How do you scale a CBitmap object?

Question

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?

Answer 1

1. Select your CBitmap obj into a memDC A (using CDC::SelectObject())
2. Create a new CBitmap with desired sized and select it into another MemDC B
3. Use CDC::stretchblt(...) to stretch bmp in MemDC A into MemDC B
4. Deselect your CBitmap objects (by selecting what was returned from the previous calls to SelectObject)
5. Use your new CBitmap

Answer 2

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;
}