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POHow to draw 32-bit alpha channel bitmaps?
 

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  1. POHow to draw 32-bit alpha channel bitmaps?
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    <p>I need to create a custom control to display bmp images with alpha channel. The background can be painted in different colors and the images have shadows so I need to truly "paint" the alpha channel.</p> <p>Does anybody know how to do it?</p> <p>I also want if possible to create a mask using the alpha channel information to know whether the mouse has been click on the image or on the transparent area.</p> <p>Any kind of help will be appreciated!</p> <p>Thanks.</p> <p>Edited(JDePedro): As some of you have suggested I've been trying to use alpha blend to paint the bitmap with alpha channel. This just a test I've implemented where I load a 32-bit bitmap from resources and I try to paint it using AlphaBlend function:</p> <pre><code>void CAlphaDlg::OnPaint() { CClientDC dc(this); CDC dcMem; dcMem.CreateCompatibleDC(&amp;dc); CBitmap bitmap; bitmap.LoadBitmap(IDB_BITMAP); BITMAP BitMap; bitmap.GetBitmap(&amp;BitMap); int nWidth = BitMap.bmWidth; int nHeight = BitMap.bmHeight; CBitmap *pOldBitmap = dcMem.SelectObject(&amp;bitmap); BLENDFUNCTION m_bf; m_bf.BlendOp = AC_SRC_OVER; m_bf.BlendFlags = 0; m_bf.SourceConstantAlpha = 255; m_bf.AlphaFormat = AC_SRC_ALPHA; AlphaBlend(dc.GetSafeHdc(), 100, 100, nWidth, nHeight, dcMem.GetSafeHdc(), 0, 0,nWidth, nHeight,m_bf); dcMem.SelectObject(pOldBitmap); CDialog::OnPaint(); } </code></pre> <p>This is just a test so I put the code in the OnPaint of the dialog (I also tried the AlphaBlend function of the CDC object).</p> <p>The non-transparent areas are being painted correctly but I get white where the bitmap should be transparent.</p> <p>Any help???</p> <p>This is a screenshot..it's not easy to see but there is a white rectangle around the blue circle: <a href="http://img385.imageshack.us/img385/7965/alphamh8.png">alt text http://img385.imageshack.us/img385/7965/alphamh8.png</a></p> <p>Ok. I got it! I have to pre-multiply every pixel for the alpha value. Someone can suggest the optimized way to do that?</p>
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      1. POHow to draw 32-bit alpha channel bitmaps?
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