I've loaded a CBitmap object from a resource ID, and I'm now wanting to scale it to 50% its size in each dimension. How might I go about this?
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There are 3 answers
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BEST ANSWER
- Select your CBitmap obj into a memDC A (using CDC::SelectObject())
- Create a new CBitmap with desired sized and select it into another MemDC B
- Use CDC::stretchblt(...) to stretch bmp in MemDC A into MemDC B
- Deselect your CBitmap objects (by selecting what was returned from the previous calls to SelectObject)
- Use your new CBitmap
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Here's a worked out implementation of @Smashery's answer.
I use this to scale based on DPI, but it should be easy to adapt to arbitrary scales.
std::shared_ptr<CBitmap> AppHiDpiScaleBitmap (CBitmap &bmp)
{
BITMAP bm = { 0 };
bmp.GetBitmap (&bm);
auto size = CSize (bm.bmWidth, bm.bmHeight);
CWindowDC screenCDC (NULL);
auto dpiX = screenCDC.GetDeviceCaps (LOGPIXELSX);
auto dpiY = screenCDC.GetDeviceCaps (LOGPIXELSY);
auto hiSize = CSize ((dpiX * size.cx) / 96, (dpiY * size.cy) / 96);
std::shared_ptr<CBitmap> res (new CBitmap ());
res->CreateCompatibleBitmap (&screenCDC, hiSize.cx, hiSize.cy);
CDC srcCompatCDC;
srcCompatCDC.CreateCompatibleDC (&screenCDC);
CDC destCompatCDC;
destCompatCDC.CreateCompatibleDC (&screenCDC);
CMemDC srcDC (srcCompatCDC, CRect (CPoint (), size));
auto oldSrcBmp = srcDC.GetDC ().SelectObject (&bmp);
CMemDC destDC (destCompatCDC, CRect(CPoint(), hiSize));
auto oldDestBmp = destDC.GetDC ().SelectObject (res.get());
destDC.GetDC ().StretchBlt (0, 0, hiSize.cx, hiSize.cy, &srcDC.GetDC(), 0, 0, size.cx, size.cy, SRCCOPY);
srcDC.GetDC ().SelectObject (oldSrcBmp);
destDC.GetDC ().SelectObject (oldDestBmp);
return res;
}
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void ResizeBitmap (CBitmap &bmp_src, CBitmap& bmp_dst, int dstW, int dstH)
{
BITMAP bm = { 0 };
bmp_src.GetBitmap (&bm);
auto size = CSize (bm.bmWidth, bm.bmHeight);
CWindowDC wndDC(NULL);
CDC srcDC;
srcDC.CreateCompatibleDC(&wndDC);
auto oldSrcBmp = srcDC.SelectObject(&bmp_src);
CDC destDC;
destDC.CreateCompatibleDC(&wndDC);
bmp_dst.CreateCompatibleBitmap (&wndDC, dstW, dstH);
auto oldDestBmp = destDC.SelectObject (&bmp_dst);
destDC.StretchBlt(0, 0, dstW, dstH, &srcDC, 0, 0, size.cx, size.cy, SRCCOPY);
}