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?

score 6 · Accepted Answer · edited May 07 '10 at 00:21

6

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

edited May 07 '10 at 00:21

Smashery

57,848
30
97
128

answered May 05 '10 at 05:42

SysAdmin

5,455
8
33
34

1

I'm not really certain about how to use all of those functions; in which order, using which version of the function, and how a CDC becomes usable for a CBitmap. Could you please provide some code? – Smashery May 06 '10 at 00:58
And which of the Raster Operation codes to use? http://msdn.microsoft.com/en-us/library/dd183370(v=VS.85).aspx – Smashery May 06 '10 at 06:57
@Smashery - SRCCOPY would do fine – SysAdmin May 06 '10 at 07:27
The problem was with Deselecting the CBitmap object. I've edited your answer to note this. – Smashery May 07 '10 at 00:23

Frank Krueger · Answer 2 · 2016-06-23T20:18:37.247

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;
}

score 1 · Answer 3 · answered Apr 16 '20 at 12:07

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);
}

How do you scale a CBitmap object?

3 Answers3