rotate transparent bitmap

"[email protected] [wtl]" <[email protected]> 22 Apr 2016 01:21:45 -0700
Newsgroups gmane.comp.windows.wtl
Message-ID <[email protected]>
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<p><span>I made a pic previewer based on CZoomScrollWindowImpl (I modified the sample in message 11073). I even convinced it to paint alpha transparent bitmaps using AlphaBlend. Now I am working on simple clockwise rotation. It works, but not for transparency. Here&#39;s the main idea</span></p><p><span><br></span></p><p><span>1.&nbsp;start with a DIB</span></p><p><span>2. cretate GDI+ bitmap</span></p><p><span>3. rotate using RotateFlip</span></p><p><span>4. retrieve HBITMAP and continue painting with GDI (plain)</span></p><p><span><br></span></p><p><span>HBITMAP hStraight = &lt;something&gt;, bmTurned = 0;</span></p><p><span>Gdiplus::Bitmap gdibm(hStraight, 0);</span></p><p><span>gdibm.RotateFlip(Rotate90FlipNone);</span><span><br>gdibm.GetHBITMAP(Gdiplus::Color::Transparent, &amp;bmTurn
 ed);</span></p><p><span>BitBlt(bmTurned);</span></p><p><br></p><p>that works as expected for plain bitmaps. If I continue cycling the operation, feeding the resulting bitmap back to step 1, it keeps on turning 90 degrees.</p><p><br></p><p>If I try the above steps with a PARGB (premultiplied as needed by AlphaBlend), the transparency is lost. The bitmap turns but instead of transparency I get the black background.</p><p><br></p><p>I found a solution that creates a PixelFormat32bppPARGB gdi+ bitmap in step 2 as follows:</p><p><br></p><p>BITMAP bmpInfo;<br>Gdiplus::Bitmap newBitmap(bmpInfo.bmWidth, bmpInfo.bmHeight, bmpInfo.bmWidthBytes, PixelFormat32bppPARGB, (BYTE*)bmpInfo.bmBits);</p><p><br></p><p>using this instead of the HBITMAP constructor, the rotation takes into account the alpha ch
 annel, and even paints ok, however if I keep on feeding the resulting turned bitmap to the procedure, instead of getting all turn angles (90, 180, 270, 360=0) it gets stuck in the cycle 90-180-90-180... </p><p><br></p><p>The 180 degree turned pic should turn into 270, but instead it goes to 90 degrees (!). Very weird<br></p><p><br></p><p>btw, it doesn&#39;t have to do anything with&nbsp;CZoomScrollWindowImpl, because I used a normal paint DC (not anisotropic) and it still has the same problems</p><p><br></p><p>what can be wrong?</p><p><span><br></span><span><br></span></p><p><span><br></span></p>



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