><code> byte* bits[1000];// = new byte[w * 4]; BITMAPINFO bmi; memset(&amp;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**)&amp;bits, &amp;bmi, DIB_RGB_COLORS); //bits[3] == alpha of topleft pixel; //delete[] bits; </code></pre>
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